Immediate Foreshock Sequences of Oceanic Transform Earthquakes on the East Pacific Rise

Foreshock sequences provide one of the few windows into the earthquake nucleation process. Until recently it was impossible to study foreshock sequences of oceanic earthquakes owing to the low detection thresholds of standard seismic catalogs for the regions near spreading ridges. In the last few years autonomous hydrophone arrays operated by the National Oceanic and Atmospheric Administrationʹs Pacific Marine Environmental Laboratory have lowered the detection threshold in several oceanic regions to about magnitude 3. We investigate the foreshock sequences of large oceanic transform fault earthquakes using the seismicity catalogs derived from the hydrophone data. Transforms on the equatorial East Pacific Rise regularly have foreshocks within 1 hr before a magnitude 5–6 earthquake, while those on the Juan De Fuca and Mid-Atlantic Ridges do not. The foreshocks typically occur in the period between 100 and 1000 sec before the mainshock, suggesting that this timescale is important for large-earthquake nucleation in this region.