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Chapter 10: Earthquakes and Earth’s Interior. 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.
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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 (1) • 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 (2) • 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 (1) • 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 (2) • 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 (1) • 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 (2) • 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 (3) • 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 (4) • 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 (5) • 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. • 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 (1) • 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.
Surface Waves (2) • 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 (1) • 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 (2) • 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 (1) • 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 (2) • 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 (3) • 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). • Several large earthquakes jolted central and eastern North America in the nineteenth century (New Madrid, Missouri, 1811 and 1812).
Earthquake Disasters (1) • 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 (2) • 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 (1) • 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 (2) • 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). • Earthquakes generate seismic sea waves, called tsunami, which have been particularly destructive in the Pacific Ocean.
Modified Mercalli Scale • 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.
First-Motion Studies Of The Earthquake Source • If the first motion of the arriving P wave pushes the seismometer upward, then fault motion at the earthquake focus is toward the seismometer. • If the first motion of the P wave is downward, the fault motion must be away from the seismometer. • S-waves and surface waves also carry the signature of earthquake slip and fault orientation and can provide independent estimates of motion at the earthquake focus.
Earthquake Forecasting And Prediction (1) • Forecasting identifies both earthquake-prone areas and man-made structures that are especially vulnerable to damage from shaking. • Earthquake prediction refers to attempts to estimate precisely when the next earthquake on a particular fault is likely to occur.
Earthquake Forecasting And Prediction (2) • Earthquake forecasting is based largely on elastic rebound theory and plate tectonics. • The elastic rebound theory suggests that if fault surfaces do not slip easily past one another, energy will be stored in elastically deformed rock, just as in a steel spring that is compressed. • Currently, seismologists use plate tectonic motions and Global positioning System (GPS) measurements to monitor the accumulation of strain in rocks near active faults.
Earthquake Forecasting And Prediction (3) • Earthquake prediction has had few successes. • Earthquake precursors: • Suspicious animal behavior. • Unusual electrical signals. • Many large earthquakes are preceded by small earthquakes called foreshocks (Chinese authorities used an ominous series of foreshocks to anticipate (the Haicheng earthquake in 1975).
