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).