Possible link found between earthquakes along the Cascadia and San Andreas faults

Goldfinger and his colleagues collected core samples that cover the past 10,000 years, and the next step involves analyzing this data for further evidence of a corollary relationship between the plate boundary faults for earlier periods of time. “This type of relationship doesn’t just happen accidentally. We expect the temporal relationship, if correct, to show itself over the longer period of time,” said Goldfinger.

Perhaps the most thoroughly studied seismic event, the 1906 quake continues to fascinate seismologists. BSSA’s special section considers the landmark event, which was initiated along the San Andreas Fault just off the San Francisco coast on April 18, 1906. The strong shaking caused widespread damage along the 300 miles of the fault in northern California, reducing much of San Francisco to rubble.

“The directivity of the ruptures, north to south, which is implied by this study, will have significant meaning for seismic hazard models for San Francisco,” said Goldfinger. The 1906 earthquake, which is an exception to the pattern over the past 3000 years, ruptured in both directions, but mostly from south to north.

“Lessons from the 1906 earthquake should apply to similar faults and earthquakes elsewhere,” writes Brad T. Aagaard, a research geophysicist at the USGS Menlo Park and co-author of the introduction to the special section and two papers that focus on ground motion. “As our understanding of earthquakes evolves and the technology to increase our knowledge develops, there is much to be gained by revisiting older events.

In 1906, approximately 600,000 people lived in the greater Bay Area, about 10 percent of today’s population. Today’s cities have high rise buildings, people travel by car, and five major bridges connect the major cities around the San Francisco Bay.

The special section features new research that characterizes the earthquake source, refines assessments of ground shaking that support higher intensities, and explores the possible effects of a repeat of the 1906 earthquake, or similar-sized earthquakes on the San Andreas Fault.

Research by Aagaard et al., demonstrates how the variability in strong shaking over the San Francisco Bay area observed in 1906 can be attributed to the geologic structure and rupture characteristics. More importantly, by considering other possible rupture scenarios, the Aagaard et al., conclude that future large earthquakes along the San Andreas Fault may subject the San Francisco Bay Area to stronger shaking than occurred in the 1906 earthquake.