The Wave that Wasn’t
Tsunami Warning Cancelled After Earthquake Off the Coast
• • • • • • • • • • December 2024 • • • • • • • • • •
Alarge, off-coast earthquake earlier this morning triggered a tsunami warning for parts of coastal Northern California and Southern Oregon, including the San Francisco Bay Area. Bay Area residents received warnings about the event and were, in some cases, evacuated from neighborhoods right off the coast. Thankfully, residents received another message less than an hour later canceling the warning, allowing some to cautiously return to their homes.
A large, off-coast earthquake earlier this morning triggered a tsunami warning for parts of coastal Northern California and Southern Oregon, including the San Francisco Bay Area. Bay Area residents received warnings about the event and were, in some cases, evacuated from neighborhoods right off the coast. Thankfully, residents received another message less than an hour later canceling the warning, allowing some to cautiously return to their homes.
According to the US Geological Survey, a 7.0 magnitude earthquake on the San Andreas fault took place at 10:44 am off the coast of Mendocino, causing the US Tsunami Warning System (TWS) to issue alerts for Port Orford, Brookings, and Charleston in Oregon, and Fort Bragg, Crescent City, and the San Francisco Bay Area in Northern California. Both San Francisco and West Berkeley issued evacuation orders for residents in the coastal neighborhoods.
The Department of Emergency Management sent out a message warning San Franciscans to stay away from Ocean Beach and other coastal areas, although as many people ignored the alert as those who took it to heart.”
TWS issued a warning statement ten minutes after the 7.0-m seismic event: “A tsunami threat exists for parts of the Pacific located closer to the earthquake.” It issued a second statement at 11:24 am specifying which areas of the West Coast were most likely to be affected and when; San Francisco was estimated to be hit at 12:10 pm.
The San Francisco Department of Emergency Management sent out a message warning San Franciscans to stay away from Ocean Beach and other coastal areas, although as many people ignored the alert as those who took it to heart. Surfers and pedestrians were recorded walking along the beach and Great Highway, looking out over the ocean as if straining to see the impending wave.
MUNI bus lines on the west side were temporarily suspended, as well as BART service to and from Oakland, as transit officials feared the trans-bay tunnel was in danger of being flooded. There were also multiple accounts of people leaving the Embarcadero and Financial District in an effort to avoid possible danger. One expert, Prof. Ross Stein of Stanford University, later spoke about the “high vulnerability of the San Francisco waterfront” and the tenuousness of the existing seawall – built during the Gold Rush – in another situation like this. The City is currently in the process of rebuilding and fortifying the seawall, which he called “a good step” towards protecting the City from other possible tsunamis.
In a relieving turn of events, TWS issued a third message at 11:54 am canceling the tsunami warning for the Bay Area. Seismologists explained that at the time of the first warning, the type of tectonic activity had been misidentified as vertical motion, an event in which the tectonic plates thrust upward, disturbing the water around it and resulting in increased wave activity.
They soon after determined that the type of tectonic activity that caused the earthquake was actually “strike-slip,” an event in which the edge of one tectonic plate pushes “slips” over the edge of its neighboring plate. Strike-slip earthquakes very rarely cause tsunamis as the direction of motion is horizontal, creating less disturbance in the water. This new information, in addition to visual observations, led TWS to cancel their prior alert.
While there have been reports of damage to shops and restaurants along the Northern California coast, there have been no fatalities. Dozens of aftershocks were recorded off the coast, the strongest measured 5.0 magnitude. The most significant “tsunami” recorded wasn’t measured at much more than 2 – 3 feet. San Franciscans have taken the event in stride. Some city officials consider the event to be good preparation for when it happens again. It seems, however, that more work will have to be done to dissuade the City’s surfers from catching that future wave.
Maura Corkery lives and works in West Portal.
December, 2024
