Cascadia Subduction Zone

We model tsunami runup and inundation along the Northern California Coast caused by hypothetical earthquakes on the Cascadia Subduction Zone (CSZ). The CSZ forms the boundary between the Pacific and North American tectonic plates and extends from Northern California to Southern British Columbia and is believed capable of producing great earthquakes with moment magnitudes (Mw) 9.0 or greater. A suite of CSZ rupture scenarios based on the geological characteristics of the southern part of the CSZ were simulated with the tsunami propagation and inundation model MOST. CSZ inundation projections were compared and related to historical accounts and modeled data for two teletsunamis at Crescent City: the tsunami of 1 April 1964 that caused 12 deaths and more than 17 million in damages to Northern California, and the 15 November 2006 Kuril Islands tsunami that caused nearly 10 million in damages to the Crescent City harbor. Our modeling suggests that near field tsunamis from the CSZ present a hazard greater than the 1964 event for the northernmost part of the State. However, south of Shelter Cove tsunamis generated by great earthquakes along the Alaska – Aleutians subduction zone are potentially more damaging.

During the Holocene, numerous great earthquakes have occurred along the west coast of North America. Enormous seismic waves, triggered by these colossal forces of nature, have episodically inundated vulnerable regions of the Cascadia coastal lands to elevations of up to 20 m above mean sea level. The destructive paths of tsunamis leave behind important geological evidence like anomalous sheets of gravel and sand containing marine fossils and terrestrial detritus marking a distinct disturbance in the depositional environment. Such tsunamigenic deposits can be found in low-elevation lakes; depositional environments which allow one to determine the distribution and inland range of the tsunami wave run-up but also ideal environments because they tend to preserve a longer and older sedimentological record. Multiple inferred tsunami deposits were recorded in Kakawis Lake, on the west central coast of Vancouver Island, and have provided a 14,000-year-old record of information. The tsunamig...

Episodic tremor and slip (ETS) events in Cascadia include slow earthquake phenomena such as slow slip events, tremor, low frequency earthquakes (LFEs), and very low frequency earthquakes (VLFEs; Rogers & Dragert, 2003, https://doi.org/10.1126/science.1084783). Using VLFEs detected from a grid-search centroid moment tensor inversion algorithm in the 2011 (Ghosh et al., 2015, https://doi.org/10.1002/ 2015GL063286) and 2014 (Hutchison & Ghosh, 2016, https://doi.org/10.1002/2016GL069750) ETS events as templates, we apply a matched filter algorithm to create a VLFE catalog for each ETS event. We also use the 2011 template events to create a VLFE catalog from July 2011 through the end of 2012, which encompasses both the 2012 ETS event and an inter-ETS period. The successful application matched filtering intrinsically suggests that VLFEs are repeating events, of which thousands were detected during each ETS event. The findings contained herein come shortly after the successful application of a similar matched filter methodology in Western Japan where a number of VLFEs were also successfully detected (Baba et al., 2018, https://doi.org/10.1002/2017GL076122). The high temporal resolution of these VLFE catalogs show a significant increase in VLFE activity during ETS events that drops off immediately before and after the ETS period. A comparison of VLFE activity to GPS data shows that VLFE tracks slow slip, even when tremor is not occurring or is behaving anomalously during ETS periods. Finally, we find continued VLFE activity during the inter-ETS period for all template events, though the extent of reactivation of VLFE asperities and their spatiotemporal coincidence with tremor is varied among the template events.

Source geometry and simple propagation of S waves generated by low- frequency earthquakes (LFEs) in northern Cascadia results in strong waveform coherence on horizontal channels for stations at smaller epicentral distances. As recognized by previous workers, this cross-station similarity can be exploited for detection of LFEs in tremor. We develop a cross-station correlation approach based on waveform coherence and travel-time consistency that exploits a full complement of network stations, and we demonstrate its application to separate arrays on Vancouver Island and Washington state. Our approach yields thousands of impulsive LFE detections per slow-slip episode and hundreds of events between episodes. LFE epicentral distributions reveal the presence of well-defined regions of high asperity density with those regions farther downdip that are active during interepisodic tremor and slow-slip periods. The LFE epicenters also display pronounced spatiotemporal clustering that compares favorably with independent tremor and LFE template catalogs and that defines rapid tremor reversals with complex propagation patterns. The LFE source depths can be estimated for some detections in which P waves are identified on the vertical channel through correlation with the horizontal-component S waveforms. LFE hypocenters cluster between two recently developed plate interface models below southern Vancouver Island. Our approach enables the identification of high signal-to-noise-ratio impulsive LFE detections from targeted regions on the plate boundary for generation of LFE templates to be used in studies of structure and seismogenesis in Cascadia.

During the Holocene, numerous great earthquakes have occurred long the west coast of North America. Enormous seismic waves, triggered by these colossal forces of nature, have episodically inundated vulnerable regions of the Cascadia coastal lands to elevations of up to 20m above mean sea level. The destruction paths of tsunamis leave behind important geological evidence like anomalous sheets of gravel and sand containing marine fossils and terrestrial detritus marking a distinct disturbance in the depositional environment. Such tsunamigenic deposits can be found in low-elevation lakes; depositional environments which allow to determine the distribution and inland range of the tsunami wave run-up but also ideal environments because they tend to preserve a longer and older sedimentological record. Multiple inferred tsunami deposits were recorded at Kakawis Lake, on the west central coast of Vancouver Island, and have provided a 14,000-year-old record of information. The tsunamigenic sediments pre-date AD1700 and are similar to those found in other lakes on the Island. As for a regional correlation, these tsunami deposits are likely related to at least two and possibly three of the events reported in the Atwater - Hemphill-Haley chronology, with recurrence intervals of about 400 years.