EVIDENCE FOR INTERNAL DEFORMATION OF THE INDIAN PLATE

The Ninetyeast Ridge, far from being ‘aseismic’, has historically been a region of substantial seismicity. Since 1913, 4 magnitude 7 or greater earthquakes (including one with Ms = 7.7) and 10 magnitude 6 events have occurred in this general area. We have determined the mechanisms of several of these earthquakes, which suggest that the Ninetyeast Ridge area is presently a complex zone of deformation within the Indian plate. The northern portion (3°N–10°S) of the ridge is the active seismic zone, where both vertical and strike slip motion occur, while further south the ridge is far less seismic. This transition roughly coincides with a change in the ridge's morphology from irregular en echelon blocks to a smooth flat-topped high. The strike slip motion is left lateral, which is consistent with the Indian (west) side encountering resistance due to the collision with Asia while the Australian (east) side is subducting smoothly at the Sumatra trench. South of about 9°S the style of deformation differs on the two sides of the ridge. To the east, normal faulting occurs, which may be related to the formation of grabenlike structures. To the west the topography can be interpreted as the result of NW-SE compression which takes place largely aseismically but is observed for one large earthquake. This signficant intraplate deformation may explain the difficulties that occur in attempts to close the India-Africa-Antarctica triple junction using a rigid Indian plate.

[1]  S. Uyeda,et al.  Seismotectonics and tectonic history of the andaman sea , 1979 .

[2]  S. Solomon,et al.  The relationship of source parameters of oceanic transform earthquakes to plate velocity and transform length , 1978 .

[3]  J. Peirce.  The northward motion of India since the Late Cretaceous , 1978 .

[4]  C. G. Chase.  Plate kinematics: The Americas, East Africa, and the rest of the world , 1978 .

[5]  Thomas H. Jordan,et al.  Present‐day plate motions , 1977 .

[6]  B. Luyendyk,et al.  Tectonic history of aseismic ridges in the eastern Indian Ocean , 1977 .

[7]  H. Kanamori,et al.  Magnitudes of great shallow earthquakes from 1904 to 1952 , 1977, Bulletin of the Seismological Society of America.

[8]  R. Detrick,et al.  The subsidence of aseismic ridges , 1977 .

[9]  E. Okal.  The July 9 and 23, 1905, Mongolian earthquakes: A surface wave investigation , 1977 .

[10]  S. Solomon,et al.  Apparent stress and stress drop for intraplate earthquakes and tectonic stress in the plates , 1977 .

[11]  E. Okal.  A surface-wave investigation of the rupture mechanism of the Gobi-Altai (December 4, 1957) earthquake , 1976 .

[12]  H. Kanamori.  Re-examination of the earth's free oxcillations excited by the Kamchatka earthquake of November 4, 1952 , 1976 .

[13]  D. L. Anderson,et al.  Theoretical Basis of Some Empirical Relations in Seismology by Hiroo Kanamori And , 1975 .

[14]  B. Luyendyk,et al.  Results of DSDP Leg 26 and the Geologic History of the Southern Indian Ocean , 1974 .

[15]  J. Sclater,et al.  Evolution of the East: Central Indian Ocean, with Emphasis on the Tectonic Setting of the Ninetyeast Ridge , 1974 .

[16]  T. Jordan,et al.  Numerical Modelling of Instantaneous Plate Tectonics , 1974 .

[17]  Hiroo Kanamori,et al.  Focal process of the great Chilean earthquake May 22, 1960☆ , 1974 .

[18]  D. G. Moore,et al.  Sedimentary and Tectonic Processes in the Bengal Deep-Sea Fan and Geosyncline , 1974 .

[19]  C. Bowin.  Origin of the Ninety East Ridge from studies near the equator , 1973 .

[20]  J. Ewing,et al.  Mid‐plate tectonics in the Indian Ocean , 1972 .

[21]  T. Fitch.  Plate convergence, transcurrent faults, and internal deformation adjacent to Southeast Asia and the western Pacific , 1972 .

[22]  C. G. Chase.  The N Plate Problem of Plate Tectonics , 1972 .

[23]  W. J. Morgan,et al.  Deep Mantle Convection Plumes and Plate Motions , 1972 .

[24]  H. Kanamori.  Seismological evidence for a lithospheric normal faulting — the Sanriku earthquake of 1933 , 1971 .

[25]  K. Aki,et al.  Precise focal depth determination from amplitude spectra of surface waves , 1970 .

[26]  L. Sykes.  Seismicity of the Indian Ocean and a possible nascent island arc between Ceylon and Australia , 1970 .

[27]  H. Kanamori.  Synthesis ofzce Studies–Kurile Islands Earthquake of October 13, 1963 , 1970 .

[28]  H. Kanamori.  The Alaska Earthquake of 1964: Radiation of long-period surface waves and source mechanism , 1970 .

[29]  H. Kanamori.  Synthesis of Long-Period Surface Waves and Its Application to Earthquake Source Studies -- Kurile Islands Earthquake of October 13, 1963 , 1970 .

[30]  B. Gutenberg,et al.  Seismicity of the Earth , 1970, Nature.

[31]  X. Pichon,et al.  Magnetic Anomalies in the Indian Ocean and Sea-Floor Spreading Continents , 1968 .

[32]  James N. Brune,et al.  Seismic moment, seismicity, and rate of slip along major fault zones , 1968 .

[33]  T. Francis,et al.  Seismic refraction measurements in the southern Indian Ocean , 1967 .

[34]  A. J. Wickens,et al.  Computer re-evaluation of earthquake mechanism solutions , 1967 .

[35]  C. Stover.  Seismicity of the Indian Ocean , 1966 .

[36]  C. R. Allen,et al.  Relationship between seismicity and geologic structure in the Southern California region , 1965 .