Probing the Southern Cascadia Plate Interface With the Dense Amphibious Cascadia Initiative Seismic Array

• Published in: Journal of geophysical research. Solid earth 2021-07, Vol.126 (7), p.n/a
• Main Authors: Alongi, T., Schwartz, S. Y., Shaddox, H. R., Small, D. T.
• Format: Article
• Language: English
• Subjects: Arrays; Catalogues; Clustering; Earthquakes; Fault lines; Fault location; Geological faults; Geophysics; microseismicity; ocean bottom seismometers; Plate boundaries; plate interface; Plate tectonics; Plates (tectonics); Seismic activity; Seismic arrays; Seismicity; Seismographs; seismology; Seismometers; Slip; Solifluction; Strain; strain transient; Subduction; Subduction (geology); Subduction zones; Template matching; Tsunami hazard; Weather hazards
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2021JB022180

Fault coupling is vital in determining the amount of strain that is accumulated along faults. The magnitude and location of stored elastic strain energy in highly coupled regions has important implications for understanding the full range of slip behavior at plate boundary faults, as well as earthquake and tsunami hazards. We use the temporary dense amphibious array of seismometers offered by the Cascadia Initiative to create a high‐resolution catalog of events to examine the spatio‐temporal behavior of earthquakes near the plate interface. The data show that in southern Cascadia the plate interface updip of the geodetically locked region is nearly devoid of seismicity, therefore likely highly coupled and accumulating strain. The catalog reveals events that are clustered at the downdip edge of the highly coupled megathrust that correlate in time with nearby strain transient observations. Template matching of events in the cluster using permanent stations of the Northern California Seismic Network over a 10‐year period between 2010 and 2020 indicates that this cluster is unique in space and time. Its activity only during the strain transient provides support for the utility of seismic observations in the identification of strain transients. Plain Language Summary Great megathrust earthquakes have occurred in the Cascadia Subduction Zone. The most recent occurred in 1700, and it is expected that another will happen in the next few hundred years. Although the megathrust has hosted great earthquakes, few small interplate events have been identified along this plate boundary. Instead, a portion of the convergence between the tectonic plates is released in both periodic slow slip events and (quasi) continuous fault creep. It is essential to understand the full range of plate boundary behavior to assess the hazard risk for great earthquakes. To further this understanding, we gather data using a densely spaced temporary array of seismometers deployed in southern Cascadia in 2014–2015 to increase detection as well as determine precise earthquake locations. We identify events that are on or very near the plate boundary fault. There is some clustering of earthquakes near the plate boundary, but we find an absence of earthquakes along the shallowest segment, that indicate that the fault is likely locked. When compared to other geophysical observations in the area, it appears that the cluster of earthquakes near the plate boundary is sensitive to strain transients, dem