Natural Hazards: Causes and Effects - Disaster Management Center ...

Frequency of Earthquakes
More than one million earthquakes shake the earth each year, on an average of about two each
minute. Some scientists say there may be about five million a year, counting all the
microearthquakes and tremors that are picked up only on highly sensitive seismographs.
From 1900 to 1964 there was a yearly average, the world over, of about 20 major earthquakes,
one or two of which were usually great. Since 1964, the year of the great Alaskan earthquake,
the annual total of major earthquakes has been distinctly less, with none exceeding magnitude 8
until mid-January 1971.
Effects of Earthquakes
Primary Effects
The initial effect of an earthquake is the violent ground motion. Additionally the ground often
fissures or cracks, and there can be large permanent displacements horizontally—sometimes
as much as 10-15 meters (30-50 feet).
The San Fransisco earthquake of April 18, 1906, occurred along the San Andreas fault over a
length of 470 kilometers (300 miles), displacing the lips by a few centimeters up to one meter
and causing a slip between the sides varying from 25 centimeters to seven meters. (10 inches
to 20 feet) The Mino-Avari earthquake in Japan caused changes in level of about six meters
(20 feet) and horizontal slips of two meters (seven feet) along the Neo fault, which, unlike that at
San Francisco, is not rectilinear, but divided into stepped sections. The consequences of such
ground movements are, of course, disastrous for buildings near to or on the line of a fault.
During the Tokyo earthquake (September 1923) in Japan, the coast of Sagmi Bay rose two
meters (seven feet), whereas on the north side of the bay the bottom rose 250 meters (820
feet). At the center it subsided 100 to 200 meters—even 400 meters at some points. In
addition, there was horizontal displacement of up to 4 meters (13 feet) in the land surrounding
Tokyo Bay. 7
Another primary effect is known as liquefaction. Loose sandy soils with a high moisture content
separate when shaken by an earthquake. The water moves upward, giving the surface a
consistency much like that of quicksand. Heavy structures resting on these soils slowly sink into
the ground. Large portions of Port Royal, Jamaica, were damaged in this way during the
earthquakes that struck the city in 1694 and 1970.
Secondary Effects
Often as destructive as the earthquake itself are the resulting secondary effects such as
landslides, fires, tsunamis, and floods. Landslides are especially damaging and often account
for the majority of lives lost. During the 1970 earthquake in Peru a very large portion of those
killed were swept away by a landslide that covered the town of Yungay. Similarly, in the
Guatemala earthquake of 1976, most deaths that occurred in Guatemala City were caused by
the collapse of the unstabilized hillsides where thousands of urban squatters had settled.
Of far more concern are tsunamis, the large seawaves caused by an earthquake abruptly
moving the ocean floor. The waves move at a high velocity and can cross thousands of
kilometers before they run up on shore. At sea, their low wave height gives little evidence of
their existence; however, as they reach shallower depths, their velocity decreases and their
height increases. In this way a five-meter crest moving at 600 kilometers per hour in the open