Two magnitude 5 earthquakes occurred in Northern California this month. On May 11, an M5.5 struck Plumas County near the south shore of Lake Almoner, and this past Sunday, an M5.5 struck off the coast of Cape Mendocino in Humboldt County. They occurred on either side of the complex geological framework of Northern California and were not related.
The Lake Almanor earthquake was centered nine miles southeast of Chester. More than 6,700 hair reports have been submitted to the USGS Did You Feel It site—as far south as Visalia and north to the Oregon border. The earthquake caused objects to fall off shelves and trigger rockslides, but no major injuries or damage were reported.
The Lake Almoner earthquake was caused by a rupture in a natural fault oriented north-south. Natural faults are extended, caused by stretching, the dominant pattern of faulting in Basin County and Range tectonics. The May 11 earthquake was near the western edge of the basin and range and was unrelated to the Mount Lassen volcano.
This past Sunday, May 21, a M5.6 earthquake occurred on the Mendocino Fault, 60 miles west of Cape Mendocino. It didn’t get as many felt reports as the Lake Almanor earthquake because of its offshore location, but 602 people did report on Did You Feel It, from Bodega Bay to Brookings, Oregon and inland to Chico. There were no reports of damage.
The Mendocino fault is one of the most active faults in the state of California. Thirty-nine M5.0 and larger earthquakes have been recorded on the fault since 1970, the largest being a 7.1 in 1994. The fault marks the transform plate boundary between the Gorda and Pacific plates and the magnitude 5.6-like earthquakes are showing a recent east-west strike-slip motion. Slow propagation along the Gorda Range to the west and pull of the Cascadia Subduction Zone to the east results in steady shear along the fault.
The Lake Almanor and Mendocino fault earthquakes were more than 230 miles apart and caused by different tectonic processes but one thing in common. They both ran the ShakeAlert early warning system. Relying on a dense network of seismic stations, ShakeAlert detects an earthquake in the first seconds after a rip begins. Automated algorithms calculate the epicenter, magnitude, and areas where the shaking will be felt. If the volume and strength of the vibration exceed a preset threshold, audio and text alerts will be triggered within the next few seconds, hopefully before the strongest shaking occurs.
Discovery and analysis are only the first two steps in the process. What’s more difficult is getting alerts for you. Alerts are delivered to mobile phones in three ways. They all use the same information from the West Coast ShakeAlert earthquake early warning system seismic monitors. In California, ShakeAlert is overseen by the USGS and the California Governor’s Office of Emergency Services. The message is short – an audible announcement “Earthquake, quake – drop cover and wait.” Your screen says “Shake expected earthquake vibration detected” with cover drop graphic.
Wireless Emergency Alerts (WEAs) are a national system overseen by FEMA. WEAs include Amber Alerts to locate kidnapped children; messages of the imminent threat of severe disasters such as wildfires, earthquakes, tsunamis, and severe weather; Presidential alerts. WEAs are based on pre-arranged agreements between government agencies and private cell phone companies.
No need to register to receive WEAs. When an earthquake of magnitude 5 or greater occurs and is likely to be widely felt, a WEA is issued. WEAs do not know your location; Cell towers in the area broadcast the message to all phones within a specified geographic area. If you do not wish to receive WEA alerts, you can opt out by deactivating them in your phone settings.
You can also download the free MyShake app. In addition to delivering a ShakeAlert-powered message, MyShake uses your phone’s motion sensors to record ground movement to add to the sensed database. You can also share information about your experience with vibration. MyShake uses a threshold size of 4.5 for issuing alerts. You can define a home base in MyShake that will send messages through the cell towers in the area you are most likely to feel and to your phone no matter where you are. Remember to activate location services to receive alerts.
If you have an Android mobile phone, you probably already have ShakeAlert and you don’t need to download anything. ShakeAlert on Android works in a similar way to MyShake. If you do not want the messages, you can opt out by deactivating alert delivery in your settings.
Earthquake alerts are still new in California and each event is an opportunity to improve the system. Sometimes people far away from the area receive an unnecessary alert while others in the vibration area do not receive messages. If you are near the epicenter, you will feel the shaking before the alert arrives. In rural areas, cell phone coverage can delay/complicate reception. We don’t have instruments on the sea floor, so the vibrations of marine earthquakes reach the coast at the same time as the alerts.
The most challenging part of ShakeAlert is what you do with the information. You only have a moment to react, and human brains are not very good at responding to new situations. The most important thing is not to move. If you are physically able, get off the ground. If there is a desk or table nearby – get under it. in the bed? stay here.
I got the alert last Sunday while I was working in our garden. Lie on the floor to increase the odds that I will feel the weak vibrations. It was an open area, and there was nothing close to fall on me. In this case, I didn’t feel a thing.
If I had been inside I would have sat still and covered the back of my neck with my arm. My knee is not good enough to get under the table. If I was driving, I would slow down and find a safe place to park to the side of the road. If I don’t feel anything after a minute, I’ll be back on my way. One exception – if I was in a tsunami zone, I would keep driving carefully until I got out.
At present, no products supported by ShakeAlert include Tsunami. The National Oceanic and Atmospheric Administration (NOAA) National Weather Service is responsible for tsunami alerts, not the USGS. It takes several minutes to assess the likelihood of a tsunami after an earthquake, which is much longer than the few-second window of time for the ShakeAlert app. If a tsunami warning is issued for our coast, it will alert the WEA just as ShakeAlert does for earthquakes. Always remember that the shaking itself may be the first warning of a tsunami coming from nearby.
Do not confuse earthquake alerts with county emergency notifications. All California counties have systems in place to alert you to dangerous situations. These notifications are issued after deliberation by the administrators and are sent to persons who have previously signed up to receive notifications via text, phone, and/or email. If you do not register, check with your county OES office and find out how to do so.
Learn more about ShakeAlert and the earthquake shake notification at https://www.usgs.gov/faqs/how-do-i-sign-shakealertr-earthquake-early-warning-system.
Laurie Dengler is Professor Emeritus of Geology at Cal Poly Humboldt, and an expert on tsunami and earthquake risk. Questions or comments about this column, or want a free copy of Living on Shaky Ground readiness magazine? Leave a message at 707-826-6019 or send an email to [email protected].
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