Earth and Planetary Sciences ETDs

Publication Date

Spring 4-17-2017

Abstract

A dense array of ~1,000 continuously recording, short-period geophones was deployed in the summer of 2014 within ~15 km of Mount St. Helens. Two earthquake catalogs created using reverse time imaging and template detection techniques, increase the detection rate and completeness of the earthquake catalog when compared to the permanent network, Pacific Northwest Seismic Network, catalog. An investigation into event type for ~200 of the earthquake detections leads to the discrimination of two major classification groups, volcano-tectonic and long period. Previous to this study, long period earthquakes had not been identified in the upper crust during a volcanically inactive period and our results lead us to believe they are detected but routinely misidentified as surface-generated signals and consequently, removed from the Pacific Northwest Seismic Network earthquake catalog. Earthquake locations from the reverse time imaging and template detection catalogs are mainly distributed in a volume directly beneath the summit crater between 0–6 km below sea level with a width of ~1–3 km. Volcano-tectonic and long period earthquakes occur in the same source volume, suggesting multiple source processes, and have no apparent temporal relationship. Their location is of particular interest because it is between the main upper crustal magma reservoir (~5–15 km below sea level) and the volcanic edifice (0–2.5 km above sea level), as well as directly above the location where melt last equilibrated (~5-12 km below sea level) during the 1980 eruption. Future efforts should focus on creating LP template detections to prevent their misidentification during normal monitoring periods and enable tracking the occurrence of LP seismicity through time. Additionally, dense three-component recordings will be necessary to constrain the source mechanism of the upper crustal LP earthquakes and potential relationships to magmatic processes.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Brandon Schmandt

Second Committee Member

Steven Hansen

Third Committee Member

Tobias Fischer

Fourth Committee Member

Mousumi Roy

Language

English

Keywords

seismology, volcano, Mount St. Helens, long period earthquakes, microseismicity

Document Type

Thesis

Download

Share

COinS