Episodic creep triggering seismic slip at the Blanco Transform Fault, Blanco Ridge segment, NE Pacific

Ocean Transform Faults (OTFs) slip via aseismic creep and seismically during earthquakes up to magnitude 6 < M < 7. These earthquakes appear to have predictable behavior both in the long-term (quasi-periodicity) and short-term perspective (foreshocks), which is likely related to the aseismic fault motion. However, the exact mode of OTF slip remains unclear. We present results of the Blanco Transform Fault OBS Experiment, which included a 1-year deployment of 55 three-component seismometers co-located with differential pressure gauges on and around the Blanco Transform Fault Zone (BTFZ), in the North East Pacific. Here we concentrate on the Blanco Ridge segment of the BTFZ, a 130-km-long seismically highly active strike-slip fault that spans between the extensional Cascadia and Gorda depressions. We determined precise hypocentral parameters of about 1,600 events with an automatic detection (using BRTT Antelope) and a relative relocation algorithm (hypoDD). Earthquakes nucleate predominantly in the mid-to-lower crust between 3 and 7 km of depth and in the mantle from 9 km to the down-dip limit of seismicity at 13 km depth, which roughly coincides with the 600°C isotherm. The crustal seismicity is dominated by temporally unclustered events and an aftershock sequence after the strongest earthquake in the catalogue, the ML 5.5 event on January 30th, 2013. The mantle seismicity is notably different consisting of short, up to 4-hours-lasting seismic swarms. Based on hypocentral migration rates of swarm events, swarms likely are a direct response to episodic creep in the upper-most mantle. We observe a small mantle swarm preceding the ML 5.5 earthquake by less than an hour. We interpret the precursory swarm (foreshock sequence) as a response to an episode of mantle creep shortly preceding the mainshock. The mantle creep is thus not only loading the fully coupled, brittle crust in a long-term perspective, but also acts as a short-term direct trigger of large OTF earthquakes.