Asperity distributions and large earthquake occurrence in subduction zones

Abstract

Ruff, L.J., 1992. Asperity distributions and large earthquake occurrence in subduction zones. In: T. Mikumo, K. Aki, M. Ohnaka, L.J. Ruff and P.K.P. Spudich (Editors), Earthquake Source Physics and Earthquake Precursors. Tectonophysics, 211: 61–83. Plate tectonics and the seismic gap hypothesis provide the framework for long-term earthquake forecasting of plate boundary earthquakes. Unfortunately, detailed examination reveals that earthquake recurrence times and rupture length vary between successive earthquake cycles in the same subduction zone. Furthermore, larger coseismic slip is commonly associated with larger rupture length. Hence, large earthquake occurrence in subduction zones is characterized by variability in: (1) recurrence times, (2) rupture length, and (3) coseismic slip. These facts, plus many other observations, indicate that there are significant spatial variations in the “strength” of the plate interface. One simple description of these variations and their role in the earthquake cycle is the asperity model, where the large strong regions of the plate interface are called asperities, and the large earthquakes occur when the large asperities break. The asperity model of earthquake occurrence is able to qualitatively explain several features of large plate boundary earthquakes. To go beyond general qualitative notions. I pose the following scientific test: are the observed asperity distributions and a simple model of their interaction self-consistent with the above three observed features of large earthquake occurrence? The distribution of the major asperities along plate boundary segments has now been determined for several subduction zones. Rupture process studies of adjacent large and great earthquakes have provided reliable estimates of the along-strike asperity lengths and separations for several adjacent asperities in the Kurile Islands, Colombia, and Peru subduction zones. The simplest mechanical model for asperity interaction is to idealize two adjacent asperities as frictional sliders that are connected by main springs to the upper plate, by a coupling spring to each other, and maintain frictional contact with a conveyer belt (the lower plate) that moves with a constant velocity. An “earthquake” occurs when the net force on the asperity frictional slider reaches some specified level. The failure force and spring constants are determined by the observed asperity distribution and simple models of elastic interaction. Two different macroscopic failure criteria are used. This simple mechanical model displays a remarkable range of behavior from simple to complex. When the two asperities are identical in all their properties, sequences of identical “earthquakes” are produced. For the more realistic case of non-identical asperities, “earthquake” sequences show great variety. Using system variables from the observed asperity distributions, the “earthquake” sequences typically display: (1) variable recurrence times, (2) variable rupture length, i.e. a combination of single-asperity and double-asperity failures, and for one of the failure criteria (3) larger coseismic slip for double-asperity failures. Statistical summaries of thousands of simulated “earthquake” sequences for asperity pairs in the Kuriles, Colombia, and Peru subduction zones are broadly consistent with the observed features of large earthquake occurrence in these subduction zones. The main conclusion is that the asperity model provides a self-consistent explanation for: fault zone heterogeneity, the rupture process, and recurrence times and rupture mode of large earthquake sequences via a simple model for adjacent asperity interaction. In addition, a conclusion independent of any particular model for fault zone heterogeneity is that simple deterministic models of fault zone interaction can explain complex patterns of large earthquake occurrence in subduction zones.