Seismological evidence for selectivity in slip planes under down.dip extension or compression

Abstract

A selectivity in slip planes under down-dip extension or compression in seismic zones is suggested from recent analyses of multiplets. A fault plane solution cannot distinguish the slip plane from the auxiliary plane. Analysis of a multiplet can remove this ambiguity. The results of the analyses of more than twenty deep multiplets are synthesized in reference to the mutual relationship in dip anllle between the slip planes and their auxiliary planes. The slip planes tend to dip more steeply than their auxiliary planes in seismic zones where either down-dip extension or compression is predominant. The external forces applied below and above the sinking slab of lithosphere could cause either extension or compression but cannot produce such a selectivity in slip planes. The idea of gravitational sinking of the lithosphere provides the simplest explanation for this selectivity. The explanation is based on the fact that faulting must do some work against gravity. A similar selectivity may exist for shallow lithospheric earthquakes beneath trenches.