Dynamically Triggered Changes of Plate Interface Coupling in Southern Cascadia
Ph.D. work of Kathryn Materna
In Southern Cascadia, precise 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 about 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 dynamic shaking near the tremor region may provide a mechanism. Observations of transient coupling changes are rare and challenging to explain mechanistically but have important implications for earthquake processes on faults.
Figure 1: a) Tectonic setting of the Mendocino Triple Junction, located at the intersection of the San Andreas Fault (SAF), the Mendocino Fault Zone (MFZ), and the Cascadia Subduction Zone(CSZ). The black velocity vectors show the PBO velocity field in the Sierra Nevada Great Valley (SNGV) reference frame, converted using the Euler Pole from Dixon et al. (2000). b and c) Velocity changes (T3-T2 and T4-T3) from the PBO solutions with least-squares seasonal removal, shown with 1-sigma confidence ellipses. (d) Time series of east component for station P160 (red arrow in (b)). Time series after removal of coseismic offsets is shown in black and the detrended and de-seasonalized east component is shown in red, with dashed lines indicating the periods used for slope estimation. e) Residuals to linear fits in Figure 1d.
This work was supported in part by a grant from the National Science Foundation's GeoPRISMS program, "Collaborative Research: Improving models of interseismic locking and slow slip events in Cascadia and New Zealand" (award #1551929). Kathryn Materna was additionally supported by a National Science Foundation Graduate Research Fellowship.