Abstract:

Recent great earthquakes and tsunamis in Sumatra, Chile, and Japan underscore the importance of understanding subduction zone rupture patterns and recurrence intervals. The 50 states in the USA are home to two subduction zones, the Cascadia subduction zone from northern California to British Columbia, Canada, and the Alaska-Aleutian subduction zone that encompasses the Aleutian Island chain in the west to the Gulf of Alaska in the east. These two subduction zones vary in the level of knowledge that we have on rupture timings, magnitude, and extent.

The entire Cascadia subduction zone ruptured in AD 1700, and three decades of research has led to a general understanding of the timing of past earthquakes. However, little progress has been made on understanding the details of past ruptures, including the possibility of serial ruptures separated by decades or full margin rupture separated by centuries. Recently, our understanding of possible segmentation of this subduction zone has improved through the analysis of turbidite deposits. Microfossil-based paleogeodesy from onshore sites has the potential to further improve our understanding of the amount and extent of rupture, as well as identifying regions of substantial vertical deformation.

In contrast to Cascadia, little is known about the rupture history of the Alaska-Aleutian subduction zone, with the exception of sites within the 1964 rupture area The Alaska-Aleutian subduction zone is the source of frequent great earthquakes and accompanying destructive tsunamis (e.g., 1946, 1957, 1964, 1965). GPS identifies sections that are highly coupled in the interseismic interval but it is unknown how accumulated strain will be released in future ruptures, because GPS instrumentation only spans a fraction of the earthquake cycle. For example, historical evidence suggests the 1788 earthquake may have propagated through a 600-km-long swath of the Alaska-Aleutian subduction zone from Sanak Island to Kodiak, although portions of this area are currently identified as creeping by GPS. Paleoseismology studies are the only way to test the persistence of creeping patches and rupture boundaries through time.

In this presentation, I will present data compiled from paleoseismology studies undertaken during the last seven years from sites in Cascadia and Alaska, identify how they have improved our understanding of past rupture and megathrust behavior, and identify areas where further progress might be made using the paleoseismic record.