On Gofar Transform Fault on the East Pacific Rise, the largest earthquakes (6.0 ≤ MW ≤ 6.2) have repeatedly ruptured the same portion of the fault, while intervening fault segments host swarms of microearthquakes. These long‐term patterns in earthquake occurrence suggest that heterogeneous fault zone properties control earthquake behavior. Using waveforms from ocean bottom seismometers that recorded seismicity before and after an anticipated 2008 MW 6.0 mainshock, we investigate the role that differences in material properties have on earthquake rupture at Gofar. We determine stress drop for 138 earthquakes (2.3 ≤ MW ≤ 4.0) that occurred within and between the rupture areas of large earthquakes. Stress drops are calculated from corner frequencies derived using an empirical Green's function spectral ratio method, and seismic moments are obtained by fitting the omega‐square source model to the low frequency amplitude of the displacement spectrum. Our analysis yields stress drops from 0.04 to 3.2 MPa with statistically significant spatial variation, including ~2 times higher average stress drop in fault segments where large earthquakes also occur compared to fault segments that host earthquake swarms. We find an inverse correlation between stress drop and P wave velocity reduction, which we interpret as the effect of fault zone damage on the ability of the fault to store strain energy that leads to our spatial variations in stress drop. Additionally, we observe lower stress drops following the MW 6.0 mainshock, consistent with increased damage and decreased fault strength after a large earthquake.

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Variations in stress drops of earthquakes associated with the April and May 2015 eruption of Axial Seamount, on the Juan de Fuca Ridge, suggest a reduction in crustal strength as a result of the eruption. Seismicity during the inflation and deflation periods was well recorded by ocean bottom seismometers located within and along the caldera. We use these nearby recordings and an empirical Green's function spectral ratio method to obtain corner frequencies for stress drops of earthquakes on caldera ring faults. We find stress drops from 0.6 to 43 MPa for 423 ring fault earthquakes (1.6 ≤ MW ≤ 3.6) and an average stress drop two times higher during the inflation period (6.4 MPa) prior to the eruption, than during the subsequent deflation (3.2 MPa). Stress drops also correlate with spatially varying shear wave speed, possibly reflecting a region of pervasive cracking in the northern caldera.

[1] We compute apparent stress for 114 aftershocks (0.9 ≤ M L ≤ 3.7) of the 1999 M w = 6.9 Quepos, Costa Rica, thrustfaulting earthquake to examine the influence of subducting plate topographic complexity near the Osa Peninsula on earthquake rupture. Using seismic coda techniques, we find a heterogeneous distribution in apparent stress of 0.1-2.5 MPa (mean 0.6 MPa) for these aftershocks. Mean aftershock apparent stress is more than twice the global mean for thrust-faulting earthquakes at oceanic subduction zones and 1.5 times the mean for events just northward along the margin near the Nicoya Peninsula where the subducting plate has lower relief. We also find constant source scaling for the Osa aftershocks. The variation in apparent stress found near the Osa Peninsula, and high mean as compared to global and regional values, suggest areas of stress concentration in the region of bathymetric complexity in the subduction zone.

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