Geology Reference
In-Depth Information
FIGURE 6.3
Seismic acceleration, expressed as a percent of gravity, that can be expected during a 50 year period. Higher numbers
indicate greater potential for shaking (From Peterson and others, 2008).
earthquakes around the world. The Richter scale is a log-
arithmic scale, which means that each increase in num-
ber, for example from M5 to M6, represents a 10-fold
increase in amplitude (and about a 30-fold increase in
actual energy released by the earthquake).
storm, and, staggering and bewildered, I vaulted into the
saddle and my terrified horse started, as eager as I was to
get out of the vicinity." (Eisman, 1972)
Intensity:
QUESTIONS (6, PART A2)
1.
Following are some historical descriptions of earth-
quakes (a-d). Such statements, made to scientists or reporters
or recorded in diaries or on survey forms distributed by
government agencies, allow scientists to determine the
intensity of an earthquake. Using the Modified Mercalli
Intensity Scale (Table 6.1), assign each of the quakes an
intensity number. Pick the lowest number exhibiting the
characteristics given. The first quotation describes the
observations of an eyewitness to a California earthquake
around 1913. The second, third, and part of the fourth
descriptions are from data gathered by the U.S. Coast and
Geodetic Survey after the Daly City, California, earthquake
of 1957 (Richter magnitude 5.3).
a.
"There was a keen frost, and when we reached the
water-hole a thin film of ice was seen upon the water. I dis-
mounted and led my horse by the bridle, and walked to the
edge of the water. Just as I reached it, the ground seemed to
be violently swayed from east to west. The water splashed
up to my knees; the trees whipped about and limbs fell on
and all around me. I was affected by a fearful nausea, my
horse snorted and in terror struggled violently to get away
from me, but I hung to him, having as great a fear as he had
himself. The lake commenced to roar like the ocean in a
b.
"The shock seemed to be a sort of gentle swaying back
and forth, causing hanging fixtures to swing, but doing
no damage." (Iacopi, 1971)
Intensity:
c.
"The earthquake was very intense... a heavy oak china
cabinet and massive table moved 2 to 3 inches away from
original positions; kitchen stove moved 2 inches; furnace
in basement moved two inches off base and water heater
tilted off base." (Iacopi, 1971)
Intensity:
