^* FIGURE 9-7 The relationship between the distribution of earthquake epicenters and plate boundaries. Approximately 80% of earthquakes occur within the circum-Pacific belt, 15% within the Mediterranean-Asiatic belt, and the remaining 5% within the interiors of plates or along oceanic spreading ridge systems. Each dot represents a single earthquake epicenter.
*   l    *    A                                              -----------------------------___________________
Convergent                  Divergent                   Transform
boundary                     boundary                   boundary
Wave front
Fault
^ FIGURE 9-6 The focus of an earthquake is the location where rupture begins and energy is released. The place on the Earth's surface vertically above the focus is the epicenter.
Original position                                                          Deformation
Rupture and release of energy                     Rocks rebound to original undeformed shape
(a)
^ FIGURE 9-3   (#) According to che elastic rebound theory, when rocks are deformed, th
store energy and bend. When the inherent strength of the rocks is exceeded, they
rupture, releasing the energy in the form of earthquake waves that radiate outward in all
directions. Upon rupture, the rocks rebound to their former imdeformed shape.
(b) During the 1906 San Francisco earthquake, this fence in Marin County was displaced
2.5 m.
														
	■	Hjt					ÉtÉÉÉÉ	ééiiéé	ÉÉfÉjÉ		IMM*L		44:	- •
			i i						1   í:					
				i			i	:						
					.	i    !    1     ''    M						'■" i   i   1   i •		"ill
ŕ"	pí													
(a) Undisturbed material
c #
i
it
í i
i
i
/
ď
,oc
$
o°
Undisturbed
material
(b) Primary wave
Direction of wave mover™
fe-
(c) Secondary wave
Distance from focus (km)
8> FIGURE 9-I2 A time-distance graph sh owing the a ver ace ;ravel times for P- and S-waves. The farther away a seismograph station is from the focus of an earthquake, the longer the nterval between the arrivals of the P- and S-waves, and hence :he greater the distance between the curves on the time-distance 2;raph as indicated by the P-S time interval.
^ FIGURE 9-13  Three seismograph stations are needed to locate the epicenter of an earthquake. The P-S time interval is plotted on a time-distance graph for each seismograph station to determine the distance that station is from the epicenter. A circle with that radins is drawn from each station, and the intersection of the three circles is the epicenter of the earthquake.
Support
^P    /Cable
Base anchored into bedrock and moves with it
Suspended mass Marker
Rotating drum
(b)
Support
Base anchored into bedrock and
moves with it
Spring
Suspended mass
Rotating drum
(c)
^> FIGURE 9-1 I   A schematic seísmogram showing the arrival order and pattern produced by I3-, S-, and L-waves. When an earthquake occurs, body and surface waves radiate outward from the focus at the same time. Because P-waves are che fastest, they arrive at a seismograph first, followed by S-waves, and then by surface waves, which are the slowest waves. The difference between the arrival times of ehe P- and the S-waves is the P-S time interval; it is a function of the dištance of the seismograph station from the focus.
Body waves
Surface waves
Arrival of P-wave
Background noise
Time marks
Arrival of S-wave
Arrival of -wave
P-S time interval i
Time
^ FIGURE 9-23 Tsumani travel times within the Pacific Ocean basin to Honolulu, Hawaii.
USSR
$> FIGURE 9-27 The relationship between dilatancy and various other earthquake precursors. The onset of dilatancy matches a change in each of the precursors illustrated. For example, a drop in seismic wave velocity corresponds to the onset of dilatancy and the development of cracks.
Seismic   + wave
velocity    -
Uplift and + tilting of ground    -
Emission of radon
+
Electrical
resistivity  _
Number of
seismic events
Precursor stages
Stage IV
earthquake
Stage
Deformation
of crustal
rocks
Stage II
Dilatancy and
development of cracks
Stage III
Unstable deformation in fault zone and influx of water
Stage V
Sudden drop
in stress followed by
aftershocks