This blog post breaks down the Monday, April 20 sequence of Northern California earthquakes. It highlights the strong 4.7-magnitude main shock off the Petrolia coast, the aftershocks, the quick ShakeAlert warnings, and what residents from Marin County to Eureka should know about safety and preparedness.
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What happened on Monday, April 20
The magnitude 4.7 mainshock hit about 52 kilometers west of Petrolia at around 3:41 a.m. The quake originated at a depth of 13.6 kilometers.
ShakeAlert sent out warnings within seconds, first estimating magnitudes between 3.6 and 3.8 as new data came in. Nine people filed felt reports, and two smaller quakes followed—a 3.2 southeast of Johannesburg and a 2.5 south-southwest of Maricopa. Both were shallow and produced only light vibrations.
Shaking, aftershocks, and affected communities
The U.S. Geological Survey gave the mainshock a Level III intensity, which means light shaking. Five aftershocks followed, ranging from magnitude 3.6 to 2.3. Authorities warned that more aftershocks could still pop up near the main event.
People across the region reported feeling the shaking, especially in towns to the north and inland. So far, official tallies show no reported damage, which is honestly a relief.
In this northern California swarm, folks in Ferndale, Rio Dell, Fortuna, Hydesville, Pine Hills, Humboldt Hill, Eureka, Arcata, Ukiah, and McKinleyville all felt the tremors. Marin County readers probably didn’t see much impact, but the sequence shows how even offshore quakes can ripple across the state and spark aftershocks days later.
- There’s a 16% chance of two or more aftershocks of magnitude 3 or higher in the next week.
- The probability of a magnitude 4+ aftershock sits at about 2%.
- Chances of a damaging magnitude 5+ aftershock are under 1%. No one expects a 6.0–7.0 event in this sequence.
Officials say this is a typical pattern for offshore-to-land quakes along the Pacific Coast. They urge the public to stay ready, not alarmed.
For Marin County—San Rafael, Mill Valley, Novato, Larkspur, Sausalito, Tiburon, and Corte Madera included—the message remains: tremors can travel, but damage is unlikely if you’re prepared and a bit cautious.
Forecasts and safety reminders
Forecasts point to a continued, though uncertain, chance of more aftershocks in the coming days. Local emergency planners in Marin and nearby counties will keep a close eye on instrumentation and reports. They’ll coordinate with state and federal agencies to update residents as new information comes in.
Honestly, it’s pretty simple: stay informed, have a plan, and practice basic earthquake safety—even if the tremors feel mild in your neighborhood.
Marin County readiness: how to stay prepared
For families and businesses in Marin—whether you call San Rafael home, you’re in the heart of San Anselmo, or you operate near Novato’s shops and schools—it’s probably a good time to refresh emergency plans.
Here are some practical steps to keep you ready during the current aftershock window and, honestly, for whatever else comes next:
- Keep an emergency kit in your home and car. Pack water, nonperishable food, flashlights, a first-aid kit, and something like a whistle or battery-powered radio.
- Secure heavy furniture and appliances. Fasten shelves or cabinets to walls so they don’t shift during aftershocks.
- Know where to drop, cover, and hold on in each room. Practicing really does make it second nature when a quake hits.
- Find safe spots in Marin homes and workplaces—under sturdy desks or tables, away from windows and glass. Rehearse evacuation routes if you’re outdoors or near tall structures.
- After a quake, check for hazards. Inspect utilities if you smell gas or sense a leak, and try to use your phone only for essential communications so first responders have a clear network.
Whether you live in Fairfax, Ross, Point Reyes Station, or down in Tiburon, staying calm and prepared can help the community recover faster when the next aftershock arrives. It might feel like overkill, but a little readiness really goes a long way.
Here is the source article for this story: Three Earthquakes Rattle California Within Hours, Largest Measuring Magnitude 4.7
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