Earthquakes - Geotechnical Engineering Investigation - page 899

Plastic Yielding
At a depth of about 3 mi or so, the lithostatic pressure is approximately equal to the
strength of massive rock at the temperature (500

C) and pressure present. Rock deforma-
tion under stress, therefore, would be expected to be plastic yielding rather than the brit-
tle rupture needed for a large release of energy. The cause of earthquakes that originate
with deeper foci is not clearly understood. The dilantancy theory has been used to explain
rupture at substantial depths ( Section 11.2.8) .
Deep-focus earthquakes appear to be generally associated with tectonic plates and
spreading seafloor movements.

°

Volcanic Activity
The worldwide distribution of volcanic activity is shown in Figure 11.1 , where it is seen
that volcanoes are generally located near plate edges. Large earthquakes were at one time
attributed to volcanic activity but there is usually a separation of about 120 mi (200 km) or
more between belts of active volcanoes and major tectonic activity. The seismic shocks
occurring before, during, and after eruptions are referred to by Richter (1958) as volcanic
tremors. The volcano hazard is described in Section 11.3.6.

Other Natural Causes
Minor Earth shaking over a relatively small surface area can occasionally be attributed to
the collapse of mines or caverns, to large slope failures such as avalanches, or to meteorites
striking the Earth.

Human-Induced Causes
Reservoirs
Filling reservoirs behind dams, forming lakes of the order of 100 m or more in depth, cre-
ates stress changes in the crust which may be of sufficient magnitude over a large area to
induce earthquakes, especially where faults are near, or within, the reservoir area. The
cause of reservoir-induced earthquakes is not clearly understood but seems to be more
closely associated with an increase in pore- and cleft-water pressures in the underlying
rocks than with the reservoir weight. Artificial reservoirs associated with seismic activity
and some of their characteristics are given in Table 11.2. Of the 52 reservoirs over 100 m in
height in the United States (1973), only about 20% caused seismic activity from water
impounding (Bolt et al., 1975). Over 10,000 shocks have been recorded in the area of Lake
Mead behind Hoover Dam ( H 221 m) since its impoundment in 1935, with the largest

having an intensity of MM IV occurring in 1939. The 236-m-high Oroville Dam in

California had not caused detectable seismicity within 10 km from the date of its
impoundment in 1968 through early 1975. Following a series of small shocks, an event of
magnitude M 5.7 occurred on August 1, 1975.

At Nurek Dam, Tadzhikistan, U.S.S.R., during construction in a seismically active area,
the number of yearly events increased significantly since 1972 when the water level
reached 116 m (ENR, 1975). Completed in 1980, Nurek Dam is the world's highest at 315
m. Seismic events are being monitored and studied. At the Hsinfengkaing Dam ( H

105
m), l60 km from Canton, China, constructed in an area that had no record of damaging
earthquakes, a shock of M

6.1 occurred 7 months after the reservoir was filled, causing a
crack 82 m long in the upper dam structure.
Accelerographs ( Section 11.2.3) are used to instrument large dams to monitor reservoir-
induced seismicity. The practice is also being applied to dams lower than 100 m where
there is substantial risk to the public if failure should occur.