For example, multibeam sonar and chirp systems both use high frequency sound to create detailed views, respectively, of the seafloor and features beneath the seafloor. Scientists bounce sound off the seafloor to image the bottom (bathymetric mapping), or image the layers of sediment and rock beneath the seafloor (seismic mapping). ![]() The USGS is collecting higher-resolution offshore geophysical data to better characterize these faults. Rocks are notably upwarped and folded adjacent to the fault. Perspective view of seafloor offshore of Half Moon Bay, showing scarp (arrows) along the eastern strand of the San Gregorio fault zone. ![]() New geophysical evidence suggests the Hayward and Rodgers Creek faults may be directly connected north of San Pablo Bay-resolving a long-standing debate among scientists. East of the San Andreas fault, the Hayward and Rodgers Creek faults are considered the most likely faults in the San Francisco Bay area to have a damaging (magnitude greater than 6.7) earthquake in the next 30 years. West of the San Andreas fault, the 400-kilometer-long San Gregorio-Hosgri fault extends primarily offshore between Point Conception and Bolinas, and sits within 3 nautical miles (in state waters) of the Diablo Canyon Power Plant. For 300 kilometers between Pacifica and Cape Mendocino, about 60 percent of the trace of the San Andreas fault lies beneath the ocean floor. Each of these fault zones has important offshore sections that, until recently, were not mapped in great detail. The most significant faults within the plate boundary in central and northern California include the San Andreas, San Gregorio-Hosgri, and Hayward-Rodgers Creek fault zones. By applying consistent mapping techniques with state-of-the art tools, scientists can vastly improve knowledge about faults, and compare data collected at different times and in different regions. Mapping efforts in the past have been patchy and used a variety of instruments that produced lower-resolution data. Documenting the offshore portions of significant active faults in central and northern California contributes crucial information to the national earthquake-hazard assessment effort. Such assessment depends on accurate descriptions of faults, including their location, length, geometry, slip rate, and rupture history, as well as the connections between faults. The USGS plays a prominent role in assessing earthquake hazards, providing information that informs building codes, insurance rate structures, relicensing of nuclear power plants, risk evaluations, and public policy. This event resulted in about 3,000 deaths and destroyed more than three quarters of San Francisco. One important example-the Great 1906 San Francisco earthquake (magnitude 7.8)-occurred along the San Andreas fault a few kilometers off the coast of San Francisco. The same forces that create the high coastal mountains and control the paths of coastal rivers also lead to devastating earthquakes that endanger coastal populations and infrastructure. Much of this stunning landscape is shaped by movements along active faults between the North American and Pacific tectonic plates on the U.S. The Central California coast is known for its natural beauty. Main faults along the northern and central California Coast. What she found from the very detailed images beneath the seafloor confirmed suspicions that the Hayward fault actually heads toward the Rodgers Creek fault, a relationship that could generate a larger magnitude earthquake-even stronger than the 1989 magnitude 6.9 Loma Prieta earthquake-if both rupture together. So, USGS geophysicist Janet Watt used a small boat that could operate in shallow waters to launch seismic equipment called a “chirp” floated on pontoons. However, San Pablo Bay averages only 2 meters deep-too shallow for a large ship-and gas just beneath the seafloor interferes with the imaging. ![]() In San Pablo Bay, for example, USGS scientists wanted to pin down the location of an important segment of the Hayward fault-a fault considered most likely to produce the next large earthquake in the San Francisco Bay area. In marine environments, a ship can send sound through water and sediment and back to produce a clear image, but other problems may arise. Researching earthquake hazards on land has its challenges fault lines can run right through cities, where it’s difficult to use sound-generating equipment to image the underground fractures. The chirp is attached to pontoons to keep the equipment from running aground in the shallow waters of San Pablo Bay, California. USGS scientist Jackson Currie deploys a chirp sub-bottom profiler (in the center) from research vessel Parke Snavely.
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