The seismic moments observed for low‐frequency earthquakes (LFEs) vary over multiple orders of magnitude, even where the LFEs occur within families of similar events. Although this variability is typically interpreted to record a scale‐limited process at the LFE source, neither the slip per LFE nor the rupture area can be determined from seismological constraints. Here, we examine incrementally developed slickenfibers that have been proposed to record LFEs in exhumed subduction zones. These structures form through repeated, micron‐scale slip events across dilational irregularities in the fault plane, which are punctuated by cementation and sealing in the interstitial space. By statistically analyzing the geometry of inclusion trails delineating slip‐parallel mineral‐growth increments, we constrain the variability in slip per inferred LFE and test end‐member hypotheses regarding the controls on LFE moments. We find that that the slickenfibers exhibit characteristic slip increments, favoring a “slip‐limited” model that requires large variability in LFE rupture areas.
We identify and characterize four different low‐frequency earthquake (LFE) families (LFEs 1–4, ordered updip to downdip) that span the width of the transition zone in the Cascadia Subduction Zone beneath western Washington State. We find that LFE swarm duration, recurrence interval, and event size decrease systematically with increasing depth. LFE moments are observed to follow an exponential distribution rather than a power law distribution, allowing us to determine a characteristic event size for each family. Absolute locations of these LFE families place them very near the inferred Juan de Fuca‐North America plate interface. Relative relocation of individual LFEs within each family reveal elongation parallel to the plate convergence direction and narrow depth distributions with dips corresponding to plate interface models. We interpret the behavior of our LFE families as indicative of slow slip at those locations on the plate interface. The tidal response of LFE activity at LFEs 1, 2, and 3 during a large episodic tremor and slip (ETS) event follows a previously observed evolution, in which tidal response during the initial 1 to 2 days of slip at an LFE patch is much weaker than that during the next few days of slip. The brief bursts of activity at LFE4, although little affected by large episodic tremor and slip events, respond strongly to tidal stresses. Our observed along‐dip contrasts in recurrence behavior and tidal response provide evidence for stress transfer and increased frictional strength with position updip through the tremor zone.
more » « less- PAR ID:
- 10461263
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 20
- Issue:
- 2
- ISSN:
- 1525-2027
- Page Range / eLocation ID:
- p. 1202-1217
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Long‐term slow slip events have been observed at several subduction zones around the globe, where they play an integral part in strain release along megathrust faults. Nevertheless, evidence for long‐term slow slip has remained elusive in the Cascadia subduction zone. Here we conduct a systematic analysis of 13 years of GNSS time series data from 2006 to 2019 and present evidence of at least one low‐amplitude long‐term slow slip event on the Cascadia subduction zone, with the possibility of others that are less resolved. Starting in mid‐2012, a 1.5‐year transient is observed in southern Cascadia, with a group of coastal GNSS stations moving ∼2 mm to the west. The data are modeled as a Mw 6.4 slow slip event occurring at 15–35 km depth on the plate interface, just updip of previously recognized short‐term slow slip and tremor. The event shares many characteristics with similar long‐term transient events on the Nankai subduction zone. However, the total fault slip amplitude is an order‐of‐magnitude smaller in Cascadia when compared to large events elsewhere, making long‐term slow slip detection challenging in Cascadia. While there are other westward long‐duration transients in the refined data set, the surface displacements are below the level of the noise or are limited spatially to a few neighboring stations, making interpretation unclear.
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