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Seismotectonics of the 1857 Mw7.9 Fort Tejon Earthquake Rupture Zone Along the San Andreas Fault, Southern California

Egill Hauksson, Lucile M. Jones, Joann M. Stock, & Allen L. Husker

Submitted September 10, 2023, SCEC Contribution #13077, 2023 SCEC Annual Meeting Poster #038

The 9th of January 1857 Mw7.9 Fort Tejon earthquake ruptured the Southern San Andreas Fault (SSAF) with an average slip of 5 m for a distance of 350 km, from Parkfield in the north to Wrightwood near Cajon Pass in the south (Sieh, 1978a). It was felt across the Western US and northern Baja California (Wood, 1955; Agnew & Sieh, 1978b). Since 1981 or for the last 42 years, the relocated seismicity adjacent to the 1857 rupture zone does not clearly outline the principal slip surfaces (PSSs) of the fault. This sparse seismicity is spatially clustered but it has no obvious relationship to other features of the SSAF such as geometrical shape, the segment boundaries, or geodetic locking depth. Seismicity depth cross sections across several segments show absence of small earthquakes within ± 2 km of the steeply dipping mapped SSAF trace. The SSAF is also a boundary between lithological blocks of different geological composition, which accommodate seismicity mostly at their edges. Such adjacent blocks may have different 95% depth of the seismicity as well as different spatial distribution with different seismicity rates. Their depth distributions often form an apparent step in the 95% depth of the seismicity across the fault, and the spatial scattering has a different texture on one side versus the other. The focal mechanisms of the SAF adjacent events are also heterogeneous and rarely have strike and dips that are consistent with slip on the PSSs. Thus, a time period of 166 years has not been sufficiently long to recover a stress state that might trigger elevated seismicity, as for instance, is observed along the Parkfield segment or the San Jacinto Fault. This absence of small earthquakes on the PSSs suggests that only minor, or possibly none, residual stresses were left by the 1857 event within the fault zone.

Key Words
1857 Earthquake San Andreas Fault

Hauksson, E., Jones, L. M., Stock, J. M., & Husker, A. L. (2023, 09). Seismotectonics of the 1857 Mw7.9 Fort Tejon Earthquake Rupture Zone Along the San Andreas Fault, Southern California. Poster Presentation at 2023 SCEC Annual Meeting.

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