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A Catalog of Automated Focal Mechanisms for Microearthquakes at Axial Seamount Based on Waveform Cross-Correlation

Axial Seamount is a submarine volcano at the intersection of the Cobb hotspot and the Juan de Fuca Ridge. The last eruption in 2015 was recorded by 7 ocean bottom seismometers on the Ocean Observatories Initiative Regional Cabled Array. A small number of manual composite focal mechanisms (FMs) obtained by Levy et al. (2018) confirmed that the motion on an outward dipping ring fault beneath the summit caldera reversed during the 2015 eruption. Our earthquake catalog for Axial Seamount comprises ~150,000 hypocenters from 2015-2024. A large data set of FMs would provide constraints on the geometry and deformation of the caldera ring faults, the impacts of magma movement and hydrothermal process on faulting, and the evolution of crustal stresses through the eruptive cycle. To generate FMs automatically, we incorporate the methodology of Shelly et al. (2016) which determines polarities by maximizing congruence with signed waveform correlation coefficients. We first tested the approach on subsets of 5000 earthquakes from the east and west walls of the caldera. We obtained 53,000 polarities which show a 95% consistency with the sign of the cross-correlation coefficients and a 99% accuracy rate when benchmarked against 3,000 manual picks. Considering just earthquakes with at least five polarities and 5 S/P amplitude ratios, we created 680 groups of ~15 earthquakes with fully consistent polarities and similar locations and S/P ratios. Finally, we used the HASH program to generate composite FMs. The hypocenters beneath the west wall mostly fall on an outward dipping ring fault and most FMs are normal faults consistent with this fault plane. The hypocenters in the test data set beneath the east wall have a more complex distribution and the focal mechanism similarly show more variation, consistent with the influence of ring faulting and deformation in the regions where dikes are injected during spreading events. We are presently extending the approach to the entire catalog and based on the number of earthquakes with sufficient polarities and S/P ratios, expect to obtain ~7,000 composite FMs.

Session: Seismoacoustic, Geodetic and Other Geophysical Investigations of Active Volcanoes - I

Type: Oral

Room: Kahtnu 2

Date: 5/3/2024

Presentation Time: 08:45 AM (local time)

Presenting Author: Maochuan Zhang

Student Presenter: Yes

Additional Authors