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.