A magnitude 7 earthquake off the Humboldt County coast Thursday morning triggered a West Coast tsunami warning that put Bay Area residents in evacuation mode until it was canceled about an hour later.
The warning impacted the entire Bay Area, including Richmond, Berkeley and Oakland, said Brayden Murdock, a National Weather Service meteorologist. It was predicted that waves could reach San Francisco “around noon.”
The warning was lifted after more details about the type of earthquake showed it was unlikely to cause large amounts of water to move.
Amy Williamson, a research scientist at the UC Berkeley Seismology Lab who previously worked for the National Tsunami Warning Center, said the earthquake “likely” occurred on a strike-slip fault, in which two tectonic plates slide past each other. (The San Andreas Fault is also a strike-slip fault. The Hayward Fault, which runs through Richmond and other East Bay cities, is a right-lateral strike-slip fault, which means it moves horizontally.)
Compared to a subduction-zone earthquake, in which two tectonic plates collide and one is pushed under the other, earthquakes on strike-slip faults are less likely to cause a tsunami.
But depth can play a role as well. Earthquakes that occur in shallow water are more likely to cause a tsunami; Thursday’s quake was in “fairly shallow” water, within the top mile, Williamson said.
The decision to issue a tsunami warning for the Northern California and Southern Oregon coastline was in line with protocol, according to Williamson.
A tsunami warning is immediately issued for the surrounding coastline following any earthquake with a magnitude of 7.0 or higher.
What to do before, during and after a tsunami
Here are some resources compiled by the state to help residents prepare for a tsunami.
- Contra Costa County Emergencies Services website
- Ready.gov: Tsunamis summarizes how to prepare now, survive during and be safe after a tsunami.
- TsunamiZone.org offers suggestions and resources for your family or organization to “know your zone” and to learn how to be safe.
- The Earthquake Country Alliance develops resources and organizes activities to improve earthquake and tsunami preparedness, mitigation and resiliency. The ECA offers a host of free booklets and other materials in multiple languages.
When that happens, the National Tsunami Warning Center leaps into action, pulling in data from its offshore gauges and sensors and running forecast models to determine things such as what time hazardous waves are anticipated to reach shore. They need to act fast, given that tsunamis travel at roughly the speed of a passenger jet and can push house-high walls of water into coastal areas.
A phone alert is sent out and information is then passed along to weather service groups and the state emergency management offices responsible for notifying local authorities.
“If there is a tsunami, it may reach coastlines within minutes or tens of minutes, so they want to provide a warning fast,” Williamson said.
Once forecasters determined the quake would not create a tsunami, the warning was canceled.
When a tsunami warning occurs, authorities warn the public to move away from the coast as waves generated by the quake could wash over low-lying areas. It’s recommended that residents evacuate at least a mile away from the shoreline and get to higher ground, at least 100 feet above sea level.
It’s unclear how many people in Richmond, home to 32 miles of shoreline, began evacuating. One business on Macdonald Avenue, CoBiz, where Richmondside has an office, shut down while the warning was in effect, saying in an email blast “safe than sorry.”
The National Tsunami Warning Center says: “If you are in a tsunami warning area: Evacuate inland or to higher ground above and beyond designated tsunami hazard zones or move to an upper floor of a multi-story building, depending on your situation.”
The state Department of Conservation tsunami hazard map shows south Richmond and part of the Iron Triangle neighborhood along Interstate 580 to be particularly at-risk but points out that tsunamis are rare events and the map is representative of “several extreme, infrequent, and realistic tsunami sources.”
BART service was stopped in the Transbay Tube due to the earthquake.
Richmond Port Director Charles Gerard said that as of 11:36 a.m. port operations had not been affected.
“We are monitoring the situation and are keeping an eye on Tsunami.com,” Gerard said. “That’s where we are at the moment.”
The San Francisco Bay Ferry public information manager Alexis Matsui said they monitored the tsunami warning and that staff and crews were standing by if an emergency had occurred. (Passengers are encouraged to sign up for real-time alerts here.)
There are several types of tsunami alerts, according to the National Weather Service, including a watch, an advisory and a warning. A predicted water rise above 3 feet prompts a warning, which is the highest level of risk.
In tsunami terms, anything under 1 foot usually isn’t dangerous. But a higher rise raises eyebrows — and attention — from emergency responders, especially for potential impacts on people or structures in the water or in low-lying areas along the shore. In tsunami science lingo, it’s called the inundation zone.
Generally speaking, tsunamis pose more danger to the Pacific coast than to the bayshore. With the position of San Francisco and Marin counties, destructive waves only infrequently strike the East Bay. But if they do reach the shore, they can cause hundreds of thousands of dollars’ worth of damage within moments.
The 8.9-magnitude earthquake that struck Japan in 2011 touched off a tsunami surge that hit the Berkeley Marina, thrashing boats and docks.
Reporter Joel Umanzor and Richmondside Editor Kari Hulac contributed to this report.
