Earthquakes

CVE 3205Engineering Geology

Wong Jee Khai

Introduction– When the Earth quakes, the energy stored in

elastically strained rocks is suddenly released.– The more energy released, the stronger the

quake.– Massive bodies of rock slip along fault surfaces

deep underground.– Earthquakes are key indicators of plate motion.

How Earthquakes Are Studied

– Seismometers are used to record the shocks and vibrations caused by earthquakes.• All seismometers make use of inertia, which is the

resistance of a stationary mass to sudden movement.– This is the principal used in inertial seismometers.

– The seismometer measures the electric current needed to make the mass and ground move together.

How Earthquakes Are Studied

– Three inertial seismometers are commonly used in one instrument housing to measure up-down, east-west, north-south motions simultaneously.

Earthquake Focus And Epicenter

– The earthquake focus is the point where earthquake starts to release the elastic strain of surrounding rock.

– The epicenter is the point on Earth’s surface that lies vertically above the focus of an earthquake.

– Fault slippage begins at the focus and spreads across a fault surface in a rupture front.

– The rupture front travels at roughly 3 kilometers per second for earthquakes in the crust.

Seismic Waves– Vibrational waves spread outward initially from

the focus of an earthquake, and continue to radiate from elsewhere on the fault as rupture proceeds.

Seismic Waves– There are two basic families of seismic waves.• Body waves can transmit either:

– Compressional motion (P waves), or– Shear motion (S waves).

• Surface waves are vibrations that are trapped near Earth’s surface. There are two types of surface waves:– Love waves, or– Rayleigh waves.

Body Waves– Body waves travel outward in all directions from

their point of origin.– The first kind of body waves, a compressional

wave, deforms rocks largely by change of volume and consists of alternating pulses of contraction and expansion acting in the direction of wave travel.• Compressional waves are the first waves to be

recorded by a seismometer, so they are called P (for “primary”) waves.

Body Waves– The second kind of body waves is a shear wave.• Shear waves deform materials by change of shape,• Because shear waves are slower than P waves and

reach a seismometer some time after P waves arrives, they are called S (for “secondary”) waves.

Body Waves• Compressional (P) waves can pass through

solids, liquid, or gases.• P waves move more rapidly than other seismic

waves:– 6 km/s is typical for the crust.– 8 km/s is typical for the uppermost mantle.

Body Waves– Shear (S) waves consist of an alternating series of

side-wise movements.• Shear waves can travel only within solid matter.• A typical speed for a shear wave in the crust is 3.5

km/s, 5 km/s in the uppermost mantle.

– Seismic body waves, like light waves and sound waves, can be reflected and refracted by change in material properties.

– When change in material properties results in a change in wave speed, refraction bends the direction of wave travel.

Body Waves• For seismic waves within Earth, the changes in

wave speed and wave direction can be either gradual or abrupt, depending on changes in chemical composition, pressure, and mineralogy.

Body Waves• If Earth had a homogeneous composition and

mineralogy, rock density and wave speed would increase steadily with depth as a result of increasing pressure (gradual refraction).– Measurements reveal that the seismic waves are

refracted and reflected by several abrupt changes in wave speed.

Surface Waves• Surface waves travel more slowly than P waves

and S waves, but are often the largest vibrational signals in a seismogram.

• Love waves consist entirely of shear wave vibrations in the horizontal plane, analogous to an S wave that travels horizontally.

• Rayleigh waves combine shear and compressional vibration types, and involve motion in both the vertical and horizontal directions.

Figure 10.7

Surface Waves• The longer the wave length of a surface wave,

the deeper the wave motion penetrates Earth. Surface waves of different wave lengths develop different velocities. This Behavior is called Dispersion

Determining The Epicenter• An earthquake’s epicenter can be calculated

from the arrival times of the P and S waves at a seismometer.– The farther a seismometer is away from an

epicenter, the greatest the time difference between the arrival of the P and S waves.

Determining The Epicenter• The epicenter can be determined when data

from three or more seismometers are available. – It lies where the circles intersect (radius =

calculated distance to the epicenter).• The depth of an earthquake focus below an

epicenter can also be determined, using P-S time intervals.

Earthquake Magnitude• The Richter magnitude scale is divided into

steps called magnitudes with numerical values M.

• Each step in the Richter scale, for instance, from magnitude M = 2 to magnitude M = 3, represents approximately a thirty fold increase in earthquake energy.

Earthquake Frequency• Each year there are roughly 200 earthquakes

worldwide with magnitude M = 6.0 or higher.• Each year on average, there are 20

earthquakes with M = 7.0 or larger.• Each year on average, there is one “great”

earthquake with M = 8.0 or larger.

Earthquake Frequency• Four earthquakes in the twentieth century

met or exceeded magnitude 9.0.– 1952 in Kamchatka (M = 9.0).– 1957 in the Aleutian Island (M = 9.1).– 1964 in Alaska (M = 9.2).– 1960 in Chile (M = 9.5).

Earthquake Frequency• The nuclear bomb dropped in 1945 on the

Japanese city of Hiroshima was equal to an earthquake of magnitude M = 5.3.– The most destructive man-made devices are small

in comparison with the largest earthquakes.

Earthquake Hazard• Seismic events are most common along plate

boundaries.• Earthquakes associated with hot spot

volcanism pose a hazard to Hawaii.• Earthquakes are common in much of the

intermontane western United States (Nevada, Utah, and Idaho).

Earthquake Disasters• In Western nations, urban areas that are

known to be earthquake-prone have special building codes that require structures to resist earthquake damage.– However, building codes are absent or ignored in

many developing nations.• In the 1976 T’ang Shan earthquake in China,

240,000 people lost their lives.

Earthquake Disasters• Eighteen earthquakes are known to have

caused 50,000 or more deaths apiece.• The most disastrous earthquake on record

occurred in 1556, in Shaanxi province, China, where in estimated 830,000 people died.

Earthquake Damage• Earthquakes have six kinds of destructive

effects.• Primary effects: – Ground motion results from the movement of

seismic waves.– Where a fault breaks the ground surface itself,

buildings can be split or roads disrupted.

Earthquake Damage• Secondary effects:– Ground movement displaces stoves, breaks gas

lines, and loosens electrical wires, thereby starting fires.

– In regions of steep slopes, earthquake vibrations may cause regolith to slip and cliffs to collapse.

– The sudden shaking and disturbance of water-saturated sediment and regolith can turn seemingly solid ground to a liquid mass similar to quicksand (liquefaction).

Modified Mercalli Scale– Earthquakes generate seismic sea waves, called

tsunami, which have been particularly destructive in the Pacific Ocean.

• This scale is based on the amount of vibration people feel during low-magnitude quakes, and the extent of building damage during high-magnitude quakes.

• There are 12 degrees of intensity in the modified Mercalli scale.

World Distribution of Earthquakes

• Subduction zones have the largest quakes.• The circum-Pacific belt, where about 80

percent of all recorded earthquakes originate, follows the subduction zones of the Pacific Ocean.

• The Mediterranean-Himalayan belt is responsible for 15 percent of all earthquakes.

Depth of Earthquake Foci• Most foci are no deeper than 100 km. down in

the Benioff zone, that extends from the surface to as deep as 700 km.

• No earthquakes have been detected at depths below 700 km. Two hypotheses may explain this.– Sinking lithosphere warms sufficiently to become

entirely ductile at 700 km depth.– The slab undergoes a mineral phase change near

670 km depth and loses its tendency to fracture.

Earthquakes Influence Geochemical Cycles

• Earthquakes play an important role in the transport of volatiles through Earth’s solid interior.

• Earthquakes facilitate the concentration of many important metals into ore deposits.

• In the mantle, the carbon and hydrologic cycles are fed when the subducting slab releases water, CO2, and other volatiles at roughly 100-km depth beneath the overriding plate.

Earthquakes Influence Geochemical Cycles

• Some seismologists speculate that water released from the slab helps cause brittle fracture in the slab itself, and that water may be necessary for deep earthquakes to occur in the Benioff zone.