Distribution of stresses in the descending lithosphere from a global survey of focal‐mechanism solutions of mantle earthquakes

TLDR

Analysis of 204 deep and intermediate‑depth earthquake focal mechanisms shows that descending lithosphere acts as slab‑like stress guides, aligning stresses parallel to inclined seismic zones, and that a double‑couple shear‑dislocation model adequately describes all data. The study compiles almost all WWSSN focal‑mechanism solutions from 1962–1968 and selected pre‑1962 events, covering nearly every mantle‑earthquake region worldwide. At intermediate depths, extensional stresses parallel to the dip dominate in zones with seismic gaps or no deep earthquakes, while compressional stresses parallel to the dip prevail below about 300 km, indicating that the lithosphere sinks into the asthenosphere but is resisted below that depth, with slab contortions and disruptions observed, yet simple bending stresses appear unimportant for sub‑crustal earthquake generation.

Abstract

A region‐by‐region analysis of 204 reliable focal‐mechanism solutions for deep and intermediate‐depth earthquakes strongly supports the idea that portions of the lithosphere that descend into the mantle are slablike stress guides that align the earthquake‐generating stresses parallel to the inclined seismic zones. At intermediate depths extensional stresses parallel to the dip of the zone are predominant in zones characterized either by gaps in the seismicity as a function of depth or by an absence of deep earthquakes. Compressional stresses parallel to the dip of the zone are prevalent everywhere the zone exists below about 300 km. These results indicate that the lithosphere sinks into the asthenosphere under its own weight but encounters resistance to its downward motion below about 300 km. Additional results indicate contortions and disruptions of the descending slabs; however, stresses attributable to simple bending of the plates do not seem to be important in the generation of subcrustal earthquakes. This summary, intended to be comprehensive, includes nearly all solutions obtainable from the World‐Wide Standardized Seismograph Network (WWSSN) for the period 1962 through part of 1968 plus a selection of reliable solutions of pre‐1962 events, and it includes data from nearly every region in the world where earthquakes occur in the mantle. The double‐couple or shear dislocation model of the source mechanism is adequate for all the data.