: Upper Plate Deformation Across Active Faults in the Southern Cascadia Subduction Zone, Northern California: Geodetic Slip Rate Estimates Compared with Geologic Rates
Abstract
Fault slip rates in the North America plate are a key factor when considering northern California seismic hazard. Crustal faults in the north coast region are capable of producing > M 7earthquakes. Observations here may help us understand fault behavior elsewhere, in regions of higher population density. The majority of slip rates for these faults are geologic slip rates, based on uplifted marine terraces and offset Tertiary and older geologic units.
We incorporate secular geodetic observations (tide gage, GPS, level analyses) to compare with known geologic rates. We use tide gage data obtained from NOAA, as well as 'campaign' style tide gages, to infer interseismic vertical land motion (VLM). We autocorrelate tide gage data from Crescent City to Humboldt Bay, California and, after regional sea level is removed, we estimate rates of VLM. We also use first-order leveling data collected by the National Geodetic Survey, along with USGS Global Positioning System permanent site data, to evaluate vertical interseismic deformation. Land subsidence in and around Humboldt Bay contributes to sea-level rise up to 2-3 times greater than anywhere else in California. These observations confirm land is subsiding in Humboldt Bay, in contrast to Crescent City, to the north, where land is rising. Rates of sea-level rise are 5.84 mm/yr in South Humboldt Bay, 3.76 mm/yr at Fields Landing, 4.61 mm/yr at the North Spit, 2.53 mm/yr at Samoa, and 3.39 mm/yr in Arcata Bay. An East-West trending variation in VLM is primarily due to the Cascadia subduction zone. We associate a heterogeneous North-South trend in VLM to crustal fault related strain. We use offsets in VLM rates across these crustal faults, primarily thrusts with associated folds, to derive estimates for rates of vertical separation. Cumulative vertical separation slip deficit across the Humboldt Bay region from geologic slip rates is 5.3 mm/yr, while the geodetic slip deficit is 4.9 mm/yr. Including estimates for the Russ fault system near Cape Mendocino (or to the south), the geodetic vertical separation slip deficit increases to 8.3 mm/yr. Offsets in geodetic VLM derived vertical separation rates match well with the geologic vertical separation rates, even though these are calculated over different time periods.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T13H0324P
- Keywords:
-
- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICSDE: 8170 Subduction zone processes;
- TECTONOPHYSICSDE: 8180 Tomography;
- TECTONOPHYSICSDE: 8185 Volcanic arcs;
- TECTONOPHYSICS