Thomas J. Fitch

Fault plane solutions of shallow earthquakes and contemporary tectonics in Asia

Abstract The tectonics of Asia are interpreted as a result of convergence of the Indian and Eurasian plates. The Indian shield bends down and underthrusts the Himalayas to the northeast along a shallow dipping fault plane while the Eurasian plate underthrusts the Pamir mountains, and therefore presumably the Indian Plate, to the south. The convergence of the Indian and Eurasian plates appears to cause relatively high stress to be transmitted across a broad area, north and east of the Himalayas, and this stress in turn causes earthquakes and renewed tectonic activity in some of the ancient Paleozoic and Mesozoic fold belts that separate more stable, aseismic blocks in Asia.

Compressional velocity in source regions of deep earthquakes: An application of the master earthquake technique

Abstract The technique of earthquake location relative to a master event is used to estimate near-source velocity and take-off angles for rays travelling to selected stations. Computations of a reconnaissance nature were carried out with arrival times of P and pP from deep earthquakes beneath the northwest corner of the Fiji plateau, the Peru-Brazil border region and the basin separating Fiji from the Tonga arc. These data yield estimates of compressional velocity of 11.2 ± 0.4, 11.4 ± 0.7 and 10.7 ± 0.3 km/sec respectively. Each of these velocities and the other parameters of each model space are essentially independent of their starting values. The corresponding depth ranges are 600–660, 580–650 and 540–600 km. These in-situ velocities are 5–10% higher than those of the Helmberger and Wiggins model. To account for such high velocities by a thermal effect alone would require an improbably high thermal contrast of 1000°C between “normal” mantle and the cooler earthquake zones. Spinels of proposed mantle composition would have compressional velocities of about 10.4 km/sec at temperatures that are taken as normal for these depths. If the high values of near-source velocity are explained by the addition of a post-spinel assemblage, then by implication this transformation occurs at shallower depths in those seismic zones than in the “normal” mantle.

Mechanisms of Australian earthquakes and contemporary stress in the Indian ocean plate

Orientation of contemporary stress within plates of lithosphere, inferred mainly from earthquake mechanism solutions, is an important constraint to be satisfied by proposed driving mechanisms for global tectonics. Axes of maximum compressive stress in western Australia, India and the intervening oceanic region are nearly horizontal. In the NW part of this region, adjacent to the Burmese, Andaman and western Sunda arcs, axes of minimum compressive stress are also horizontal. Here stress axes are oriented in such a way as to suggest interaction of two stress fields; one generated by continental collision along the Himalayas and the other generated by body forces acting on parts of this plate that are descending into the upper mantle. We speculate that release of tectonic stress within large plates is to some extent localized by epirogenic processes some of which may operate independently of mechanisms that drive the plates.