Revisiting the Earthquake Catalog of Mount St. Helens 2004–2008 Eruption

Session: Waveform Cross-Correlation-Based Methods in Observational Seismology
Type: Oral
Date: 4/30/2020
Time: 09:15 AM
Room: 230 + 235
Description: 

The 2004-2008 eruption of Mount St. Helens was characterized by a high seismicity rate and a relative absence of deep events (i.e., 2 km beneath sea level). However, the accuracy and completeness of the earthquake catalogs are challenged by station coverage and data availability in the earlier stage of eruption. There were <10 seismic stations within 20 km and most of them are single component instruments. Due to the limited human labor and computer power, the earthquake catalogs for the first few months of the eruption could be less complete than that before/after. In this study, we revisit the continuous data for the decade (2001-2010) and re-examine the seismic catalog using cross-correlation based methods and parallel computing. We first refine the hypocenter locations of ~100 PNSN reported events (M0.3-2) using hand-picked P arrivals and a recent 3D velocity model from the iMUSH active source experiment. To ensure template diversity and coverage, these events are selected based on 1) previous case studies of the earthquake clusters and 2) waveform similarity at a near-crater station. Preliminary results generally suggest shallower depths than the reported values. For example, a M0.7 earthquake (on Oct 4, 2004) reported at a depth of 8.8 km with an epicenter outside the crater is relocated to 1 km depth directly beneath the crater. Using network cross-correlation, we identify over 200,000 earthquakes during the decade, which is hundreds of times larger than from the PNSN catalog. Most of the new detections belong to the first month of eruption (Oct, 2004). In addition, most multiples are detected within weeks of their templates, suggesting short longevities of earthquake families. Only a small portion of the multiples are detected months after their template event. Finally, we will calibrate the magnitudes of the detected events and moment rate to better inform future geomechanical modelling. Overall, the new comprehensive catalogue with accurate source parameters is fundamental to understanding the magma motion before/during the recent eruption of Mount St. Helens.

Presenting Author: Ruijia Wang

Authors