Deadliest Earthquakes

• Earthquakes rank among the largest and most powerful natural hazards.
• Earthquakes and the resulting fires, tsunamis, and landslides led to over 2 million deaths in the 20th century.

Deadliest Earthquakes

• In 1556, Shaanxi, China experienced an earthquake of magnitude 8.0, resulting in 820,000-830,000 fatalities.
• In 1976, Tangshan, China experienced an earthquake of magnitude 7.5 causing fatalities numbering 242,419-779,000.
• In 525, Antioch, Turkey was hit by an earthquake of magnitude 8.0, causing ~250,000 fatalities.
• In 1920, Ningxia-Ganso, China experienced an earthquake of magnitude 7.8 resulting in 235,502 fatalities.
• In 2004, Sumatra, Indonesia was hit by an earthquake of magnitude 9.1, leading to 230,000 fatalities.
• In 1138, Aleppo, Syria experienced an earthquake of magnitude 8.5 leading to ~230,000 fatalities.
• In 2010, Haiti was struck by an earthquake of magnitude 7.0, causing 222,570 fatalities.
• In 1923, the Kanto region, Japan was hit by an earthquake of magnitude 7.9 resulting in 142,000 fatalities.

What is an Earthquake?

• Earthquakes are sudden and violent ground shaking events caused by a sudden energy release.
• Earthquakes are commonly caused by fault movement, but also by volcanoes, rock falls, or human activity.
• Earthquakes are usually felt at the ground surface and can result in great destruction

How Different are Earthquakes?

• Earthquakes can vary significantly based on the tectonic setting, energy released, depth of the energy source, and ground composition between the source and observer.

Why Earthquakes Happen

• Earthquakes occur from sudden friction release along a tectonic plate boundary.
• Faults are created when rock in an area breaks.
• The focus of an earthquake is where the fault begins and generates seismic waves.
• Seismic waves are vibrational energy forms that shake the ground.
• Ground shaking is referred to as the earthquake (quake).
• Transform faults, convergent, and divergent boundaries are common earthquake locations.
• The San Andreas Fault is an example of a transform fault boundary.
• Ocean-continent zones like the Cascades or Andes are convergent boundaries.
• Divergent boundaries, such as Iceland, the East African Rift zone, or mid-ocean ridges, release pressure and mostly occur in the ocean, often going unnoticed.
• Plate boundaries store and release energy proportional to their surface area, resulting in friction and forces at the boundary.
• More frequent energy release results in less energy released per event.
• Fewer quakes can indicate more dangerous quakes.

Areas at Risk

• Most earthquakes happen at or near tectonic plate boundaries.
• Densely populated areas increase the risk factor.
• Countries along the Pacific Ring of Fire, the Himalaya, many Middle East countries, and southern Europe face the greatest risk.

Earthquake Cycle

• The earthquake cycle is also known as the elastic rebound theory and the seismic cycle.
• Pressure (stress) increases as tectonic plates move.
• Rock deforms elastically with increasing pressure; release allows rocks to return to their original state.
• Increased pressure will eventually break rocks, creating a fault.
• Some energy breaks rock, while the rest moves surrounding materials.
• Rock starts accumulating stress after it is released.
• Locations on plate boundaries experience earthquakes in cycles of hundreds to thousands of years.
• A four-stage cycle is recognized in large fault systems that experience quakes.
• Stage 1: Interseismic Stage involves long periods of inactivity where rocks are locked and do not move.
• Stage 2: Preseismic Stage involves accumulated elastic strain causing smaller earthquakes.
• Foreshocks (small to moderate size) can occur hours or days before a large earthquake, but do not always occur.
• Stage 3: Coseismic Stage is the main earthquake period.
• Seismic waves are produced by rapid fault motion.
• Stage 4: Postseismic Stage involves aftershocks occurring from minutes to years after the quake, before returning to the Interseismic Stage.

Earthquake Shaking

• Shaking severity and damage are determined by the earthquake magnitude and energy released.
• Other factors include the distance to the epicenter, focal depth, energy lost or redirected, fault rupture direction, and local soil and rock conditions.
• The direction of fault rupture amplifies seismic waves and shaking.
• Local soil and rock conditions influence the amount of shaking felt.

Earthquake Connections to Hazards

• Earthquake injury, death, and property damage also come from the hazards that quakes cause.
• Primary effects include ground shaking and rupture.
• Secondary effects include ground liquefaction, land-level changes, landslides, fires, tsunamis, disease, and groundwater disruption.
• The impacts of an earthquake are related to the level of development in the affected area.

Destructive Earthquake Effects

• Amplitude, duration, and damage increase in poorly consolidated materials.
• Aftershocks occur as the Earth adjusts to its resulting state and can last for a month or more.
• Larger quakes result in more and larger aftershocks.
• Aftershocks are usually 0.1 to 3.0 units smaller than the original mainshock.

Negative Earthquake Effects

• Negative impacts include shaking and ground rupture, soil liquefaction, groundwater interruption, landslides, fires, floods, tsunamis, and human effects (injury, death, trauma).

Positive Earthquake Effects

• Not all effects are bad, as some useful effects could only happen because of quakes.
• Groundwater is often transmitted through faults.
• Oil and gas are often trapped by structural action associated with earthquakes.
• Quake energy causes rock alteration to produce valuable secondary minerals.
• Seismic energy aids in interpreting earth structure through geophysics.

Measuring Earthquakes

• Two measures are used when comparing earthquake sizes.
• Intensity was initially used to describe ground shaking at a specific location or how much damage occurred.
• Intensity is affected by the location, not just the earthquake itself, and can vary from place to place for the same event.
• Intensity depends on proximity to the earthquake, earth material types, and building material strength.
• Risk and impact depend on the specific conditions of an earthquake.
• The Modified Mercalli Intensity Scale measures intensity based on California building standards.
• The scale has 10-12 levels, ranging from not felt to total damage.
• The Modified Mercalli scale measures intensity, while the Richter scale measures magnitude.

Richter Scale

• The Richter Scale is a logarithmic scale measuring movement and energy released, ranging from levels 1-10.
• Each level shows 10x more movement and 32x more energy.
• A magnitude 4 earthquake is 32 times stronger than a magnitude 3 earthquake.

Largest Earthquakes

• In Valdivia, Chile, in 1960, there was an earthquake of magnitude 9.5.
• In Prince William Sound, Alaska USA, happened in 1964, an earthquake with magnitude 9.2.
• In Sumatra, Indonesia, happened in 2004, an earthquake with magnitude 9.1.
• In Kamchatka, Russia, happened in 1952, an earthquake with magnitude 9.0.
• In Sendai, Japan, happened in 2011, an earthquake with magnitude 9.0
• In Maule, Chile, occurred in 2010, an earthquake with magnitude 8.8
• In Ecuador-Colombia, occurred in 1906, an earthquake with magnitude 8.8
• In Sumatra, Indonesia, happened in 1833, an earthquake with magnitude 8.8
• In the Pacific Ocean, Japan, USA, and Canada, in 1700, there was an earthquake with a magnitude from 8.7-9.2.

Seismic Waves

• Seismic waves transfer energy through the Earth, with the material's response determined by the earthquake's energy release.
• There are two types of seismic waves: body waves (P and S) and surface waves (R and L).

Body Waves

• P (primary) waves are the fastest and travel through solids, liquids, and gases as compressional waves where material moves in the same direction as the wave.
• S (secondary) waves are slower, travel only through solids, and are shear waves in which material moves perpendicular to wave movement.

Earthquake Epicenter

• P waves arrive first, followed by S waves, and then R and L waves.
• Average wave speeds are known, so the difference in arrival times after an earthquake can be used to calculate the distance from the seismograph to the epicenter.
• The focus is the point of origin for earthquake energy.
• Damage is affected by distance from the focus.
• The epicenter is the Earth's surface nearest to the energy origin, making it the most vulnerable area for damage.

Surface Waves

• When body waves reach the surface, some of their energy transfers to surface waves.
• Rayleigh waves (R) are characterized by vertical ground motion that creates a rolling motion like waves.
• Love waves (L) are characterized by horizontal ground motion.
• Surface waves travel at or just below the ground's surface.
• Surface waves are slower than body waves and involve rolling and side-to-side movement, which damages buildings.

Earthquakes and Human Influence

• Several unintentional factors cause earthquakes.
• Building dams and flooding valleys can result in quakes ranging from 2-3 (small reservoirs) to 6 (large reservoirs).
• The weight of water loads the crust, causing fractures.
• Deep waste disposal from injecting fluids can lubricate fault lines and cause slippage causing quakes ranging between 2-6.
• Pumping oil or gas in western Alberta and northern British Columbia can cause Hydraulic Fracturing (Fracking).
• Injecting highly pressurized fluids with grit keeps fractures open for hydrocarbons and can cause small to larger quakes ranging in magnitude from 2-3.
• Underground Nuclear Testing in Nevada in the 1950s triggered M 6.3 quakes.
• Western European seismologists have recorded test blasts from Russia that caused earthquakes.

Minimizing Earthquake Hazards

• Earthquakes are unpredictable, making limiting damage, injury, and loss of life difficult. However, its possible to predict one will happen in a particular area, or on a given fault segment.
• Forecasts help officials responsible for seismic safety, but they do not help residents.
• The Geological Survey of Canada and the US Geological Survey develop programs to reduce hazards by operating national seismograph networks and alerting seismologists to earthquakes via email.
• These programs also aim to develop an understanding of earthquake sources through classifying active fault movement rates to determine earthquake potential and predict effects on buildings and structures.
• Estimating losses from various magnitude quakes provides feedback on models.
• Communicating research results helps educate people to reduce risk.

Earthquake Warning Systems

• Earthquake warning systems tell of a quake that has already happened.
• Radio waves are faster than seismic waves, which could be used in an early-warning system such system.
• Japanese systems warn bullet trains of earthquakes to prevent derailment.
• Derailments can result in hundreds of deaths.
• These systems provide 15 seconds to 1 minute of warning, which is enough time to shut off gas values, machinery, and computers, depending on the distance from the quake epicenter.

Community Adjustment to Earthquake Hazards

• As it is not possible to prevent people from living in seismically active areas, steps must be taken to minimize the risks.
• The situation is similar to people living on a flood plain and minimizing flood risks.
• Although people know the steps needed to minimize risk, few areas have done so.
• Is southern California an exception?

Reducing Earthquake Risk

• Hospitals, schools, police and fire stations, communication systems and power plants must be located in safe zones based on site-specific investigations to demonstrate their degree of safety.
• Structural protection involves that buildings, pipelines, and bridges must be built to withstand at least moderate shaking through research and building codes.
• Potential education mechanisms include drills, pamphlets, media announcements, and demonstrations at all levels and places.
• Insurance and medical relief is vital at all levels of government and must be delivered quickly and efficiently, requiring huge amounts of money.

Readiness

• Most places are not prepared for an earthquake.
• Older structures are impacted the most strongly.
• Building codes vary with location and that most large cities feature only partial good construction.
• Wood is surprisingly flexible, and some buildings feature 'shock-absorbing structures'.

Turkey-Syria Earthquake

• Turkey and Syria are earthquake-prone due to their location at the intersection of the Anatolian, Arabian, and African tectonic plates.
• Feb 6, 2023, at 4:17 am, a magnitude 7.8 earthquake led to >40,000 deaths as of Feb 14.
• The earthquake had a depth of 24.1 km (14.9 mi).
• Arabia is bumping into Eurasia, pushing Anatolia westwards, and Syria is bumping into Europe, squeezing out Turkey.