Cascadia onshore-offshore site-response, submarine sediment slope-failures and flows, and earthquake recurrence
Joan Gomberg, USGS Seattle / University of Washington
Monday, July 24, 2017 at 10:30 AM
- Building 3, Rambo Auditorium
To better understand how earthquake-generated shaking mobilizes and redistributes submarine sediments I used seismic data from the 2011-2015 offshore Cascadia Initiative and permanent onshore seismic networks to measure three simple site-response measures (PGV, PGA and energy density), in low (.02-1 Hz) and high (1-10 Hz) frequency passbands. These metrics show similar patterns, and vary by factors of almost 10 to 60 for low and high frequencies, respectively, across all of Cascadia. Site-response variability is primarily longitudinal, with the caveat that few observations sample the continental shelf in southern Cascadia. Offshore site-response estimates have nearly opposite patterns at low and high frequencies, which broadly may be attributed to sediment resonance and attenuation. However, the patterns are not visually or statistically fully consistent with the mapped sediment distribution and only weakly consistent with peaks in horizontal to vertical spectral ratios often attributed to resonance. This undoubtedly partly reflects sediment thickness model inaccuracies, but the topography likely has a profound impact on the shaking, evident in an abrupt change in the longitudinal trend of site-response along most of the Cascadia margin. This change coincides with the base of the steepest slopes that border the shelf. Offshore site-response exceeds that offshore in both passbands, which should be accounted for when extrapolating from onshore ground motions to estimate those offshore. The patterns of site-response alone do not shed light on what frequency passband is most important for triggering slope failures and remobilizing sediments, but if low frequencies are the most effective triggers, suggest that where slopes are steepest site-response facilitates triggering. Finally, site-response patterns neither support nor permit rejection of the hypothesis that the southerly decrease in great earthquake recurrence intervals estimated by Goldfinger et al., [2012, 2013, 2016] and Priest et al.  is attributable to inherently stronger shaking going southward, though significant north-south variations in site-response are not apparent.