Late Holocene Tectonics and Paleoseismicity, Southern Cascadia Subduction Zone

Abstract

Holocene deformation indicative of large subduction-zone earthquakes has occurred on two large thrust fault systems in the Humboldt Bay region of northern California. Displaced stratigraphic markers record three offsets of 5 to 7 meters each on the Little Salmon fault during the past 1700 years. Smaller and less frequent Holocene displacements have occurred in the Mad River fault zone. Elsewhere, as many as five episodes of sudden subsidence of marsh peats and fossil forests and uplift of marine terraces are recorded. Carbon-14 dates suggest that the faulting, subsidence, and uplift events were synchronous. Relations between magnitude and various fault-offset parameters indicate that earthquakes accompanying displacements on the Little Salmon fault had magnitudes of at least 7.6 to 7.8. More likely this faulting accompanied rupture of the boundary between the Gorda and North American plates, and magnitudes were about 8.4 or greater.

Rapid detection and characterization of large earthquakes using quasi-finite-source Green's functions in continuous moment tensor inversion

“…Its seismicity is mostly located offshore: along the Mendocino Transform Fault (MTF), within the highly deformed Gorda plate, and along the Cascadia Subduction Zone (CSZ) (Figure 1). Paleoseismic evidence of great earthquakes along the CSZ [Atwater, 1987], including radiocarbon dating and several offshore turbidite episodes of deformation in the last 2000 years [Clarke and Carver, 1992], indicate that the CSZ is capable of multiple M8+ earthquakes, as large as M9 [Goldfinger et al, 2008;Satake et al, 1996;Heaton and Hartzell, 1987].…”

Establishing the Inundation Distance and Overtopping Height of Paleotsunami from the Late-Holocene Geologic Record at Open-Coastal Wetland Sites, Central Cascadia Margin

“…Tsunami sand layers exist in coastal bays and wetlands at a number of sites along the Cascadia Margin in the Pacific Northwest (PNW) (Figure 2). These have included tidal marshes at Humboldt Bay, California (Clarke and Carver, 1992), Coos…”

Do cataclastic deformation bands form parallel to lines of no finite elongation (LNFE) or zero extension directions?

“…The McKinleyville fault has experienced an average slip rate 184 of 0.6 ± 0.2 mm/yr over the last ca. 200 ka (Petersen et al, 1996), whilst Clarke and Carver (1992) 185 have reported at least two Holocene slip events, each of which resulted in ca. 3.5 m of dip slip along 186 the McKinleyville fault.…”