Stratigraphic Evidence of Close Temporal Occurrence of Northern San Andreas and Southern Cascadia Earthquakes: Partial Synchronization?

Session: Crustal Stress and Strain and Implications for Fault Interaction and Slip [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Age models constrained by precise bomb-carbon ¹⁴C ages at Trinidad Canyon suggest the youngest event beds in southern Cascadia likely include the 1980 Eureka, the 1992 Petrolia and the 1906 San Andreas earthquakes. This evidence suggests the lower limit for recording of such events lies near Mw 7.0 locally. The presence of a likely 1906 bed from the NSAF earthquake is the first evidence that NSAF earthquakes are recorded in Cascadia. We also find a bed of ~ 1700 CE in in cores at Noyo Canyon, offshore the NSAF. The NSAF also had an earthquake at about that time recorded at multiple land sites. On both sides of the triple junction, this bed is a unique doublet, with two fining upward sequences and no other significant beds within a ~700 year span. This relationship is represented in multiple cores and results in an unusual inverted coarsening upward doublet sequence. We suggest that the observed doublet may be event beds from both faults stacked closely together. There no sedimentary evidence of intervening time, but the ~ 1700 doubled has a clear erosional unconformity between the two units. At other times that the Cascadia and NSAF earthquake sequences coincide closely based on radiocarbon at multiple sites, there are inverted doublet beds observed rather than distinct multiple beds. In the past 2800 years, there are 8 examples of inverted doublet beds where pairs of beds from both faults are expected. In five other cases, the NSAF 1906 bed being one of them, there are beds recorded in one or both systems that clearly represent single events with thinner fining upward beds that taper rapidly away from the Mendocino Triple Junction. All of the doublet beds show little or no time intervening between the two main elements and are observed in multiple cores. A stress-triggering relationship was previously proposed based on radiocarbon evidence alone, and this hypothesis is now supported by the new stratigraphic evidence. The stratigraphic and radiocarbon evidence together suggest an interval of partial synchronization of the two plate boundary faults.

Presenting Author: Chris Goldfinger

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