Cataloging Tectonic Tremor Energy Radiation in the Cascadia Subduction Zone

• Published in: Journal of geophysical research. Solid earth 2021-10, Vol.126 (10), p.n/a
• Main Author: Wech, Aaron G.
• Format: Article
• Language: English
• Subjects: Amplitudes; Cascadia; Catalogues; Dimensions; Distribution; Earthquakes; Energy; Energy distribution; Geophysics; Identification; Locating; Monitoring systems; Plate boundaries; Radiation; Seismic activity; Seismic energy; Slip; Subduction; Subduction (geology); subduction zone; Subduction zones; Tectonics; tremor; Tremors
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2021JB022523

For the past ∼12 years the Pacific Northwest Seismic Network has been automatically detecting and locating tectonic tremor across the Cascadia subduction zone, resulting in a catalog of more than 500,000 tremor epicenters to date, which has served as a valuable resource for tremor and slip research. This manuscript presents an updated methodology for routine tremor detection in Cascadia and a new catalog of over 180,000 tremor epicenters including amplitudes detected along the subduction zone margin from 2017 to 2021. The events are detected via cross‐correlation of continuous vertical envelope data of 128 stations from northern California to northern Vancouver Island. The modified approach results in less scatter and a 55% increase in detected epicenters than previously observed, as well as a newly identified tremor source offset updip from the main tremor and slip region at the southern edge of the subduction zone. Radiated seismic energy in the 1.5–5 Hz band is used to assign epicenters an energy magnitude (MeL), which is calibrated to the ML of local earthquakes. Southern Cascadia is most active, but the highest tremor energy rates occur in northern Cascadia. Tremor in central Cascadia is systematically weaker and less frequent. Individual epicenter magnitudes range from ∼0.5–2 and spatiotemporally cluster into 1,060 swarms with cumulative MeL ranging from ∼0.8 to 3.7. The swarms reflect underlying slow slip events and occur with an earthquake‐like energy distribution with a b value ∼1. Tremor epicenters, however, follow a tapered Gutenberg‐Richter distribution with high b values, suggesting individual tremor bursts and their constituent low‐frequency earthquakes are fault‐dimension limited. Plain Language Summary Tectonic tremor is a continuous, low‐frequency seismic signal associated with slow fault motion at major plate boundaries. In order to better understand how tremor occurs and how it relates to the underlying slow slip, scientists need a detailed catalog of tremor activity. In the Cascadia subduction zone, tremor has been routinely monitored and cataloged for the past 12 years. But, that catalog lacks information about tremor amplitude, which limits its utility and the physical insights it can provide. Here I present an update to that monitoring system to include estimates of tremor magnitudes from 2017 to present. The results identify a new tremor source and show margin‐wide trends in total radiated seismic energy and energy rates that have n