Seismicity of the Indian Ocean

Abstract

The seismicity of the Indian Ocean is primarily confined to the mid‐ocean ridge system and its associated rift valley; however, a broad NW‐SE band of 29 epicenters is confirmed in the northeastern Indian Ocean—11 of the 29 earthquakes of magnitude 6 or higher. The location of seismic activity on fractures that cut the ridge is shown by the epicenters along the Owen, Prince Edward, and Amsterdam fracture zones and by the epicenters on the fracture at 41°S, 81°E. The seismic activity of the mid‐ocean ridge, as shown by the rate of energy release, has been relatively constant since 1922, except for the years 1942 and 1951 when large earthquakes occurred. A solution of the equation Log N = a + b(8 − M), based on data from 76 earthquakes over a period of thirty years, resulted in the values a = −1.78 and b = 0.91 for the mid‐ocean ridge system. A table of frequency of occurrence based on this solution shows the frequency of earthquakes to range from fifty years for a magnitude of 8 to one year for a magnitude of 6–6¼.

Compressional Deformation of Oceanic Lithosphere in the Central Indian Ocean: Why It Is Where It Is

“…The larger events, which were mainly located in the lithospheric mantle, reflect continuing displacement on thrust and strike-slip faults in response to northsouth directed compression (Fig. 5;Stover, 1966;Sykes, 1970;Stein and Okal, 1978;Bergman andSolomon, 1980, 1985;Wiens and Stein, 1983;Wiens, 1986;Petroy and Wiens, 1989). In addition, the Chagos-Laccadive Ridge tends to be associated with large normal-fault earthquakes, consistent with the predictions of Wortel (1985, 1986).…”

Magnetic anomalies in the Indian Ocean and sea-floor spreading

“…The Amsterdam fracture zone, as shown in the physiographic diagram of I-Ieezen and Tharp, is a complex of echelon fractures that pro,b•bly accommodates most of the differential movement between the two ridges. The fractures found near the crest of the ridge south of Australia and the intense seismic activity al'ong the crest [Stover, 1966] In uppermost Miocene (anomaly 5), the movement of spreading resumed its previous speed, which has apparenfiy been constant to the present. It is during this latest phase of spreading that the central troughs of the Red Sea and Gulf of Aden were formed.…”

Intraplate earthquakes and the state of stress in oceanic lithosphere

“…The association of many oceanic intraplate earthquakes with the Ninetyeast Ridge, a subject explored further by Stein and Okal [1978], was also noted by Gutenberg and Richter. With a larger and more accurate set of earthquake locations, Sykes [1970a] noted a pattern in the seismicity of the northern Indian Ocean which he Clearly, care must be exercised in the interpretation of short-term (i.e., decades) patterns in oceanic intraplate seismicity, but these and other studies [e.g., Stover, 1966;Rothe, 1969] In this chapter we present source mechanisms (obtained by inversion of long-period P and SH waveforms, as described in Chapter 3)…”