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The Cascadia subduction zone and the San Andreas fault on North America’s West Coast may be more closely connected than previously thought, with major earthquakes on the two systems sometimes occurring within minutes or hours of each other, according to a new study.
Researchers reached the conclusion after examining about 3,100 years of earthquake history preserved in deep-sea sediment cores.
The study, led by marine geologist Chris Goldfinger of Oregon State University, found evidence suggesting that major earthquakes along the Cascadia and northern San Andreas fault systems have occasionally occurred in close succession.
Goldfinger said the findings suggest that the “Big One” associated with Cascadia may not represent the worst-case scenario if a major earthquake there is followed shortly by a rupture on the San Andreas.
The researchers studied layers of sediment known as turbidites, which can form when underwater landslides send large amounts of sediment down the seafloor. Major earthquakes can trigger such landslides, leaving deposits that can provide a record of past seismic activity.
By comparing turbidite layers linked to the two fault systems, the researchers found similarities in their timing and structure. Although it is difficult to determine the exact interval between earthquakes that occurred centuries ago, the team identified three events over the past 1,500 years in which Cascadia and the northern San Andreas appear to have ruptured within minutes to hours of each other.
The most recent apparent episode occurred around 1700.
The researchers said closely timed earthquakes on both fault systems could pose a major challenge for emergency responders because the events could affect several major population centres along the West Coast at nearly the same time.
An earthquake on either fault could place heavy demands on emergency resources. If major ruptures occurred on both systems within a short period, cities including San Francisco, Portland, Seattle and Vancouver could face emergencies simultaneously, Goldfinger said.
Scientists have long considered whether separate fault systems can influence or synchronize with one another, but direct evidence has been limited. One previously documented example involved earthquakes in Sumatra that occurred three months apart in 2004 and 2005.
The latest research has its origins in an unexpected discovery during a 1999 ocean expedition. Goldfinger and his colleagues were collecting sediment cores from the Cascadia subduction zone when a navigational error took their vessel about 55 miles south of Cape Mendocino, California, into the San Andreas fault region.
The researchers decided to collect a sediment core from the area rather than abandon the detour.
The core contained an unusual sediment sequence. Normally, coarse material settles first, followed by progressively finer sediment. In this sample, however, fine silty material was found beneath coarse sandy sediment.
The researchers interpreted the unusual layering as evidence of two separate seismic events occurring in rapid succession. They concluded that the lower layer may have been deposited after a Cascadia earthquake, while the upper layer could have resulted from subsequent movement on the nearby San Andreas fault.
The team then used radiocarbon dating to estimate the ages of turbidite deposits from the core and other samples collected north and south of Cape Mendocino.
The location is particularly significant because Cape Mendocino lies near the area where the northern San Andreas system and the Cascadia subduction zone converge.