Magnitudes and moment‐duration scaling of low‐frequency earthquakes beneath southern Vancouver Island

Abstract

We employ 130 low‐frequency earthquake (LFE) templates representing tremor sources on the plate boundary below southern Vancouver Island to examine LFE magnitudes. Each template is assembled from hundreds to thousands of individual LFEs, representing over 269,000 independent detections from major episodic‐tremor‐and‐slip (ETS) events between 2003 and 2013. Template displacement waveforms for direct P and S waves at near epicentral distances are remarkably simple at many stations, approaching the zero‐phase, single pulse expected for a point dislocation source in a homogeneous medium. High spatiotemporal precision of template match‐filtered detections facilitates precise alignment of individual LFE detections and analysis of waveforms. Upon correction for 1‐D geometrical spreading, attenuation, free surface magnification and radiation pattern, we solve a large, sparse linear system for 3‐D path corrections and LFE magnitudes for all detections corresponding to a single‐ETS template. The spatiotemporal distribution of magnitudes indicates that typically half the total moment release occurs within the first 12–24 h of LFE activity during an ETS episode when tidal sensitivity is low. The remainder is released in bursts over several days, particularly as spatially extensive rapid tremor reversals (RTRs), during which tidal sensitivity is high. RTRs are characterized by large‐magnitude LFEs and are most strongly expressed in the updip portions of the ETS transition zone and less organized at downdip levels. LFE magnitude‐frequency relations are better described by power law than exponential distributions although they exhibit very high b values ≥∼5. We examine LFE moment‐duration scaling by generating templates using detections for limiting magnitude ranges (MW<1.5, MW≥2.0). LFE duration displays a weaker dependence upon moment than expected for self‐similarity, suggesting that LFE asperities are limited in fault dimension and that moment variation is dominated by slip. This behavior implies that LFEs exhibit a scaling distinct from both large‐scale slow earthquakes and regular seismicity.

Seismic Moment, Seismic Energy, and Source Duration of Slow Earthquakes: Application of Brownian slow earthquake model to three major subduction zones

“…These pulses would be recognized as isolated LFEs if they were observed in nature. This result is similar to that reported by Bostock et al (2015). Each spike is produced by a rare impulsive fluctuation characterized by the long tail of a power law type distribution, and the amplitude soon decreases without a subsequent increase.…”

“…One example is the observation of LFEs with almost constant duration (Bostock et al, 2015;Thomas et al, 2016). This fluctuation can be regarded as either the start or end of the isolated LFEs, or the acceleration or deceleration of slip radiating from continuous tremor.…”

“…A family of LFEs shares near-constant source time functions (Bostock et al, 2015;Thomas et al, 2016). A family of LFEs shares near-constant source time functions (Bostock et al, 2015;Thomas et al, 2016).…”

Using Low‐Frequency Earthquake Families on the San Andreas Fault as Deep Creepmeters

“…The catalog includes over one million earthquakes recorded since 2001; however, we limit our analysis to between 2006 and May of 2016 to minimize the effect of afterslip from the 2004 M6 Parkfield earthquake. Additionally, estimated LFE source dimensions of hundreds of meters (e.g., Bostock et al, 2015;Thomas et al, 2016) are typically less than LFE location uncertainties of 1-2 km (Shelly, 2017). The time series of cross correlations for each template are then summed over each station and channel to create a network cross-correlation time series.…”

“…Bartlow et al, 2014;Bletery et al, 2017;Bostock et al, 2015;Brodsky & Mori, 2007;Hawthorne et al, 2016;Ide et al, 2007;Ito & Obara, 2006; long timescale) Episodic (short timescale) Continuous (short timescale) (a) versus median slip per episode. (b) versus median slip between episodes.…”