A new study has raised fears that a major earthquake in the Pacific north-west could trigger a secondary quake on California's San Andreas fault, leading to an unprecedented disaster. Researchers found that the Cascadia subduction zone, capable of producing magnitude 9 earthquakes, has triggered temblors on the San Andreas fault in the past.
Published on 29 September, the study analysed deep-sea sediment cores containing thousands of years of geologic history. Lead author Dr Chris Goldfinger, a marine geologist and geophysicist, said the findings came after decades of puzzling over data. The researchers initially thought submarine landslide deposits were from San Andreas quakes, but realised they were from two separate events.
By studying sandy structures within turbidites, the scientists determined that some ruptures occurred just hours or minutes apart. They found evidence of 18 synchronized earthquakes over the last 3,000 years, suggesting such back-to-back quakes are common. The most recent occurrence was linked to a magnitude 9 earthquake in 1700, which sent a tsunami across the Pacific and levelled homes on Vancouver Island.
The researchers found that a second quake, up to magnitude 7.9, probably struck northern California shortly after the first. A magnitude 9 earthquake on the Cascadia fault could produce an 80ft tsunami within 30 minutes, affecting over 140,000 square miles across three US states and parts of Canada. A similar analysis of a magnitude 7.8 quake on the southern San Andreas estimated over 1,800 deaths, 50,000 injuries and $200bn in damage.
Goldfinger warned that if both faults rupture together, cities including San Francisco, Portland, Seattle and Vancouver could face simultaneous emergencies. However, he noted that a Cascadia quake could serve as a natural early warning system for a subsequent San Andreas event. "Our preparedness level is poor," he said, "We have a long way to go."
