Ch 10: Earthquakes

What is a fault system?

A plate boundary can rarely be described as a single fault; rather, the zone of deformation between two moving plates usually comprises a network of ineracting faults—a fault system

How does a seismograph work?

A seismograph works by suspending a weight from bedrock, which remains motionless during an earthquake. A rotating drum that is affixed to the moving bedrock moves as the rock moves, recording the relative displacement between the stationary weight and the rotating drum. A seismograph can measure P waves, S waves, and surface waves.

What are building codes?

Building codes specify the forces a structure must be able to withstand, based on the maximum intensity of shaking expected in an area. In the aftermath of an earthquake, engineers study buildings that were damaged and recommend modifications to building codes that could reduce future damage from similar earthquakes.

Besides earthquakes, what other disturbances can cause tsunamis?

Disturbances of the seafloor caused by landslides or volcanic eruptions can also produce tsunamis. For example, the 1883 explosion of the Indonesian volcano Krakatau generated a tsunami that reached 40 m in height and drowned 36,000 people on nearby coasts.

What should you do during an earthquake?

Drop, Cover, Hold on! 1. Drop to the floor 2. Take cover: under a sturdy desk or table 3. Hold on! so it doesn't move away from you Wait until the shaking stops before you move!

If you live on the coast, the best warning system is a very simple one. What is it?

If you feel a strong earthquake, move quickly away from the coast lowlands to higher ground!

How long does it take for computers to pinpoint an earthquake's focus?

In a fraction of a minute, computers can pinpoint the earthquake's focus, measure its magnitude, and determine its fault mechanism.

Where are tsunamis most commonly caused by megathrusts events?

Tsunamis caused by megathrust events are most common in the Pacific Ocean, which is ringed with very active subduction zones.

Why do most active faults (including the San Andreas) NOT conform to elastic rebound theory?

1. All the strain accumulated since the last earthquake may not bereleased in the next earthquake (so the rebound may not be complete). 2. The stress on one fault may change because of earthquakes on nearby faults. 3. Over the longterm, the strength of the fault rocks themselves may change (these are only SOME of the reasons why earthquakes are so difficut to predict!)

List the 4 personal steps to earthquake safety BEFORE an earthquake

1.identify potential hazards in your home 2. create your disaster plan 3. create your disaster kit 4. identify your building's potential weaknesses & begin to fix them

What is the "Big Bend" in the San Andreas fault? And why has the "Big Bend" caused recent earthquakes?

A bend in the fault (the "Big Bend") creates compressive forces that cause thrust faulting just north of Los Angeles. Thrust faulting in this "Big Bend" was responsible for two recent deadly earthquakes: San Fernando (1971, mag 6.6, 65 dead); Northridge (1994, mag 6.7, 58 dead). Over the past several million years, this thrust faulting has raised the San Gabriel mountains from 1800 to 3000 meters.

How does the Basin and Range province relate to the San Andreas fault system?

A complication in the San Andreas fault system is the plate extension east of California in the Basin and Range province, which spans the states of Nevada, Utah, and Arizona. This broad zone of deformation is connected with the San Andreas fault system through a series of faults running along the eastern side of the Sierra Nevada and the Mojave Desert.

What is a disaster plan (to be followed after an earthquake?)

A disaster plan is what you will do before, during, and after an earthquake. It should include safe spots you can go during the shaking (under sturdy desks and tables); safe spot outside your home where you cdan meet others after the shaking stops; contact numbers, including someone OUTSIDE the area who cdan be called to relay information in case local communications are disrupted.

What is a fault scarp, and how is it useful for field seismologists?

A fault scarp is a slope that is created by motion along a fault line. Seismologists can sometimes determine the fault mechanism from observing fault scarps in the field. However, most ruptures are too deep to break the surface, so they must use seismograms to deduce the fault mechanism.

Define TSUNAMI

A tsunami is a fast moving and potentially extremely destructive sea wave that is generated by an earthquake that lifts the seafloor and propagates across the ocean, increasing in size when it reaches the shore. Tsunamis are by far the deadliest and most destructive hazards associated with the world's largest earthquakes: the megathrust events that occur in subduction zones.

According to elastic rebound theory, what is rebound? And what does the rebound cause?

Accoring to elastic rebound theory, the energy that is slowly built up by elastic strain as the two blocks (on each sie of the fault) are pushed in opposite directions is like the elastic energy stored in a rubber band when it is stretched ==> the sudden release of energy when a fault slips is the rebound that occurs, like when the rubber band breaks, some of which is radiated in seismic waves, causing violent shaking many kilomenters away from the fault.

What should you do after an earthquake?

After the shaking stops ==> check for damage and injuries that need attention. Take care of yourself first: get to a safe location and remember your disaster plan ==> if trapped, protect your mouth, nose & eyes; signal for help (cell, whistle, or knocking loudly); check for injuries & treat others. Check for fires, gas leaks, damaged electrical systems, spills. Stay away from damaged structures. Then follow your disaster plan.

Define aftershocks

Aftershocks follow the triggering event (or mainshock) in sequences, and their foci are distributed in and around the rupture surface of the mainshock

If all other factors are equal, what is the main factor that determines the intensity of the seismic waves produced by an earthquake, and therefore its potential destructiveness?

All other factors being equal, the main factor that determines the intensity of the seismic waves produced by an earthquake, and therefore its potential destructiveness, is its MAGNITUDE

In what way do aftershocks exemplify the complexities of earthquakes that cannot be described by simple elastic rebound theory?

Although fault slipping during the mainshock decreases the stress along most of the rupture surface, it can increase the stress on parts of the fault surface that did not slip, or where the slip was incomplete. Aftershocks happen where that stress exceeds the rock strength.

What was the main difference between the experience of those in the 2004 Sumatra (Indian Ocean) Tsunami and the Tohoku, Japan, Tsunami of 2011?

Although there were over 20,000 deaths in Japan, many thousands of lives were saved due to the early warning system. And in fact, due to the Pacific Tsunami Warning Center in Hawaii, the Pacific Islands (including Hawaii) and the western coastlines of the Americas all evacuated prior to the tsunami. No such warning system existed in the Indian Ocean, so the 2004 tsunami struck with essentially no warning, killing tens of thousands.

An earthquake occurs when rocks under ________ suddenly fail along a _________ _________.

An earthquake occurs when rocks under stress suddenly fail along a geological fault.

What is an intraplate earthquake?

An intraplate earthquake, rather than occurring at plate boundaries, originates within plate interiors. This is only a small percentage of earthquakes, and they are relatively shallow. Most occur on continents, but among them are some of the most famous in American history: one near New Madrid, Missouri (1811); one in Charleston, SC (1886); one in Cape Ann, MA (1755). Many occur on old faults that were once ancient plate boundaries.

Charles Richter (1935) devised the Richter magnitude scale, in which two earthquakes at the same distance from a seismograph differ by one magnitude unit if the peak amplitude of the ground movement they produce differs by a factor of ________.

Charles Richter (1935) devised the Richter magnitude scale, in which two earthquakes at the same distance from a seismograph differ by one magnitude unit if the peak amplitude of the ground movement they produce differs by a factor of TEN.

What are transform-fault boundaries characterized by?

Earthquake activity at transform-fault boundaries are most often found at mid-ocean ridges. These earthquakes have strike-slip fault mechanisms, and the slip direction is left-lateral where the ridge crest steps right, and right-lateral where it steps left. The San Andreas fault is a right-lateral transform fault.

How do earthquake early warning systems work?

Earthquake early warning systems detect strong shaking near an earthquake's epicenter and transmit alerts ahead of the seismic waves. These warning times depend primarily on the distance between the user and the earthquake epicenter.

Which countries already have early warning systems deployed? Is the United States one of these countries? Which country(ies) provide(s) public alerts?

Earthquake early warning systems have been deployed in these countries: Japan, Romania, Taiwan, Turkey, and a prototype in the United States (not yet deployed); Japan is the only country with a nation-wide system that provides public alerts. A national seismic network of nearly 1000 seismographs is used to detect earthquakes and issue warnings, via the Internet, satellite, cell phones, and authomated control systems that stop trains, and place sensitive equipment in safe mode.

Why is it that earthquake magnitude by itself does not describe seismic hazard?

Earthquake magnitude alone does not describe seismic hazard, because destruction generally weakens with distance from fault rupture. Therefore a mag 8 earthquake in a remote area may cause far less (or even no human or economic) loss whereas a mag 6 earthquake in an unprepared city such as Christchurch 2011 may be devastating.

Explain what is meant by the "chain reaction" of earthquakes

Earthquakes proceed as a chain reaction in which the primary effects of earthquakes are faulting and ground shaking. These trigger secondary effects, which include landslides, tsunamis, collapsing structures, and fires. Examples of this are the devastating fires of the 1906 San Franciso earthquake and the deadly tsunamis of the Indian Ocean in 2004 & Tohoku Japan in 2011.

Can you explain the elastic rebound theory, proposed by Henry Fielding Reid (1910) after studying the Great 1906 San Francisco Earthquake?

Elastic rebound theory explains the build up and release of stress in rocks before, during and after earthquakes. Rocks on either side of a fault are locked in place by friction and will slowly deform over time. When the stress exceeds the strength of the rock, the rocks will slip suddenly in a rupture starting at one point but quickly extending over a section of the fault, causing an earthquake. After the earthquake, the fault locks up again, and the stress slowly builds again: the earthquake cycle repeats.

Why is estimating seismic risk such a complex job?

Estimating seismic risk is complex because exposure to seismic damage involved not just seismic hazard but also exposure to seismic damage, which varies according to population and density of buildings and other infrastructure. Additionally, it looks at fragility: vulnerability of its built enbironment to seismic shaking. (EX: Recent earthquake in Turkey in part so devastating because of fragility of buildings!)

What do seismologists believe about the very largest, most extreme earthquakes (magnitude 10)?

Even the largest subduction zone megathrusts are too small to create magnitude 10 earthquakes, so seismologists believe that events of such extreme size do not follow the rule of earthquake frequency: they believe that these extreme events are even less frequent than once per century.

How do land-use policies impact exposure to seismic hazard?

Exposure of buildings and other structures to seismic hazard can be dramatically reuced through policies that restrict land uses in high-hazard areas. For example, these policies can prohibit or restrict erecting buildings on active faults.

T or F: Most of the damaging earthquakes in Southern California have occured on the main San Andreas "master" fault

False! In fact, most of the damaging earthquakes in Southern California during the past century have occurred on the subsidiary faults, not the "master" fault!

Explain how fires can erupt as a result of earthquakes

Fires can erupt as a result of the nature of the built environment itself: for example, ignited by ruptured gas lines or downed electric power lines. In addition, damage to water mains in the earthquake can make firefighting all but impossible—this circumstance contributed to the burning of San Francisco after the 1906 earthquake. Most of the 140,000 fatalities of the 1923 Kanto earthquake resulted from the fires following the earthquake in Tokyo and Yokohama.

In what ways are GPS receivers useful in measuring lithospheric plate movements?

GPS receivers can record: a) the slow movements of lithospheric plates b) the strain that builds up from the movement of lithospheric plates c) the sudden slipping on a fault when it ruptures in an earthquake

How is GPS useful in analyzing silent earthquakes (also called slow-slip events)?

GPS receivers have found surface movements at convergent plate boundaries that reflect short-lived creep events (silent earthquakes). Even though these silent earthquakes do not trigger destructive seismic waves, they can release large amounts of stored strain energy.

Generally, the longer the fault rupture, the ___________ the fault slip.

Generally, the longer the fault rupture, the greater the fault slip

Geologists can reliably predict which faults are likely to produce earthquakes over the ________ term, but not at all precisely when a particularly fault will rupture at a particular time in the _______ term.

Geologists can reliably predict which faults are likely to produce earthquakes over the long term, but not at all precisely when a particularly fault will rupture at a particular time in the short term.

What past event alerted geologists to the danger of an earthquake and tsunami in the Pacific Northwest (the Cascadia subduction zone)?

Geologists found evidence of a mag 9 earthquake in 1700, before any written historical accounts of the area existed, that caused major ground subsidence along the Cascadia coastline and left a record of flooded, dead coastal forest. A tsunami at least 5 m high hit Japan.

Why are death tolls especially high in densely populated areas of developing countries?

In developing countries, buildings are often constructed from bricks and morar without steel reinforcement. EX: Haiti's 2010 mag 7 earthquake: destroyed 250,000 homes and 30,000 commercial buildings, killed more than 230,000 people. Brookie, this is in contrast with Los Angeles, where buildings are constructed with STRICT building codes and with steel reinforcement (including the building you live in!

How can aftershocks be more dangerous than the mainshock?

In some cases, in a populated region, the shaking from large aftershocks can compound the damage caused by the mainshock. For example, the mag 7 earthquake just outside Christchurch, New Zealand (2010) ==> extensive damage, but noboy killed; however, mag 6.3 aftershock struck right beneath center of Christchurch (2011) ==> collapsed builings and killed 185 people ==> damages several times greater than mainshock

In Los Angeles, what recent event stimulated a state response of changing policy and planning to protect future damage?

In the 1971 San Fernando earthquake, a fault ruptured under a densely populated area of Los Angeles, and destroyed almost 100 structures. The state of CA responded in 1972 with a law restricting construction of new buildings across an active fault. Also, if an existing residence sits on or very near a fault, this must be disclosed to potential buyers.

How high did the Tohoku Tsunami (2011) reach?

In the coastal city of Miyako, the water mass reached an astounding 38 m (123 ft!) above normal sea level, destroying nearly everything in its path. It also traveled across the Pacific, attaining heights of more than 2 m along the coast of Chile, 16,000 km away.

How has the U.S. fared compared to the rest of the world in terms of deaths due to earthquakes in the last decade? How about in terms of disruption of economies and damages?

In the last decade, earthquakes worldwide have killed more than 700,000 people. But in the U.S., only about 120 people have died in that time from earthquakes. The two earthquakes on the San Andreas fault (1989 Loma Prieta & 1994 Northridge) amounted to $50 billion in damages, but the damages in worldwide earthquakes were far greater: eg: 2011 Tohoku (20,000 dead; the most expensive natural disaster in recorded history)

What have we recently (in the past couple of decades) come to appreciate about the Pacific Northwest and seismic hazard?

In the past couple of decades, we have learned that an earthquake in the Cascadia subduction zone (N. CA through OR and WA to British Columbia) could produce a Tsunami as large and devastating as the 2004 Indian Ocean or 2011 Tohoku Tsunamis.

Intermediate-focus earthquakes occur in the ___________ ____________ . Deep focus earthquakes ALSO occur in the _________ __________ .

Intermediate-focus earthquakes occur in the descending slab. Deep focus earthquakes ALSO occur in the descending slab.

Large shallow earthquakes occur mainly on __________ faults at the plate boundary.

Large shallow earthquakes occur mainly on thrust faults at the plate boundary.

What are megathrusts?

Megathrusts are the huge thrust faults that form the boundaries where one plate is subducted beneath another.

Even though "Richter Scale" is the commonly used term for measuring earthquakes, what is the preferred measurement for seismologists direclty relating to the physical properties of the faulting that causes earthquakes?

Moment magnitude: the seismic moment of an earthquake is a number proportional to the product of the area of faulting and the average fault slip. The corresponding moment magnitude increases by about one unit for every tenfold increase in the area of faulting.

Most earthquakes are ( very small / very big ) with rupture dimensions (much less / much more) than the depth of the focus so that the rupture ( never breaks / usually breaks ) the ground surface.

Most earthquakes are very small with rupture dimensions much less than the depth of the focus so that the rupture never breaks the ground surface.

What is the range of depth of most earthquakes in continental crust? How deep CAN they get? How about in subduction zones?

Most focal depths of earthquakes in continental crust range from 2 to 20 km. Below 20 km is rare because under the high temps and pressures found below 20 km, continental crust behaves ductily, not brittly. In subuction zones, however, where cold oceanic lithosphere plunges into the mantle, earthquakes can originate at depths of almost 700 km.

Most large earthquakes are caused by ruptures of ( new / preexisting ) faults. When the fault slips, the stress is ( reduced / increased ); after the earthquake, the stress ( decreases / increases ) over time again. The faults involved in this repeated earthquake cycle are called (passive / active) faults.

Most large earthquakes are caused by ruptures of preexisting faults. When the fault slips, the stress is reduced; after the earthquake, the stress increases over time again. The faults involved in this repeated earthquake cycle are called active faults.

While some of the stored energy is released as seismic waves traveling outward from the rupture during an earthquake, what happens to most of the stored energy?

Most of the stored energy is converted to frictional heat in the fault zone or dissipated by rock fracturing

Most seismographic stations measure what three components of ground movements?

Most seismographic stations measure vertical, horizontal east-west, and horizontal north-south ground movements. Modern seismographs can detect ground oscillations of less than a billionth of a meter~displacements of atomic size!

What is the modified Mercalli intensity scale?

Named for Giuseppe Mercalli (1902), the modified Mercalli intensity scale assigns a value (Roman numerals I-XII) to the intensity of the shaking at a particular location. The scale ranges from I: not felt except by a very few ... to XII: Damage is total. Waves are seen on ground surfaces. Lines of sight and level are distorted. Objects are thrown upward into the air.

Which do we have more control over: seismic hazard or seismic risk? Why?

Not much can be done about seismic hazard, because we have no way to prevent or control earthquakes. However, there are many important steps that society can take to reduce seismic risk if the hazard is properly characterized.

What error in building judgement of the nuclear facilities in Japan led to disastrous results in 2007 and 2011?

Nuclear power plants should be built to avoid seismic and tsunamic hazards, including the need for water to cool the reactors; but in Japan, two nuclear facilities along the Japanese coast have been severely damaged in earthquakes: Kashiwazaki-Kariwa (2007) & Fukushimi-Daiichi (2011)

Define P waves

P waves: primary waves, similar to sound waves in the air, travel through solid rock, liquid, or gaseous materials ==> compressional waves, travel in a succession of compressions and expansions (push-pull waves); They pull particles of matter in the direction of their path

How do we calculate recurrance interval, according to elastic rebound theory?

Recurrance interval (according to elastic rebound theory) is calculated by dividng the fault slip in each rupture by the long-term slip rate. For example, the long-term slip rate on the San Andreas fault is about 30 mm/yr, so earthquakes with 4 m of slip should occur with a recurrance of about once every 130 years.

What is the recurrance interval?

Recurrance interval is the average time between large earthquakes at a particular location; according to the elastic rebound theory, the time required to accumulate the strain that will be released by fault slipping in a future earthquake.

Please compare the Richter Scale and Moment Magnitude

Richter Scale: older, lesser-used, determines magnitude based on logarhythm of the amplitude of waves recorded on seismographs. Valid for only certain frequency & distance ranges Moment Magnitude: measures the area the fault has slipped & how far it moved ==> converts to a value that can be compared to other earthquake magnitudes; Used universally as is more accurate and reliable measure of earthquake size

Define S waves

S waves: secondary waves, travel through solid rock at a little more than half the velocity of P waves; they are shear waves ==> displace material at right angles to their path of travel. S waves cannot travel through liquids or gases

Exaplain how seismic waves can be hazardous.

Seismic waves can shake structures so hard that they collapse. In fact, the ground accelerations near the epicenter of a large earthquake can approach and even exceed the acceleration of gravity, so an object lying on the ground surface can be thrown into the air. Very few manmade structures can survive such severe shaking, and those that do are severely damaged. The collapse of buildings and other structures is the leading cause of of casualties and economic

What are travel-time curves used for?

Seismologists use networks of sensitive seimographs around the world and highly accurate clocks to time the arrival of seismic waves from earthquakes AND from underground nuclear test sites to construct travel-time curves, which show how long it takes seismic waves of each type to travel a certain distance. They use these travel-time curves to determine the distance from the seismograph to the focus. Using the distances from three or more seismographic stations, they can locate the focus

What is a "ShakeMap"?

ShakeMaps are automated systems that deliver automated maps in nearly real time showing where the ground shaking is strong enough to cause significant damage, and help emergency managers and other officials deploy equipment and personnel as quickly as possible to save people trapped in rubble and to reduce further economic losses from fires and other secondary hazards.

Shallow earthquakes coincide with ___________ faulting at divergent boundaries and with ___________ faulting at transform-fault boundaries.

Shallow earthquakes coincide with normal faulting at divergent boundaries and with strike-slip faulting at transform-fault boundaries .

What are silent earthquakes?

Silent earthquakes are slow-slip events; the rocks slide past (or creep past) each other slowly over time (days, weeks, years, or even decades) and the energy is dissispated very, very slowly. Silent earthquakes slowly deform structures and crack pavements that cross the fault trace.

What are surface waves?

Surface waves are confined to earth's surface and outer layers (like ocean waves). Their velocity is slightly less than S waves. There are two types: 1) rolling motion in the ground 2) shakes the ground sideways. Surface waves are usually the most destructive waves in a large, shallow-focus earthquake

What is the International Decade of Natural Disaster Reduction?

The International Decade of Natural Disaster Reduction was an effort during the 1990's on the part of the United Nations to reduce the effects of natural disasters in general worldwide, and included the first global seismic hazard map. But because it was based on historical earthquakes, it may underestimate the hazard in some regions where the historical record is short.

Are you on MyShake? If you are not, please download it now :).

The MyShake App sends a warning to mobile phone users that shaking is about to occur. The system uses ground-motion sensors to detect earthquakes that have already started and estimates their size, location, and impact. It also has helpful ideas about preparedness for earthquakes. I have it and so should you! (I'll help you set it up so it doesn't wake you up for little earthquakes and make you unnecessarily anxious. My brother-in-law did this for me.)

What is the story of The Tsunami Stone of Aneyoshi?

The Tsunami Stone of Aneyoshi (on the Tohoku coastline of northeastern Honshu in the fishing hamlet of Aneyoshi) is a monument inscribed with Japanese characters warning that no houses should be built below it, as a result of two tsunamis (1896 & 1933) that wiped out all but six people in the villages ~ a reminder to live on higher ground.

How did the The Tsunami Stone of Aneyoshi become prophecy?

The Tsunami Stone of Aneyoshi became prophecy after the devastating mag 9.1 Tohoku Earthquake and Tsunami of 2011. The rupture started 100 km southeast of Aneyoshi; the deathtole along the Tohoku coastline was almost 20,000. While the enormous wave was 39 m (128 ft) above the shoreline, the residents above the Aneyoshi stone were safe.

What is the leading cause of casualties and economic damage during earthquakes?

The collapse of buildings and other structures is the leading cause of of casualties and economic damage during earthquakes. In cities, most casualties are caused by falling buildings and their contents.

Describe the intraplate earthquake near Bhuj, India (2001)

The deadly intraplate earthquake near Bhuj, India in the state of Gujarat (2001) occurred on a previously unknown thrust fault between the Indian and Eurasian plates. It was a result of compressive stresses from ongoing collision of these two plates. An estimated 20,000 people died. This intraplate earthquake shows that strong crustal forces can develop and cause devastating faulting far from modern plate boundaries.

Large earthquakes occur much less frequently than smaller ones. The frequency number decreases by a factor of ? for each magnitude unit.

The earthquake frequency decreases by a factor of 10 for each magnitude unit. Hence there are about 100,000 earthquakes with magnitudes greater than 3, about 1000 with magnitudes greater than 5, and about 10 with magnitudes greater than 7 each year. This frequency is expressed with the simple relationship in the graph shown here.

What is the fault mechanism, and how does it effect the pattern of ground shaking?

The fault mechanism is two things: the orientation of the fault rupture and the direction of slipping. Together, these two tell us whether the rupture was on a normal, reverse, or strike-slip fault. If the rupture was on a strike-slip fault, the fault mechanism also tells us whether the movement was right-lateral or left-lateral.

What is the fault slip?

The fault slip is the distance of the displacement of the two blocks of rock on either side of a fault that occurs during an earthquake

The first seismic risk map in the U.S. was published when? And what does it show in terms of difference between seismic risk and seismic damage in this country?

The first seismic risk map was published in 2001. Though seismic hazard levels in AK and CA are both high, CA's exposure to seismic damage is much greater, leading to much higher total risk. CA leads the nation in seismic risk (75% of total national risk).

Define the focus and the epicenter of the earthquake

The focus is the point along the fault at which slipping initiates the earthquake, and the epicenter is the geographic point on earth's surface directly above the focus of the earthquake

What determines the number and sizes of aftershocks?

The number and sizes of aftershocks depend on the magnitude of the mainshock, and their frequencies decrease with time after the mainshock. EG: the aftershocks of a mag 5 earthquake might last only a few weeks, where a mag 7 earthquake occurs over a much larger region and can continue for several years. Generally the size of the largest aftershock is about one mag unit smaller than the mainshock.

Why does this axiom by a seismologist make sense: "The longer the time since the last big quake, the sooner the next one will be"?

The recurrence interval—the time required to accumulate the strain that will be released by fault slipping in a future earthquake—can be calculated two ways: from the rate of relative plate movement and the expected fault slip, and by estimating the intervals between large earthquakes up to several thousand years in the past by finding and dating soil layers offset by fault displacements.

What are the three forces that deform brittle crustal rocks? HINT: SSS! Can you define these three forces?

The three forces are stress, strain, & strength: stress: local force per unit area that causes rocks to deform strain: relative amount of deformation (% of distortion) strength: rock's resistance to permanent deformation by flow or fracture into two or more parts

What does the time interval between the arrival of P waves and the arrival of S waves indicate to seismologists?

The time interval between the arrival of P waves and S waves depends on the distance the waves have traveled from the focus: the longer the interval, the longer the distance the waves have traveled. Seismologists use networks of sensitive seimographs around the world and highly accurate clocks to time the arrival of seismic waves from earthquakes AND from underground nuclear test sites to construct travel-time curves

The velocities of P and S waves are higher when resistance to their movement is ( lesser / greater ). It takes more force to compress solids than to shear them, so P waves always travel ( slower / faster ) than S waves through a solid. This physical principle also explains why S waves cannot travel through air, water, or Earth's liquid outer core: gases and liquids put up no resistance to shear.

The velocities of P an S waves are higher when resistance to their movement is greater. It takes more force to compress solids than to shear them, so P waves always travel faster than S waves through a solid. This physical principle also explains why S waves cannot travel through air, water, or Earth's liquid outer core: gases and liquids put up no resistance to shear.

The largest earthquakes in the last hundred years occurred at convergent boundaries. Can you remember where they were? And what caused them?

The world's largest earthquakes occur at convergent boundaries, including Tohoku (2011, mag 9.0); Sumatra (2004, mag 9.1); Alaska (1964, mag 9.2); Chile (1960, mag 9.5 ==> largest of all: The fault mechanisms of these great earthquakes show that they were caused by horizontal compression along megathrusts. All of these earthquakes displaced the seafloor, generating tsunamis that devastated coastlines.

Please describe convergent boundaries. Where are they most often found, and what are some of their characteristics?

The world's largest earthquakes occur at convergent boundaries. Earth's deepest earthquakes also occur at convergent boundaries ==> almost all earthquakes originating below 100 km rupture the descending plate in a subduction zone. The deepest earthquakes take place in the oldest (therefore coldest) descending plates, incluing beneath South America, Japan, and the island arcs of the Western Pacific.

What is the "blind zone"?

There is a blind zone near an earthquake epicenter where early warning is not feasible. However, at more distant sites, warnings can be issues from a few seconds up to about one minute prior to strong ground shaking

Why is using foreshocks to predict earthquakes so difficult and unreliable?

Unfortunately, it is difficult to distinguish foreshocks from other small earthquakes that occur randomly and frequently on active faults. Also, a foreshock (such as the magnitude 7.2 in Japan 2011) may be understood to BE the main event; it was not until after the magnitude-9 Tohoku earthquake and Tsunami hit Honshu, Japan that seismologists understood that the mag 7.2 was actually a foreshock and not the mainshock.

Using the Richter magnitude scale, the ground movement of a magnitude 3 earthquake would be ____ times that of a magnitude 2; a magnitude 6 earthquake produces ground movements that are _____ times that of a magnitude 4. A magnitude 7 earthquake releases _____ times that of a magnitude 6 and _____ times that of a magnitude 5.

Using the Richter magnitude scale, the ground movement of a magnitude 3 earthquake would be 10 times that of a magnitude 2; a magnitude 6 earthquake produces ground movements that are 100 times that of a magnitude 4. A magnitude 7 earthquake releases about 1000 times times that of a magnitude 5 (32 X 32)

What is meant by "know thy enemy" in terms of seismic hazard?

We have a good understanding of seismic hazard in some parts of the world—esp the U.S. and Japan—but much less in others. The more we learn about seismic hazard, the better we can prepare and reduce the seismic risk.

How does a tsunami happen?

When a megathrust ruptures, it can push the seafloor landward of the deep-sea trench upward by as much as 10 m, displacing a large mass of the overlying ocean water ==> this disturbance flows outward in waves that travel across the ocean as fast as 800 km/hour. While in the deep sea, a tsunami is barely noticeable, when it approaches shallow coastal waters, the waves slow down and pile up and can reach heights of tens of meters ==> The "run-up" can propagate inland for hundreds of meters and even kilometers, depending on the land surface's slope!

How do earthquakes cause landslides? And why are these dangerous? What is soil liquification?

When seismic waves shake water-saturated soils, those soils can behave like a liquid and become unstable. The ground simply flows away (soil liquification), taking buildings, bridges, and everything else with it. This soil liquification is what destroyed the residential area of Turnagain Heights near Anchorage Alaska in the 1964 earthquake and in the Nimitz Freeway near San Francisco (1989).

While earthquake intensities in general are highest near the fault rupture, they also depend on the local geology. For example, when sites at equal distance from the rupture are compared, the shaking tends to be (less / more) intense on soft sediments than on hard basement rock.

While earthquake intensities in general are highest near the fault rupture, they also depend on the local geology. For example, when sites at equal distance from the rupture are compared, the shaking tends to be more intense on soft sediments than on hard basement rock.

Have building codes been successful in the United States?

Yes! Building codes have been largely very successful in the U.S. preventing loss of life! In the 20-year period from 1981 to 2012, 146 people died in 11 severe earthquakes in the western United States. In contrast, more than 1 million people died by earthquakes worldwide!

Can more be done to improve earthquake protection in the U.S.?

Yes, more can be done: 1. Retrofitting older, more vulnerable structures to be seismically safe 2. Using specialized construction materials and advanced engineering methods in new construction 3. Putting entire buildings on movable supports to isolate them from seismic shaking! (this is news to me!)

EXTRA CREDIT QUESTION: Who do you know very well whose mother's house slid down the hill due to soil liquification in the 1989 Loma Prieta Earthquake?

Your beloved Monique's mom's house in "the summit" (Santa Cruz Mountains) slid down the hill, along with 12 other houses, in the 1989 earthquake and were condemned. No one was hurt. My mom's boyfriend and my sister's husband had to sneak in at night to get my mom's 'treasures' because otherwise she would have lost literally everything; they were not allowed by law to reenter the house ever again. Thank god they snuck in because everything I have from Croatia and Africa is thanks to their bravery!!!

What should be in your disaster kit? How about your home kit?

Your disaster kit should contain: 1. medications 2. a first-aid kit 3. a whistle 4. sturdy shoes 5. high energy snacks 6. a flashlight with extra batteries 7. personal hygiene supplies 8. At home: fire extinguisher, wrenches to turn off gas, drinking water, portable radio, food supplies, extra clothing

What is a foreshock? How do seismologists use foreshocks?

a foreshock is a small earthquake that occurs shortly before a mainshock; seismologists use foreshocks to help predict large earthquakes.

What is the difference between seismic hazard and seismic risk?

seismic hazard: the frequency and intensity of earthquake shaking and ground disruption that can be expected over the long term at some specified location—expressed by a seismic hazard map seismic risk: damage expected over the long term in a specified region (county or state), measured in terms of both casualties and economics per year. A region's risk depends both on seismic hazard and on exposure to seismic damage