Planet earth earthquake notes

Prof. C.ValentiPlanet Earth Earthquake Notes 1

Earthquakes Defined

Earthquakes are vibrations of the earth caused by the rupture and sudden movement of rocks that have been strained beyond their elastic limit. If a strained rock breaks, it then snaps into a new position and, in the process of rebounding, generates vibrations called seismic waves. The rocks on opposites sides of the fault move with respect to each other, typically distances ranging from millimeters to many meters.

Earthquakes occur when energy stored in elastically strained rocks is suddenly released. This release of energy causes intense ground shaking in the area near the source of the earthquake and sends waves of elastic energy, called seismic waves, throughout the Earth.The source of an earthquake is called the focus, which is an exact location within the Earth were seismic waves are generated by sudden release of stored elastic energy. The epicenter is the point on the surface of the Earth directly above the focus. Sometimes the media get these two terms confused.

Origin of EarthquakesMost natural earthquakes are caused by sudden slippage along a fault zone. Within the Earth rocks are constantly subjected to forces that tend to bend, twist, or fracture them. When rocks bend, twist or fracture they are said to deform or strain (change shape or size). The forces that cause deformation are referred to as stresses.

The elastic rebound theory suggests that if slippage along a fault is hindered such that elastic strain energy builds up in the deforming rocks on either side of the fault, when the slippage does occur, the energy released causes an earthquake. This theory was discovered by making measurements at a number of points across a fault. Prior to an earthquake it was noted that the rocks adjacent to the fault were bending. These bends disappeared after an earthquake suggesting that the energy stored in bending the rocks was suddenly released during the earthquake.Earthquakes are an energy release in the form of seismic waves caused by the sudden rupture of strained rocks. Strain is deformation of rocks resulting from stress (e.g., tectonic forces). Mechanism of fault movement is explained by the Elastic Rebound Theory which states that strain energy (deformation and

Planet Earth Earthquake Notes


friction) builds up on rocks as they are forced in different directions. Rocks on either side of a fault undergo elastic strain as they are stressed by tectonic forces. When the stress exceeds the strength of the rock, it breaks and the rocks abruptly slip past one another along the rupture (fault). When slippage and rupture occur along the fault the stored energy is released as seismic waves that radiate out in all directions and the rocks "rebound" to their original undeformed shape.

Stages of Elastic Rebound Model EQ Cycle Demo: bend a stick until it snaps. Energy is stored in the elastic

bending and is released if rupture occurs, causing the fractured ends to vibrate and send out sound waves.

Fault is a break in a rock. Tectonic forces push on two rock slabs in different directions.

Friction between the rocks along the fault causes the rocks to remain put.

Continual forces 'deform the rock' and causes the rocks to build up energy (deformation is called strain).

Eventually the forces pushing on the rocks exceed the strength of friction between the rocks along a fault, the rocks break, are displaced (slippage along the fault occurs) and seismic energy is released. The deformed rock snaps back to its original shape; strain is relieved.

Pressure builds up again and repeats the process.

Buildup of strain may occur over hundreds of years and then suddenly released in one good shot.

Sometimes faults creep along gradually releasing energy in much shorter periods of time.

The recurrence interval is the time it takes to accumulate sufficient elastic strain to cause the next EQ.



Fracture of Brittle RocksFaults - Faults occur when brittle rocks fracture and there is an offset along the fracture. When the offset is small, the displacement can be easily measured, but sometimes the displacement is so large that it is difficult to measure.

Types of FaultsFaults can be divided into several different types depending on the direction of relative displacement. One division of faults is between dip-slip faults, where the displacement is vertical, and strike-slip faults where the displacement is horizontal.

Dip Slip Faults - Dip slip faults are faults that have an inclined fault plane along which the relative displacement or offset is vertical. Note that in looking at the displacement on any fault we don't know which side actually moved or if both sides moved, all we can determine is the relative sense of motion.

For any inclined fault plane we define the block above the fault as the hanging wall block and the block below the fault as the footwall block.

Normal Faults - are faults that result from horizontal tensional stresses in brittle rocks and where the hanging-wall block has moved down relative to the footwall block.

Horsts & Gabens - Due to the tensional stress responsible for normal faults, they often occur in a series, with adjacent faults dipping in opposite directions. In such a case the down-dropped blocks form grabens and the uplifted blocks form horsts. In areas where tensional stress has affected the crust, the grabens may form rift valleys and the uplifted horst blocks may form linear mountain ranges.

The East African Rift Valley is an example of an area where continental extension has created such a rift. The basin and range province of the western U.S. (Nevada, Utah, and Idaho) is also an area that has recently undergone crustal extension. In the basin and range, the basins are elongated grabens that now form valleys, and the ranges are uplifted horst blocks.



Earthquakes at Diverging Plate Boundaries. Diverging plate boundaries are zones where two plates move away from each other. Earthquakes along divergent plate boundaries are caused by tensional stress in rift valleys along MOR and within continental plates (Mid-Atlantic Ridge, African Rift Valley). Earthquakes that occur along such boundaries show normal fault motion and have low Richter magnitudes due to the tendency of rocks to break easily under tensional stress. Rupture usually occur before great stress can build in the rocks. The earthquakes tend to be shallow focus earthquakes with focal depths less than about 20 km because the brittle lithosphere is relatively thin along these diverging plate boundaries. Examples - all oceanic ridges, Mid-Atlantic Ridge, East Pacific rise,

and continental rift valleys such as the basin and range province of the western U.S. & the East African Rift Valley.



Reverse Faults - are faults that result from horizontal compressional stresses in brittle rocks, where the hanging-wall block has moved up relative the footwall block.

A Thrust Fault is a special case of a reverse fault where the dip of the fault is less than 15o. Thrust faults can have considerable displacement, measuring hundreds of kilometers, and can result in older strata overlying younger strata.

Earthquakes at Converging Plate Boundaries. Convergent plate boundaries are boundaries where two plates run into each other. Thus, they tend to be zones where compressional stresses are active and thus reverse faults or thrust faults are common. There are two types of converging plate boundaries. (1) subduction boundaries, where oceanic lithosphere is pushed beneath either oceanic or continental lithosphere; and (2) collision boundaries where two plates with continental lithosphere collide.

Subduction boundaries -At subduction boundaries cold oceanic lithosphere is pushed back down into the mantle where two plates converge at an oceanic trench. Because the subducted lithosphere is cold it remains brittle as it descends and thus can fracture under the compressional stress. The subduction of the cold oceanic lithosphere produces a continuum of stress along the subduction zone and generates earthquakes that have progressively deeper foci in the direction of subduction beneath the overriding plate. This zone of earthquakes is called the Benioff Zone. The shallow focus earthquakes are near the oceanic trench. Focal depths of earthquakes in the Benioff Zone can reach down to 700 km. Rocks are strong under compression and can store large amounts of strain energy before they rupture. Therefore, these earthquakes are very powerful.

Examples - Along coasts of South American, Central America, Mexico, Northwestern U.S., Alaska, Japan, Philippines, Caribbean Islands.



Subduction Zones* - generate the largest EQ's (M >8.5); trigger other natural disasters. 1960 southern Chile (Mw = 9.5)* largest EQ ever recorded 1964 Alaska (Mw = 9.2)** 2nd largest EQ recorded 1985 Mexico City (Ms = 8.1)

Collision boundaries - At collision boundaries two plates of continental lithosphere collide resulting in fold-thrust mountain belts. Earthquakes occur due to the thrust faulting and range in depth from shallow to about 200 km. Examples - Along the Himalayan Belt into China, along the

Northern edge of the Mediterranean Sea through Black Sea and Caspian Sea into Iraq and Iran. 1998 Afghanistan (Ms = 6.1) 1990 Western Iran (M = 7.7) 1988 Armenia (M = 7.0)



Strike Slip Faults - are faults where the relative motion on the fault has taken place along a horizontal direction. Such faults result from shear stresses acting in the crust. Strike slip faults can be of two varieties, depending on the sense of displacement. To an observer standing on one side of the fault and looking across the fault, if the block on the other side has moved to the left, we say that the fault is a left-lateral strike-slip fault. If the block on the other side has moved to the right, we say that the fault is a right-lateral strike-slip fault. The famous San Andreas Fault in California is an example of a right-lateral strike-slip fault. Displacements on the San Andreas fault are estimated at over 600 km.

Transform-Faults are a special class of strike-slip faults. These are plate boundaries along which two plates slide past one another in a horizontal manner. The most common type of transform faults occur where oceanic ridges are offset. Note that the transform fault only occurs between the two segments of the ridge. Outside of this area there is no relative movement because blocks are moving in the same direction. These areas are called fracture zones. The San Andreas fault in California is also a transform fault.

Earthquakes at Transform Fault Boundaries. Transform fault boundaries are plate boundaries where lithospheric plates slide past one another in a horizontal fashion. The plates undergo shear stress as they move horizontally past each other in a bind and lurch pattern. Strain builds as the plates are pressured to move past one another, but friction binds them. The longer the fault segments are locked the greater strain accumulates and the energy released. Earthquakes along these boundaries tend to be shallow focus earthquakes with depths usually less than about 100 km. Richter magnitudes can be large.



World Distribution of EarthquakesThe distribution of earthquakes is called seismicity. Seismicity is highest along relatively narrow belts that coincide with plate boundaries. This makes sense, since plate boundaries are zones along which lithospheric plates move relative to one another.

When lithospheric plates move relative to one another their movement is slowed by friction. As a result, rocks along the boundary undergo strain (deformation resulting from stress). When the stress on the rocks exceeds their strength, the rocks rupture, forming a fault. Earthquakes occur along faults (fracture along which rocks have been displaced). The sudden rupture of rocks along faults produces earthquake waves, or seismic waves, that shake the ground.

Earthquakes and Plate TectonicsThe majority of earthquakes (90%) are caused by rocks rupturing in response to tectonic stresses at active plate margins. Earthquakes along plate boundaries can be divided into shallow focus earthquakes that have focal depths less than about 100 km and deep focus earthquakes that have focal depths between 100 and 700 km.Smaller earthquakes occur in association with volcanic eruptions, land fills, and infection of fluids into deep fractured rock and intraplate due to mantle plumes.

Intraplate Earthquakes - These are earthquakes that occur in the stable portions of continents that are not near plate boundaries. Many of them occur as a result of re-activation of ancient faults, although the causes of some intraplate earthquakes are not well understood. Examples - Basin and Range in Southwestern U.S. is the result of

tensional stress of the continental rocks, or rifting. These result in horst/graben, mountain formation. Continuing tension leads to recurring vertical movements along normal faults.

1811-1812 New Madrid sequence (four EQ's Mw ~7.8 and 8.3) 1886 Charleston, SC (Ms = 7.7)

Human Induced Earthquakes: Fracture zones (faults) are activated by increased load of water on

land and increased water pressure in rocks below the reservoir



(The construction of Hoover Dam on the Colorado river in Arizona and Nevada produced several hundred minor tremors, up to magnitude 4 and 5, Construction of a reservoir in India caused a magnitude 6 earthquake which killed 200 people).

Setting off underground nuclear explosions produces aftershocks that relieve stress in the crust. (Nevada nuclear test site produced earthquakes of magnitude 5-6).

Deep waste disposal of material injected into wells increases the fluid pressure in fractures and facilitates slippage along the fractures. (Rocky Mountain Arsenal in Colorado).



Seismic Waves and Ground ShakingWhen a fault ruptures, rocks break apart suddenly and violently, releasing energy in the form of seismic waves. It is the propagation of these waves through the earth (ground) that is felt during an earthquake.

TYPES OF SEISMIC WAVES Four types of waves are generated simultaneously from the focus during an earthquake, traveling in different ways at different speeds

Body Waves - penetrate the earth and travel through it. They emanate from the focus and travel in all directions through the body of the Earth. There are two types of body waves: P -waves and S-waves:

P - waves – primary (push-pull) compression/tension in direction of wave travel fast ~5-6 km/sec

Travel through solids, liquids, or gases.

S-Waves – secondary (shear) perpendicular to the direction of wave travel ~3-4 km/sec

shear movement within the horizontal.Travel only through solids.

P waves can travel through solids, liquids and gasses; S waves only travel through solids (liquids and gasses cannot shear).

Surface Waves - Surface waves differ from body waves in that they do not travel through the Earth, but instead travel along paths nearly parallel to the surface of the Earth. Surface waves behave like S-waves in that they cause up and down and side to side movement as they pass, but they travel slower than S-waves and do not travel through the body of the Earth. Surface waves are often the cause of the most intense ground motion during an earthquake. They move



along the surface of the ground at a rate of ~2km/sec. Slowest and most damaging.

Love surface - complex horizontal motion. perpendicular/transverse up/down. slow; perpendicular to wave propagation but in the vertical (shear).

Raleigh surface - rolling or elliptical motion in the vertical plane, like a waves on the ocean surface, a little slower than Love waves

As these different types of waves move at different speeds away from the focus, they become organized into groups of waves travelling at similar velocities. However, near the source of the earthquake, there is not time for this wave segregation to take place, and the shaking may be severe and complex. In addition, waves traveling through rocks are both reflected and refracted across boundaries between different earth materials and along the surface of the earth, amplifying the waves and the resultant shaking and damage. For these reasons damage tends to be greatest at the epicenter.

Recording Seismic Waves Earthquakes are detected from all points on the globe when

their seismic waves reach earthquake monitoring stations. The detecting instrument is a seismograph which times and records the incoming waves on a seismogram (Overhead).

The record of an earthquake, a seismogram, as recorded by a seismograph, will be a plot of vibrations versus time. On the seismogram, time is marked at regular intervals, so that we can determine the time of arrival of the first P-wave and the time of arrival of the first S-wave.



EARTH'S INTERIOR No body has ever been through the crust let alone the center of the

earth. Knowledge comes from seismic waves. Seismic body not only can

travel through a medium, but can also be reflected and refracted. Reflection is the bouncing of a wave off the surface between two media. Refraction is the change in velocity when a wave passes from on medium to another; causes the wave to bend. Reflection and refracton of seismic body waves are the way information was gained about the different compositional layers in the earth.

Seismic Wave Rules Velocity depends on the type of wave, density of material (the

higher the density, the greater the speed), elasticity of the material (more elastic, the faster it springs back), and ductility of the material (more ductile, the slower the speed).

Velocity increases with depth because pressure and density increases with depth. Closer packing of molecules.

P waves travel through solids, liquids and gasses. S waves (shear) doesn't; water does not vibrate at right angles;

liquids have no shear strength. P waves travel faster in any material compared to other waves

traveling through the same material. When waves pass through materials of different densities they

bend (refracted); like light traveling through water and air. P waves travel faster in solids>liquids>gasses.

Layers of different compositionIf the earth had a homogenous composition, rock density would increase steadily with depth as a result of increasing pressure and therefore body wave velocity would also increase steadily. However this is not the case. Body waves are abruptly refracted and reflected at several depths inside the Earth. This means that density does not increase smoothly, and within the earth there must be some boundaries separating materials having distinctly different densities. Compositional/density. Crust, Mantle, Core (Inner core is solid,

outer core is liquid), core is iron and nickel. Mantle has high iron and magnesium, crust has higher silica and oxygen.

Moho discontinuity: Boundary between the crust and the mantle. Crust velocity of P waves is 6 km/sec, mantle velocity of P waves is 8 km/sec. Moho measured these velocity changes and assumed that if there is this big of a difference in velocity there must be a great difference in composition between the two. He observed that seismographs recording P waves 800 km from the epicenter recorded



two sets of P waves that arrived at the seismograph at different times. He concluded that the set that arrived first must have been refracted into a denser layer (mantle), causing it to travel faster and arrive at the seismograph first. The second set must have traveled from the focus to the station by a direct path through the lower density crust. He concluded therefore that a distinct compositional boundary separates the crust from this underlying zone of denser composition. This boundary, referred to as the Moho discontinuity or moho, marks the base of the crust from the mantle.

Core/Mantle Boundary: Seismic waves are sent out in all directions but not all stations receive the waves. P wave shadow zone: P waves are absent between 103 and 143

degrees from the focus. (P wave shadow zone). When P waves reach the core mantle boundary they are refracted so strongly that the boundary is said to cast a P wave shadow, which is an area where no P waves are observed.

S wave shadow zone: S waves are absent from 105-105 degrees from the focus. When S waves reach the core-mantle boundary they stop because S waves cannot travel through liquid. This casts a very large S-wave shadow zone on the earth’s surface opposite the epicenter where no S waves are observed.

Lehmann Discontinuity; the inner-outer core boundary: Said there was a core within a core because P waves travel faster in the inner core than the outer core so it must be a solid.



Layers of Different StrengthRock strength has a marked effect on the speed of body waves. Rock strength is an expression of the elasticity. This can be equated to the tendency of a rock to fracture (called brittleness; high brittleness = high elasticity) as opposed to a tendency to deform and flow (called ductility or plasticity; higher plasticity lower brittleness = lower elasticity) The more elastic the material the faster the waves will travel (more elastic, the faster it springs back), and the more ductile or plastic the material the slower the waves will travel.

Three strength boundariesThe lithosphere-asthenosphere boundary at a depth of 100km below the Earth’s surface. The asthenosphere-mesosphere boundary at a depth of 350km below the Earth’s surfaceThe outer core-inner core boundary at a depth of ~5100km below the Earth’s surface.



S-wave and P-wave Velocities versus Depth in Earth.Figure Handout1. P waves travel through the crust increasing velocity with depth due

to the increase in pressure with depth. At the Moho, rapid increase in velocity (jump) because of the differences between crust and mantle materials.

2. Pressure increases with depth through the rest of the lithosphere so velocity increases gradually.

3. Asthenosphere, the low velocity layer. Zone of partial melting where the lithosphere 'rides' on top of the asthenosphere. Here, velocity of waves slow down because material is partially melted so it has lower density.

4. Velocity increases with depth through the rest of the asthenosphere.

5. Asthenosphere ends, increase in pressure, increase the stiffness, lower mantle is more rigid so there is a jump in velocity here.

6. Mesosphere increases pressure with depth so increase in velocity with depth.

7. Outer core - mantle boundary: Jump back! Rapid decrease in velocity without change in depth because material is liquid therefore velocity decreases because density of the material decreases.

8. Pressure increases with depth in the outer core so does velocity, increases with increasing depth.

9. Outer-inner core boundary: Jump fast! Rapid increase in velocity without changing depth because the inner core is solid, more dense and velocity is much faster.

10. Pressure increases with depth in the inner core so velocity increases with depth.

11. S waves start out at a lower velocity than P waves because they travel slower. Same process happens as in 1-6 for S waves.

At the outer core boundary, S waves stop! They cannot travel through liquids therefore they can't make it through to the inner core (even if it is solid)!



EARTHQUAKE MAGNITUDE AND INTENSITY 1. Richter Magnitude (energy, power) Scale: (from 0 to infinity;

values of >9 are unlikely) Measure of the energy released. The Richter Magnitude involves

measuring the amplitude (height) of the largest recorded wave at a distance of ~100km from the earthquake focus.

Richter Magnitude is a scale of earthquake size developed by a seismologist named Charles Richter.

Earthquake magnitudes can be calculated from the amplitude of the waves (vertical distance between the crest and the trough) and the distance from the epicenter. Quantitative. The greater the amplitude, the greater the energy released.

Symbols: ML = Local magnitude regardless of wave used (P, S, or surface).

Mb = magnitude based on body waves (P or S). Ms = magnitude based on surface waves (Love or Raleigh).

The Richter scale is not linear. Each whole unit on the Richter scale represents a ten-fold increase in wave amplitude and a thirty-one fold increase in the energy released by the earthquake. Base-10 logarithmic scale; thus M=7 EQ has wave amplitude on a seismograph 10x larger than M=6 EQ. A magnitude 7 earthquake releases 31 times more energy than a magnitude 6 earthquake. A magnitude 8 earthquake releases 31 x 31 or 961 times as much energy as a magnitude 6 earthquake. Note that the Richter scale is an open ended scale with no

maximum or minimum. The largest earthquakes are probably limited by rock strength, although meteorite impacts could cause even larger earthquakes.

Note that it usually takes more than one seismographic station to calculate the magnitude of an earthquake. Thus you will hear initial estimates of earthquake magnitude immediately after an earthquake and a final assigned magnitude for the same earthquake that may differ from initial estimates, but is assigned after seismologists have had time to evaluate the data from numerous seismographic stations.



2. Modified Mercalli Intensity (Damage) Scale: (ranges from I to XII)

Note that the Richter magnitude scale results in one number for the size of the earthquake. Maximum ground shaking will occur only in the area of the epicenter of the earthquake, but the earthquake may be felt over a much larger area. The Modified Mercalli Scale was developed in the late 1800s to assess the intensity of ground shaking and building damage over large areas.

2) The scale is applied after the earthquake by conducting surveys of people's response to the intensity of ground shaking and destruction. Thus, a given earthquake will have zones of different intensity all surrounding a zone of maximum intensity.

3) The Modified Mercalli Scale is shown in the table below. Note that correspondence between maximum intensity and Richter Scale magnitude only applies in the area around the epicenter.Subjective measure of the kind of damage and human reaction. The Mercalli Scale describes the Intensity of an earthquake based on the damage caused and its effects as perceived by people.

It is Qualitative. Derived from interviews and observation of damage. Provides approximate location and size of EQ, as well as the effects of

local geology and building construction. Plotting Mercalli intensity values on a map gives an informative pattern of the destructive effects as one moves away from the epicenter.

Problems: (1)Subjectivity of interviewer, interviewee, data analyst, (2)Geology:rock types-wave amplitude and destruction are greater in soft, unconsolidated material than in dense rock, (3)building structure and material, (4)population density, (5)time of day, etc.

Intensity

Characteristic Effects Richter Scale

Equivalent

4)

I People do not feel any Earth movement <3.4 5)II A few people notice movement if at rest and/or on upper floors of

tall buildings6)

III People indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring

4.2 7)

IV People indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. Feels like a heavy truck hitting walls.

4.3 - 4.8 8)



Some people outdoors may feel movement. Parked cars rock.V Almost everyone feels movement. Sleeping people are awakened.

Doors swing open/close. Dishes break. Small objects move or are turned over. Trees shake. Liquids spill from open containers

4.9-5.4 9)

VI Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls may crack. Trees and bushes shake. Damage slight in poorly built buildings.

5.5 - 6.1 10)

VII People have difficulty standing. Drivers feel cars shaking. Furniture breaks. Loose bricks fall from buildings. Damage slight to moderate in well-built buildings; considerable in poorly built buildings.

5.5 - 6.1 11)

VIII Drivers have trouble steering. Houses not bolted down shift on foundations. Towers & chimneys twist and fall. Well-built buildings suffer slight damage. Poorly built structures severely damaged. Tree branches break. Hillsides crack if ground is wet. Water levels in wells change.

6.2 - 6.9 12)

IX Well-built buildings suffer considerable damage. Houses not bolted down move off foundations. Some underground pipes broken. Ground cracks. Serious damage to Reservoirs.

6.2 - 6.9 13)

X Most buildings & their foundations destroyed. Some bridges destroyed. Dams damaged. Large landslides occur. Water thrown on the banks of canals, rivers, lakes. Ground cracks in large areas. Railroad tracks bent slightly.

7.0 - 7.3 14)

XI Most buildings collapse. Some bridges destroyed. Large cracks appear in the ground. Underground pipelines destroyed. Railroad tracks badly bent.

7.4 - 7.9 15)

XII Almost everything is destroyed. Objects thrown into the air. Ground moves in waves or ripples. Large amounts of rock may move.

>8.0 16)



Location of Epicenter Lab.The difference in arrival times of the first recorded P wave and S waves (the lag time) is used to locate an earthquake’s epicenter.

The difference in arrival time of a P and S wave from the seismic station is proportional to the distance from the epicenter. The greater the lag time, the greater the distance from the epicenter.

The time lag between seismic wave arrivals indicates the distance, but not the direction, from the recording station to the epicenter. The direction of the epicenter is determined by

using seismograms from three or more stations. By creating circles around the stations with radii equal to the distance to the epicenter, the epicenter can be pinpointed to where the three circles intersect.



Hazards Associated with EarthquakesAnd Earthquake Risk

1. Ground Motion - Shaking of the ground caused by the passage of

seismic waves, especially surface waves, near the epicenter of the earthquake are responsible for the most damage during an earthquake. The intensity of ground shaking depends on:

Magnitude of the Earthquake. In general, the larger the earthquake, the more intense is the shaking and the duration of the shaking.

Distance from the Epicenter. Shaking is most severe near the epicenter and drops off away from the epicenter. The distance factor depends on the type of material underlying the area. There are, however, strange exceptions. For example, the 1985 Mexico City Earthquake (magnitude 8.1) had an epicenter on the coast of Mexico, more than 350 km to the south, yet damage in Mexico City was substantial because Mexico City is built on soft unconsolidated sediments that fill a former lake (see Liquefaction, below).

Local geologic conditions in the area. In general, loose unconsolidated sediment is subject to more intense shaking than solid bedrock. Material amplification. The amplitude of shaking (vertical movement) of earth materials tends to increase as you go from solid bedrock to unconsolidated, water saturated sediments.

Factors to consider…Damage to structures from shaking depends on the type of construction Concrete and masonry structures are brittle and thus more

susceptible to damage. Wood and steel structures are more flexible and thus less

susceptible to damage. Buildings have natural vibration frequencies in the same range as

earthquake waves which facilitates shaking of buildings when the frequency of the building is close to the frequency of the waves produced by the earthquake.

High frequency (short period) P and S waves cause short buildings to oscillate while low frequency (long period) surface waves cause tall buildings to oscillate. "Swaying" or resonance of buildings amplified so much that building may collapse.

2. Faulting and Ground Rupture - Ground rupture generally occurs only along the fault zone that moves during the earthquake. Vertical and Horizontal movement of the earth’s surface. Rupture - surface cracking usually accompanied by horizontal and/or vertical displacements. Large displacements during large EQ's (e.g. 1-10 meters) can cause



direct destruction of man-made structures. Destroys buildings, roadways, pipelines across the fault.

3. Aftershocks - These are usually smaller earthquakes that occur after a main earthquake, and in most cases there are many of these (1260 were measured after the 1964 Alaskan Earthquake). Aftershocks occur because the main earthquake changes the stress pattern in areas around the epicenter, and the crust must adjust to these changes. Aftershocks are very dangerous because they cause further collapse of structures damaged by the main shock.

4. Fire - Fire is a secondary effect of earthquakes. Because power lines may be knocked down and because natural gas lines may rupture due to an earthquake, fires are often started closely following an earthquake. The problem is compounded if water lines are also broken during the earthquake since there will not be a supply of water to extinguish the fires once they have started. - Caused by broken gas and electrical lines - Broken water lines and impassable roads and highways hinder

fire fighting capability. - Fires can account for a large percentage of the death and

destruction.- 1906 San Francisco- 80-90% of damage was caused by

associated fires.- 1923 Tokyo- 70-100% of damage was caused by fire, 40% of

deaths resulted from fire.

5. Landslides - triggered by seismic vibration. In mountainous regions subjected to earthquakes ground shaking may trigger landslides, rock and debris falls, rock and debris slides, slumps, and debris avalanches. 1970 Peru- ~30% of deaths caused by giant landslide in the Andes Mtns.

6. Liquefaction - Liquefaction is a process that occurs in water-saturated unconsolidated sediment due to shaking. In areas underlain by such material, the ground shaking causes the grains to lose grain to grain contact, and thus the material is liquefied and flows. Forms quicksand, sand ridges, sand volcanoes, quickclays. Man-made structures collapse or sink.

7. Tsunamis - Tsunamis are giant ocean waves that can rapidly travel across oceans. They are caused by sudden displacement of sea floor or submarine landslide triggered by EQ's.



When part of the sea floor drops the water drops with it. Almost immediately, water from the surrounding area rushes in to fill the depression, forming a flat (~1m), high speed (up to 700km/hr), spread out wave (with a wavelength measuring 10 to 100km). When the wave approaches shore, its base drags against the ocean floor and slows down to <100 km/hr. This compresses the wave and the distance between successive crests decreases as the wave height increases to up to ~65 m in height (commonly only ~20 m for a large tsunami).

Dangerous for shorelines populations. Mostly restricted to the Pacific Ocean.



Earthquake Risk and "Prediction"Long-Term PredictionPaleoseismology - the study of prehistoric earthquakes. Through study of the offsets in sedimentary layers near fault zones, it is often possible to determine recurrence intervals of major earthquakes prior to historical records. If it is determined that a fault has a recurrence intervals of say 1 every 100 years, and there are no records of earthquakes in the last 100 years, then a long-term forecast can be made and efforts can be undertaken to reduce seismic risk.

Seismic gaps - A seismic gap is a zone along a tectonically active area where no earthquakes have occurred recently, but it is known that elastic strain is building in the rocks. If a seismic gap can be identified, then it might be an area expected to have a large earthquake in the near future. Example - The San Francisco, Loma Prieta, and Parkfield Seismic

Gaps. Shown below are two cross-sections along the San Andreas Fault in northern California. The upper cross section shows earthquakes that occurred along the fault prior to October 17, 1989. Three seismic gaps are seen, where the density of earthquakes appears to be lower than along sections of the fault outside the gaps. To the southeast of San Francisco is the San Francisco Gap, followed by the Loma Prieta Gap, and the Parkfield Gap. Because of the low density of earthquakes in these gaps, the fault is often said to be locked along these areas, and thus strain must be building. On Oct. 17, 1989 a magnitude 7.1 earthquake occurred in the Loma Prieta gap, followed by numerous aftershocks. Note how in the lower cross-section, this earthquake and its aftershocks have filled in the Loma Prieta Gap. This still leaves the San Francisco and Parkfield gaps as areas where we might predict a future large event.

Short-term Prediction (<1 yr. down to days; based on

precursor events)



Despite the array of possible precursor events that are possible to monitor, successful short-term earthquake prediction has so far been difficult to obtain. This is likely because:

the processes that cause earthquakes occur deep beneath the surface and are difficult to monitor.

earthquakes in different regions or along different faults all behave differently, thus no consistent patterns have so far been recognized.

Among the precursor events that may be important are the following: Ground deformation:

Ground Uplift and Tilting. Rapid increase in ground deformation in the focal area resulting from stress along the fault – may be deformation or fault slip.Surface Bulging. Caused by the production of cracks in rocks that are under stress. The cracks increase the rock volume, causing the surface to bulge. This cracking in the rocks may lead to small earthquakes called foreshocks.

Foreshocks. Small earthquakes that precede a large quake by a few seconds to a few weeks. Analogy: bending a stick slowly may produce a few cracking sounds before the final snap. 50% of large earthquakes are preceded by foreshocks.

Water Level in Wells - As rocks become strained in the vicinity of a fault, changes in pressure of the groundwater (water existing in the pore spaces and fractures in rocks) occur. This may force the groundwater to move to higher or lower elevations, causing changes in the water levels in wells.

Emission of Radon Gas - Radon is an inert gas that is produced by the radioactive decay of uranium and other elements in rocks. Because Radon is inert, it does not combine with other elements to form compounds, and thus remains in a crystal structure until some event forces it out. Deformation resulting from strain may force the Radon out and lead to emissions of Radon that show up in well water.

Strange Animal Behavior - Since the beginning of recorded history, virtually every culture in the world has reported observations of unusual animal behavior prior to earthquakes, but conventional science has never been able to adequately explain the phenomenon. Nonetheless, the Chinese and Japanese have employed such sightings for hundreds of years as an important part of a nationally-orchestrated earthquake warning systems, with some success.



Prior to a magnitude 7.4 earthquake in Tanjin, China, zookeepers reported unusual animal behavior. Snakes refusing to go into their holes, swans refusing to go near water, pandas screaming, etc. This was the first systematic study of this phenomenon prior to an earthquake.

Perhaps most significantly, on February 4, 1975 the Chinese successfully evacuated the city of Haicheng several hours before a 7.3 magnitude earthquake-- based primarily on observations of unusual animal behavior. 90% of the city's structures were destroyed in the quake, but the entire city had been evacuated before it struck. Nearly 90,000 lives were saved.

Nocturnal animals come out in the daytime, snakes come out of hibernation in winter, rats climb to power lines, etc. These precursors are hard to quantify but should not be dismissed and are being studied. Although other attempts have been made to repeat a prediction based on animal behavior, there have been no other successful predictions. (One California man claimed to predict earthquakes based on the number of lost-pet notices in the newspaper).


Technology

Planet earth earthquake notes