Geology: Earthquakes
What is the difference between seismic hazard and seismic risk?
Seismic Hazard: Potential for damaging effects caused by Earthquakes (magnitude, distance to fault, underlying geology) Seismic Risk: Chance of injury, damage or loss resulting from seismic hazards (population, building codes)
How do tsunamis differ from normal wind-driven waves?
Compared to ordinary wind-driven ocean waves, tsunamis have the following characteristics: Very long wavelength (100 to 500 km) - time from one peak to the next in the open ocean. Very long period (10 minutes to 2 hours apart) Drag on the ocean floor everywhere Move very fast (as fast as 800 km/hour) Can be low amplitude (height) in the open ocean (just a few feet) Slow dramatically in shallower water (coast) (to 150 to 300 km/hour) Amplitude (wave height) increases dramatically as the waves approach the shore (up to many meters, depending on local conditions)
What should you do before and after an earthquake?
Before: Secure your space by identifying hazards and securing moveable items Plan: create a disaster plan and decide how you will communicate in an emergency Emergency kit: organize disaster supplies in convenient locations Minimize financial hardships by organizing important documents, strengthening your property, and considering insurance. After: If you are near the coast, move uphill or inland as soon as you can Evacuate if necessary Major earthquakes will be followed by aftershocks, so try to prevent further damage Communicate
What types of earthquake hazards and levels of damage would one predict during a future large magnitude earthquake in the Bay Area?
Brick buildings and gas/stoves due to it being the main cause of fires throughout the city. Liquefaction caused mud volcanoes where muddy water was squeezed up to the surface. Liquefaction contributed to broken gas mains and toppled power lines, causing numerous fires. Water lines were broken, making fire-fighting difficult During a quake, regions in the Bay Area underlain by weak, muddy sediment will experience much greater damage than those areas underlain by bedrock due to the wave amplification effect
displacement (offset)
the amount of movement or slip across a fault plane
Focus / Hypocenter
the point within the earth where an earthquake rupture starts
Fault?
(geology) a crack in the earth's crust resulting from the displacement of one side with respect to the other
What were the moment magnitudes of the 1989 Loma Prieta and 1906 San Francisco Earthquakes? Based on these magnitudes, approximately how much stronger shaking (wave amplitude) and how much more total energy release would one predict for the 1906 San Francisco earthquake?
1906 SF Earthquake: M 7.9 Total Fault ruptured was 476 km (296 miles) Felt in LA; 3000 dead, mainly because of fires Mercalli intensity up to XI (extreme) 1989 Loma Prieta M 6.9 10x less wave amp, 32x less energy 62 fatalities $6 billion damage
What is an aseismic creep and what part of the San Andreas is exhibiting this type of behavior?
Aseismic creep is the measurable surface displacement along a fault in the absence of notable earthquakes. An aseismic creep exists along the Calaveras fault in Hollister, California.
What are mid-ocean ridges, continental rifts, subduction zones, and continental collision zones? You should be able to associate the earthquakes (and regions) we discussed in class with one of these plate boundaries (including transform plate boundaries).
Divergent Plate Boundaries: Mid-ocean Ridges: Oceanic plates pull apart, mantle wells up to fill the gap, and as the hot material rises it melts and erupts. Many earthquakes and volcanic activity occur at mid-ocean ridges. Ex: The mid-Atlantic ridge Continental rifts: Continental crust pulls apart at continental rifts. Ex: East African Rift valley. Convergent Plate Boundaries: Subduction zone: Where an oceanic plate collides with another plate, one plunges into the interior and the other overrides. The largest earthquakes ever recorded have occurred at subduction zones. Ex: Japan trench, Alaska, Peru-Chile trench (S. America), Sumatra, Cascadia (Western US) Wadati-Benioff zone of earthquakes marks the path of the subducted slab as it descends into the mantle The plate bends as it turns into the interior, creating a deep trench in the ocean Ex: Marianas Trench, the deepest point in the ocean Major mountain belts and the volcanic chains occur in this tectonic setting Continental Collision zone: Where continents converge, the buoyant continental crust builds up, creating mountain belts. Ex: Indian plate colliding with the Eurasian plate to create the Himalayas. Transform Plate Boundaries: Ex: San Andreas fault system (California USA); North Anatolian fault system (Turkey) Offset features can provide information about the recurrence interval.
What are the three major types of plate boundaries and how do plates behave (style of deformation) at each of these boundaries?
Divergent: two plates pull apart; new plate forms at this type of boundary. Convergent: two plates collide; one plunges into the interior and the other overrides it. The plate is destroyed at this type of boundary. Transform: two plates slide by one another; plate area is conserved- neither formed nor destroyed - at this type of plate boundary.
How does the Earth's layered structure vary in terms of composition and material strength? In answering this question, you should be able to describe the crust, mantle, core, lithosphere, and asthenosphere.
Earth's structure characterized by composition: Crust: buoyant, rich in silicate rocks. Very thin (6 km in the oceans, 36 km in the continents) Mantle: Made up of solid silicate rocks, but with a different composition than the crust. Thickness is about half the radius of the Earth (2900 km) Core: Metal: mostly iron and nickel, with similar amounts of other elements. The source of the earth's magnetic field. Described in terms in of strength of material: Lithosphere: strong, outer layer. Consists of crust and some of the upper mantle. This is what makes up the tectonic plates Asthenosphere: hotter, and weaker, than the lithosphere. Although it is solid, crystalline, under the elevated temperatures of the earth's deep interior, it flows like a fluid. Core: Consists of a solid inner core and a liquid outer core.
When is a tsunami warning issued and what techniques do scientists use to decide about issuing a warning?
Earthquake monitoring Dart Buoys Tide gauges Satellites Record of arrival
Across the globe, how are the majority of earthquakes and volcanoes distributed? Are the locations of earthquakes and volcanoes correlated?
Earthquakes and volcanoes are concentrated along linear trends across the earth. Earthquakes occur around the Pacific rim, along with the middle of the Atlantic ocean and across south Asia, earthquakes tend to be clustered along bands following plate boundaries. Volcanoes are also concentrated along some of the same lineaments as earthquakes. Volcanoes can also occur along mid-ocean ridges (divergent plate boundaries).
What methods can society use to enhance earthquake resilience?
Estimating the probability of earthquakes in a given location (forecasting) Earthquake early warning (not the same as the prediction) Earthquake resistant codes and construction practices. Earthquake preparedness.
What determines the size of tsunamis generated by earthquakes?
For tsunamis that are generated by underwater earthquakes, the amplitude of the tsunami is determined by the amount by which the seafloor is displaced.
What is a foreshock and why is it often difficult to classify earthquakes as foreshocks immediately after they occur?
Foreshocks: If larger events follow an earthquake then the preceding event is RECAST as a foreshock. Earthquakes are difficult to classify as foreshocks because they can occur seconds to days before the MAIN shock (or not at all.)
In what locations does seismic and volcanic activity take place far away from plate boundaries?
Hotspots: location of anomalous volcanic activity and earthquakes not associated with a plate boundary Ex: Hawaii, Galapagos, Iceland Intraplate seismic activity Ex: New Madrid fault zone
What hypothesis was the Parkfield Prediction Experiment testing and what observations did scientists use to develop this hypothesis? What was the outcome of this prediction experiment? (PART 2)
In 1993 the prediction window closed without an earthquake September 28, 2004: The magnitude 6.0 earthquake occurred! Magnitude 6.0 - as predicted No foreshock or other precursors observed 38 yrs after the last earthquake (longer than average recurrence interval) Although the lack of precursory phenomena was disappointing, the experiments yielded a great deal of information about the earthquake.
What type of fault and plate boundary did the Kashmir quake of 2005 occur on? What factors contributed to the large loss of life in this earthquake?
Kashmir is located at the juncture of the Eurasian and Indian tectonic plates—the collision of which caused the formation of the Himalaya Mountains—making it prone to intense seismic activity. Oblique-Slip Fault: A fault which has a component of dip-slip and a component of strike-slip is termed an oblique-slip fault. Nearly all faults have some component of both dip-slip and strike-slip, so defining a fault as oblique requires both dip and strike components to be measurable and significant.
What occurs during seismic wave amplification and liquefaction? How do these processes related to the damage during earthquakes in the Bay Area, including the 1906 San Francisco and 1989 Loma Prieta Earthquake?
Liquefaction is a process caused by seismic shaking in which water-saturated, muddy soil takes on the characteristics of a dense liquid rather than a wet solid mass. Clay flakes and sand grains in wet unconsolidated sediment lose cohesion and form a slurry of sediment and water during seismic shaking. Structures built above liquefied ground lose stability and tilt downward where underlain by a weak slurry of sand or clay. Liquefaction caused mud volcanoes where muddy water was squeezed up to the surface. Liquefaction contributed to broken gas mains and toppled power lines, causing numerous fires. Water lines were broken, making fire-fighting difficult.
How is the magnitude of an earthquake calculated and what is the difference between the Richter magnitude, Moment magnitude, and Modified Mercalli Intensity scale?
Magnitude is a measure of the size of an earthquake as a function of the maximum ground motion measured by a seismograph Richter magnitude is a measure of absolute "strength" of an earthquake and each quake has only one Richter magnitude
What type of fault and plate boundary did the "Mexico City" earthquake of 1985 occur on? Was the earthquake epicenter close to Mexico city and why was there significant damage in Mexico city during this earthquake?
Mexico is located on one of Earth's subduction zones, where the ocean floor of the Cocos tectonic plate is forcing its way down ("subducting") beneath the continental edge of the North American plate (and Pacific Plates) Mexico City is about 140 miles away (semi close) from the epicenter and since the city is "built" on a "waterbed" / historic lake, the prevailing silt and volcanic clay sediments amplify seismic shaking. Damage to structures is worsened by soil liquefaction which causes the loss of foundation support and contributes to a dramatic settlement of large buildings.
Where do most major earthquakes occur in terms of geography and depth? Where do the largest earthquakes on Earth occur and stress state are they associated with? What is the Wadati-Benioff zone and how does this relate to where the largest earthquakes on Earth occur?
Most major earthquakes occur on the Pacific rim and they are shallow- focus earthquakes (in the upper 20 km of the crust) The largest earthquakes on record occur in subduction zones. Compressional stresses are associated with subduction mega-thrusts Wadati-Benioff zone: a planar zone of seismicity corresponding with the down-going slab in the subduction zone. Different motions along this zone cause earthquakes
How often do major tsunamis occur and where are most of these tsunamis concentrated?
On average, two tsunamis occur per year throughout the world which inflicts damage near the source. Approximately every 15 years a destructive, ocean-wide tsunami occurs. The earthquake must cause significant vertical deformation of the seafloor in order for a tsunami to occur.
What is the definition of a recurrence interval? What factors control the size of earthquake recurrence intervals? Why are most observed earthquake occurrence intervals almost never regular? What observations and tools do geologists use to estimate earthquake recurrence intervals on a given fault strand?
Recurrence Interval: Average time period between repeated geologic events such as earthquakes, floods, or volcanic eruptions We estimate recurrence intervals for earthquakes through the historical record of seismicity. And paleoseismology (in which researchers identify and establish the date of ancient earthquakes to determine the recurrence interval) Complexity and irregular recurrence intervals can occur due to the geometry or interaction of fault segments. (Seismographs)
What are seismic waves, how are they generated and how do they travel away from the focus/epicenter? Does the total amount of seismic energy increase or decrease as they move away from the focus?
Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. They are the energy that travels through the earth and is recorded on seismographs. Seismic waves are usually generated by movements of the Earth's tectonic plates but may also be caused by explosions, volcanoes, and landslides. Seismologists use seismographs to record the amount of time it takes seismic waves to travel through different layers of the Earth. The amplitude recorded on seismograms will decrease with increasing distance from the earthquake
What are seismographs and seismograms? What information is recorded onto a seismogram and what is the relative order in which different seismic waves arrive at a seismograph station (and are recorded onto a seismogram)? What properties of an earthquake can the information on a seismogram be used to determine?
Seismograph: Earthquake- recording instrument Seismogram: a record of seismic waves on a seismograph. This is a recording of the relative movement between the moving Earth and the instrument. P waves always arrive first, then s waves and surface waves. The properties of an earthquake can be used to determine: 1. The time of the earthquake 2. The distance from the epicenter of the quake 3. The magnitude of the quake
In the Bay Area, do most of the major faults within the San Andreas fault system have the same sense of motion?
Si
Does the Southern San Andreas system have a high probability of producing a large magnitude earthquake (> 6.7 or greater) in the next few decades?
Similar to the SAF system in the Bay Area, all faults in the SAF system in Southern California have the same sense of motion (the block to the east of any fault in the system will always move southeasterly during an earthquake on that fault) - all of these faults collectively soak up accumulated tectonic strain and are thus seismically active - They commonly pop off magnitude 2 and 3 earthquakes (generally too small to be felt) and all are capable of generating large-magnitude quakes Many of the strike-slip and blind thrust faults that underlie Southern California are capable of M7 earthquakes
What are the main sources of damage and destruction in an earthquake? How would you expect these sources of damage and destruction to vary between strike-slip (transform) and subduction (convergent) and continental collision (convergent) plate boundaries?
Sources of destruction in earthquakes: Liquefaction Soft-story construction/failure Broken gas lines lead to fires Wave amplification due to soft soil shaking Unreinforced masonry (brick and mortar) earthquake -triggered landslides
What is stick-slip motion and how does it relate to the earthquake cycle?
Stick-slip refers to the fast movement that occurs between two sides of a fault when the two sides of the fault become unstuck. The rock becomes distorted, or bent, but holds its position until the earthquake occurs. When the rock snaps back into an unstrained position it is called elastic rebound. Stick-slip displacement on a fault radiates energy in the form of seismic waves, creating an earthquake.
What are the different hazards that can cause significant damage and loss of life in a subduction zone megathrust earthquake? What different factors in the Japanese earthquake of 1923 (Tokyo), 1995 (Kobe) and 2011 (Tohoku) lead to significant loss of life?
Stress builds upon the part of the megathrust that is "locked". Then a tectonic plate is forced underneath the other. These earthquakes can be most destructive because the can exceed Mw 9.0. Also, a big factor that leads to significant loss of life in the Japanese earthquake of 1923 (Tokyo), 1995 (Kobe) and 2011 (Tohoku) were the Tsunami Run-up that reaches over 38.9 meters.
Despite being relatively close in time, location and magnitude, why were there far fewer deaths in the 1975 Haicheng earthquake than in the 1976 Tangshan earthquake? How does this relate to the events preceding the 1975 Haicheng earthquake?
Tangshan: Struck with no warning, foreshocks, or other precursors (250,000 deaths and massive destruction) Haicheng: Proceeded by many signs of unusual behavior Several small to moderate earthquakes (foreshocks) Groundwater changes, random gas release Reports of unusual animal behavior
What series of steps and processes does elastic rebound theory invoke for how earthquakes occur and faults behave over time? In answering this question, you should be familiar with the terms strain, elastic, rebound, locked, friction, strain energy and seismic waves.
Tectonic plate motion generates stress on rocks which are the push, pull or shear that material experiences. Rocks also accumulate elastic strain energy, which is the change in shape of the material. On either side of the fault under stress the rock bends as an elastic material, meaning that once the stress is relaxed, the rock will rebind to its original shape. The fault will remain locked by the friction of the rocks against each other. Strain energy then builds until the frictional strength of the rocks is overcome, and rocks rupture along fault planes. The rupture then migrates along the fault plane and releases strain energy as seismic waves.
What does the Gutenberg-Richter relationship illustrate between the number and size of earthquakes both globally and on a specific fault strand? How does this relationship relate to aftershocks?
The Gutenberg-Richter relationship between size and number of earthquakes: there are many more small events than large events (aftershocks) For each M 8 earthquake there are: 10 Magnitude 7 earthquakes 100 M 6 1000 M 5 10,000 M 4 etc.
Is there a high probability that a large (magnitude 6.7 or greater earthquake) will occur in the Bay Area in the next 30-40 years? Within the Bay Area San Andreas fault system, which fault is considered the most likely to have a large magnitude event in this time frame and why is it considered especially dangerous?
The Hayward fault is considered the most dangerous fault in North America, partly because of the size of potential quakes on it, and partly because of the 2-3 million people living along it. The last quake on the Hayward was in 1868 when the Bay Area was very undeveloped (estimated to have been an M6.9). The massive amount of development since the last major quake on the Hayward ensures that the next quake will be costly. Structures west of the Hayward are built on soft mud and bay fill that will shake violently during an earthquake, enhancing the probability of liquefaction and structural failure.
How does the amplitude on a seismogram and amount of energy-related vary on the Richter magnitude scale? You will need to know the exact numbers for the test.
The Richter scale is logarithmic means that the amplitude on a seismogram for an M5 is 10 times higher than the measured amplitude for an M14 In terms of the total energy released, each whole number increase on the Richter scale translates to a 30 times increase in the amount of energy released
What is the "Big Bend" in the Southern San Andreas Fault system and how does it influence both the state of stress and types of faults/earthquakes in Southern California? What topographic features are associated with the big bend?
The San Andreas takes on more of an east-west trend north and east of Los Angeles - called the 'Big Bend' - the North American and Pacific plates grind against each other along the Big Bend of the SAF, with a major component of the compression ('pushing' stress) added to the shear stress. This combination of compression plus shear (translational motion) is called transgression. Transpression along the Big Bend squeezes a rock, folding and faulting them, and raising them upward as the east-west oriented Transverse Ranges. The Transverse Ranges (e.g., Santa Ynez, San Gabriels, Santa Monica, San Bernardino, San Jacinto) are among the most rapidly rising mountains in the world
Along the length of the San Andreas Fault system, what regions contain simple linear fault segments verse broad networks of sub-parallel faults?
The San Andreas, Hayward, Rodgers Creek, Calaveras, and San Gregorio faults. Faults in the SAF system generally have the same relative sense of motion (the block to the east of any fault in the system moves southeasterly during an earthquake on that fault) All of the faults in the Bay Area collectively accumulate tectonic strain and are seismically active Much smaller, less active (as far as we know) faults occur within the broad zone of big, long faults
Epicenter?
The point on Earth's surface directly above an earthquake's focus
What is the theory of plate tectonics and how does it relate to the majority of major geologic processes, including the majority of seismicity and volcanism? In answering this question, you should know the definition of the terms plate and tectonics.
The theory of plate tectonics is the theory that Earth's outer shell is divided into several plates that glide over the mantle. Plate tectonics: The driving mechanism for most major geologic processes. Plate: Refers to the shape of the strong moving sections of the earth's outer surface Tectonics: refers to the large scale movement and deformation of the earth's outer surface.
How is the epicenter of an earthquake located?
The time gap between the arrival of the p wave and the arrival of the s wave varies with distance from quake epicenter. So, the longer the time gap, the greater the distance between the seismograph and the epicenter Need at least 3 stations to triangulate to determine epicenter location
How can geological maps of the Bay Area help predict the location of seismic wave amplification and liquefaction during future earthquakes?
There has been extensive development and population growth since 1906 the 1906 quake and much of it has occurred directly on top of the SAF system - this virtually guarantees that the next large earthquake in the area will be far more devastating than either the 1906 or 1989 quakes Catastrophic damage would occur by the combined effects of severe ground shaking, landslides on steep slopes, wave amplification and liquefaction. Liquefaction in low-lying areas underlain by wet, muddy sediment, and fires triggered by broken infrastructure.
What hypothesis was the Parkfield Prediction Experiment testing and what observations did scientists use to develop this hypothesis? What was the outcome of this prediction experiment?
There may be a characteristic earthquake at Parkfield (M 6.0, recurrence interval ~22years....) The hypothesis must be tested by making specific, clear, measurable predictions. 1985: US Geological Survey issued an official prediction: Magnitude 5.5 to 6.0 expected (90% chance) on the Parkfield fault segment between 1985 and 1993 Extensive instrumentation was installed to monitor the fault and "catch" the earthquake (and any precursors) Scientists worked with state and federal emergency response agencies to establish an alert system
Is stick-slip behavior on faults periodic or chaotic and unpredictable?
They are periodic, think of the example about the fence moving but not too noticeable over time.
On what type of faults did the 2010 Haiti and 1999 Izmit (Turkey) earthquakes occur? Are there likely to be large earthquakes in the future on these fault systems? What are the different factors that lead to the amount of infrastructure damage and loss of lives in these earthquakes? For example, how did the relative size of the earthquakes, underlying geology, infrastructure, and post-earthquake rescue efforts play a role?
They occurred on strike-slip faults. It is likely for there to be other large earthquakes in the future on these fault systems. In Haiti, most deaths occurred in concrete or cinder block buildings without reinforcing steel structure and in Izmit "pancaked," concrete buildings caused a great deal of the destruction.
What type of fault did the 1971 San Fernando and 1994 Northridge earthquakes occur on? Why makes this type of fault particularly dangerous?
This earthquake occurred on the San Fernando fault zone, a zone of thrust faulting which broke the surface in the Sylmar-San Fernando Area. Occurred on previously unrecognized blind thrust. Blind thrusts are by nature buried beneath the surface and are thus difficult to see - we commonly don't know they exist till they rupture, causing an earthquake. We are "blind" to them. Thus blind thrusts create a very dangerous hazard in Southern California, adding significantly to the hazard from the San Andreas strike-slip fault system
What type of deformation is associated with shear stress and what types of faults form along plate boundaries dominated by shear stresses?
Transpression: Combination of compression plus shear stress is transpression -The North American and Pacific plates grind against each other along the Big Bend of the SAF, with a major component of compression ('pushing' stress) added to the shear stress
What type of events are capable of generating a large tsunami?
Tsunamis from Volcanic Eruptions Tsunamis from Landslides Tsunamis from Asteroid Impacts Tsunamis from Earthquakes
Relatively, how good are we at predicting where earthquakes are likely to occur, how large an earthquake a given fault segment is likely to produce and when earthquakes are likely to occur?
We are pretty good at predicting WHERE earthquakes are likely to occur. We are moderately good at predicting HOW LARGE an earthquake a given segment of a fault is likely to produce (Not really for large earthquakes). However, we are NOT good at predicting WHEN an earthquake will occur because of the limited data, complicated physics, the nonlinear system, and no faults are the same.
What are aftershocks and how do they relate to earthquakes and the earthquake cycle?
When an earthquake occurs some of the energy released from the sudden fracturing of rock is transferred to the rocks nearby, which adds to the pushing, pulling, and twisting stresses already placed on them. When these stresses are too much for the rocks to bear, they break as well, releasing a new round of pent-up energy and creating new faults in the rock. Aftershocks tend to be the most severe and happen more frequently in the hours and days that follow an earthquake. However, their magnitude and frequency decrease over time.
What are the topographic features of fault traces? Which of these features are likely to vary between a strike-slip fault and normal (extension) or thrust (compression) fault?
linear features across large distances such as linear valleys and ridges •sag ponds and elongate lakes •linear alignments of springs (often marked by lines of trees) •distinctive offsets in features that cross the fault (such as stream channels) •sharp scarps (abrupt ramps or steps in the topography) from recent vertical movement •the geology will commonly be different on either side of a fault, as well. One rock type will be juxtaposed against a completely different rock type on either side of a fault, the result of continuous stick-slip fault behavior over time