Cascadia Great Earthquakes: Riddle of the Sands

Chris Goldfinger

Director
Active Tectonics and Seafloor Mapping Laboratory
College of Oceanic and Atmospheric Sciences,Oregon State University

Abstract:

Cascadia has become the poster child for paleoseismology in subduction zones. Evidence of great earthquakes was first discovered by Brian Atwater in the late 1980’s. Since then, numerous investigators both onshore and offshore have pulled together a remarkable story of past earthquakes that reveals the long-term behavior of what was once the world’s most enigmatic subduction zone.

Paleoseismic data suggest that the Cascadia subduction zone is segmented, with at least four seismic segments that have been active in the Holocene. Along the northern margin, we find 19 large earthquakes with a recurrence averaging ~500-530 years. The central Oregon to northern California margin comprises at least three segments that experienced all of the northern ruptures as well as ~22 smaller events of restricted latitude range that are correlated between multiple sites. At least two northern California sites probably also record numerous small sedimentologically or storm triggered turbidites during the early Holocene. The shorter extents and thinner turbidites of the southern margin correspond well with timing and spatial extents interpreted from the onshore paleoseismic record. Based on 41 events, a Holocene recurrence interval of ~240 years is estimated for the southern Cascadia margin. Time-independent probabilities for segmented ruptures range from 7-9% in the next 50 years for full margin ruptures, to ~18% in 50 years for a southern segment rupture. Time dependant failure analysis suggests the probability of an event by 2060 of ~25% for the northern margin and ~80% for the southern margin. The long paleoseismic record also indicates a pattern of clustered earthquakes that includes 4-5 cycles that are more robust in the later Holocene. The next Cascadia event is most likely to be a segmented rupture along one or both of the southern segments.

The Holocene Cascadia earthquake series affords uncommon opportunities to examine recurrence models, clustering and long term strain history. We attempt to address the issue of energy management over multiple earthquake cycles through the temporal record of inter-seismic intervals and a proxy for magnitude of the earthquakes. Plate convergence between earthquakes is assumed to increase elastic strain energy in proportion to inter-event time. We propose that co-seismic energy may be modeled as proportional to the mass of turbidites triggered in seismic shaking. We infer that turbidite mass is a suitable proxy for energy release because of its consistency along strike at multiple sites. We scale turbidite mass (energy release) to balance plate convergence (energy gain) to generate a 10ka energy time series for Cascadia. The pattern reveals that the earthquake clusters apparent in the time series have variable behavior. What is apparent is that some events release less while others release more energy than available from plate convergence (slip deficit). Those that are larger may have borrowed stored energy from previous cycles. Cycle variations may explain mismatches between deformation models based on inter-event times in the last 4600 years and coastal paleoseismic data. Long term energy cycling may negate attempts to fit earthquake recurrence into “time-predictable” or “slip-predictable” models.


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