FALL 2004EASA-130 Seismology and Nuclear Explosions 1 Earthquakes as Seismic Sources Lupei Zhu

FALL 2004EASA-130 Seismology and Nuclear Explosions2 Topics F Earthquakes as seismic sources F How do earthquakes happen? –Rupture on faults; stress buildup F Locating earthquakes –Hypocenter, epicenter; how to locate? F Earthquake magnitudes –Richter’s local magnitude –Body wave magnitude and surface wave magnitude –Moment magnitude F How often do earthquake happen? –Gutenburg-Richter Law

FALL 2004EASA-130 Seismology and Nuclear Explosions3 Seismic Sources F Any radiator of seismic waves –Earthquakes –Explosions –Landslides F Parameters to describe a seismic source –Location –Occurrence time –Source dimension –Time duration –Strength

FALL 2004EASA-130 Seismology and Nuclear Explosions4 How Earthquakes happen F Earthquakes happen when rocks somewhere underground break along a surface called fault and the two sides pass each other in a sudden and violent motion. See Faults.Faults. F The cause of this sudden faulting is due to a gradual buildup of stress by the long-term plate-tectonics process inside the Earth. This explains why earthquakes are concentrated at plate boundaries. F Faults that were ruptured previously are weak zones and therefore are likely to be broken again in the future (Earthquake cycle). The time interval between earthquakes, however, is very irregular. F New faults can also be produced (so no one is 100% safe)

FALL 2004EASA-130 Seismology and Nuclear Explosions5 Locating Earthquakes F The starting point of an earthquake rupture is called the earthquake hypocenter, which is given by the latitude and longitude of its projection on the surface (called epicenter) and depth. F Seismologists use arrival times of P and S waves at different seismic stations to locate an earthquake and determine its occurrence time. F At least three stations are needed to determine the epicenter and occurrence time (three unknowns). One more station is needed if the depth is included. F Earthquake depth trades off with its occurrence time and is more difficult to get accurately. F Modern Seismic Networks usually use hundreds of stations.

FALL 2004EASA-130 Seismology and Nuclear Explosions6 Travel Time F Travel time, T, is defined as T = distance / velocity F Since P-waves travel faster than S-wave, the time separation between the two is larger at greater distances.

FALL 2004EASA-130 Seismology and Nuclear Explosions7 A “Rule of Thumb” F Because of the structure of Earth, for distance ranges between about 50 and 500 km, we can use a formula to estimate the distance from the observed S-arrival time minus the P-arrival time: distance = 8  (S-P arrival time)

FALL 2004EASA-130 Seismology and Nuclear Explosions8 Example F If the arrival time of an S wave is 09:30:15.0 (GMT) and the arrival time of a P wave is 09:29:45.0 (GMT), then the time difference is 30 s. Thus, the earthquake is located about 240 km away from the seismometer. F But in which direction ???

FALL 2004EASA-130 Seismology and Nuclear Explosions9 Distances and Circles F In this case, if you know the distance the earthquake is from the seismometer, you know the earthquake must be located on a circle centered on the seismometer, with a radius equal to the distance.

FALL 2004EASA-130 Seismology and Nuclear Explosions10 Triangulation F With three or more stations, you can locate the earthquake using triangulation.

FALL 2004EASA-130 Seismology and Nuclear Explosions11 Richter’s Local Magnitude F Another important parameter is the magnitude of an earthquake. It is a measure of the energy it released in the form of seismic wave. F Charles Richter in 1935 first developed a magnitude scale based on the peak amplitude A of the seismogram recorded by a particular type of seismometer  km away from the epicenter M L = Log A Log  F Richter’s scale is a logarithmic scale. Earthquakes of 1 magnitude difference produce 10 times amplitude difference. F Since the Richter Scale is defined for a old type of seismometer, it is rarely used today. But it is still widely and mistakenly quoted in news press.

FALL 2004EASA-130 Seismology and Nuclear Explosions12 Other Magnitude Scales F The two most common modern magnitude scales are: u M S, Surface-wave magnitude (Rayleigh Wave) u m b, Body-wave magnitude (P-wave)

FALL 2004EASA-130 Seismology and Nuclear Explosions13 Problem with Ms and mb F It was found that these two magnitudes saturate when earthquakes are large than certain levels (6 for mb and 7-8 for Ms).

FALL 2004EASA-130 Seismology and Nuclear Explosions14 What Causes Saturation? F The rupture process. Large earthquakes rupture large areas and are relatively depleted in high frequency (short wavelength) seismic signals which the Ms and mb are measured with.

FALL 2004EASA-130 Seismology and Nuclear Explosions15 The Best Magnitude  The best magnitude should be based on the actual ruptured area and the amount of slip. This is how the seismic moment M 0 is defined: M o = (rigidity)  (rupture area)  (slip) The rigidity is a measure of how strong the rock is. Rock rigidity is ~30 GPa. Water’s rigidity is zero. M 0 has units of force*distance (Nm)  The moment magnitude Mw is defined as M W = 2/3 log M to tie it to the surface magnitude. It will never saturate.

FALL 2004EASA-130 Seismology and Nuclear Explosions16 Some Magnitude Examples F For an earthquakes like the 1991 Landers, California, Earthquake that ruptures a fault of 100 by 10 km 2 with an offset of 3 m, the Mw is 7.3. F The hypothetical largest earthquake on Earth would to rupture the upper 100 km of the Earth around the globe, which corresponds to a magnitude of ~ F An example of magnitude zero earthquake would be a 3 cm slip on a one square meter area. F So there are earthquakes of negative magnitudes (such as tearing a piece of paper, ~ -6).

FALL 2004EASA-130 Seismology and Nuclear Explosions17 Seismic Energy and Magnitude F Seismic energy E is the energy released from the source in the form seismic waves. F It is only a small portion of the total energy released during the earthquake. A large portion (more than 90%) is spent on breaking rocks and producing permanent deformation in the source region. F It is directly related to magnitude log E (in joule) = 1.5 M F The seismic energy for a magnitude 6 earthquake is J (20 kt TNT, a Hiroshima type nuclear bomb), which is = 32 times greater than from a magnitude 5 earthquake.

FALL 2004EASA-130 Seismology and Nuclear Explosions18 Magnitude-Frequency Law F Gutenberg and Richter did statistics on number of earthquakes of different magnitudes in a given time. They found a universal law ( the Gutenberg-Richter Law) log N = a - M or N = N M F Globally, every year there are about two magnitude 8 earthquakes, 20 magnitude 7’s, 200 magnitude 6’s, … F The largest earthquake ever recorded is the 1960 Chile Earthquake of magnitude 9.8. According to the Gutenberg- Richter law, earthquake of this size happens every 50 years (we are almost there). F The parameter N 0 varies from region to region, depending on local geology and stress environment.

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