Earthquake Stress Drop Before, During, and After the 2015 Eruption at Axial Seamount

Abstract:

The volcanic eruption in April and May, 2015 of Axial Seamount on the Juan de Fuca Ridge was well documented by geophysical, chemical, and biological sensors as part of the Ocean Observatory Initiative Cabled Array. Seven ocean bottom seismometers (OBSs) at 200 Hz sample rate located within and along Axial caldera recorded seismicity associated with magmatic processes before, during, and after the eruption. These processes include the gradual inflation of the caldera with increasing seismicity in the months leading up to the eruption, rapid deflation accompanied by intense seismicity along caldera wall faults, and down-rift seismicity associated with dike emplacement. The extensive amount of seismic data recorded during the eruption cycle on Axial provides a unique opportunity to investigate the evolution of earthquake properties that reflect changes in seafloor deformation and hydrothermal fluid flow associated with submarine volcanism.

We use techniques refined from our previous work with OBS data and calculate stress drop from corner frequency derived using an empirical Green's function spectral ratio method. We focus our analysis on earthquakes with M_L ≥ 2.0 located within the caldera to ensure sufficient signal to noise and azimuthal station coverage for reliable corner frequency measurements. Our initial analysis with nine earthquakes in the southeast corner of the caldera occurring in April, 2015 finds stress drops from 0.9 to 6.4 MPa. We continue our analysis to determine spatial and temporal variations in stress drop in response to variations in crustal strength or fluid pressure during the eruption period. We plan to compare stress drop for earthquakes located on wall faults to those within the hydrothermal fracturing zone to obtain a more complete picture of the coupled earthquake, magma flux, and hydrothermal fluid system during the 2015 eruption of Axial Seamount.