Paleoseismic interpretation and a preliminary geologic slip rate for the Parkfield segment of the San Andreas Fault
Abstract
The Parkfield segment of the San Andreas Fault (SAF) is the northwest terminus of the great 1857 Fort Tejón earthquake. Slip budgets accounting for creep and moderate magnitude earthquakes suggest a slip deficit of 5 m along the extent of the 1857 rupture and a smaller deficit waning into the Parkfield segment. However, a geologic slip-rate for the Parkfield segment has not been established and it is unclear how slip is partitioned between the SAF and nearby sub-parallel faults. In 2007, we excavated three trenches at the Miller's Field paleoseismic site. This effort followed a similar paleoseismic campaign in 2004. Here we present new radiocarbon dating from our previous excavations across a sag pond (MST04) and a pressure ridge (PT04) and interpretations of the new paleoseismic exposures on these same geomorphic features (MST07 and PT07). We also opened two pilot trenches along the Southwest Fracture Zone (SWFZ). The Miller's field paleoseismic site is a Holocene terrace of the Little Cholame Creek. The exposed stratigraphy is overbank deposits of sand and silt. Some units are separated by thin charcoal-rich horizons that we interpret to have settled out during the waning of paleofloods. Along the SAF scarp, several buried soil horizons are deformed by the fault zone and clay-rich layers have accumulated within a sag pond. Deformation within the sag is partitioned among several zones of faulting with multiple splays. Many splays extend to the surface of the trenches and some are oriented obliquely to the trend of the SAF, consistent with formation from en-echelon surface cracking similar to the 2004 M6 Parkfield earthquake rupture. Interpreting ground rupturing paleoearthquakes at Parkfield must be cautionary because the SAF is creeping and numerous moderate magnitude earthquakes have occurred there historically. Upward terminating offsets could be formed on creeping fault splays. Despite this caution, we did observe three upward terminating offsets in the MST04 trench. This is weak evidence for two events between 898 A.D. and 1483 A.D. and one event prior to 898 A.D. Also two sets of upward terminating offsets and three lobes of reworked material (possibly fissures or colluvial wedges) within the PT exposures provide weak evidence for five ground rupturing events prior to 1857. However, we did not observe any geomorphic or stratigraphic evidence for a multi-meter rupture extending this far northwest in 1857. New radiocarbon dates allow us to estimate the sag pond began to form between 898 A.D. and 1036 A.D. Using the sag pond scarp formation as an initiation time for the channel that is apparently offset 17 to 24 meters across the MST site, we estimate a minimum slip rate of 16 to 25 mm/yr across a 10 meter aperture of the SAF. A slip rate of ~20 mm/yr for the main SAF at Parkfield indicates that another 10-15 mm/yr are accommodated by structures in a wider aperture of the fault zone. The SWFZ could be responsible for a significant percentage of this slip, but probably not all of it because other splays have experienced slip in the most recent Parkfield earthquakes and nearby faults such as the Buzzard Canyon and White Canyon faults display tectonic signals in their geomorphology.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.T41A1955T
- Keywords:
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- 7221 Paleoseismology (8036);
- 8111 Continental tectonics: strike-slip and transform;
- 8118 Dynamics and mechanics of faulting (8004);
- 8150 Plate boundary: general (3040);
- 8155 Plate motions: general (3040)