California's central coast earthquake hazards: New information about recently identified faults

Seismologists are re-evaluating the earthquake potential of the Central Coast, a very complex tectonic region located west of the San Andreas Fault, between Monterey Bay and the Western Transverse Ranges. This area of increasing population growth ranks as one of the top 40 U.S. metropolitan areas with significant earthquake risk. Speakers from the US Geological Survey, PG&E and academia will compare fresh data to illuminate the complexity of faulting in the central California coastal region.

Three talks will use separate datasets to focus on the California Central Ranges, Hosgri Fault Zone and nearby faults:

• Fault structure of the California Central Coast: Jeanne Hardebeck, US Geological Survey, will present and interpret new earthquake relocations and focal mechanisms for earthquakes occurring along the central California coast, including the offshore region near San Luis Obispo. A prominent newly-observed feature is a 25 km long linear trend of seismicity running just offshore and parallel to the coast-line in the region of Point Buchon. This seismicity trend is accompanied by a linear magnetic anomaly, and both the seismicity and the magnetic anomaly are truncated where they obliquely meet the Hosgri Fault. Focal mechanisms indicate that this feature is a vertical strike-slip fault.
• Geophysical characterization of the Hosgri Fault zone: High-resolution marine magnetic and seismic-reflection data collected offshore Point Buchon show that the Hosgri Fault represents a complex zone of steeply dipping faults that varies significantly in character along strike. The boundary of a northwest-trending linear magnetic anomaly off Point Buchon corresponds to a linear trend of small earthquakes, suggesting an active fault. Continued interpretation and geophysical modeling of magnetic, seismic reflection, and seismicity data will help determine whether or not the magnetic boundaries are fault boundaries, and if so, how these structures relate to the Hosgri Fault Zone.
• Constraints on 3-dimensional structure from gravity and magnetic data: V. E. Langenheim, US Geological Survey, will present analysis based on a new physical dataset that is sensitive to magnetic properties of rock, mapping fault boundaries. Her research suggests complex, non-linear features with intersecting faults. Fault and basin geometry will be important for estimating shaking potential of scenario earthquakes.

Source: Seismological Society of America