Possible focal mechanism change during reactivation of a previously ruptured subduction zone and stress tensor implications

Abstract

Body waveform inversion methods usually assume a constant double‐couple source, or a combination of such subevents. Previous studies suggest that the source rupture of mature sources in terms of coupling along smooth subduction zones are usually resolved on this assumption. However, for events where the source area overlaps previously ruptured zones, the conventional inversion methods fail to resolve the source process. For such events the subduction coupling may vary between the mature source area and the previously ruptured zone, causing a variation in the focal mechanism too. We study the implications on the stress tensor of the coexistence of two such focal mechanisms using a major aftershock of the 1985 Chilean earthquake (M2= 6.0), the 1978 Kurile (M2= 7.6) and 1980 Santa Cruz Islands (M2= 7.7) earthquakes. For the first event, we use the two different sets of focal parameters obtained by a study which describes a focal mechanism change during the process. The epicentres of the latter two large events are located in the spots which had remained unbroken in the preceding large events, e.g. the 1963 and 1969 Kurile earthquakes and the 1966 Santa Cruz Is. earthquake, respectively. For each event, one of the focal mechanism solutions shows a substantial strike‐slip component. The stress tensors are examined under the constraints that the shear stress is parallel to the slip vector and a friction law is satisfied. Two particular families of stress tensors examined are: (1) the tensors that are optimally orientated with respect to the mature source area and (2) the tensors that have a vertical principal stress direction. The first type of tensors can fully account for the observed slip orientation only for the Chile aftershock, but it comes close to the observed one in the case of the Kurile Island event, and would indeed reactivate the decoupled area at a reasonably low stress level. The second type of tensors can be compatible with the two observed slip orientations in all cases. However, for the Santa Cruz Is. and Kurile Is. earthquakes, the friction law which should be satisfied on each plane requires too high a stress deviation. Copyright © 1989, Wiley Blackwell. All rights reserved