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Earthquake Seismology
Earthquakes and Faulting Faulting and earthquakes are two aspects of the same dynamical system:
the former is its long-timescale manifestation and the latter its short timescale manifestation. Earthquakes
are a result of an instability in faulting; most of the slip on most faults occurs during earthquakes.
History of Ideas In the middle of the first millennium BC, Anaximenes of ancient Miletus suggested
that earthquakes are due to sudden subterranean volume changes, such as cave collapses. This view
prevailed until the middle of the 19th Century. In 1857, Robert Mallet (born Dublin, educated TCD)
showed that earthquakes are restricted to particular belts around the globe. He said that earthquakes
result from elastic waves which propagate through the earth and invented the term seismology for the
study of these waves. In 1884, Gilbert recognised that geological faults which have large offsets are the
result of repeated earthquakes, each of which generate a small offset. The 1906, San Francisco earthquake
provided direct observations of co-seismic slip and rupture length. These led to Reid’s elastic rebound
theory and to the idea that major faults can accommodate very large offsets. During the 1960s, the
observations of limited spatial earthquake distribution and shearing slip on faults led to the idea of plate
boundaries.
Earthquake Terminology
Epicentre: point on the Earth’s surface directly above the hypocentre of an earthquake.
Focus: point where earthquake rupture or fault movement originates.
Hypocentre: calculated location of the focus of an earthquake.
Seismic Waves Several types of wave are produced when a fault ruptures. Body waves travel through
the interior of the Earth. They can be sub-divided into P-waves (primary/compressional waves), which
have vibrations parallel to the propagation direction, and S-waves (secondary/shear waves), which have
vibrations perpendicular to the propagation direction. Surface waves travel round the surface of the
Earth. They can be sub-divided into Rayleigh waves, characterised by retrograde elliptical ground motion,
and Love waves, which have horizontally polarised ground motion. It is the surface waves which do the
damage in earthquakes; body waves have smaller amplitudes. Body waves are first to arrive because
seismic velocity increases with depth within the Earth; surface waves travel slower and arrive later.
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Detecting Earthquakes Earthquakes are measured and located using seismometers. Accurate of earthquake measurement is possible because there is now a dense network of seismometers all around the world.
The original network was the World Wide Standardized Seismographic Network (WWSSN), set up to
monitor the Nuclear Test-Ban Treaty. It is now being replaced by the Global Seismographic Network
(GSN).
Locating Earthquakes The difference between arrival times of the P and S waves is a function of angular distance between earthquake and receiver. Given
such distance determinations from at least three seismic stations, the earthquake’s position can be pinpointed.
Earthquake Magnitude is a measure of the energy released during the earthquake. It is determined
from the logarithm of the maximum amplitude of the earthquake signal as seen on the seismogram, with
a correction for the distance between the focus and the seismometer. The distance correction is applied
because the amplitude of a seismic wave decreases with distance from the earthquake (attenuation); it
means that earthquake magnitude is constant in space and does not describe the variations in ground
motion from place to place. Several different magnitude scales exist, based on the different types of seismic
wave. The Richter local magnitude (ML ) is defined to be used for ‘local’ earthquakes up to 600 km away
and is calculated using high-frequency data from nearby stations. Surface wave magnitude (Ms ) is based
on the maximum amplitude of the surface waves and body wave magnitude (Mb ) is calculated from the
body waves. Although these magnitudes can be calculated at any epicentral distance, Ms is usually used
for observations near the earthquake epicentre where the surface wave is larger than the body wave and
Mb is usually used at larger distance from the epicentre (amplitude attenuation of body waves is less than
that of surface waves). Moment magnitude (Mw ) is considered the most reliable measure of earthquake
size, especially for the largest events since Ms saturates at about magnitude 8. It is calculated by
analysis of the frequency spectra of the earthquake. Note that since all magnitude scales are logarithmic,
an increase of 1 magnitude point represents a 10-fold increase in ground motion and a 30-fold increase in
energy released.
Earthquake Intensity describes the degree of shaking caused by an earthquake at a given place and
decreases with distance from the earthquake epicentre. Whilst magnitude measurement requires instru-
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mental monitoring for its calculation, intensity can be assigned based on testimony of the population.
Several scales exist; the Modified Mercalli scale is commonly used in the US. The US Geological Survey compiles qualitative ground motion reports submitted online by thousands of volunteers to make
Community Intensity Maps.
Monitoring Nuclear Test-Ban Treaty Earthquakes have double-couple sources: explosions do not.
This means that the first motion of P wave arrivals from an explosion is always compressional, but the
first motion of P waves from an earthquake can be either compressional or dilational depending where
you are in relation to the earthquake. Also, the ratio Ms /Mb differs between earthquakes and explosions.
Historical Earthquakes Prior to deployment of the WWSSN and GSN, seismic data was collected at
only a few locations using different instrument types. For older information, we have to rely on historical
records; the best ones are from the Middle East and they go back several millenia. Study of intensity
and the production of isoseismal maps, contouring areas of equal intensity, is particularly important for
the study of historical earthquakes which occurred prior to instrumental monitoring. Geologists and
historians work together to compile intensity maps from ancient records.
Earthquake Hazards Ground movement — earthquakes don’t kill people: buildings do! Fires are often
a problem when gas mains are ruptured. Tsunamis are large waves which can be caused by shallow-focus
submarine earthquakes and/or submarine landslides. Landslides can be triggered by earthquakes. They
can cause buildings to fall down intact. Surface waves can cause Liquefaction of loose sediments. Flooding
can result if the dams are broken or if river courses are altered by the earthquake.