Dynamically Triggered Changes of Plate Interface Coupling in Southern Cascadia

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.

A Long‐Term View of Episodic Tremor and Slip in Cascadia

“…The tremor catalog used here is a modified version of the publicly available catalog found online (pnsn.org/tremor). The catalog has been reprocessed from the start of 2015 to the start of 2018 for only the southernmost part of Cascadia near the Mendocino Triple Junction to increase the rate of detection during a period in which many seismic stations were unavailable, and additionally to remove false detections (Materna et al, ). Many more detections were added than were removed.…”

The Transient and Intermittent Nature of Slow Slip

“…It is tempting to appeal to different physical mechanisms for slow slip in different tectonic contexts, but, so far, our geophysical evidence of all of these phenomena is quite comparable. For instance, observations of time‐varying megathrust coupling in subduction zones (e.g., Materna et al, ) are in fact not so different from the major slow slip events that regularly occur in Cascadia or Japan. A recent study (Frank, ) found that the collection of slow slip cycles in one subduction segment could explain nearly 100% of the geodetically inferred lack of slip deficit (i.e., coupling).…”

Earthquake Swarms, Slow Slip and Fault Interactions at the Western‐End of the Hellenic Subduction System Precede the Mw 6.9 Zakynthos Earthquake, Greece

“…Furthermore, the acceleration of the vectors observed prior to both slow‐slip events was recorded in all near‐field stations to last for about 2 months in each case (Figures 2e and 7 and Movie ). This acceleration may be indicative of deep active processes related to changes in slab pull force (Bedford et al., 2020) and/or to a dynamic increase of locking along the plate‐interface zone prior to seismic or aseismic slip events (Materna et al., 2019). The described SSEs of this study are the first to be reported in the HSS.…”