SEISMICITY OF THE GALAPAGOS 95.5°W PROPAGATING RIFT

Abstract

An array of ocean bottom seismometers was deployed for 21 days about the 95.5°W Galapagos propagating rift tip to obtain seismic evidence for the propagating rift hypothesis and to investigate the dynamic effects of rift propagation. Seismicity was fairly constant (10–17 events per day) with no indication of swarm activity. Hypocenters were determined for 304 earthquakes, the largest of which is estimated to be mb = 2.3, with a seismic moment of 2.9 × 1020 dyn cm. The cumulative seismic moment released by all local earthquakes is estimated to be 6.6 × 1020 dyn cm. A b value of 0.77 ± 0.08 was estimated for the region, implying a stress regime dominated by a broad tectonic stress field. The distribution of earthquakes is consistent with the propagating rift hypothesis. Most earthquakes occurred south of the propagating rift within the De Steiguer Deep. Seismicity within the transform zone is clustered in such a way as to suggest that two or three discrete fault segments may be active at the same time. The seismicity observed along the propagating rift is diffuse. Earthquakes were observed at the propagating rift tip, but not in front of it. Almost all earthquakes occur south of the northern pseudofault and its extension west of the rift tip; that is, earthquakes occur almost exclusively in lithosphere that has been or is being overtaken by the propagating rift. Six composite focal mechanisms are consistent with the propagating rift hypothesis. First motions from earthquakes located on the propagating rift indicate extension, whereas motions associated with the failing rift area indicate compression. A compressional regime is also indicated west of the transform zone. Twelve earthquakes from the transform zone produce a poorly constrained focal mechanism consistent with right‐lateral strike‐slip motion along a fault plane that is roughly parallel to the isochrons of the transferred lithosphere.