TABLE 2.1 Size Measures of Some Important Earthquakes

Date

Location

MS

MW

M0 (1018 N-m)

April 18, 1906

San Francisco

8.25

8.0

1,000

Sept. 1, 1923

Kanto, Japan

8.2

7.9

850

Nov. 4, 1952

Kamchatka

8.25

9.0

35,000

March 9, 1957

Aleutian Islands

8.25

9.1

58,500

May 22, 1960

Chile

8.3

9.5

200,000

March 25, 1964

Alaska

8.4

9.2

82,000

June 16, 1964

Niigata, Japan

7.5

7.6

300

Feb. 4, 1965

Aleutian Islands

7.75

8.7

12,500

May 31, 1970

Peru

7.4

8.0

1,000

Feb. 4, 1975

Haicheng, China

7.4

6.9

31

July 28, 1976

Tangshan, China

7.9

7.6

280

Aug. 19, 1977

Sumba

7.9

8.3

3,590

Oct. 28, 1983

Borah Peak

7.3

6.9

31

Sept. 19, 1985

Mexico

8.1

8.0

1,100

Oct. 18, 1989

Loma Prieta

7.1

6.9

27

June 28, 1992

Landers

7.5

7.3

110

Jan. 17, 1994

Northridge

6.6

6.7

12

June 9, 1994

Bolvia

7.0a

8.2

2,630

Jan. 16, 1995

Hyogo-ken Nanbu, Japan

6.8

6.9

24

Aug. 17, 1999

Izmit, Turkey

7.8

7.4

242

Sept. 20, 1999

Chi-Chi, Taiwan

7.7

7.6

340

Oct. 16, 1999

Hector Mine

7.4

7.1

60

Jan. 13, 2001

El Salvador

7.8

7.7

460

Jan. 26, 2001

Bhuj, India

8.0

7.6

340

NOTE: All events are shallow except Bolivia, which had a focal depth of 657 km. Moment magnitude MW computed from seismic moment M0 via Equation 2.7.

aBody-wave magnitude.

SOURCES: U.S. Geological Survey and Harvard University.

Unless otherwise noted, all magnitudes given throughout the remainder of this report are moment magnitudes.

Beginning in the 1950s, arrays of temporary seismic stations were deployed to study the aftershocks of large earthquakes. Aftershocks are caused by subsidiary faulting from stress concentrations produced by the main shock, owing to inhomogeneities in fault slippage and heterogeneities in the properties of the nearby rocks. Omori’s work on the 1891 Nobi earthquake had demonstrated that the frequency of aftershocks decayed inversely with the time following the main shock (97). In its modern form, “Omori’s law” states that the aftershock frequency obeys a power law of the form

n(t) = A(t + c)p, (2.8)



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