Earthquakes - Geotechnical Engineering Investigation - page 893

11.1.2 Geographic Distribution

Worldwide

General Distribution
The relationship between earthquake zones and tectonic plates (see Appendix A.3) is
given in Figure 11.1. It is seen that concentrations are along the boundaries of subducting
plates and zones of seafloor spreading, the concept of which is illustrated in Figure 11.2.
The great Precambrian shields of Brazil, Canada, Africa, India, Siberia, and Australia are
generally aseismic although their margins are subjected to earthquake activity.
Earthquake occurrence predominates in three major belts: island chains and land masses
forming the Pacific Ocean; the mid-Atlantic ridge; and an east-west zone extend-
ing from China through northern India, Turkey, Greece, Italy, and western North Africa
to Portugal. Countries with a high incidence of damaging earthquakes include Chile,
China, Greece, India, Indonesia, Italy, Iran, Japan, Mexico, Morocco, Peru, the
Philippines, Rumania, Spain, Turkey, Yugoslavia, the East Indies, and California in
the United States.
Important worldwide events are summarized in Table 11.1. Selection is based on consider-
ation of the number of deaths; the effect on the land surface in terms of faulting, subsi-
dence, or landslides; and the contribution to the knowledge of seismology and earthquake
engineering. The table also serves to illustrate the partial distribution of events by city and
country.
During the 20 years since the first edition of this topic there have been many significant,
damaging earthquakes. Of particular note, outside of the United States, were the Izmit,
Turkey event of 1999 ( Section 11.3.1 ) and the Kobe, Japan (Hanshin event), of January 17,
1995. With M 6.9, Hanshin resulted in the deaths of 5500 persons. The deep focus depth

was 10 mi, located along the Nojima strike-slip fault, and the duration less than 1 min.
Major damage occurred where structures were located over soft ground; elevated high-
way and rail lines and buildings generally five to ten stories collapsed. Liquefaction
destroyed port facilities and caused other buildings to collapse.

Two General Classes
Plate-edge earthquakes: The boundaries of the lithospheric plates are defined by the princi-
pal global seismic zones in which about 90% of the world's earthquakes occur (Figure 11.1
and Figure A.2). Note that in Figure A.2 additional plates have been identified, such as the
Juan de Fuca plate in Northern California and Oregon.
Intraplate earthquakes: Areas far from the plate edges are characterized by fewer and
smaller events, but large destructive earthquakes occur from time to time such as those of
New Madrid, Charleston, and northern China (Table 11.1). These events and others indi-
cate that the lithospheric plates are not rigid and free of rupture.

Continental United States
The distribution of the more damaging earthquakes in the continental United States
through 1966 is given in Figure 11.3. California has by far, the largest incidence of damag-
ing events. The largest events recorded, however, include New Madrid, Missouri (1811
and 1812), Charleston, South Carolina (1886), and Anchorage, Alaska (1964), all of which
had magnitudes estimated to be greater than 8. The New Madrid quakes had tremendous
effects on the central lowlands, causing as much as 15 to 20 ft of subsidence in the
Mississippi valley, forming many large lakes, and were felt from New Orleans to Boston
(Guttenberg and Richter, 1954).