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ISNS 4359 EARTHQUAKES AND VOLCANOES
Spring 2005
Lecture 7. Seismology; First Motions, Man-made seismicity; Tsunami
Steve Bergman, Instructor
When a seismic wave meets a discontinuity surface (any change in rock property), part of the wave could be reflected and
part refracted (bent), depending on the incident angle. Every reflection or refraction generates additional waves, producing an
incredibly complex situation and seismograms that are confusing and difficult to interpret. The Earth can be thought of as
being made up of an infinite number of layers, generally increasing in density with increasing depth. This results in an infinite
number of refractions and is responsible for the general curved nature of the paths of seismic waves through the Earth. Travel
times for P and S waves depend primarily on the distance they travel and therefore the depth to which they penetrate into the
Earth (velocity also generally increases with depth). Depth to a discontinuity can be calculated using the velocity and timing of
arrivals of reflected and refracted waves at known distances from source.
Major boundaries occur within the Earth: the atmosphere/crust interface is the surface boundary layer. Going down:
Moho (Mohorovicic) discontinuity: at the crust-mantle boundary (depths of ~7km [oceans] or ~30-80 km [continents]),
Transition Zone: upper mantle-lower mantle phase transitions (400-670 km),
Gutenberg discontinuity: at the mantle-outer core boundary (~2900 km), and
outer-inner core boundary (~5000 km). In addition, there are many minor discontinuties, such as crustal layers, and the
LVZ (Low S-wave Velocity Zone, caused by small amounts of distributed melt) near the top of upper mantle asthenosphere.
Seismic prospecting methods:
Explosions, vibrations and dropped objects produce artificial seismic waves. Basic procedure is to generate seismic waves
and time their arrivals at known distances. The waves may travel along direct paths, or may be refracted or reflected. The
first arrivals of P-waves are generally used, regardless of the path taken. Two commonly used methods:
Seismic reflection & Seismic refraction.
Seismic reflection: the most widely used and valuable geophysical exploration method and one of the easiest to interpret
qualitatively. Seismic waves traveling down from a source are reflected upward from each interface encountered.
Interfaces are not necessarily boundaries between layers but could be any of a number of rock-type changes that cause
velocity/density contrasts. Reflections from a single shot are usually recorded by groups of geophones - as many as 96.
After reflections have been identified, they are timed, using the trough of the first wave, and the depth to the interface can
be easily calculated using the travel time, the distance between the shot point and the receiver, and the average velocity in
the section above the interface. Reflection surveys can be 2-D or 3-D.
Seismic refraction: Can be used to determine thicknesses and dips of layers and seismic velocities in each layer,
permitting identification of rock types and their structural geometries.
Fault Plane solutions (first motions) use P- wave polarity to determine the focal mechanism, the geometry of offset on
a fault associated with an earthquake. Beachball-like lower hemisphere stereo-net projections shade the quadrants
receiving compressional first arrivals black or some color, whereas the quadrants receiving dilational first arrivals are not.
Man-made seismicity: first recognized in S Africa gold and German coal mines over 100 years ago. In addition to natural
processes such as lithospheric strain, volcanism, landslides, and collapse of caverns, earthquakes can result from man-made
events by changing local shear stresses. Nuclear explosions were first detected on seismographs around the world during the
24July1946 Bikini Atoll underwater explosion in the Pacific. P & S waves are more symmetrical in nuclear explosions than
in natural earthquakes. Nuclear explosions produce smaller surface waves than natural earthquakes with the same P-wave
amplitude (mb), resulting in higher seismic moments (Mo) for natural earthquakes. Various Test Ban Treaties were signed in
1963, 1974, & 1996 and over a hundred seismographs in 60 countries are currently in use. Underground nuclear tests and
mine/quarry explosions can trigger earthquakes (although none have caused many deaths), as well as waste injection wells,
oil field pumping, and reservoir filling. The Rocky Mountain Arsenal Injection well (3.7 km) induced up to M4.8 EQ at <8
km depths in the 1960’s. Humans have induced or triggered earthquakes by filling reservoirs (30-200 m deep; by weight of
water & injection) in Asia (1967 Koyna, India, M6.3; 30 cm offset on fault 5 km deep, killed 200; 1962 Xinfengjiang, China,
M~6.1) caused hundreds of deaths and damaged dams. Other examples: 1975 Manic-3 Canada (M~4.1, during filling), 1975
Oroville CA (M~5.7, 8 years after filling, 5 cm offset), 1981 Aswan Dam, Egypt (M5.3, 17 years after filling). [DW
Simpson, 1986, AnnRevEPSci, 14:21]. Gupta (1992 Reservoir Induced EQ): M>3 EQ are associated with ~25% of
reservoirs >150m deep.
Tsunami: (abiki, yota, rissaga, Seebär; maremoto; Japanese for “harbor wave”) are caused by earthquakes, submarine
landslides, volcanic eruptions, caldera collapse, meteorite impacts, or any process that suddenly changes local seafloor
elevation or sea level. Tsunami have wavelengths of 100-800 km, periods of ~0.1-1 hr, deep-water amplitudes of ~1 m, and
velocities of 100-200 m/sec. For comparison, wind-blown ocean waves have wavelengths of 30-600 m, periods of ~5-20 sec,
amplitudes of 1-5 m, and velocities of 8-30 m/sec. Tsunami can be deadly, eg. 15 June 1896 tsunami (~38 m runup height) in
Japan killed ~27,000 people and destroyed ~10,000 homes. The eruption of Krakatau in 1883 created a tsunami that killed
>30,000 people living within 2 km of the coast near the Sunda Straits. 26Dec2004 M9 Sumatra EQ & tsunami killed>275,000.