Earthquakes and Earth’s Interior

Earthquakes are vibrations of the earth caused by the rupture and sudden movement of rocks that have been strained (deformed) beyond their elastic limit.

Earthquakes occur along faults. Faults are breaks in the lithosphere where regions of rock move past each other. Most major faults occur along tectonic plate boundaries.

The focus is the point on the fault where the rupture begins. The epicenter is the point on the earth’s surface directly above the focus. When the fault ruptures, waves of energy spread out in all directions.

Elastic rebound theory states that the waves of energy from an earthquake result from the sudden release of stored up strain energy in rock as it deforms. Much like a rubber band stretched past its breaking point, the rock on either side of a fault snaps suddenly to a new position, releasing energy in the process.

Types of FaultsThe majority of earthquakes (90%) are caused by rocks rupturing in response to tectonic stresses at active plate margins.Faults can be divided depending on the direction of relative displacement. There are 2 main categories.

Dip-slip faults Strike-slip faults

Relative displacement is largely a function of the type of tectonic stress the rock is under. Tensional Stress Compressional Stress Shear Stress

Types of Tectonic StressDip-Slip Faults - Normal Faults• Normal faults result from tensional stresses along divergent boundaries.• The hanging wall block moves down relative to the footwall block. • Low Richter magnitudes due to the tendency of rocks to break easily under tensional stress.• Shallow focus (less than 20 km) because the lithosphere is relatively thin along diverging plate boundaries.

Dip-Slip Faults - Reverse Faults• Reverse faults are faults that result from horizontal compressional stresses where the hanging wall block has moved up relative to the footwall block. • Reverse faulting occurs along convergent boundaries.There are two types of converging plate boundaries.

1. Subduction boundaries where oceanic lithosphere is pushed beneath either oceanic or continental lithosphere.

2. Collision boundaries where two plates with continental lithosphere collide.

Subduction Boundaries• At subduction boundaries there is a continuum of stress along the subducting plate. Shallow focus earthquakes can be generated near the trench, but focal depths can reach down to 700 km as earthquakes are generated along the subducting plate.• Rocks are strong under compression and can store large amounts of strain energy before they rupture. Therefore, these earthquakes can be very powerful.

– 1960 Southern Chili = 9.5– 1964 Alaska = 9.2

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.

• Example: The Himalayas from the collision of India with Asia

Strike-Slip Faults - Transform Faults• Strike-slip faults where the relative motion on the fault has taken place along a horizontal direction due to shear stresses acting on the lithosphere.• Can be right lateral or left lateral.• Earthquakes along these boundaries tend to be shallow focus with depths usually less than about 100 km. Richter magnitudes can be large.

Earthquake Seismic WavesBody waves travel through the interior (body) of the earth as they leave the focus. They include P-waves and S-waves.

P - waves“Primary” wavesPush-pull waves

S – waves“Secondary” waves Shear waves

Surface waves travel parallel to the earth’s surface. They are the slowest and most damaging. They include Love and Rayleigh Waves.

Love Waves - complex, horizontal motionRayleigh Waves - Rolling or elliptical motion.

Seismographs are instruments that detect and record ground shaking produced by earthquake waves. Due to their different speeds, the different waves arrive at the seismograph at different times: first P-waves arrive, then S-waves, then surface waves.

Seismogram - the record of an earthquake as recorded by a seismograph. It is a plot of vibrations versus time.

Remember: P-waves are faster than S-waves. Therefore the time gap between their arrival at a seismograph increases precisely with distance from the quake. Therefore, lag time is proportional to distance traveled.We can use the lag time between the P-waves and S- waves to calculate the distance to an earthquake! If we do this for a minimum of three different seismic stations, we can precisely locate the epicenter. In the figure, each circle has a radius equal to the distance to the earthquake from three separate seismic stations. The three circles intersect at only one point -- the epicenter!

Earthquake MeasurementRichter Magnitude scale- ML; based on the highest amplitude wave measured on a seismogram, corrected for distance from the seismograph to the epicenter- ranges from 1.0 (smallest) to infinity, but 9.0 is typically the highest possible value for an earthquake.- logarithmic scale: each whole unit on the Richter scale represents a ten-fold increase in wave amplitude (ground shaking) and an ~ thirty fold increase in the energy released.

Modified Mercalli scale- based on people’s reported perceptions of shaking, and the type and extent of damage produced- ranges from I (not felt by people) to XII (catastrophic destruction)

Fault CreepNot all fault movements result in violent earthquakes. Some faults move slowly and fairly continuously, a movement called fault creep. Fault creep never killed anyone, but it can cause damage to roads or other structures.

Earthquake Hazards and MitigationNow that you are familiar with some important concepts related to earthquakes and their measurement, we shall now consider the specific types of hazards generated by earthquakes, and

the specific steps people can take to mitigate (reduce) those hazards. The hazards we will review are:

• ground shaking• liquifaction• uplift or subsidence of land • fire• tsunamis

Ground ShakingAn old saying among geologists is that “earthquakes don’t kill people, buildings do”. The vast majority of deaths in earthquakes occur when ground shaking from earthquake waves (particularly S-waves and surface waves) causes buildings or other structures collapse, killing the people inside.

Most damage and collapse of structures like buildings, bridges, and roads occurs due to sideways movement of the ground from earthquake waves. This process is called horizontal ground acceleration, or base shear.

Base shear causes the building to deform from a rectangle into a parallelogram and causes buildings constructed on so-called “cripples” to fall sideways, causing damage

The most deadly type of failure from base shear is “story-shift”, in which the sideways acceleration causes floors to shift and collapse onto one another -- a situation called pancaking. Few or no occupants survive such collapses.

In addition to buildings, highway overpasses, bridges, and multi-decked freeways also suffer major damage from base shear. Collapse of freeways is most commonly caused by failure of the concrete supporting columns

LiquifactionLiquifaction occurs in water-saturated soils and rock. The shaking of earthquake waves causes the soil or rock to turn into a weak, fluid-like mass. Structures built on areas that liquify may simply fall over

Land Uplift and SubsidenceAreas right next to the fault can experience direct damage from the ground shifting upward (called uplift) or downward (called subsidence).

FiresFires commonly break out during quakes due to ruptured gas lines or downed electrical lines. In some urban quakes, fires have caused more damage than the ground shaking itself.

Tsunamis Tsunamis are waves generated by physical disturbances of the ocean. Shifting of the sea floor

during an earthquake is the most common cause. Undersea volcanic eruptions, landslides, or even meteorite impacts can also cause tsunamis.

When part of the sea floor drops the water drops with it. Almost immediately, water from the surrounding are rushes in to fill the depression, form a flat (~1m), high speed (up to 700km/hr), spread out wave with a wavelength measuring 10 to 100 km. In deep water tsunamis waves are nearly undetectable. But as the leading waves of a tsunami approach a shoreline, friction with the sea floor slows the waves down (100km.hr), This compresses the wave and the distance between successive crests decreases as the wave height increases. The waves surge onto shore typically as a rapidly rising flood of water with great destructive power.

Most destructive tsunamis occur in Pacific Ocean. This is clearly related to plate tectonics: the borders of the Pacific Ocean are dominated by active subduction zones that produce frequent violent earthquakes (as well as undersea volcanic eruptions and landslides).

Earthquake Prediction / ForecastingMillions of dollars, and great research effort, has gone toward finding a reliable system for predicting earthquakes in the short term (several hours to days before the event). The assumption of this research has been that large earthquakes produce precursors -- some type of “signal” before they happen.

Ground deformation: Measurements taken in the vicinity of active faults sometimes show that prior to an earthquake the ground is uplifted or tilts due to the strain building on the fault.

Foreshocks: Small earthquakes that precede a large quake by a few seconds to a few weeks. The pattern and intensity of foreshocks usually increase in magnitude and may cluster or migrate down a fault to the place where the main shock will eventually occur.

Abnormal Animal Behavior.

Alas, no reliable short-term precursors have been found. Therefore research today focuses on longer-term warnings or forecasts. In this approach, geologists attempt to identify regions where large earthquakes are likely to occur within the next several years or decades. While this does not provide short-term warnings, it is useful for long-range planning for building codes and emergency response services.

Statistical Methods use the history of past earthquakes in a region -- the recurrence interval -- to predict the magnitude and frequency of future quakes.

Recurrence interval is the expected time interval between events of a given magnitude. The theory here is that faults should behave in the future like they behaved in the past,

producing a characteristic number of quakes of particular sizes over a given time interval. The Statistical Method allows us to calculate the probability of an earthquake of a certain

magnitude occurring in a region over a certain time interval.

Geophysical Methods attempt to identify seismic gaps along faults where strain may be building up….strain that may be released in a future earthquake. The theory here is that if a portion of a fault has been “locked” for some time (i.e. has not had an earthquake in a long time), then strain may have built up to especially high levels there, and a large quake may occur in the near future.

Earth’s Interior

• No one has ever been through the crust let alone the center of the earth. How do we know what is going on there?

• Recall, density increases with depth in the earth and seismic waves travel faster as density increases. Waves will travel faster through DENSER materials because atoms are packed closer together.

• P waves travel through solids, liquids and gasses, whereas S waves only travel through solids.• Let’s assume the earth was homogenous with respect to depth, pressure and temperature. Seismic waves originating at the top of the earth would penetrate the earth in a straight pattern.• We do know that density increases with depth. If waves travel through material of higher densities, they will speed up and bend (refract). Refraction (bending) occurs when energy travels through different density materials. Example: why does the pencil look ‘bent’ when you put it in the glass of water? Light travels slower in air than water. When the light hits the water, light waves speed up (water is denser) and bend. This is why the image you see (pencil) appears bent.

Crust-Mantle Boundary• Mohorovicic noticed that seismic waves reach distant seismograph stations BEFORE closer stations even though deeper waves travel further distance. Waves must increase in velocity with depth because density increases with depth.• He estimated a boundary approximately 50 km deep as the boundary between the crust and mantle known as the Moho.

Mantle-Core Boundary• Gutenberg discovered that P waves that travel through the earth are not recorded between 105 and 140 degrees from the epicenter.• P waves refract and bend abruptly when they travel from solid mantle to liquid outer core. • S waves do not travel through the outer core (liquid).• He concluded there must be a boundary between the mantle and core.• The area that would not receive P waves is called the P-wave shadow zone.

Inner Core-Outer Core BoundaryInge Lehmann recognized that seismic P waves travel FASTER if they go deeper into the earth (inner

core is solid!)So P waves that hit the inner core speed up and refract again.

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)!