Seismic Imaging and Structure of the West Napa Fault Near Calistoga, California
Description:
The 24 August 2014 Mw 6.0 South Napa, California, earthquake caused extensive structural damage in the City of Napa, California. The causative fault, the West Napa Fault (WNF), continues northward from the City of Napa to Saint Helena, California, and post-earthquake investigations suggest an additional fault section extending past Calistoga. We evaluated the structure and geometry of the WNF near Calistoga, California by conducting an active-source, high-resolution (2-m shot and receiver spacing) seismic imaging survey across the suspected trace of the fault. We deployed two parallel seismic arrays using 3-component nodal seismometers and recorded P- and S-waves generated from vertical sledgehammer impacts on a steel plate and horizontal sledgehammer impacts to an aluminum block, respectively. We used refraction tomography to develop 2-D VP, VS, VP/VS, and Poisson’s ratio models, and Multichannel Analysis of Surface Waves (MASW) to develop additional VS(Rayleigh- and Love-wave) models. The VPmodel for the southern array shows a low-velocity zone with velocities between 1900 and 2200 m/s near the suspected trace of the fault. We observe an increase in depth of the 1500 m/s VPcontour (top of groundwater) coincident with the near-surface location of the low-velocity zone, which suggests the fault acts as a barrier to groundwater flow. Our VStomographic model for the southern array shows lower VS west of the suspected location of the fault. Based on our refraction tomography and surface-wave models, we interpret a steeply west-dipping main fault and possibly a minor splay approximately 40 m to the east of the near-surface trace.
Session: Cryptic Faults: Advances in Characterizing Low Strain Rate and Environmentally Obscured Faults [Poster Session]
Type: Poster
Date: 5/1/2024
Presentation Time: 08:00 AM (local time)
Presenting Author: Mark
Student Presenter: No
Invited Presentation:
Authors
Joanne Chan Corresponding Author jchan@usgs.gov U.S. Geological Survey |
Rufus Catchings catching@usgs.gov U.S. Geological Survey |
Mark Goldman Presenting Author goldmark@usgs.gov U.S. Geological Survey |
Belle Philibosian bphilibosian@usgs.gov U.S. Geological Survey |
Robert Sickler rsickler@usgs.gov U.S. Geological Survey |
Coyn Criley ccriley@usgs.gov U.S. Geological Survey |
Jessie Vermeer jvermeer@usgs.gov U.S. Geological Survey |
Delton Samuel dsamuel@usgs.gov U.S. Geological Survey |
Ahkayla Gomez agomez@usgs.gov U.S. Geological Survey |
Nick Cunetta ncunetta@usgs.gov U.S. Geological Survey, Mountain View, California, United States |
Karimah Comstock kcomstock@usgs.gov U.S. Geological Survey, Mountain View, California, United States |
Jenna Chaffeur jchaffeur@usgs.gov U.S. Geological Survey, Mountain View, California, United States |
Seismic Imaging and Structure of the West Napa Fault Near Calistoga, California
Session
Cryptic Faults: Advances in Characterizing Low Strain Rate and Environmentally Obscured Faults