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South Island could be in most quake-prone part of the Pacific coast: study

A new study has found the section of the Cascadia Subduction Zone that includes the south Island and Washington state could be the most susceptible to a major earthquake

Southern Vancouver Island could be in the most dangerous part of the Cascadia Subduction Zone, a 1,000-kilometre-long area stretching from B.C. to California prone to earthquake activity.

A newly released study has found that the zone is ­composed of four segments rather than a single structure, and the ­segment taking in the Island and Washington state is potentially the most susceptible to a major earthquake.

Natural Resources Canada seismologist John Cassidy, who was not involved in the study, said state-of-the-art equipment ­produced data that has created a more precise view of what the entire zone contains than ever before.

“It’s like we were looking at the subduction zone with foggy glasses and we have a much clearer picture now,” he said. “But there’s still more work to do.”

The study used extensive data from 2021 gathered with high-tech waterproof cables and microphones laid out in the ocean, Cassidy said.

Prior to that, lower-quality images from the 1980s were all that was available. They were generated from cables about three kilometres long, while the current information was collected with vastly improved technology and cables up to 15 kilometres in length.

The study will improve assessment of both earthquake and tsunami risk, Cassidy said. “It allows for better models of tsunami generation that emergency planners would use, for example, and it allows for better estimates of ground shaking.”

The latter can be used in determining building codes, he said.

According to an article from the Columbia Climate School, whose marine geophysicist Suzanne Charbotte was involved in the research, the study suggests the earthquake-prone area in the segment could extend under the Olympic Peninsula.

That might magnify shaking on land and “could mean the ­difference between alarming and catastrophic” for places like Tacoma and Seattle, University of Washington geophysicist ­Harold Tobin said in the article.

Cassidy said the new images show that the segment skirting the Island has a very smooth sea floor, indicating there could be increased “slip” there when an earthquake hits.

More slip, in turn, generally translates into stronger ground motions, Cassidy said.

The smoothness could also mean that the entire segment ruptures during an earthquake, rather than being curtailed by various formations at the bottom of the ocean.

Cassidy noted that the ­Cascadia Subduction Zone ­contains a “megathrust fault” that has produced earthquakes up to a magnitude of 9.0 or more — the most recent in January 1700.

Information about that ­earthquake comes from ­Indigenous accounts and through written records from Japan, which the resulting ­tsunami reached, he said.

Paleoseismology — the ­examination of ancient rocks and sediments — shows there have been 19 such 9.0-plus-­magnitude quakes in the past 10,000 years.

Sometimes the gap between large-magnitude quakes is about 250 years, while other times it’s up to 850 years, said Cassidy, noting the last one was 324 years ago.

Those large-magnitude quakes are rare events and easy to forget about, “but they did happen in the past and will ­happen again,” he said, noting a federal earthquake early-warning system is in place in B.C., as are others in the private sector.

The volume of data collected will lead to more findings and further analysis, he said.

“This is just the first set of results,” he said. “These will continue to come in over the next months and even the next few years.”

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