Man's physical universe

xanabras

Tennessee.

DIASTROPHISM AND VULCANISM 157

The great San Andreas fault, six hundred miles long, was

the location of the shifting in the earth's surface which produced the

earthquake which caused the San Francisco fire.

The main stresses in the earth's surface today lie along fractures

bordering the Pacific Ocean and a path that reaches through the

Mediterranean, the Himalayas, and the East Indies.

The earthquakes produced by faults, called "tectonic earthquakes,"

may shake entire continents. Less extensive earthquakes may be produced

by the explosive eruption of volcanoes. Tokachi caused great

damage in the nearby town of Menegama in Japan as a result of the

earthquake accompanying the explosion of this volcano in 1925.

A few minor earthquakes have been produced by the collapse of the

roofs of large underground caverns, landslides in mountain regions, or

the slumping on delta slopes.

The Seismograph Is One of the Geologist's Most Useful Instruments.

The seismograph is a delicately adjusted instrument extremely

sensitive to vibrations, whose amplitude it records.

There are about 350 seismographs located at various stations on the

earth's surface to record earthquakes. The seismograph enables the

scientist to determine the exact location

of the larger earthquakes, even when

their centers are thousands of miles

away. Earthquake vibrations are transmitted

through rock by different types

of wave motion. The majority of earthquakes

originate in the earth's outer

crust, but the resulting vibrations travel

throughout the globe. Some travel

around the surface, while others travel

Fig. 44. Three observatories

cooperate in locating an earthquake.

A circle is drawn around

each station as a center, with a

radius equal to the estimated

distance to the earthquake. The

intersection of the three circles

is the desired location.

to the center where they are deflected

by the dense cores. The time intervals

between the arrivals of the different

waves are noted at three different

stations. Inasmuch as these time intervals

are determined by the distance

from the earthquake center, the earthquake center may thus be

precisely located as shown in Fig. 44.

There are two general types of seismographs in common use.

The

first consists of a heavy pendulum, which holds a delicate penpoint

against a moving sheet of smoked paper and makes waves in the lines

thus formed according to the amplitude of the vibrations. The second

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