Earthquake Mechanism PDF

**PCS2-M** Professional Course- Specialized 2 (Earthquake Engineering) **Earthquake Mechanism: A written report** **Submitted to:** **Engr. Edmundo C. Dela Cruz** **Earthquake Mechanism** I. **Introduction** [Earthquake] is ground shaking as a result of which structures connected to...

**PCS2-M** Professional Course- Specialized 2 (Earthquake Engineering) **Earthquake Mechanism: A written report** **Submitted to:** **Engr. Edmundo C. Dela Cruz** **Earthquake Mechanism** I. **Introduction** [Earthquake] is ground shaking as a result of which structures connected to the ground are set in vibration. The mechanism of generation of earthquake, ground motion, structural vibration, and the destruction of buildings (Celebi and Brown, 1994) are described below in detail. II. **Stress and Forces** - **Compression** squeezes rocks together, causing rocks to fold or fracture (break). Compression is the most common stress at [convergent plate boundaries] - Rocks that are pulled apart are under **tension/extensional stress**. Rocks under tension lengthen or break apart. Tension is the major type of stress at [divergent plate boundaries]. - When forces are parallel but moving in opposite directions, the stress is called **shear**. Shear stress is the most common stress at [transform plate boundaries]. - A deeply buried rock is pushed down by the weight of all the material above it. Since the rock cannot move, it cannot deform. This is called **confining stress**. 1. Pressure -- increases with depth 2. Temperature -- make rocks more flexible 3. Mineral Composition -- varies depending on the composition of rocks. +-----------------------------------+-----------------------------------+ | - **Elastic Deformation** | | | | | | - The rock returns to its | | | original shape when the | | | stress is removed. | | | | | | - Reversible | | | | | | - Not permanent | | +===================================+===================================+ | - **Plastic Deformation / |  | | Ductile Deformation** | | | | | | - The rock does not return | | | to its original shape | | | when the stress is | | | removed | | | | | | - Irreversible | | | | | | - Permanent | | +-----------------------------------+-----------------------------------+ | - **Brittle Fracture** | | | | | | - The rock breaks | | +-----------------------------------+-----------------------------------+ +-----------------------------------+-----------------------------------+ | 3. **Monocline** -- Monocline is |  | | a simple bend in the rock | | | layers so that they are no | | | longer horizontal. | | +===================================+===================================+ | 1. **Anticline** -- An anticline | | | is a fold that arches upward. | | | The rocks dip away from the | | | center of the fold. The | | | oldest rocks are at the | | | center of an anticline and | | | the youngest are draped over | | | them | | +-----------------------------------+-----------------------------------+ | 2. **Syncline**- A syncline is a |  | | fold that bends downward. The | | | youngest rocks are at the | | | center and the oldest are at | | | the outside | | +-----------------------------------+-----------------------------------+ **Types of Stress** **Fault** **Boundary** --------------------- ------------- -------------- --------------------------- ---------------- Tension Normal Divergent Move away from each other Ridge Compression Reverse Convergent Move towards each other Mountain Range Shear Strike-Slip Transform Move horizontally III. **Types of Earthquakes** **WHAT IS AN EARTHQUAKE?** There are many types of earthquakes. Each type of earthquake behaves uniquely. An earthquake is the ground shaking caused by the sudden energy release in the earth's crust or upper mantle of the Earth. Earthquakes can differ in length from those that are so small that they are barely felt to those that are so powerful that they propel folks into the air, wreaking havoc on entire cities. The seismicity, or seismic activity, of an area, is the frequency, type and size of earthquakes experienced over a period of time. Seismometer measurements are used to measure earthquakes. For the whole globe, the moment magnitude is by far the most prevalent scale about which earthquakes bigger than roughly magnitude five are registered. **SEISMIC WAVES** **TWO TYPES OF SEISMIC WAVES:** - **[BODY WAVES]** - Body waves are of two types: ***primary (P) waves*** and ***secondary (S) waves***. P- and S- waves are called *\"body waves\"* because they can travel through the interior of a body such as the Earth\'s inner layers, from the focus of an earthquake to distant points on the surface. The Earth\'s molten core can only be traveled through by compressional waves. - **[SURFACE WAVES]** - Surface waves, in contrast to body waves can only move along the surface. They arrive after the main P and S waves and are confined to the outer layers of the Earth. They cause the most surface destruction. Earthquake surface waves are divided into two different categories: ***Love Waves*** and ***Rayleigh Waves***. - **[PRIMARY WAVES (P-WAVES)]** - P waves, or Primary waves, are the first waves to arrive at a seismograph. P waves are the fastest seismic waves and can move through solid, liquid, or gas. They leave behind a trail of compressions and rarefactions on the medium they move through. P waves are also called *pressure waves* for this reason. Certain animals, such as dogs, can feel the P waves much before an earthquake hits the crust (surface waves arrive). Humans can only feel the ramifications it has on the crust. - **[SECONDARY WAVES (S-WAVES)]** - S waves, or secondary waves, are the second waves to arrive during an earthquake. They are much slower than P waves and can travel only through solids. It is after studying the trajectory of S waves through the layers of earth, scientists were able to conclude that the earth's outer core is liquid. - **[LOVE WAVES]** - Named after British ***mathematician Augustus Edward Hough Love***, who worked out the mathematical model for this wave type in 1911. Love waves have a particle motion, which, like the S-wave, is transverse to the direction of propagation but with no vertical motion. Their side-to-side motion (like a snake wriggling) causes the ground to twist from side to side, that\'s why Love waves cause the most damage to structures. Love waves produce entirely horizontal motion. The amplitude is largest at the surface and diminishes with greater depth. - **[RAYLEIGH WAVES]** - The other kind of surface wave is the Rayleigh wave, named for *John William Strutt*, known as ***Lord Rayleigh***, who mathematically predicted the existence of this kind of wave in 1885. A Rayleigh wave rolls along the ground with a more complex motion than Love waves. Although Rayleigh waves appear to roll like waves on an ocean, the particle motion is opposite of ocean waves. Because it rolls, it moves the ground up and down, and forward and backward in the direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves. Like Love waves, the amplitude of the wave decreases dramatically with depth. **4 TYPES OF EARTHQUAKE** - **TECTONIC EARTHQUAKE** - **[DIVERGENT PLATE BOUNDARIES]** - This is where two plates move away from each other. Molten rock from the mantle erupts along the opening, forming new crust. The earthquakes that occur along these zones, called *spreading centers*, are relatively small. The *Great Rift Valley in Africa*, the *Red Sea* and the *Gulf of Aden* all formed as a result of divergent plate motion. - **[CONVERGENT PLATE BOUNDARIES] -** This occurs when plates move towards each other and collide. When a continental plate meets an oceanic plate, the thinner, denser, and more flexible oceanic plate sinks beneath the thicker, more rigid continental plate. This is called subduction. ***Subduction*** causes deep ocean trenches to form, such as the one along the west coast of South America. ***Collision*** on the other hand is when two tectonic plates are both strong enough to create a mountain-like shape in plates after colliding. The rocks pulled down under the continent begin to melt. Sometimes the molten rock rises to the surface, through the continent, forming a line of volcanoes. About 80% of earthquakes occur where plates are pushed together, called convergent boundaries. - **[TRANSFORM PLATE BOUNDARIES] -** Causes a fault between two plates of the lithosphere, which will slide past one another. This motion does not create or destroy crust and will cause earthquakes, but no volcanoes. A transform boundary occurs when two tectonic plates move past one another. ***San Andreas Fault*** is the best example. - **VOLCANIC EARTHQUAKE** Volcanic earthquakes possess different characteristics due to the deep and tectonic seismic events that occur along great faults. They are considerably less devastating and may continue at a detectable level for weeks or months before the eruption. It is generated by the abrupt opening of channels in crustal rocks, excessive accumulation of gas pressure in the crust, rapid changes of motion of magma, roof collapses of subterranean channels emptied of magma. Earthquakes related to volcanic activity may produce hazards which include ground cracks, ground deformation, and damage to manmade structures. There are two general categories of earthquakes that can occur at a volcano: ***volcano-tectonic earthquakes*** and ***long period earthquakes***. 2. **GENERAL CATEGORIES OF VOLCANIC EARTHQUAKE:** - **[VOLCANO-TECTONIC EARTHQUAKE]** - Earthquakes produced by stress changes in solid rock due to the injection or withdrawal of magma (molton rock) are called volcano-tectonic earthquakes (Chouet, 1993). These earthquakes can cause land to subside and can produce large ground cracks. These earthquakes can occur as rock is moving to fill in spaces where magma is no longer present. Volcano-tectonic earthquakes don\'t indicate that the volcano will be erupting but can occur at anytime. - **[LONG PERIOD EARTHQUAKES]** - typically occur at greater depths (10--35 km, or about 6--22 miles) than VT events and are often located near or below what is called the "brittle-ductile transition", which is the area where the crust stops behaving in a brittle manner (fracturing) and starts to flow (like Silly Putty).These earthquakes are a result of pressure changes during the unsteady transport of the magma. When magma injection is sustained a lot of earthquakes are produced (Chouet, 1993). This type of activity indicates that a volcano is about to erupt. 2. **TYPES OF VOLCANIC EARTHQUAKE:** - **[A-TYPE]** - These are earthquakes originated from the bases of volcanoes or from the depths of about 1--20 km. These earthquakes take place previous to and during the first stage of eruptive activity and occur in swarms. They are generally less than 6 in magnitude. The nature of earthquake motions cannot be distinguished from those of general shallow earthquakes (tectonic earthquakes). The P-phase and S-phase of seismic waves also are clearly defined. A close-up of a graph Description automatically generated - **[B-TYPE]** - The hypocenters are limited to an area of about 1 km in radius around the active crater. The hypocenters of these earthquakes are shallower than those of the A-type volcanic earthquakes. These earthquakes take place in swarms in the shallow part ranging from the earth's surface to depths of several hundred meters. Their magnitudes are generally extremely small. The surface waves are predominating and the S-phase is not clear.  - **EXPLOSION EARTHQUAKE** - **[STROMBOLIAN ERUPTION]** - Strombolian eruptions are moderately explosive eruptions of basaltic magma with moderate gas content. Strombolian eruptions consist of intermittent, discrete explosive bursts which eject pyroclasts as high as hundreds of feet into the air in firework-like incandescent rooster-tails. Strombolian eruptive activity can be very long-lasting because the conduit system is not strongly affected by the eruptive activity, so that the eruptive system can repeatedly reset itself. - **[VESUVIAN/PLINIAN ERUPTION]** - Plinian/Vesuvian eruptions are marked by columns of volcanic debris and hot gases ejected high into the stratosphere, the second layer of earth\'s atmosphere. The key characteristics are the ejection of a large amount of pumice and very powerful continuous gas-driven eruptions. Short eruptions can end in less than a day, but longer events can continue for several days or months. The longer eruptions begin with production of clouds of volcanic ash, sometimes with pyroclastic surges. The amount of magma ejected can be so large that it depletes the magma chamber below, causing the top of the volcano to collapse, resulting in a caldera. Fine ash and pulverized pumice can be deposited over large areas. Plinian eruptions are often accompanied by loud noises.  - **COLLAPSE EARTHQUAKE** A collapse earthquake in geography is a type of earthquake that occurs due to a landslide or collapse of material below the earth\'s surface. Collapse earthquakes can occur in many regions of the world and can cause considerable damage to buildings, infrastructure and the surrounding environment. This happens when the induced stress around the mine working because large masses of rock fly off the mine face explosively, producing seismic waves. Collapse earthquakes are also produced by massive land sliding. In general, collapse earthquakes occur in mountainous areas that have steep slopes and are often hit by heavy rainfall. Also, Collapse earthquakes occur when buildings or other structures collapse. These earthquakes are typically very small but can be deadly if they occur in a populated area. This can happen due to a variety of reasons, including poor construction, severe weather, or an earthquake. **EXAMPLES OF COLLAPSE EARTHQUAKE** - **[7.9 MAGNITUDE IN SICHUAN REGION OF CHINA (2008)]** - This 7.9 magnitude earthquake was caused by the collapse of mountains around the Wenchuan region. The earthquake left more than 69,000 people dead or missing and damaged thousands of buildings and infrastructure. It also caused landslides and flooding that threatened the safety of local communities. - **[7.4 MAGNITUDE IN CENTRAL JAVA (2014)]** - In Indonesia, earthquake collapses have also occurred, such as in the Banjarnegara region of Central Java in 2014. This earthquake was caused by the collapse of a cliff on a mountainside following heavy rainfall in the area. This collapse earthquake caused damage to several buildings and claimed dozens of lives. IV. **Categories of Earthquake** Earthquake magnitude, energy release, and shaking intensity are all related earthquake measurements that are frequently confused with one another. Their interdependence and relationships can be complex, and even one of these concepts on its own can be confusing. In this report, we are to look at each of these interconnectedness and dependencies. **Magnitude** **Intensity** **Modified Mercalli Scale** **MERCALLI INTENSITY** **MAGNITUDE** **WITNESS OBSERVATIONS** ------------------------ --------------- ----------------------------------------------------------------------------------------------------------------------------------- I 1 to 2 Felt by very few people; barely noticeable. II 2 to 3 Felt by a few people, especially on upper floors. III 3 to 4 Noticeable indoors, especially on upper floors, but may not recognized as an earthquake. IV 4 Felt by many indoors and few outdoors. May feel like heavy truck passing by. V 4 to 5 Felt by almost everyone, some people awakened. Small objects moved. Tress and poles may shake. VI 5 to 6 Felt by everyone. Difficult to stand. Some heavy furniture moves, some plaster falls. Chimneys may be slightly damaged. VII 6 Slight to moderate damage in well built, ordinary structures, Considerable damage to poorly built structures. VIII 6 to 7 Little damage in specially built structures. Considerable damage to ordinary buildings, severe damage to poorly built structures. IX 7 Considerable damage to specially built structures, buildings shifted off foundations. Ground cracked noticeably. Landslides. X 7 to 8 Most masonry and frame structures and their foundations destroyed. Ground badly cracked. Landslides. XI 8 Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks in ground. Waves seen on ground. **Richter Scale vs Seismograph** How Are Earthquakes Measured? (Video)  **Earthquake Magnitude Scale** **Earthquake Magnitude Scale** -------------------------------- ----------------------------------------------------------------------- -------------------------------- **Magnitude** **Earthquake Effects** **Estimated Number Each Year** 2.5 or less Usually not felt but can be recorded by seismograph. Millions 2.5 to 5.4 Often felt, but only causes minor damage. 500,000 5.5 to 6.0 Slight damage to buildings and other structures. 350 6.1 to 6.9 May cause a lot of damage in very populated areas. 100 7.0 to 7.9 Major earthquake. Serious damage. 10-15 8.0 or greater Great earthquake. Can totally destroy communities near the epicenter. One every year or two V. **Effects of Earthquake** a. **Landslides** 1. 2. 3. a. b. 4. a. b. c. - **Natural Causes of Landslides** 1. **Earthquakes and Volcano Eruptions** - Seismic activities have always been a main cause of landslides throughout the world. Any time plate tectonics move the soil that covers them moves with it. When earthquakes occur on areas with steep slopes, many times the soil slips causing landslides. 2. **Heavy Rainfall -** When sloped areas become completely saturated by heavy rainfall many times landslides can occur. Without the aid of mechanical root support, the soil simply runs off when it contains too much water. - **Human Causes of Landslides** 3. **Clear Cutting** - Method of timber harvesting which completely removes all old growth timber from the area. This method is hazardous because it destroys the existing mechanical root structure in the area. 4. **Mining** - Mining operations that use blasting techniques often cause other areas that are at the risk of sliding to slide due to vibrations under the soil. b. **Human Impact** Earthquakes have several impacts on the lives of the people: 1. **Destruction** -- Common impacts of earthquakes include *structural damage to buildings.* As earthquake hits structures, it generates inertia forces which could be greatly destructive causing deformations and, horizontal and vertical shaking, resulting in failure or perhaps total collapse. 2. **Injuries and Death** -- One of the biggest and worst effects of an earthquake is the cost to human life since many buildings, hospitals, schools, etc. are destroyed due to it and a lot of people get trapped, killed and injured. 3. **Cost** - Massive costs hit communities and nations when an earthquake happens. Victims and survivors of the earthquake need immediate help from rescue teams. 4. **Psychological effects** -- Victims of earthquake events often suffer psychological problems which can last for days, years or an entire lifetime. 5. **Environmental impacts** -- Earthquakes not only destroy human life, but have a devastating effect on animals and plants. This also causes surface faulting, tsunamis, soil liquefactions, ground resonance, landslides, and ground failure due to its shaking. c. **Tsunamis** - **Local Tsunami** - its destructive effects are experienced on coasts within 100 km from the source of the tsunami.  In such cases, the travel time for the tsunami is generally less than one hour. A local tsunami is usually generated by an earthquake but can also be caused by a landslide or a pyroclastic flow from a volcanic eruption. - **Regional Tsunami** - A tsunami capable of destruction in a particular area which lies between 100 km - 1,000 km from the source of the tsunami.  Regional tsunamis can take between 1-3 hours to reach the affected [shoreline](http://weready.org/tsunami/index.php?option=com_glossary&Itemid=13&id=60&letter=S). The most destructive tsunamis can be classified as local or regional. - **Distant Tsunami** - Also referred to as a ***tele-tsunami*** or ***ocean-wide tsunami***, distant tsunamis originate from a faraway source (more than 1000 km away) and generally take more than 3 hours to arrive at affected coasts. \ When a tsunami is formed, the waves generally radiate and move in opposite directions. In this case, a local tsunami can impact on coastlines which are close to the [tsunami source](http://weready.org/tsunami/index.php?option=com_glossary&Itemid=13&id=84&letter=T). d. **Liquefaction** 1. 2. 3. 4. e. **Fires** - Fires caused by earthquakes are not directly related to the shaking itself but often result from secondary effects of the seismic event. The primary ways earthquakes can lead to fires include: 1. 2. 3. 4. - To reduce the risk of fires caused by earthquakes, it\'s important for communities to implement the following me asures: 1. **Building Codes and Retrofitting:** Implement and enforce building codes that enhance the earthquake resilience of structures. Retrofitting older buildings to meet modern seismic safety standards can prevent structural failures that might lead to fires. 2. **Emergency Preparedness:** Educate the public about earthquake preparedness, including having emergency kits, evacuation plans, and awareness of how to safely shut off utilities. 3. **Vegetation Management:** Implement proper vegetation management practices to reduce the risk of wildfires, especially in areas prone to earthquakes. 4. **Emergency Response Planning:** Develop and regularly update emergency response plans to address the immediate aftermath of earthquakes, including firefighting strategies and evacuation procedures. f. **Flood** - Earthquakes can lead to floods through several mechanisms, although the relationship is indirect. The primary ways earthquakes can contribute to flooding include: 1. 2. 3. - To mitigate the risk of flooding associated with earthquakes, it\'s essential to consider the following: 1. **Dam Safety Measures:** Implement and enforce strict safety measures for dams and levees, including regular inspections, maintenance, and upgrades to ensure they can withstand seismic activity. 2. **Early Warning Systems:** Develop and maintain early warning systems that can provide alerts about potential dam failures, landslides, or other earthquake-induced events that may lead to flooding. 3. **Land-Use Planning:** Implement land-use planning strategies that take into account the potential for landslides, river channel changes, and other geological impacts that may result from earthquakes.