Crustal Strength Variations Inferred from Earthquake Stress Drop at Axial Seamount Surrounding the 2015 Eruption

The Ocean Observatory Initiative Cabled Array at Axial Seamount, on the Juan de Fuca Ridge, provides a unique opportunity to investigate geophysical processes associated with submarine volcanism using seismic data captured from the seafloor. Within the first few months of installation, seismometers located in and along Axial caldera recorded nearly 200,000 earthquakes leading up to, and associated with, the most recent eruption at Axial in April and May, 2015. We focus on earthquakes in Axial's ring-fault system recorded by the cabled array, and investigate temporal and spatial variations in crustal strength inferred from changes in earthquake stress drop as a result of volcanic driven deformation surrounding the 2015 eruption.

We find stress drops from 0.6 to 43 MPa for 423 ring-fault earthquakes (1.6 ≤ M_W ≤ 3.6) located within the caldera near the cabled array using an empirical Green's function spectral ratio method to estimate corner frequency (a parameter inversely related to source radius) for stress drop. Average stress drop is twice as high during the inflation period (6.4 MPa) prior to the eruption than during the subsequent deflation (3.2 MPa), suggesting decreased fault strength as a result of the eruption. Stress drops also correlate with spatially varying shear wave speed, possibly reflecting a region of pervasive cracking in the northern caldera. We interpret our statistically significant temporal and spatial changes in stress drop to reflect both a temporary reduction in crustal strength as a result of increased fracturing caused by the eruption, and a correlation with a reduction in shear wave speed in a region of increased crustal damage.