Stress Drops on the Blanco Oceanic Transform Fault from Interstation Phase Coherence

Abstract

Oceanic transform faults display a wide range of earthquake stress drops, large aseismic slip, and along-strike variation in seismic coupling. We use and further develop a phase coherence-based method to calculate and analyze stress drops of 61 M ≥ 5.0 events between 2000 and 2016 on the Blanco Fault, off the coast of Oregon. With this method, we estimate earthquake rupture extents by examining how apparent source time functions (ASTFs) vary between stations. The variation is caused by the generation of seismic waves at different locations along the rupture, which arrive at different times depending on station location. We isolate ASTFs at a range of stations by comparing seismograms of co-located earthquakes and then use the inter-station ASTF coherence to infer rupture extent and stress drop. We examine how our analysis is influenced by various factors, including poor trace alignment, relative earthquake locations, focal mechanism variation, azimuthal distribution of stations, and depth phase arrivals. We find that as alignment accuracy decreases or distance between earthquakes increases, coherence is reduced,

Seismotectonics of the Blanco Transform Fault System, Northeast Pacific: Evidence for an Immature Plate Boundary

“…Teleseismic analysis (e.g., Dziak et al, 1991) shifted from the bathymetric features (e.g., Williams et al, 2019). The U.S. Navy's Sound Surveillance System (SOSUS) enabled earthquake detection using hydro-acoustic tertiary (T-) waves (e.g., Dziak et al, 2000Dziak et al, , 2003Géli et al, 2014), which significantly improved the precision of event locations.…”

“…Teleseismic analysis (e.g., Dziak et al, 1991) inferred the seismic coupling strength is different along the west and east segments of BTFS, although the routine event locations along the BTFS are scattered and systematically shifted from the bathymetric features (e.g., Williams et al, 2019). The U.S. Navy's Sound Surveillance System (SOSUS) enabled earthquake detection using hydro-acoustic tertiary (T-) waves (e.g., Dziak et al, 2000Dziak et al, , 2003Géli et al, 2014), which significantly improved the precision of event locations.…”