Possible Stratigraphic Evidence of Stress Triggering of the Northern San Andreas Fault Following Southern Cascadia Earthquakes

The Cascadia offshore turbidite record contains multiple events stratigraphically younger than the dated CE 1700 event that have not previously been investigated in detail. Age models constrained by precise bomb-carbon ¹⁴C ages suggest the most robust upper beds may be the 1992 Petrolia earthquake, and the 1906 San Andreas earthquake. The record may also include the 1980, 1873, and other historic events in the Mendocino Triple Junction region and suggest the lower limit for recording of such events lies below Mw 7.0 locally. The CE 1700 event bed in multiple cores in and around Trinidad Plunge Pool and the Eel Channel is a stratigraphically distinctive unit that appears correlative to a bed of similar age found in cores at Noyo Canyon, offshore the Northern San Andreas Fault. Noyo Canyon likely contains mostly Northern San Andreas event beds based on temporal correspondence with onshore paleoseismic sites at Vedanta marsh and Lake Merced in San Francisco. Noyo Canyon, at 90 km range, may also lie within range of triggering of turbidity currents from Cascadia. The distinctive CE 1700 Cascadia bed interpreted in Noyo Canyon is directly overlain by a robust sandy bed with no evidence of intervening time, and with a clear erosional unconformity between the two units. This relationship is represented in multiple cores, and results in an unusual inverted coarsening upward sequence. A similar relationship is observed for multiple other Cascadia events younger than 2800 years. These beds may also have Noyo Canyon counterparts, and are similarly directly overlain by a coarse sandy top and have an inverted, coarsening upward stratigraphy. Radiocarbon ages at the bases of these inverted units are compatible with both late Holocene Cascadia earthquakes, and NSAF events. In one instance, a separation of ~ 100 years is observed, but in all other cases, time separation between the pairs is not observed. Present data cannot resolve time separation, if any, because basal erosion may have removed intervening time intervals. Previously, a stress triggering relationship was suggested based on temporal correspondence between the faults. We suggest these inverted stratigraphic pairs may represent direct evidence of triggering of the NSAF by Cascadia during the late Holocene, with minimal time separation between the two ruptures.