Earthquake doublets in the Solomon Islands

Large, shallow, thrust earthquakes in the Solomon Islands region tend to occur in closely related pairs. Two recent sequences are July 14, 1971 (M_S = 7.9) and July 26, 1971 (M_S = 7.9) and 14^h37^m, July 20, 1975 (M_S = 7.9) and 19^h54^m, July 20, 1975 (M_S = 7.7). The mechanism of these seismic doublets has important bearing on the triggering mechanism of earthquakes in subduction zones. Detailed analysis of the seismic body waves and surface waves were performed on the 1971, 1974, and 1975 doublets, providing a better understanding of: (1) the mechanics of seismic triggering, (2) the state of stress on the fault plane, and (3) the nature of subduction between the Pacific and Indian plates. The results indicate that although the geometry of the subduction zone in the Solomon Islands is complicated by the presence of several sub-plates, the slip direction of the Indian plate with respect to the Pacific plate is relatively uniform over the entire region. The large seismic moments of the 1971 sequence (1.2 · 10^28 and 1.8 · 10^28 dyne cm) indicate that these events directly represent the underthrusting of the Indian and Solomon plates beneath the Pacific plate. The body waves from these doublets, recorded on the WWSSN long-period seismograms, are remarkably impulsive and simple compared with those from events of comparable seismic moment in other subduction zones. In addition, the source dimensions of the body waves are 30–70 km in length, substantially smaller than the overall rupture surfaces radiating the surface waves which are 100–300 km in length. These facts suggest the existence of relatively large, isolated high-stress zones on the fault plane. This type of stress distribution is distinct from other regions which have more heterogeneous stress distribution on the fault plane, and this is proposed as the principal characteristic of this region responsible for the occurrence of the doublets and for the apparent efficiency of triggering in the Solomon trench. Prior to the 1971 sequence, similar sequences have occurred in the same area in 1919–1920 and 1945–1946. From the amount of slip (1.3 m) determined for the 1971 sequence and the apparent recurrence interval of 25 years, a seismic slip rate of 5 cm yr^−1 is determined. This value is a significant portion of the convergence rate between the Indian and Pacific plates indicating that the plate motion here is taken up largely by seismic slip.

[1]  J. W. Curtis.  The spatial seismicity of Papua New Guinea and the Solomon Islands , 1973 .

[2]  J. Arthur Snoke,et al.  Slow earthquakes and stress redistribution , 1978, Nature.

[3]  J. Milsom.  Woodlark Basin, A minor center of sea‐Floor spreading in Melanesia , 1970 .

[4]  T. Nagamune.  TIME CHARACTERISTICS OF CRUSTAL DEFORMATION OF THE YUNNAN EARTHQUAKE OF MAY 29, 1976, AS INFERRED FROM TILT RECORDING , 1977 .

[5]  P. J. Coleman,et al.  The Melanesian Borderlands and India — Pacific plates' boundary , 1976 .

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

[7]  M. Odegard,et al.  Seismic studies on the Ontong Java Plateau, 1970☆ , 1976 .

[8]  K. Macdonald,et al.  Heat flow and plate boundaries in Melanesia , 1973 .

[9]  P. Molnar,et al.  Distribution of stresses in the descending lithosphere from a global survey of focal‐mechanism solutions of mantle earthquakes , 1971 .

[10]  H. Kanamori.  Mechanism of tsunami earthquakes , 1972 .

[11]  Ari Ben-Menahem,et al.  Radiation of seismic surface-waves from finite moving sources , 1961 .

[12]  K. Macdonald,et al.  Rifting History of the Woodlark Basin in the Southwest Pacific , 1973 .

[13]  P. Molnar,et al.  Focal mechanisms and plate tectonics of the southwest Pacific , 1972 .

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

[15]  I. Ripper.  Part 6. Present plate boundary seismic, volcanic and kinematic processes: Seismicity and earthquake focal mechanisms in the New Guinea Solomon Islands region , 1975 .

[16]  H. Kanamori,et al.  Back-arc opening and the mode of subduction , 1979 .

[17]  Xavier Le Pichon,et al.  Sea‐floor spreading and continental drift , 1968 .

[18]  J. W. Curtis.  Plate tectonics and the Papua—New Guinea—Solomon Islands region , 1973 .

[19]  J. Connelly.  Tectonic Development of the Bismarck Sea Based on Gravity and Magnetic Modelling , 1976 .

[20]  D. Denham.  Distribution of earthquakes in the New Guinea-Solomon Islands Region , 1969 .

[21]  G. Pascal.  Seismotectonics of the Papua New Guinea—Solomon Islands region , 1979 .

[22]  H. Kanamori,et al.  A slow earthquake , 1979 .

[23]  J. Connelly.  A structural interpretation of magnetometer and seismic profiler records in the Bismarck Sea, Melanesian Archipelago , 1974 .

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

[25]  Hiroo Kanamori,et al.  Seismological aspects of the Guatemala Earthquake of February 4, 1976 , 1978 .

[26]  M. Furumoto,et al.  Mechanism of large earthquakes along the eastern margin of the Japan sea , 1975 .

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

[28]  C. G. Chase.  Tectonic History of the Fiji Plateau , 1971 .

[29]  J. Dewey,et al.  Mountain belts and the new global tectonics , 1970 .

[30]  Hiroo Kanamori,et al.  Mode of the strain release along the Gibbs fracture zone, Mid-Atlantic ridge , 1976 .

[31]  M. Wyss,et al.  The Alaska earthquake of 28 March 1964: A complex multiple rupture , 1967 .