Mapping offshore faults near Santa Catalina Island in July of 2009.
Catalina or Bust: USGS Group Maps Faults Offshore of Los Angeles
This article is part of the September 2009 issue of the Sound Waves newsletter.
Despite mercurial motherboards, an overheating minisparker system, and having to avoid cargo ships and weekend yacht traffic, the intrepid crew of the research vessel Parke Snavelyconducted a successful cruise to study faults offshore of Santa Catalina Island, which lies about 25 mi southwest of Los Angeles, California.
The survey, part of the Earthquake and Tsunami Hazards Project, collected high-resolution seismic-reflection profiles (cross-sectional views of sub-seafloor sediment layers) to assess the recentness of fault motion and the potential for earthquake and tsunami hazards. The high-resolution data were collected with two systems that emit pulses of sound energy that penetrate beneath the seafloor and reflect off the boundaries of sub-seafloor layers:
- a 50-tip minisparker as the sound source and a short, single-channel hydrophone (underwater microphone) streamer to record the echoes; and
- an Edgetech 512i subbottom-profiling system composed of a high-frequency "chirp" sound source and a small hydrophone array, all housed within a 190-kg "fish" towed 3 to 5 m below the sea surface.
Thanks to the skills of skipper Pete Dal Ferro and engineering whiz kid Gerry Hatcher, we collected 208 km of minisparker data and 45 km of chirp data over a 5-day period in July 2009. Generally smooth seas and sunny days assisted the effort, which was led by co-chief scientists Jamie Conrad and Holly Ryan with assistance from lead geophysicist Ray Sliter.
Our study focused on the connection between the San Diego Trough fault and the San Pedro Basin fault offshore in the Southern California Continental Borderland. These faults are of particular interest as both earthquake and tsunami hazards, especially to the Port of Los Angeles. The San Diego Trough fault is a major strike-slip fault that extends about 150 km, from south of the Mexican border near Punta Santo Tomás to near Crespi Knoll, which is about 40 km offshore of San Clemente, California. The San Pedro Basin fault had been previously mapped as extending from off of Santa Catalina Island northward to near Point Dume. Multichannel seismic-reflection data collected by the USGS several years ago, together with high-resolution data collected in 2008 east of Santa Catalina Island, suggest that the San Diego Trough fault and the San Pedro Basin fault might connect at depths of about 100 to 200 m below the seafloor. One of our goals in collecting higher resolution data during the July 2009 cruise was to determine whether such a connection was apparent at or near the seafloor, which would indicate recent offset along this fault segment. Initial results suggest that the San Pedro Basin fault does not extend far enough southward to make a simple connection with the San Diego Trough fault. Instead, the San Diego Trough fault appears to terminate in a series of complex folds and faults, possibly transferring some slip eastward onto the Avalon Knoll fault or the Palos Verdes fault. How slip might transfer onto the San Pedro Basin fault to the north remains unclear.
We thank the Southern California Marine Institute (SCMI) for logistical assistance and the use of their dock facilities in Los Angeles, and the Philip K. Wrigley Marine Science Center (University of Southern California) for housing both the Snavely and the crew on Santa Catalina Island.