“Detached” deep earthquakes: are they really?

Abstract

We use primarily the generation of acoustic T waves into the ocean by deep seismic sources to investigate the propagation of high-frequency seismic energy from the bottom of subduction zones to the shoreline at the earth’s surface. Conversion from shear waves to oceanic acoustic waves can be used as a proxy for the existence of a continuous slab featuring low anelastic attenuation. With the help of other techniques, such as the estimation of Q from S-to- P spectral amplitude ratios, we examine systematically a number of regions where earthquakes have been described as “detached”. We establish the mechanical continuity of the slab to the hypocenters of the 1990 Sakhalin and 1982 Bonin events, which occurred several hundred kilometers in front of the mainstream seismic zone. The study of the 1989 Paraguay shock is inconclusive, probably due to its much smaller size. The vertical continuity of the South American slab through its aseismic depth range is verified, and a similar situation probably exists in Java. Attenuation data suggests that the deep Spanish earthquakes occur within a vertically large segment of colder material, and a similar situation may exist in Colombia. The only clearly detached deep events with no mechanical connection to the surface make up the Vityaz cluster, under the North Fiji Basin. Based on a variety of geophysical evidence, the small deep earthquakes under New Zealand are likely to take place in a detached blob at least 350 km below the termination of mainstream seismicity. These results support a model integrating buoyancy forces over a long continuous slab as the source of the down-dip compressional stresses observed in large earthquakes at the bottom of the transition zone.