Megathrust earthquakes and their associated tsunamis cause some of the worst natural disasters. In addition to earthquakes, a wide range of slip behaviors are present at subduction zones, including slow earthquakes that span multiple orders of spatial and temporal scales. Understanding these events may shed light on the stress or strength conditions of the megathrust fault. Out of all types of slow earthquakes, very low frequency earthquakes (VLFEs) are most enigmatic because they are difficult to detect reliably, and the physical nature of VLFEs are poorly understood. Here we show three VLFEs in Cascadia that were dynamically triggered by a 2009 Mw 6.9 Canal de Ballenas earthquake in the Gulf of California. The VLFEs likely locate in between the seismogenic zone and the Cascadia episodic tremor and slip (ETS) zone, including one event with a moment magnitude of 5.7. This is the largest VLFE reported to date, causing clear geodetic signals. Our results show that the Cascadia megathrust fault might slip rapidly at some spots in this gap zone, and such a permissible slip behavior has direct seismic hazard implications for coastal communities and perhaps further inland. Further, the observed seismic sources may represent a new class of slip events, whose characteristics do not fit current understandings of slow or regular earthquakes.
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.
more » « less- PAR ID:
- 10386188
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 22
- Issue:
- 3
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
Abstract—In this study, we model geodetic strain accumulation along the Cascadia subduction zone between 2007.0 and 2017.632 using position time series from 352 continuous GPS stations. First, we use the secular linear motion to determine interseismic locking along the megathrust. We determine two end member models, assuming that the megathrust is either a priori locked or creeping, which differ essentially along the trench where the inversion is poorly constrained by the data. In either case, significant locking of the megathrust updip of the coastline is needed. The downdip limit of the locked portion lies * 20–80 km updip from the coast assuming a locked a priori, but very close to the coast for a creeping a priori. Second, we use a variational Bayesian Independent Component Analysis (vbICA) decomposition to model geodetic strain time variations, an approach which is effective to separate the geodetic strain signal due to non-tectonic and tectonic sources. The Slow Slip Events (SSEs) kinematics is retrieved by linearly inverting for slip on the megathrust the Independent Components related to these transient phenomena. The procedure allows the detection and modelling of 64 SSEs which spatially and temporally match with the tremors activity. SEEs and tremors occur well inland from the coastline and follow closely the estimated location of the mantle wedge corner. The transition zone, between the locked portion of the megathrust and the zone of tremors, is creeping rather steadily at the long-term slip rate and probably buffers the effect of SSEs on the megathrust seismogenic portion.more » « less
-
more » « less
Abstract 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
Abstract Current earthquake forecasting approaches are mainly based on probabilistic assumptions, as earthquakes seem to occur randomly. Such apparent randomness can however be caused by deterministic chaos, rendering deterministic short‐term forecasts possible. Due to the short historical and instrumental record of earthquakes, chaos detection has proven challenging, but more frequently occurring slow slip events (SSE) are promising candidates to probe for determinism. Here, we characterize the SSE signatures obtained from GNSS position time series in the Hikurangi Subduction Zone (New Zealand) to investigate whether the seemingly random SSE occurrence is governed by chaotic determinism. We find evidence for deterministic chaos for stations recording shallow SSEs, suggesting that short‐term deterministic forecasting of SSEs, similar to weather forecasts, might indeed be possible over timescales of a few weeks. We anticipate that our findings could open the door for next‐generation SSE forecasting, adding new tools to existing probabilistic approaches.
-
more » « less
Abstract In Southern Cascadia, precise Global Navigation Satellite System (GNSS) measurements spanning about 15 years reveal steady deformation due to locking on the Cascadia megathrust punctuated by transient deformation from large earthquakes and episodic tremor and slip events. Near the Mendocino Triple Junction, however, we recognize several abrupt GNSS velocity changes that reflect a different process. After correcting for earthquakes and seasonal loading, we find that several dozen GNSS time series show spatially coherent east‐west velocity changes of ~2 mm/yr and that these changes coincide in time with regional
M > 6.5 earthquakes. We consider several hypotheses and propose that dynamically triggered changes in megathrust coupling best explain the data. Our inversions locate the coupling changes slightly updip of the tremor‐producing zone. We speculate that fluid exchange surrounding the tremor region may be important. Such observations of transient coupling changes are rare and challenging to explain mechanistically but have important implications for earthquake processes on faults.
