Temporal variation of large intraplate earthquakes in coupled subduction zones

Abstract

The focal mechanisms of intraplate earthquakes within subducting lithosphere are frequently used to infer large-scale stress regimes induced by slab-pull, bending or unbending, and lateral segmentation and undulations of the slab. Numerous studies have further postulated that the intraplate activity is influenced by transitory regional stress regimes such as those associated with interplate thrust events. Temporal variations of the latter type may potentially play an important role in assessing regions of uncertain seismic potential, and possibly even in earthquake forecasting. A systematic analysis of 1130 focal mechanisms for intraplate earthquakes with m_b ≥ 5.0 in the depth range 0–300 km is conducted for nine circum-Pacific subduction zones, all of which are known to have large interplate thrust events. The spatial and temporal relationships of the earthquakes within the subducting slab to the large thrust events in each region are appraised. The earthquake catalog assembled contains all published focal mechanisms, and is probably complete for m_b ≥ 6.5 for the years 1963–1986. For many of the localized regions considered in detail the catalog is complete to lower thresholds of m_b ≥ 6.0 or m_b ≥ 5.5. This analysis provides compelling evidence for a temporal link between large interplate thrust activity and intraplate seismicity. For the seismically coupled regions considered here, outer rise compressional events have occurred prior to several large thrust events or are associated with seismic gaps, while outer rise tensional events generally only follow interplate ruptures. In the intermediate depth range, large down-dip tensional events generally precede interplate thrusts, and are often concentrated at the down-dip edge of the coupled zone. A transition to down-dip compressional stress or diminished tensional activity at intermediate depth is observed after several large thrust events (e.g., 1960 Chile, 1974 Peru, 1957 Aleutian, 1971 New Britain). These examples support the notion that the intraplate stress environment responds viscoelastically to the temporally varying interplate stress regime. Assuming that this concept is correct, the seismic potential of several seismic gaps is considered on the basis of both outer rise and intermediate depth earthquake activity.