| Subject: Catalina Island tsunami hazard |
Author:
Blobrana
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Date Posted: 18:52:22 09/24/04 Fri
With a strong enough jolt - a 7.6 -magnitude earthquake - the seafloor under Catalina Island could be violently thrust upward, causing a tsunami along the Southern California coast.
There is a tsunami hazard associated with offshore faults, including one that lies under Santa Catalina Island, just 25 miles off the Los Angeles coast.
"Catalina Island itself exists due to earthquake-related uplift on a geologic structure known as a restraining bend. Although most faults offshore Los Angeles and Orange counties are mostly strike-slip - faults that move side to side - bends in the fault line produce areas where the ground is pushed up during major earthquakes. One of these regions lies directly below Santa Catalina Island."
Strike slip faults are not straight. Bends in the fault trace produce regions where earthquake stresses cause the sea floor to pop up and generate a tsunami
When a large earthquake occurs at a restraining bend, like the bend under Catalina Island, the ground is pushed up and, in turn, pushes up the entire region that has created the island and its offshore flanks.
"Future earthquakes will push the region up further, possibly resulting in a tsunami.”
"Tsunami" is a Japanese word for waves caused by large motions of the sea floor, either through earthquakes, landslides or undersea volcanoes. They are generally associated with earthquakes that occur offshore and produce significant uplifting of the sea floor.
"there is significant amplification of tsunami energy into San Pedro Bay."
San Pedro Bay’s south-facing shores are home to the largest container ports in the United States - the ports of Los Angeles and Long Beach. Billions of dollars of materials pass through these ports every day.
A large earthquake and tsunami could bring commerce to a halt, seriously impacting not only California’s economy but the nation’s economy.
"A magnitude 7.6 earthquake could cause seafloor uplift of six feet or more. That, in turn, would disturb the sea surface by the same amount, resulting in a tsunami. The shallow San Pedro shelf offshore of Long Beach focuses the waves and amplifies them by one-and-a -half times, so the original six-foot wave would build to nine feet inside the harbor."
Waves of that size could smash small boats at their moorings, possibly flood low-lying areas in the ports and push huge oil tankers and cargo ships against piers, which may not withstand the force. The destruction could create oil spills and become a serious fire hazard.
However, there is some good news: the same features that focus and amplify tsunami waves also slow the waves’ arrival.
Computer models show that depending on the source, there is anywhere from 15 to 20 minutes between the earthquake and the first significant waves in the ports.
This may give shippers enough time to evacuate dock workers and stop hazardous activities, such as cargo handling or offloading oil from tanker ships. Every second would count.
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