Chapter 13 EARTHQUAKES AND EARTH STRUCTURE. FIGURE 13.1 The elastic rebound theory explains the earthquake cycle. [Photo by G. K. Gilbert/USGS.

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1 FIGURE 13.2 Map of California, showing the segments of the San Andreas fault that ruptured in 1680, 1857, and [Southern California Earthquake Center.] Chapter 13 EARTHQUAKES AND EARTH STRUCTURE FIGURE 13.3 This photograph, taken from a balloon by George Lawrence 5 weeks after the great Jordan, The earthquake Essential Earth 1e of 2008 April by W. 18, H. Freeman 1906, and Company shows devastation of San Francisco caused by the quake and subsequent fire. View is from Nob Hill toward the business district. [CORBIS ] FIGURE 13.1 The elastic rebound theory explains the earthquake cycle. [Photo by G. K. Gilbert/USGS.] FIGURE 13.4 Irregularities in the earthquake cycle can be caused by variations in rock strength and stress accumulation. [H. Kanamori and E. E. Brosky, Physics Today (June 2001): ]

2 FIGURE 13.8 Seismographs record P-wave, S- wave, and surfacewave motions. FIG During an earthquake, fault slipping begins at the focus and spreads out along the fault surface. Panel C is just before the quake; panel D is just after, as in Fig Panels 1 4 are snapshots of the fault rupture corresponding to the numbered points on the graph. FIGURE 13.6 Aftershocks are smaller shocks that follow a large earthquake (the mainshock). Foreshocks occur near, but before, the mainshock. FIGURE 13.9 Readings from three or more seismographic stations can be used to determine the location of an earthquake s epicenter. FIGURE 13.7 A seismograph can be designed to record (a) vertical or (b) horizontal motion. Because of its inertia and its loose coupling to Earth through (a) a spring or (b) a hinge, the dense mass does not keep up with the motion of the ground. A typical observatory has seismographs set up to measure three components of ground motion: vertical, horizontal east-west, and horizontal north-south.

3 FIGURE Relationship between moment magnitude, energy release, and number of earthquakes per year worldwide. Several other large sources of sudden energy release are included for comparison. [IRIS Consortium, FIGURE The motion of the first P wave arriving at a seismographic station is used to determine the orientation of the fault surface and the direction of fault slip. The case shown here is the rupture of a right-lateral strike-slip fault. Note that the alternating pattern of pushes and pulls would remain the same if the plane perpendicular to the fault ruptured with left-lateral displacement. Seismologists can usually choose between the Jordan, two The possibilities Essential Earth 1e using 2008 by additional W. H. Freeman and information, Company such as field mapping of the fault scarp or the alignment of aftershocks along the fault surface. FIGURE Modified Mercalli intensities measured for the New Madrid earthquake of December 16, 1811, a magnitude 7.7 event near the juncture of Missouri, Arkansas, and Tennessee. Regions near the fault rupture show intensities greater than IX, and intensities as high as VI were observed 200 km from the epicenter (see Table 13.1). [Carl W. Stover and Jerry L. Cossman, USGS Professional Paper 1527, 1993.] FIGURE (a) P and S waves are reflected upward from the core-mantle boundary and can reflect off Earth s solid surface. An S wave that has reflected once off the surface is labeled SS. (b) Seismograms recorded at various distances (in angular degrees) from an earthquake in the Aleutian Islands, Alaska. The colored lines identify the arrival times of the P and S waves (in blue), the surface waves (in yellow), and the S waves reflected from Earth s solid surface (the SS waves, in green). FIGURE The three main types of fault mechanisms that initiate earthquakes and the stresses that cause them. (a) A fault before movement takes place. (b) Normal faulting due to tensional stress. (c) Reverse faulting due to compressive stress. (d) Strike-slip faulting due to shearing stress (in this case, leftlateral). FIGURE Earth s core creates P-wave and S-wave shadow zones.

4 FIGURE Danish seismologist Inge Lehmann discovered Earth s inner core in [Courtesy of Beverley Bolt.] FIGURE An estimate of Earth s geotherm, which describes the increase in temperature with depth (yellow line). The geotherm first rises above the melting curve, the temperature at which rock begins to melt (red line) in the upper mantle, forming the partially molten low-velocity zone. It does so again in the outer core, where the iron-nickel alloy is in a liquid state. The geotherm falls below the melting curve throughout most of the mantle and in the solid inner core. FIGURE Earth s layering as revealed by seismology. The lower diagram shows changes in P- wave and S-wave velocities and rock density with depth. The upper diagram is a cross section through Earth on the same depth scale, showing how these changes are related to the major layers. FIGURE A threedimensional model of Earth s mantle created by seismic tomography. [Swave velocities courtesy of G. Ekström and A. Dziewonski, Harvard University. Cross section from M. Gurnis, Scientific American (March 2001): 40. Maps courtesy of L. Chen and T. Jordan, University of Southern California.] FIGURE The structure of the mantle beneath an old ocean basin, showing S-wave velocities to a depth of 900 km. Changes in S- wave velocity mark the strong lithosphere, the weak asthenosphere, and two zones at which increasing pressure forces a rearrangement of atoms into denser and more compact crystal structures. [After D. P. McKenzie, The Earth s Mantle. Scientific American (September 1983): 66.] FIGURE Global map of seismic activity, showing the concentration of earthquakes along the boundaries between tectonic plates. The inset figures illustrate the types of faulting observed at different types of plate boundaries. [Map based on data from Harvard CMT catalog; plot by M. Boettcher and T. Jordan.]

5 FIGURE (a) A map of the fault system of Southern California (yellow lines), showing the surface traces of the San Andreas fault and its subsidiary faults. (b) Locations of earthquakes in Southern California during the period July 1970 June [Southern California Earthquake Center.] FIGURE Megathrust earthquakes may generate tsunamis that can propagate across ocean basins. [NOAA, Pacific Marine Environmental Laboratory.] FIGURE Sixteen people died in the Northridge Meadows apartment building in Los Angeles during the 1994 Northridge earthquake. The victims lived on the first floor and were crushed when the upper levels collapsed. Many more buildings would have collapsed if the newer buildings in the area had not been constructed according to stringent building codes. [Nick Ut, Files/AP Ph t ] FIGURE The tsunami caused by the 2004 Sumatra earthquake struck without warning on a beach in Phuket, Thailand. [Courtesy of David Rydevik.] FIGURE This elevated expressway, in Kobe, Japan, was overturned during an earthquake in [Tom Wagner/ CORBIS/SABA. FIGURE This small headland near Banda Aceh on the west coast of Sumatra was previously covered by dense jungle to the waterline, but was stripped clean to a height of about 15 m by the 2004 tsunami. [José Borrero, University of Southern California/ Tsunami Research Group.]

6 FIGURE Housing tracts constructed within the San Andreas fault zone, on the San Francisco Peninsula, before the state passed legislation restricting this practice. The white line indicates the approximate fault trace, along which the ground ruptured and slipped about 2 m during the earthquake of [R. E. Wallace/USGS.] FIGURE Seismic hazard map for the United States. The region of highest hazard lies along the San Andreas fault system in California, with a branch extending into eastern California and western Nevada. High hazard levels are also found along the coast of the Pacific Northwest. In the central and eastern United States, the areas of highest hazard are near New Madrid, Missouri, and Charleston, South Carolina; in eastern Tennessee; and in portions of the Northeast. [U.S. Geological Survey, FIGURE Tsunami barrier in the town of Taro, Japan. [Courtesy of Taro, Japan. FIGURE Seismic risk map for the United States. The map shows current annualized earthquake losses (AEL) on a county-by-county basis. [Federal Emergency Management Agency, Report 366, Washington, D.C., FIGURE World seismic hazard map. [K. M. Shedlock et al., Seismological Research Letters 71(2000): ] FIGURE Seven steps to earthquake safety. For further information, see Putting Down Roots in Earthquake Country, Southern California Earthquake Center, 32 pp., This pamphlet is available online at

7 FIGURE Geologist Gordon Seitz (left) examines layers of rock and peat that have been distorted by prehistoric earthquakes in a trench crossing a major strand of the San Andreas fault system in Southern California. By dating the peat using the carbon-14 method, geologists can reconstruct the history of large earthquakes on this fault. Such information helps scientists to forecast future events. [Courtesy of Tom Rockwell, San Diego State University.]