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TitleTectonic tremor localization using Bayesian inversion
AuthorBombardier, M; Cassidy, J FORCID logo; Dosso, S E; Kao, HORCID logo
SourceSeismological Research Letters vol. 93, no. 2B, 2022 p. 1363
Alt SeriesNatural Resources Canada, Contribution Series 20210551
PublisherSeismological Society of America
Mediapaper; digital; on-line
File formatpdf
ProvinceBritish Columbia
Lat/Long WENS-124.0000 -122.0000 49.0000 48.0000
Subjectstectonics; earthquakes; Nothern Cascadia
ProgramPublic Safety Geoscience Assessing Earthquake Geohazards
Released2022 04 19
AbstractPeriodically-occurring slow earthquakes in northern Cascadia produce low frequency seismic signals including low-frequency earthquakes (LFEs) and tectonic tremor. Methods to locate the sources of these two types of events often yield results that do not coincide but both are thought to originate near the subduction interface. As such, whether LFEs and tremor result from the same processes and where those processes are located remain open questions. In this study, we produce a catalogue of tremor sources from an episodic tremor and slip (ETS) event beneath Vancouver Island, Canada, using a new location method. Our method includes processes that automatically detect tremor signals, construct datasets using waveform-envelope cross-correlation, remove outliers, estimate data error statistics and use a grid-based Bayesian inversion to locate source positions and quantify uncertainties from the resulting 3D probability distributions. We first test this method on local crustal earthquakes and obtain well constrained relocations within an average of 3.5 km in epicenter and 4.5 km in depth of official catalog values. We then locate tremor during the November 2019 ETS event and find that source epicenters are highly clustered with rapid migration between clustered patches. Consistent with other studies, we show source depths peak at ~30 km depth, which is 10 km shallower than current slab surface models and LFE locations. The relatively shallow and distributed depths of well-constrained tremor sources suggest either that ETS slip occurs within a shallow vertically-distributed shear zone or that a significant portion of tremor is produced in the lower continental crust. Additionally, the shallower occurrence of tremor relative to LFEs suggests different source processes. Application of our location technique to additional ETS events will help to better understand the locations of tremor sources and thereby infer potential physical processes of ETS tremor generation.
Summary(Plain Language Summary, not published)
This study uses an improved location technique to more precisely locate seismic tremors associated with Episodic Tremor and Slip events in the Cascadia subduction zone. Of note is the improved depth estimation which allows us to determine whether these tremors originate on the subduction fault, or elsewhere. The new method is first tested on earthquake locations and depths to test, and calibrate the technique. We find that the majority of the ETS tremors originate above the subducting plate (and subduction fault), suggesting that they are related to fluids and/or fractures in the lower continental crust. This research provides new insight into the physical processes of ETS tremor generation, and will contribute to improved earthquake hazard estimation.

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