Lecture Notes on Volcanic Hazards PDF

Table of Contents {#table-of-contents.TOCHeading} ================= [V1.1 Magma properties 1](#v1.1-magma-properties) [Magma properties 2](#magma-properties) [Eruption style 3](#eruption-style) [Size of eruption 4](#size-of-eruption) [Magma 5](#magma) [V1.3 Effusive eruptions 7](#v1.3-effusive...

Table of Contents {#table-of-contents.TOCHeading} ================= [V1.1 Magma properties 1](#v1.1-magma-properties) [Magma properties 2](#magma-properties) [Eruption style 3](#eruption-style) [Size of eruption 4](#size-of-eruption) [Magma 5](#magma) [V1.3 Effusive eruptions 7](#v1.3-effusive-eruptions) [Lava flows on Hawaii 8](#lava-flows-on-hawaii) [Mount Etna 8](#mount-etna) [Lava tubes and levees 8](#lava-tubes-and-levees) [Mitigation 9](#mitigation) [Lava domes 9](#lava-domes) [V2.1 Explosive eruptions 9](#v2.1-explosive-eruptions) [Explosive eruptions 9](#explosive-eruptions) [Sizes 9](#sizes) [Dynamics 10](#dynamics) [Strombolian 11](#strombolian) [Phreato 11](#phreato) [Factors effect explosivity 13](#factors-effect-explosivity) [V2.2 PDCs and lahars 13](#v2.2-pdcs-and-lahars) [Pyroclastic Density currents 13](#pyroclastic-density-currents) [Formation 14](#formation) [Deposition 14](#deposition) [PDC classification 15](#pdc-classification) [Small volume eruption PDCs 16](#small-volume-eruption-pdcs) [Dome-collapse or dome-explosion driven 16](#dome-collapse-or-dome-explosion-driven) [Volcanic eruptions and water 16](#volcanic-eruptions-and-water) [Eruption styles 17](#eruption-styles) [Phreatic 18](#phreatic) [V3.1 Volcano monitoring 18](#v3.1-volcano-monitoring) [Monitoring 18](#monitoring) [Volcano seismicity 18](#volcano-seismicity) [Infrasound 19](#infrasound) [Volcanic gas emissions 19](#volcanic-gas-emissions) [Ground based 19](#ground-based) [Satellite based 20](#satellite-based) [Summary 20](#summary) [Geological past 20](#geological-past) [V3.3 Hazard mitigation 21](#v3.3-hazard-mitigation) [Communicating risk 22](#communicating-risk) [Controls of volcanic flows 23](#controls-of-volcanic-flows) [Summary 23](#summary-1) [Eruption and impact 24](#eruption-and-impact) [Landslides 1 24](#landslides-1) [Mass movement 25](#mass-movement) [Classification 26](#classification) [Rockfall 27](#rockfall) [Toppling 27](#toppling) [Slide 28](#slide) [Flows 29](#flows) [Slope deformation 30](#slope-deformation) [Landslides in the landscape 31](#landslides-in-the-landscape) [Landslides 2 32](#landslides-2) [Causes of landslides 32](#causes-of-landslides) [Equation 33](#equation) [Slope increase mechanisms 34](#slope-increase-mechanisms) [Resisting forces 34](#resisting-forces) [Role of water 35](#role-of-water) [Equation For Factor of safety 36](#equation-for-factor-of-safety) [Characteristics of landslides 36](#characteristics-of-landslides) [Landslide volumes and runout distance 37](#landslide-volumes-and-runout-distance) [Landslide transitions 37](#landslide-transitions) [Landslides 3 38](#landslides-3) [Landslide causes and landslide hazards 38](#landslide-causes-and-landslide-hazards) [Landslides 4 40](#landslides-4) [Magnitude frequency 40](#magnitude-frequency) [Landslide intensity 42](#landslide-intensity) [Landslide magnitude frequency 43](#landslide-magnitude-frequency) [Real data 44](#real-data) [Limitations 45](#limitations) [1.3 Flooding intro 45](#flooding-intro) [Flooding hazards 45](#flooding-hazards) [Flood intensifying factors 49](#flood-intensifying-factors) [Properties of rainfall events 49](#properties-of-rainfall-events) [Summary 50](#summary-2) [2.1 Quantifying floods 50](#quantifying-floods) [Flood Hydrograph 50](#flood-hydrograph) [Controls of peak magnitude 51](#controls-of-peak-magnitude) [Controls of time to peak 51](#controls-of-time-to-peak) [Spatial patterns 51](#spatial-patterns) [Statistical analysis of flood flows 51](#statistical-analysis-of-flood-flows) [Uses of flow duration curves 51](#uses-of-flow-duration-curves) [Modelling floods 51](#modelling-floods) [Challenges 52](#challenges) [2.2 Flood mitigation 52](#flood-mitigation) [The lower Catchment -- Defence 52](#the-lower-catchment-defence) [Context 53](#context) [Legislation 53](#legislation) [The Mid Catchment -- Storage 53](#the-mid-catchment-storage) [The Upper Catchment -- Sponge 53](#the-upper-catchment-sponge) [Balancing interventions 54](#balancing-interventions) [Lecture 15 54](#_Toc187425242) [4 phases of the earthquake 55](#phases-of-the-earthquake) [Detection 56](#detection) [Locating earthquakes 56](#locating-earthquakes) [Fault types 57](#fault-types) [Lecture 16 59](#_Toc187425247) [Aftershocks 60](#aftershocks) [Foreshocks 60](#foreshocks) [Nuclear explosions 61](#nuclear-explosions) [Body wave scale and surface wave 62](#body-wave-scale-and-surface-wave) [Magnitude and tectonic setting 63](#magnitude-and-tectonic-setting) [Lecture 17 64](#_Toc187425253) [Lecture 18 66](#_Toc187425254) [Lecture 16 67](#_Toc187425255) [Tsunami Hazards 67](#_Toc187425256) [Tsunami theory 68](#tsunami-theory) [Mitigation 69](#mitigation-1) [Direct warning/monitoring 69](#direct-warningmonitoring) [Planning 69](#planning) [Public education. Awareness 69](#public-education.-awareness) [Learning from past events 70](#learning-from-past-events) [Direct mitigation 70](#direct-mitigation) [Lecture 17 70](#_Toc187425264) V1.1 Magma properties ===================== - Volcanic hazards -- nature, processes and impacts - Hazard -- processes that can cause harm (floods, volcanoes) - Vulnerability -- engineering, economic, - Exposure -- people, buildings - Risk describes the interaction between the hazard and human populations, dependent on vulnerability/exposure http://image.slidesharecdn.com/platetectonics-introduction-100216062832-phpapp01/95/plate-tectonics-an-introduction-12-728.jpg?cb=1266301899  Magma properties ---------------- **Volcanism** -- the manifestation at a planetary surface of internal thermal processes through the emission of solid. Liquid or gaseous products. Magma is produced from the melting of the earth's mantle -- through temperature, pressure and fluids added to the mantle. - **Subduction** generates volcanism -- volcanic arcs (90%), water causes melting of peridotite at a lower temp (Flux melting). Wide composition ranges evolved, volatile rich magmas (andesites to rhyolites) http://www.columbia.edu/%7Evjd1/subd\_zone.gif - **Rift** -- Iceland, decompression melting  - **Intraplate** -- hot spots **Magma generation** -- partial melting of mantle peridotite, initial melts are basic 50% silica How magma erupts -- Polygenetic and monogenetic **Polygenetic:** large volcanic landforms through very large numbers of eruptions. **Monogenetic:** Vents may be more distributed, forming volcanic fields with multiple small volcanic constructions**.** Not always eruptive -- properties need to be specific -- volatile, chemical comp, viscosity, temperature, crystallinity. A typical **polygenetic volcano** in a volcanic arc (which may be referred to as stratovolcanoes or composite volcanoes) may have: - lifetime \>100 kyr - volume \>100 km^3^ - output flux 0.1-1 km^3^ of magma per kyr **Monogenetic example** -- Large volcanic field in a continental arc ,Active for \>100kyr. Hundreds of vents, active in single eruptions and forming monogenetic cones Eruption style -------------- - Lava flow -- Iceland - Lava dome -- Montserat - Ash plumes -- powerfull into the atmosphere - Cinder cones -- not powerfull enough so falls and forms cones. These styles can change over time  Size of eruption ---------------- Varies by many orders of magnitude -- very large spectrum Measured with the VEI (Volcanic explosivity index) VEIfigure Frequency varies as well  All volcanoes will vary in eruption style over time, so cannot predict the next eruption style. Magma ----- A silicate melt -- a polymerised mixture of silicon and oxygen atoms -- with variable proportions of other ions - Varied cations (e.g. Ca, Na, K, Fe, Mg, Al...) - Volatiles (e.g. water, CO2, halogens...) The chemistry, conditions (T, P) and physical characteristics (bubble content, crystallinity) of magma control its properties, especially its viscosity, influencing eruptive behaviour Viscosity is related to polymerisation of contents Magma is a mix of solid crystals, rock fragments and gas. **Magma ascent** -- needs overpressure to break the overlying rock, forming feeder dykes Because crustal magma bodies do not generally have an open link to the surface, continued input of fresh, deeper magma, and the exsolution of volatiles, causes overpressure to build in the system - If this overpressure exceeds the strength of the surrounding rock, a propagating fracture will form, along which magma can ascend. This is a necessary condition for eruption - This magma-filled crack, or dyke, allows magma to ascend upwards driven by the overpressure in the system. If this reaches the surface, eruption will occur **Viscosity** - A measure of a fluid's resistance to flow (it relates the shear stress in a fluid to the rate of strain) - Strongly dependent on P, T and chemical composition More polymerisation more viscous Role of Composition: - Basaltic magma (low in silica, \~50%) has fewer polymerized structures, leading to lower viscosity. - Rhyolitic magma (high in silica, \~70%) has a high degree of polymerization, with extensive silicate networks, leading to much higher viscosity.  **Factors Influencing Polymerization:** - **Silica Content:** Higher silica content increases polymerization because there are more tetrahedra available to form bonds. - **Water Content:** Water breaks bonds in silicate networks, reducing polymerization and viscosity. - http://www.alexstrekeisen.it/immagini/diagrammi/volcanicindex%282%29.jpg - **Temperature:** Higher temperatures reduce polymerization by breaking silicate bonds, lowering viscosity. - **Volatile Loss:** As volatiles (e.g., water, CO2​) escape, polymerization increases, raising viscosity. V1.3 Effusive eruptions ======================= Effusive -- lava at the surface (Montserrat + Kilauea) Montserrat -- thick lava dome forming eruption (silica rich magma) Kilauea -- Runny lava forming lava flows (silica poor magma) Effusive -- magma reach the surface as a liquid mass - **Lava** -- magma reaching the Earth's surface, in liquid form - **Tephra** -- the **fragmental** material generated by the break-up of magma during **explosive** eruptions Low viscosity magma -- this occurs if bubbles can escape and/or if there's insufficient gas to drive explosive fragmentation of the magma\ High viscosity magma -- this occurs if there is gas loss or low gas content (and slow ascent) Tephra -- fragmental material Lava flow fields -- form from one flow stopping and a new channel breaking out Lava flows on Hawaii -------------------- Viscosity determines the lava flow speed, thickness and surface morphology Pahoehoe -- ropy lava flow less viscous Aa -- rubbly lava flow more viscous Lava flows build up in pulses forming a thick layer over time, fluid sheet-like flows, flow speeds of 10 to 100m per hour. Does not pose much danger to human life. Long fissure but one point where magma breaks to the surface They have high temperatures which makes them low in viscosity Mount Etna ---------- Thicker viscous lava -- more crystalline and cooler forming blocky and rubbly front Speed -- 2.5 m per hour Lava tubes and levees ---------------------  Levees crust over and form lava tubes Mitigation ---------- Diverting lava flows -- cooling and blocking the flow, or using explosives to dig tunnels to guide it Plan a response as its slow moving Can bomb to disrupt the rate of cooling but not very successful Lava flows can be very rapid moving and will be fatal ( 10m/s) Lava domes ---------- Slow, viscous extrusion A pile of lava with a crusted surface, extruding from within (endogenous growth) Caused from degassing/ crystallisation during ascent of more silica rich magmas Main hazard of the dome - Lava domes are highly unstable structures - Major hazard is from dome collapse and associated landslides or pyroclastic flows (see later lectures) - Associated explosions are also common (pressurisation leading to short but powerful Vulcanian explosions) - Principal impact is on local populations -- unpredictability of eruption end-date and of the timing and magnitude of associated hazards Domes take a long time to grow as the lava is moving so slow Short lived explosions due to increased pressurisation (Vucanian explosions) Lava dome can grow solid lava spines -- exogenous growth V2.1 Explosive eruptions ======================== Explosive eruptions ------------------- Factors -- Volatile content, magma viscosity, mass eruption rate and ascent rate, presence of external water. Strombolian explosive style -- smaller fragments falling close to the eruption, Phreato Plinian explosive style -- Larger eruption sending fine particles into the atmosphere. Tephra -- the fragmental material generated by the break-up of magma during explosive eruptions - Tephra may include lithic clasts, pyroclastic (juvenile) material, crystals. - The grain-size of this material includes lapilli (2 -- 64 mm) and ash (\100 m/s 2. The gas/pyroclast mixture enters the atmosphere as a momentum-dominated jet 3. Subsequent interaction with the atmosphere determines the eruption style:\ If the material mixes efficiently with the air: convection and buoyant ascent of a plume leads to tephra fallout\ If mixing is insufficient, plume density remains \> atmospheric density: material falls back to the surface, generating pyroclastic density current As the plume goes into the atmosphere it gets less dense and then it spreads out when it gets to natural density. http://www.geo.mtu.edu/%7Eraman/Ashfall/Syllabus/Entries/2009/6/21\_Eruption\_Columns\_files/droppedImage.png Fall deposits are well sorted depending on wind direction and proximity to vent. Larger grain size near the vent. Ash will have the walls of the bubbles in their shape 1. Turbulent air entrainment decelerates the mixture as momentum is transferred to air 2. The air is heated and expands (transferring energy from the hot jet to the mixing air) -- expansion decreases the density of the mixture 3. If the density falls below that of the ambient atmosphere, the plume ascends through buoyancy -- this is the condition needed for plume ascent to high levels, and for widespread tephra fallout 4. Initial kinetic energy allows ascent to \~10^2-3^ m. 5. **Buoyancy is critical for higher ascent** (*sub-Plinian/Plinian eruptions*) 6. Heights of 10-30 km are typical 7. With **less air entrainment, the density remains \> atmosphere**, so material falls back to the surface (forming PDCs) 8. Additionally, larger particles may separate, forming PDCs; the remainder forming a buoyant plume -- so both processes can be seen in the same eruption, or behaviour can transition between them Sulphate aerosol -- of key importance for short term climatic consequences  Factors effect explosivity -------------------------- **Factor** **Impact on Explosivity** **Explanation** -------------------------------- ---------------------------------------------------------- --------------------------------------------------------------------------------------------------------------------------------------------------------------- **Magma Viscosity** **High viscosity → more explosive** High-viscosity magmas (rhyolitic/andesitic) trap gas, leading to pressure buildup. Low-viscosity magmas (basaltic) allow gas to escape, reducing explosivity. **Gas Content (Volatiles)** **High gas content → more explosive** High volatile concentrations (e.g., H₂O, CO₂, SO₂) lead to bubble nucleation and rapid expansion, driving explosions. **Bubble Formation & Growth** **Rapid growth and coalescence → more explosive** Bubbles that coalesce and grow rapidly increase internal pressure, fragmenting magma violently. **Magma Ascent Rate** **Faster ascent → more explosive** Rapid ascent reduces degassing time, trapping volatiles and increasing pressure at shallower depths. **Conduit Geometry** **Narrow conduits → more explosive** Narrow or constricted conduits amplify pressure buildup and impede gas escape, enhancing explosivity. **Crystallinity** **Higher crystallinity → more explosive** Crystals increase magma rigidity and enhance gas trapping, promoting fragmentation during ascent. **External Water Interaction** **Water interaction (phreatomagmatic) → more explosive** External water rapidly converts to steam, expanding violently and fragmenting magma (e.g., hydrovolcanic eruptions). **Pre-eruption Degassing** **More degassing → less explosive** Magmas that lose significant volatiles during ascent (e.g., through fractures) have reduced explosive potential. **Pressure at Depth** **Higher confining pressure → less explosive** High pressures at depth keep gases dissolved in magma, reducing explosivity until the magma nears the surface. **Temperature** **Higher temperature → less explosive** High-temperature magmas are less viscous, allowing gas to escape more easily, reducing explosive potential. **Magma Composition** **Silicic (high silica) magmas → more explosive** Silica-rich magmas (rhyolite) are more viscous and trap more gas, whereas basaltic magmas are less explosive. **Tectonic Setting** **Subduction zones → more explosive** Subduction zones produce silica-rich, gas-rich magmas, whereas mid-ocean ridges or hotspots typically generate basaltic magmas. **Gas Slug Formation** **Large gas slugs → more explosive** Formation of large, cohesive gas slugs in low-viscosity magmas can lead to Strombolian-style explosions. V2.2 PDCs and lahars ==================== Pyroclastic Density currents ---------------------------- - Hot, gravity-driven mixtures of gas and solid fragments - Inundating large areas, but with some topographic control - High-velocity (100km/hr +) and high mobility Will flow down slope until bedload is deposited, at this point remaining material will become buoyant. - Visible part of PDC is this low-concentration ash cloud, above the dense base - With sufficient air entrainment and heating, this may generate a buoyant co-ignimbrite plume and associated fall deposit Formation --------- - Buoyant column collapses or some component collapses - Buoyancy is not achieved but gas charged magma boils over - Caldera collapse - Sudden depressurization from lateral blast -- s hellens - Gravitational collapse -- lava dome - Unstable near-vent deposit collapse Deposition ---------- Largest volume explosive eruptions Rich in pumice with high ash content -- called ignimbrites  - Thick and internally complex, relative to fall deposits - Smaller area affected, relative to fall deposits (but this area can still be large, for very large eruptions) - Potential for a complex array of depositional and erosive processes (and hence bedforms and internal structures) within a single PDC deposit - Range of clast types - Diverse appearance -- from lithic block-rich to ash-dominated PDC classification ------------------ Based on particle concentration, velocity and turbulence 1. Coarse, dense, flow 2. Lower density, fines-rich turbulent flow (surge) In reality these are transitioned between PDCs have a particle concentration gradient -- dense at base They have multiple flow units, with cross cutting -- this makes them very complex Large ignimbrite-forming eruptions are associated with caldera formation Small volume eruption PDCs -------------------------- More diverse and common - Explosive column collapse/fountaining (i.e. when a buoyant plume isn't achieved) - **Phreatomagmatic** eruptions -- interaction with groundwater or surface water commonly produces high levels of fragmentation and surge-like PDC generation - Dome-collapse or dome-explosion driven PDCs - rich in dense lava blocks (block and ash flows) - Lateral blast associated PDC ### Dome-collapse or dome-explosion driven More common than the ignimbrite-forming PDCs Much smaller volume events, but a major hazard at many volcanoes Still have potential for extreme destruction Volcanic eruptions and water ---------------------------- Phreatomagmatic -- integration of magma and water - Involve water from lakes, seas, snow, ice, groundwater, near-surface rock or sediments or geothermal systems. - Heterogenous mixtures: hot and cold mixing in highly explosive eruptions with high magma fragmentation. Evidence from wide surface craters (maars). Phreatomagmatic PDCs - High fragmentation -- rich in fine ash, and local country rock - May occur in monogenetic eruptions -- pulsatory style - The highly explosive but relatively dense mixture collapses back to surface and spreads radially around vent (surge-type deposits), producing ring-like deposits around wide crater ### Eruption styles  Water increases fragmentation and explosivity Lahars -- mobilisation of erupted products by water - Slurries of water and volcanic sediment - Require water and volcanic debris Involve liquid and solid interactions so distinct from avalanches - Very high mobility due to water content - Highly destructive and erosive due to concentration of solid particles - Sed. Load in hyper concentrated flow 6-75 wt%; 75-90 wt% in debris flow - Densities reach 2000 kg/m3, can transport very large clasts Causes - Ice/ snow melt - Lake breakout - Stream interaction - Landslide into lakes - Heavy rain Deposition can form deltas offshore rapidly extending coastline, then can be eroded again transporting offshore. Phreatic -------- Explosions driven by hot fluids above a magmatic system. No juvenile magma erupted at surface (i.e. tephra composed of altered lithic grains) Small but highly explosive and unpredictable V3.1 Volcano monitoring ======================= ***Risk reduction (mitigation) requires:*** - *Detailed understanding of the **hazard** -- probability of the event (e.g. **frequency**, **magnitude**) and potential **processes/impacts*** - *In light of this, steps to **limit vulnerability*** *Improved **forecasting capability** and better constrained **hazard probability*** Long-term - Past history of behaviour (i.e. of fault, volcano\...) Detailed constraints on style/magnitude (impact) Short term - Monitoring of system Frequency vs Magnitude has to be considered, for example yellow stone is high magnitude but low frequency. Subduction zones cause large extending areas of volcanoes lots to monitor Fatality incidents show a trend -- individual events dominate the record, mainly due to increased vulnerabilities. Monitoring ---------- Ascent of magma towards the surface is a potential precursor to eruption. Approaches: - Seismicity -- Volcano tectonic - Deformation - Gas emissions -- chemical or physical changes - Ground based -- infrasound, thermal, ash cloud, surface mapping Monitoring is important before and after eruption as well as during depending on the hazards. ### Volcano seismicity Network of seismometers collecting lots of data Lots of signals are detected -- number of events and changes in types but not if they are volcanic Don't always mean a volcanic earthquake High frequency -- Volcano tectonic earthquake, brittle failure due to magma movements Low frequency -- long period earthquakes, pressure changes in fluid filled cracks Tremor -- more continuous Infrasound ---------- Low frequency of acoustic waves in the atmosphere Explosions can be picked up and the data analysed Volcanic gas emissions ---------------------- Changes in gas output can signal fresh magma input and maybe a direct hazard. They can be continuous and be lethal in short-term exposure Monitored by direct samples, continuous or crater lake and soil chemistry changes Ground based gas monitoring -- Tells us the type and concentration of gases released. Diagram of a volcano and a car Description automatically generated with medium confidence SO2 can be measured and used as proxy for eruptions. Ground based ------------ Deformation -- small changes in the shape of the ground surface can record magma movement or pressurisation in the subsurface. EDM -- electronic distance measurement, repeated measurements of 2 fixed points, indicates horizontal movement. Levelling surveys -- show changes in relative angle and thus vertical displacement Tilt meters Satellite based ---------------  Interferometric synthetic aperture radar Measures the change in ground surface and represents it as colours This shows you the movement over the whole ground surface but not showing the detail of when Gps requires individual stations that are measured Summary ------- Most powerful when all monitoring methods are used together More active volcanoes are going to be prioritized Geological past --------------- Study ancient past events recorded in the rock, that can reconstruct what happened, Frequency (but preservation is often limited to larger explosive eruptions)\ Magnitude/style\ Spatial impact\ Recent historical events, if documented, are likely to give more accurate constraints An understanding of potential styles of activity -- this draws heavily on knowledge of past behaviour (i.e. previously observed eruptions; the geological record), and is volcano-specific V3.3 Hazard mitigation ====================== A close-up of a pie chart Description automatically generated Lots of mitigation plans have been missed or too late even with monitoring. However, they might not be problematic due to being isolated location of the volcano. Blue section can cause a lack of trust due to evacuation when nothing happened. Quantifying risks remains challenging - Subjectivity or simplification - Probabilistic methods  Vulnerability -- a social factor Communicating risk ------------------ Hazard maps -- based on models, historical events, and pre-historical studies. Maps can be specific to certain hazards Can inform evacuation plans and communicate decisions Have to make decisions quick and objectively 2D maps were not very well understood better are 3D maps as they can be clearer due to topography being such an important factor. Inconsistent communication, lack of clarity on uncertainty or poorly explained decision making can damage trust and affect community response in subsequent alerts. False alarms are very bad. Communication during eruption is also very important Effective for lahars/PDC hazards. Controls of volcanic flows -------------------------- Sabo dams and channel clearing measures are widely used on volcanoes subject to lahars  Summary ------- - Assessment of risk combines the probability of a specific event (hazard) with the vulnerability - Geologists can provide both long- and short-term information relating to hazard -- but uncertainties will always remain - Volcanic events can involve multiple hazard types, which are temporally and spatially variable - This presents major difficulties for hazard assessment and mitigation, and may require rapid decision making - Although monitoring may identify eruption, forecasting the nature of an event, its duration and impact remains challenging - Large-magnitude events do not always correlate with high (human) impact (although the very largest events present severe mitigation challenges). - For decision makers, issues of warning, planning and communication, with a clear and transparent use of relevant data, are key to effective mitigation Eruption and impact ------------------- Global catastrophic risk from lower magnitude volcanic eruptions \| Nature Communications Landslides 1 ============ Definition -- Landslide is a non-specific term covering a wide range of processes that result in downward and outward movement of slope forming materials. Variables: - Driving force -- Gravity! - Gradient -- material dependent - Material -- Sediments, rocks - Size - Type of movement - Velocity -- fast failure more danger Important to study -- kills 14000 people/year, 6 billion euro a year of damage.  Destruction of properties Loss of agricultural land\# Loss of precious historical landmarks Threat to popular coastal paths Threat to key infrastructure Mass movement ------------- Mas wasting is a natural phenomenon by which rock, soil and or debris move downwards due to the action of gravity. Classification --------------  A diagram of a block flexural and a block flexural Description automatically generated ### Rockfall Detachment, fall, rolling and bouncing of rock fragments. Dynamic interactions between fragments Extremely rapid -- very dangerous Geomorphic settings -- steep, narrow gorges, steep valley flanks, upper reaches cliffs. ### Toppling Forward rotation and overturning of rock columns or plates Slow or rapid  - Movement may begin slowly and evolve to extremely rapid - Can occur at small scales (rock fall) - Can occur at large scales (within large and deep rock slides) - Requires a fractured or intensely foliated rock mass ### Slide  Irregular -- surface that generates a separation, connecting of preexisting joints via rock bridges. A diagram of a rock bridge Description automatically generated #### Slides in soil **Rotational** - Common in saturated soil with low permeability (clay and silt). - From slow to rapid depending on water content. **Planar** - Require a weak layer or a discontinuity inclined at an angle exceeding the friction angle. - Often occur where a veneer of colluvium, weathered soil, pyroclastic deposit slide over a strong substrate. - Such veneers are common in mountainous regions across the world. - In coarse material (gravel, sand) often occur on slopes between 30°- 60°. - Often occur where seepage (water) accumulates (concave slopes). - Often rapid or extremely rapid. ### Flows #### Rock avalanches - Very rapid and very large - Develop from rock slides or large rock slides - Can travel upwards on slopes - Often large (typically \>1 million m3). - Develop from rock slides or large rockfalls. - Rapid disintegration of rock mass during downslope motion. - Extremely rapid (20 to 130 m/s) with long runouts (up to tens of km) - Bulk of the rock avalanche is dry, however. - The high mobility may be explained by a cushion of saturated material entrained from the flow path and liquified under the weight of the rock debris (high pore pressures). #### Debris flows Very rapid to extremely rapid surging flow of saturated debris in a steep channel. Strong entrainment of material and water from the flow path. - Widespread in mountain terrain - Occur periodically in gullies and channels - Very erosive - Can mobilise large bounders - Deposit material on debris fans - Can mobilise large bounders #### Debris avalanches Very rapid to extremely rapid shallow flow of partially or fully saturated debris on a steep slope, without confinement in an established channel. Occurs at all scales. - - - - - ### Slope deformation #### Mountain slope deformation Large-scale gravitational deformation of steep, high mountain slopes, manifested by scarps, benches, cracks, trenches and bulges, but lacking a fully defined rupture surface. Extremely slow or unmeasurable movement rates. Extremely slow or unmeasurable movement rates mm/y High deformation Very deep in the surface of rupture Landslides in the landscape --------------------------- \` Landslides 2 ============ Causes of landslides -------------------- Always opposing forces- driving vs resting Destabilizing stresses are present within all slopes.\ Whether or not these stresses (driving stresses) are capable of triggering failure of a given\ slope at a given moment in time will depend on the relative magnitude of the stresses that\ resist the tendency for failure; these opposing stresses can be referred to as resisting\ stresses ### Equation  Factor of safety   By increasing the slope angle there is an increase in shear stress ### Slope increase mechanisms Glacier carving valleys and subsequent retreat increase the steepness of slopes, river erosion also does this. Human activity also impacts this slope angle when building houses ### Resisting forces Geologic materials have cohesion: - Roots binding regolith - Cementation of regolith - Electrostatic fooces from clay Friction - Grain size, shape - Angle of internal friction - - Rock needs fracturing to be factored in ### Role of water Water in-between the grains create fluid pressure, this pushes the grains out driving sliding More water increased risk of sliding  Water weakens slopes, Equation For Factor of safety ----------------------------- Characteristics of landslides ----------------------------- Landslide velocities  Landslide volumes and runout distance ------------------------------------- The mobility of a mass movement can be characterised in terms of if efficiency H/L, If H is bigger than L smaller landslides Efficiency Water-saturated vs. drier masses\ Larger volume flows\ Channelised flows\ Bulking (volume increase, via basal erosion and mass incorporation) ### Landslide transitions High rainfall in a few days, increasing pore pressure increasing shear stress due to increased buoyancy in the grains. Started as a slide but turned into debris flow due to a constriction in the path. A close-up of a mountain Description automatically generated Landslides 3 ============ Landslide causes and landslide hazards --------------------------------------  Preparatory and triggering factors are related A diagram of a earthquake Description automatically generated - 1 is the turning point where the slope fails - Rainfall -- water increases pore pressure but it recovers - Erosion at toe -- is a permanent change in slope safety so no recovery. - Earthquake and rain -- continue to reduce stability until it is triggered. They can cause regional scale landslides 25k in total Rainfall landslides are more channel based and shallow and fast moving. Early warning systems based on rainfall thresholds (super intense over a short period of time or long periods but less intense) Probabilistic landslide hazard assessment  Zonation of hazard risks: Divison of land in homogeneous areas or domains and ranking of these areas according to their degrees of actual or potential hazard caused by mass movement 1. Map landslide 2. Statistical, quantitative approach 3. Deterministic approach, modeling of factor of safety Landslides 4 ============ Magnitude frequency ------------------- Susceptibility map -- look at past information on landslides The likelihood of landslide occurrence in an area can be assessed based on local terrain conditions, with environmental factors serving as data layers that influence landslide susceptibility. These factors act as causal variables and are used to predict future landslide occurrence (Soeters and Van Westen, 1996; Van Westen et al., 2008). Layers Elevation (Digital Elevation Model (DEM))\ Slope angle (DEM)\ Slope aspect (DEM)\ Slope curvature (DEM)\ Geology (lithology)\ Soil type\ Soil depth\ Presence of faults/joints/fractures\ Distance from fault (as proxy for rock mass quality)\ Drainage system\ Geomorphological characteristics Map of phenomena  Statistical approach Past landslides are analysed and predictions are made based on similar conditions elsewhere. A collection of maps of different colors Description automatically generated Based on these data a map is calculated on the risk of the landslide Deterministic approach, modelling of factor of safety You can sample a single slope but over a large area that is not possible Landslide intensity -------------------  - This type of information would be ideal but it requires **monitoring** or **accurate field work** - When analysing past landslides through inventories, often the **AREA** is taken as a proxy for the volume and considered an indication of landslide magnitude - That's why you mapped accurate polygons and calculated the areas! Landslide magnitude frequency ----------------------------- The area will tell use the volume which is a proxy to the magnitude of the earthquake.  Large events are rear **Spatial Frequency**: The analysis generally looks at the number of landslides of each magnitude that occur across a landscape (like a watershed, slope, or larger region) rather than their occurrence at a single location. **Temporal Aspect**: While landslide magnitude-frequency relationships are primarily spatial, they can be assessed over a specific time frame (e.g., annually, over a decade) to capture the rate of occurrence. This allows researchers to infer risk and recurrence intervals for different magnitudes. Spatial frequencies have been calculated in the practical not temporal. Roll over -- fewer smaller landslides to a certain point Roll over caused by: - Critical mass required to initiate a slide - Geomorphological evidence of small landslides quickly erased by reworking of surface material: "landscape healing" - Undersampling of smaller events due to image resolution ### Real data "Analysis of accurate landslide inventories reveals that the abundance of landslides increases with landslide area up to a maximum value, where landslides are most frequent, then it decays rapidly along a power law."  Note the slight difference in the rollover section between an inventory based on optical images and an inventory based on another remote sensing technique (less sensitive to small landslides!) Limitations ----------- Useful because we can extrapolate the **largest landslide** areas associated with a trigger event, to give a reasonably accurate and rapid estimate of the landslide-event magnitude, along with **the total number and area of landslides** in the event. **BUT** - Affected by mapping errors (e.g., wrong boundaries, inconsistent mapping, subjective judgement) - Largely empirical technique, it requires experience and training and some systematic methodology and well-defined criteria - Resolution of imagery may not allow a representative sampling - Time elapsed (freshness of geomorphological features) - The intensity of an event is not taken into account directly 1.3 Flooding intro ================== Flooding hazards ---------------- Hydrological cycle  Global water distribution A diagram of water distribution Description automatically generated Water balance equations  More water than the area can handle results in a flood event. Need to increase the storage or evapotranspiration to handle the influx of water. Its too one dimensional, not taking into account the landscape or time involved in a flood event. Runoff processes How fast rainwater reaches the river impact the chance of a flood, when rain reaches the surface of the land it will flow or infiltrate into the subsurface. If it goes into ground sources then its not as big an impact for a flood event, unless it's a shallow subsurface flow where it will reach the river faster. A diagram of a flow of water Description automatically generated Saturated throughflow leads to lots of overland flow causing rapid flooding and happens quite quickly. If the rain eases off its will allow more soil absorption and prevent a flood, if the rate is very high then it can cause flooding even not when saturated as it will flow over and not have time to absorb. Soil type makes a difference, if its porous then more absorption, but if pipeflow is present then flow is increased.  Variable source area is where the catchment area changes due to the area around the channel network become saturated and so increases overland flow. This cause increases risk as the rain fall event continues. Diagram of a diagram of water Description automatically generated Vegetation can take out water from subsurface and increase the capacity of o the subsurface. Flood intensifying factors --------------------------  Can have intense flood event over short periods of time or more drawn-out time. Boscastle is an example of very intense rain fall over a short period of time 75mm of rain in 2 hours, was a low probability event. But the catchment type is common in Cornwall so increases the risk of the event. Valencia another type of flash flood, storms concentrated over 3 different river basins. Caused high velocity overland flows with high number of debris, sewage and other pollutants. Land was very saturated beforehand. Gloucestershire type B flood that is widespread -- 78mm of rain over 12 hours. Long prolonged rainfall over a large area. Type C mega floods -- Long duration high magnitude, for example glacial outburst flood. Glacial retreat causes a glacial lake that then bursts, this is recorded in the bedrock as its scoured by 60m. Massive amounts of water flow depths of 87m of flow. Dam collapse could be a trigger of a mega flood. ### Properties of rainfall events Intensity, duration Return period is more of a probability Flood probabilities are based on flood estimation handbook, which is a computer program For example, 1 in a 100-year event could happen tomorrow, it's more of a probability like 0.01% Size of structure based on life of structure, then take the probability of an event occurring in that life cycle and make sure that the magnitude of it will not destroy the structure. For example, hydroelectric dam, needs to withstand a very unlikely flood event as the consequences would be bad if it failed. Or a temporary flood wall would need to withstand a smaller flood event so potential needed to withstand. Summary ------- Key issues are: - Conditions - Type of event - Exposure - Warning - Mitigation 2.1 Quantifying floods ====================== Flood Hydrograph ----------------  ### Controls of peak magnitude - **Volume of rain** - **Proportion of rain that generates quickflow** - rainfall intensity - amount of saturation - *catchment geology* - *time* - **Time taken for water to** - **reach outlet** ### Controls of time to peak - Area - Slope - Shape - Drainage density Spatial patterns ---------------- Not spatially uniform in probability or magnitude Factors effecting spatial relationships: - Downstream the absolute magnitude of floods (m3s-1) increases, but relative magnitude (specific discharge -m3s km-2) decreases - Number of reasons; partly steeper slopes, higher rainfall, higher connectivity - Also scaling of storms --don't cover whole catchment? Statistical analysis of flood flows ----------------------------------- - A flow duration curve is a cumulative frequency plot - Uses river gauging data over an extended time period - Displays the percentage of time a given flow is exceeded *during the measurement period* - If gauging record is sufficiently long the frequency can be used to predict future flows. - Useful for appraising the characteristics of a drainage basin ### Uses of flow duration curves Modelling floods ---------------- Needs: - Accurate parameterisation data/boundary conditions - A computer model which is *verified*, and *validated* against known events Environment Agency has national flood modelling maps for the UK, based on the FEH parameterisation.  ### Challenges Parameterisation data vs complexity - A model might be able to replicate evapotranspiration dynamically --but do we know the vegetation types, or even know the correct parameters for them - Soil moisture may be included and used to dynamically calculate runoff --but do we know soil type, or the physical parameters for soil types - Flow may be fully hydrodynamic -but do we have accurate 3D models of the river Simple models may be better if we can't parameterise a complex one But then there are potential issues of *equifinality* 2.2 Flood mitigation ==================== The lower Catchment -- Defence ------------------------------ - Main economic impact - Highest number of people effected - Management is structural ### Context --Exposure to flood events increasing (encroachment of homes and infrastructure onto floodplains & climate change --Increasing the height and extent of structural defences is unsustainable --Although structural engineering solutions to flooding can reduce flood risk they tend to put responsibility onto the state and make individuals exposed to flood risk complacent ### Legislation --Water Framework Directive -ecological gain wherever possible when designing flood mitigation measures and to try and avoid installing structures which may damage the environment unless there is no other option --UK -Making Space for Water (2005), Floodwater Management Act (2010) takes a wider view of flood risk management than just addressing 'at-a-point' defences The Mid Catchment -- Storage ---------------------------- - Using natural floodplain to store water - River restoration - Wet woodlands slow and store water The Upper Catchment -- Sponge ----------------------------- - Woodland and logjams slowing passage of water in headwater channel network - Temporary storage ponds - Forests to increase rainfall interception, evapotranspiration Balancing interventions ----------------------- - A technocratic approach may not be able to deliver a holistic approach - What do stakeholders want? - How do we prioritise competing demands on landuse? - Holistic catchment management takes us out of the realm of the purely hydrological/engineering problem --and becomes a social conundrum - Need to engage with wider body of knowledge[]{#_Toc187425242.anchor} 1 Earthquakes intro =================== An earthquake is a sudden and substantial brittle failure along a fault Stick-slip rather than stable sliding Generates seismic waves Stick-slip earthquake model ---------------------------   Force builds up and then it slips Bend and snap model ------------------- As blocks move relative to each other, elastic strain builds up, until eventually the rocks rupture and slide past each other. The sudden release of the stored elastic energy is the earthquake and produces elastic or seismic waves that radiate outwards. This is elastic rebound. Earthquake Terminology ---------------------- **Hypocenter** (or focus) is the point within the earth where an earthquake rupture starts. This does NOT generally correspond to the point of maximum displacement during an earthquake nor the middle of the rupture **Epicenter** is the point on the Earth's surface directly above the hypocenter **Magnitude:** measure of earthquake's power on a logarithmic scale **Rupture zone**: occurs between the base of the seismogenic zone (\~ brittle crust) and surface. Only largeish earthquakes actually reach the surface =\> a **surface break** **Aftershock**: small event following main earthquake as fault zone readjusts to main slip event. Distribution can accurately define main rupture area **Seismic waves:** energy radiating out from rupture Earthquake process ------------------  4 phases of the earthquake -------------------------- P-waves, S-waves, Love-waves and Reyleigh waves Bodywaves pass deep into the earth Arrive first P-waves -- primary waves, compression and dilatations  S-waves - secondary waves, they arrive second, side to side motion (do not travel through fluid)  Surface waves Love waves -- side to side shaking at 90 degrees, most motion at the surface. Do not loose as much energy at the surface so more dangerous Rayleigh -- up and down motions  Detection --------- Using vertical or horizontal seismograph Normally have 3 vertical and horizontal for north south and west east -- gives a 3D motion The horizontal are rotated to face the EQ direction to get transverse - **Body waves**: move in 3D through the Earth, tend to arrive v steeply at distant stations: - **P-waves**: fastest, arrive first, - to-and-fro motion along raypath (direction of propagation) - **S-waves:** second fastest arrive second - Side-to-side motion perpendicular to raypath - **Surface waves**: travel along the surface only to arrive horizontally at distant stations - **Love:** bit faster than Rayleigh, arrive third - Side-side motion perpendicular to raypath - **Violent shaking** - **Rayleigh**: slowest, arrive just after Love - Elliptical motion=\> both to-and-fro motion along raypath and up-down. **Violent shaking** Locating earthquakes -------------------- Network of seismographs The difference in time in the p and s waves tells us how far away the earthquake epicentre is, distance is in degree not km A diagram of a heart beat Description automatically generated  3 stations combined to find the cross over point which is the epicentre Fault types ----------- Image result for earthquake magnitude fault types Sinistral fault moves to the left side, dextral moves to the right Compression or dilation depends on the direction of the fault movement  Beachballs ---------- A diagram of a plane Description automatically generated Each fault has a specific beachball  Working out the fault plain - **Isoseismals**: zones of equal damage/shaking elongate along direction of fault plane (1st discovered after 1906 San Francisco earthquake). Want to know for future hazard assessment - Aftershock distribution: minor earthquakes that occur after the main earthquake and cluster around the main rupture 3-4-5 Magnitude-Nuclear-Tectonic setting ======================================== Magnitude scales ---------------- Are logarithmic -- increasing the power by 1 so increasing by 10 times They allow us to see lots of magnitude scales in a compressed way ### The Richter scale There are problems with the Richter scale: - Only measures the s waves - Does not take into account the size of really long and big earthquakes - Need a scale that measures total energy ### Moment magnitude scale Uses the amount of energy released **Moment = Mo = µ A D** in N.m (or dyne-cm in cgs) µ = shear modulus \~ 32 GPa in crust (\~3.2 x 10^11^ dynes/cm^2^), \~75 GPa in mantle A = LW = rupture area\ D = average displacement during rupture. **Moment magnitude** = **Mw** (or just **M**) = 2/3 (log~10~**Mo** -- 9.05) (SI units) This makes its comparable to Richter scale Bellow we can see the difference between the different magnitudes logarithmically increasing each time  Aftershocks ----------- Smaller aftershocks occur as the crust around the displaced fault plane adjusts Happen around the slip area and can be used to get the total area slipped **Baths law** - the difference in M~W~ of the main quake and the **largest aftershock** is \~ constant and \~ 1.1-1.2. But this is really just how we define what is an aftershock and which is a "new" event and so is not very useful. **Omori's law** - The frequency of aftershocks decreases roughly with the reciprocal of time after the main shock. Rate of aftershocks: Foreshocks ---------- Small event before a big event These follow Omori's Law be instead there is an increase Sometimes they do not even occur and can be miss interpreted as the main earthquake Not well enough understood to be used for prediction Nuclear explosions ------------------ Earthquakes or Bombs? Nuclear bombs have compressional source. P wave first motion in all directions  Earthquakes -- slip on a fault p wave motion compressional and extensional A diagram of a circle and a circle with arrows Description automatically generated Seismogram  The first one has very high p-waves and not much else ### Body wave scale and surface wave Body wave magnitude is larger during earthquakes Surface wave is much smaller We can see who is testing nuclear bombs seen bellow A graph of a sound wave Description automatically generated with medium confidence Magnitude and tectonic setting ------------------------------ Magnitude is controlled by slip and by rupture area like in diagram bellow  Which faults have the largest area ? **Normal faults** Extensional and have more heat less brittle, so the rupture is smaller producing shallow moderate magnitude. \< M7 **Strike-slip fault** Shallow, large along strike extent, has magnitude 8 max Magnitude related to length that ruptured **Thrust faults** Subduction zones = megathrusts Much greater down-rip extent -- 30km depth Has cooler rock as its been moved into the earth 6 Subduction zone megathrusts ============================= Subduction zone megathrusts Megathrusts are much larger as they have a longer strike dip A blue line with red text Description automatically generated Updip limit controlled by clay dewatering and downdip controlled by hydrated mantle rocks The rupture happens out to sea not on land so reduced earthquake, but the tsunamis they make are the biggest hazards. Displacement can kill trees when salt water rises Uplift and subsidence areas  Largest tsunamis are caused by subduction zone megathrusts A screenshot of a video game Description automatically generated **Tsunami generation**: a thrust distorts the seafloor causing an asymmetric build up of water. Tsunamis results from the gravitational spread of this build-up. **Tsunami splits** into roughly equal, **asymmetric** waves travelling in opposite directions at \~up to 900 km/h (250 m/s). On the landward side of thrust is a trough initially: water withdrawal precedes inundation. On oceanward side, the first wave is a peak followed by a trough. As tsunami moves into shallower water, it slows to \~40 km/h (11m/s), the back catching up with the front =\> the **wave amplifies,** the **wavelength decreases and a wall of water forms behind a deep trough**. The tsunamis travels in a circular motion and have large cross sectional extent so cause a lot of damage as the water keeps coming. Evidence of tsunamis, is tsunamis sands that's is coarser  Ghost forests also show land subsidence evidence where salt water has encroached Turbidity currents also record earthquakes as they are triggered by earthquakes. You will see simultaneous turbidity currents merging suggesting a large earthquake triggering them all at once. 7 Intensity =========== ### Mercalli intensity scale Intensity related to ground acceleration *log~10~(A) = (X/3) - 0.5* where A is the peak ground acceleration (PGA) in the units cm/s^2^ and X is intensity. **Intensity** is the shaking of the ground defined either - by observations/descriptions/damage (the Mercalli scale) - by actual quantification of the ground movements (peak ground acceleration =\> instrumental intensity -- see forecasting) **Intensity is controlled by the earthquake itself:** - Magnitude: larger earthquakes generally induce more (and longer) shaking - Earthquake depth: by controlling the amount of surface waves **And by the location of the observer:** - Distance: the amplitude of seismic waves decreases with distance as function of... -... Regional geology affects the efficiency of transmission of the seismic waves - Local geology at the observer: -  - "soft sediment amplification", - Liquefaction. Careful engineering can reduce these influences - Mercalli intensity (particularly at the lower end) is subjective and influenced by what the observer is doing The key parameters that control intensity can be combined into a **ground motion model (next time)** that **calculates** **the intensity** (peak ground acceleration) **that** **might result from a given earthquake (magnitude, depth) as a function of distance** - Uses regional geology to predict efficiency of transmission - Uses local geology to assess soft soil amplification, liquefaction =\> predicted accelerations) - plus, the historical record (what happened during past earthquakes -- next time) 8 Forecasting ============= Prediction ---------- - Specific magnitude earthquake - Will occur in a stated place - At a stated time This might be based on precursory phenomena - Animal behaviour - Changes in seismic velocity, groundwater levels, or radon emissions: fracturing prior to the quake releases gases from depth (**potentially monitorable**), disturbs the groundwater and leads to reduced seismic velocity (not realistically monitorable) - Electromagnetic variations - **Foreshocks** (small EQs that may become more frequent before a major shock, but usually not present and always hard to identify) **However,** none have proved statistically valid, with most "predictions" being after the event and suffering from lack of statistical rigour. Forecasting ----------- - the **statistical probability** - of an earthquake ***above* a specified magnitude** - happening **in a given place** - *within* a **specific period of time** (can be years or decades) Based on 1. estimate of recurrence time 2. geodetic monitoring of strain 3. monitoring of patterns of seismic activity -- beyond scope of this course **The forecast probability is then combined with a ground motion model to give an assessment of the earthquake hazard** Recurrence time --------------- **What it is:** the time between earthquakes of a specified Mw. **Why it is useful:** If we know this and the date of the last earthquake we can better forecast (not predict) the next... **How we measure recurrence time:** - When we have a long history, we can estimate the average recurrence time - but there's a lot of scatter - When we have a short history, we estimate the recurrence time of large earthquakes from that of small ones, but this can be biased **How we go from recurrence time to forecasting:** - how we assume either that the probability of large earthquakes stays constant with time, or that it changes - How we try to relate recurrence time to physics and to monitoring **Different choices lead to different forecast hazards** Idealised elastic rebound and elastic dislocation theory -------------------------------------------------------- Seismic Gaps ------------ - Slip on neighbouring segments relieve the stress there, but transfer remaining stress onto the part that has not ruptured - Increasing risk of rupture there - Over and above the general increasing risk as recurrence time increases - May in some cases be expressed as a time progression in the location of major earthquakes Ground motion models --------------------  **Seismic Risk = Hazard** (probability, shaking intensity) **x Consequence** 9 Risk ====== Building styles --------------- - **Masonry: adobe, bricks and mortar -- very bad as very little tensile strength and are pulled apart to collapse** - **Wooden frames -- improvement** - **Concrete slabs -- bad as very little tensile strength** - **Proper ties - improvement** - **Wood and paper -- traditional Japanese (or wood and corrugated iron sheets - Haitian shacks): building may fall down but won't kill people. Plus wood has high tensile strength. Fire risk though** - **Use lots of steel -- best. Tensile strength.** Earthquake engineering ---------------------- - Earthquakes cannot be prevented nor accurately predicted. - But it is not ground shaking itself that causes life and economic loss but the collapse or damage of buildings and infrastructure that are too "weak" to resist the ground shaking. - Earthquake engineering is the application of civil engineering to reduce life and economic losses due to earthquakes, i.e to mitigate *seismic risk:* - the probability of losses occurring due to earthquakes within the lifetime of a structure; these losses can include human lives, social and economic disruption as well as material damage Seismic design -------------- Design Problem: - An earthquake usually constitutes the most severe loading to which most civil engineering structures might possibly be subjected, and yet in most parts of the world, even those that are highly seismic, there is a possibility that an earthquake may not occur during the life of the structure. - Note: Lateral loads imposed by winds = 1-3% of building weight. Lateral loads due to earthquakes = 25-30% of building weight Assessment Problem: - Most buildings that exist have not been designed to seismic codes. Can they withstand the type of earthquake that might happen in their location? What will be the damage incurred if an earthquake does occur? - Hence, engineers require seismic hazard assessments to provide not only a description of the likely seismic loads (ground shaking) to be experienced by an engineering structure, but also to attach probabilities of occurrence to these earthquake loads Solution - Normal building life assumed to = 50years - Design a building to resist an earthquake with a return period of 475yrs (i.e. the design loads will have a 10% probability of being exceeded in the structures life). - If a structure is very important (i.e. the consequences of its damage are severe) these loads will be increased: e.g. Nuclear structures designed to resist a 10,000 year return period event. - If we design for the seismic loads to be resisted by the building without damage (i.e. for the building to react elastically), the cost of construction would be prohibitive - **So we design buildings to be damaged under earthquake loading** **Design criteria** - We design buildings to be damaged, but try to control the location of the damaged areas to avoid catastrophic failure. - Choose an adequate lateral load resisting system - Maintain regularity in plan and elevation (of stiffness and mass distribution -- no weak zones or storeys) - Ensure connection between structural elements - Connecting and anchoring of reinforcement - Using appropriate materials - Avoid designing-in locations of stress concentration - Consider dynamic response in determining spacing between buildings, so as to avoid pounding - Do not build across faut zone bellow road goes on fault -  T1.1 Tsunami Hazards ==================== Form from displacement of the water column by: - Earthquakes - Slope failure - Volcanic activity - Bolide impact A diagram of a coastal erosion Description automatically generated ### Tsunami theory Water column raised up Gravitational spread of this disturbance drives tsunami  They normally have very long wave length and small heigh compared It effects the whole thickness of the sea not just the top like waves Velocity of the wave is The velocities very fast Energy loss in waves is proportional to wavelength, so they don't loose much energy over long distances As the wave reaches the coast the velocity decreases and the hight increases  Mitigation ---------- ### Direct warning/monitoring - Real time monitoring/warning systems -- DART direct detection and warning systems. Directly feed into real-time models. - Time scale -- DART works in longer time scales so close to the shoreline not the best. 2 hour is okay 1 min not. - SO in those close 1 min areas we are reliant on people knowing what to do - Awareness is required to ensure direct response ### Planning - Tsunami models - Computer simulation - 3 stages of modelling -- wave generation, tsunami propagation and inundation ### Public education. Awareness Use of maps, signs is key to directing potential responses First wave may not be the largest Potential for multiple waves A diagram of a tsunami Description automatically generated ### Learning from past events Where can megathrust earthquakes occur ? What is their repeat time ? Have we observed the worst-case scenario ### Direct mitigation In addition to warning/evaluation Infrastructure planning/protection Based on past worse case scenario Does protection affect the public response ? Fukushima was designed for not the worst case scenario = bad Summary ------- - Behaviour of seismogenic tsunamis is broadly predictable, and can be modelled accurately - Potential for real-time warning very limited in near field; and in far field requires accurate detection systems and efficient international communication and evacuation plans - Mitigation requires multiple approaches -- real time monitoring and warning infrastructure, modelled scenarios, education, use of past events (? And perhaps direct protection) - Compound tsunamis (complex sources) are far less predictable and can be highly damaging due to unexpectedly high amplitudes - Effects of such events are often localised - Volcanically generated tsunamis show a similar pattern; extreme local impacts pose a major challenge for mitigation approaches - The largest landslide-generated tsunamis can have an impact on much larger scales, potentially across ocean basins (thus with comparable reach to seismogenic events, and with more extreme heights, at least near the source) Bolide Impact ============= Bolide impacts - **Asteroid**: An inactive rocky body orbiting the Sun with diameter between 1m and 900 km. Largest discovered is Ceres (950 km, recently reclassified as a dwarf planet) and 4 Vesta (525 km diameter). - Some may be balls of rubble - **Comet**: "rocky ice" body up to \~50 km across. Ices can vapourise in sunlight forming an atmosphere (coma), and sometimes a tail, of dust, gas and ions - **Meteoroid**: A small piece from a comet or asteroid orbiting the Sun. Up to 1m across. (*Most meteoroids formed early in the history of the solar system at \~ 4.5 Ga, older than any rocks on Earth).* - **Meteor**: The light phenomena which results when a meteoroid enters the Earth\'s atmosphere and vaporises; a shooting star - **Meteorite**: A meteoroid (or rarely a small asteroid fragment) that survived its passage through the Earth\'s atmosphere and impacted the Earth\'s surface. Concerned about potential impact, very high impact velocity Asteroid types - Stoney - Nickle iron **Near Earth Asteroids (NEAs)** **Potentially Hazardous Asteroids (PHAs)** Key difference is - whether the asteroid is large enough to be hazardous (\>140m) - whether the asteroid's orbit comes within **0.05 AU (748,000 km, 20 lunar distances LD) of Earth's orbit** ("minimum orbit intersection distance" or MOID) If they hit the earth  - **Category 2, 3, 4 (Yellow)** 'Events meriting concern' -- close approaches by object that have higher collision chances than Earth typically experiences over a few decades; refinement of knowledge of orbital characteristics a high priority. - **Category 5, 6, 7 (Orange)** 'Threatening Events'. Close encounters with objects large enough to cause regional/global devastation, where the chance of collision greatly exceeds levels typical for a given century. Refinement of orbital characteristics an extreme priority. - **Category 8, 9, 10 (Red). Certain collisions with objects of sufficient size to penetrate the atmosphere.** A graph with colored text and numbers Description automatically generated with medium confidence Prevention deflection destruction **Deflect**: change bolide orbit so it misses. But might deflect so it hits instead... - "gravity tractor": position heavy object to one side of asteroid and let gravitational attraction change asteroid orbit marginally. Small bolides only, **Untested** - paint it -- changes Yarkovsky effect (heat radiation thrust) **Untested** - swift kick: crash heavy object into side of asteroid. Last year NASA successfully deflected a \~1 km asteroid - nuclear explosions to one side of asteroid to vaporise part of one side, giving it a push in the opposite direction. **Untested** **Fragment:** nuclear explosion as in Deep Impact and Armageddon. Last resort as fragments unpredictable and may be just as devastating. **Untested** **Deflection achievable for PHAs (\>140m diameter)**, as we can track the path of this size of asteroid for many orbits ahead of possible impact **Small, uncharted asteroids (e.g. Urals, Tunguska, Barringer) may provide a bigger threat. Not classified as PHAs as we cannot do much about them at the moment**