Calibrating Cascadia Paleoearthquake Magnitudes and Ground Motions From the Paleoseismic Record

Date: 4/25/2019

Time: 08:45 AM

Room: Cascade II

The Cascadia paleoseismic record is long and composed of land, lacustrine and offshore paleoseismic records. One of the elusive issues remaining is a reliable catalog of approximate magnitudes for the 10 ka record. Currently, the 1700 CE event is the only link to magnitude, and that link is somewhat tenuous given its reliance on a transoceanic tsunami model. We suggest a second calibration point exists in the deep water turbidite record, a bed that may have been the result of a turbidity current from the 1992 Mw 7.1 Petrolia earthquake. The turbidite record contains two events stratigraphically younger than the dated 1700 event that have not previously been investigated in detail. Specifically, the two beds named T0, and T0a are found in numerous southern Cascadia cores above the likely 1700 CE bed. These beds can be traced from Trinidad Plunge Pool northward variable distances, terminating at or south of the Rogue Canyon. The ages of the upper thousand years of record have been carefully refined using an age-depth model, improved analyses of the sediment intervals, and recalibration of the ages, including the youngest event using bomb-carbon methods. The model age for the likely 1700 large event bed remains close to that time, with a mean of 270 BP (1950) and 2 sigma range of 200-330 BP (1950). The three younger beds have 2 sigma model ages of T0 = 1962-2009, median 1994 CE; and T0a = 1824-1963, median 1899 CE. There are significant flooding events in 1913, 1937, 1955, 1964, 1974 and 1995 that can be considered, and further work is required to test the upper beds origin. However, we suggest a reasonable interpretation of the upper beds may be T0 as the 1992 Petrolia earthquake, and T0a as the 1906 San Andreas earthquake. The likely presence of the 1992 Petrolia event in the offshore record and the lack of evidence of flooding events allows better calibration of approximate magnitudes given upper and lower bounds are both represented, resulting in a downsizing of many of the southern Cascadia events.

Presenting Author: Chris Goldfinger

Authors