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

Abstract

Fault coupling is a critical component of the overall seismic cycle because regions of enhanced strain accumulation have frequently been spatially correlated with high coseismic slip. Relative plate motion is accommodated in earthquakes, slow slip transients, and longer term creep. The interaction of these different modes of motion are still not well understood (Walton et al., 2021). The canonical model of the subduction zone includes a highly coupled megathrust, composed predominantly of large velocity weakening asperities that are surrounded by velocity strengthening regions that may slip aseismically (Bilek et al., 2004;Lay et al., 2012). The moment of the earthquake scales with the area of slip on the fault. Cascadia has sparse seismicity and infrequent historic great earthquakes, however when great events occur, they rupture a significant area of the margin (Satake et al., 2003). It is unsurprising that in Cascadia the seismogenic section is highly coupled extending to depths of ∼20 km (

Subduction intraslab-interface fault interactions in the 2022 M w 6.4 Ferndale, California, earthquake sequence

“…They inferred these clusters occurred mostly in the uppermost Gorda slab based on phases reflected from the slab Moho. Their seismicity cluster “C2” seems to correspond with the “Fortuna” cluster examined by Alongi et al ( 23 ), who noted increased seismic activity in concert with an apparent change in coupling on the subduction interface. Notably, the aftershock sequence of the 2022 M w 6.4 rupture exhibits high rates of seismicity within the same zone as this previously studied cluster, including a linear zone of strike-slip events concentrated at 17- to 18-km depth ( Fig.…”

“…White dashed line shows the Slab2 ( 13 ) model, while the solid white line shows the model of McCrory et al ( 10 ) for the subduction interface. Black boxes show the area where the Fortuna ( 23 ), and equivalent C2 ( 15 ) clusters were observed with long-term activity before the 2022 earthquake. White boxes in (A) and (B) show the extents of zoomed views in parts (C) and (D), respectively.…”

“…Previously faulted and hydrated areas may also form localized zones of weakness with long-lived clusters of seismicity over a period of at least several years ( 15 , 23 ). Although these clusters locate near the subduction interface, evidence from focal mechanisms, locations, and waveform characteristics indicates that they likely occur within the Gorda slab rather than on the interface itself.…”

Cascadia Subduction Zone Fault Heterogeneities From Newly Detected Small Magnitude Earthquakes

“…The absence of shallow seismicity at the southern end of the margin was also interpreted as reflecting a highly coupled region by Alongi et al. (2021).…”

“…The southernmost asperity we interpret based on interplate seismicity here hugs a slow slip patch of Bartlow (2020), but the abundant seismicity implies loading on the downdip edge of strong locking; however, whether events at the southern end of the margin are truly interplate is ambiguous due to the complexity of the MTJ. The absence of shallow seismicity at the southern end of the margin was also interpreted as reflecting a highly coupled region by Alongi et al (2021).…”