M5.1 Earthquake 12km W of Petrolia, CA — July 18, 2021
2021-07-18 06:46:12 UTC (2021-07-18) · approx. 10:46 PM UTC-8 local
Felt by 751 people across surrounding communities. Maximum shaking intensity MMI 4.3 (moderate).
On July 18, 2021 at 06:46 UTC, a magnitude 5.1 shallow crustal earthquake struck 12km W of Petrolia, CA, at a depth of 30.9 km and coordinates 40.3235°, -124.4255°. The earthquake was reported felt by 751 peopleacross surrounding communities, with a maximum shaking intensity of Modified Mercalli Intensity (MMI) 4.3 (moderate). The USGS PAGER system issued a green alert level for this event, indicating no significant casualties or damage expected. This earthquake was detected by 35 seismic stations with moderate location accuracy and was assigned a USGS significance rating of 719, making it a moderate-impact event. The nearest populated place is Redway (population 1,225).
Physical scale: An earthquake of magnitude 5.1 releases seismic energy equivalent to roughly 722 metric tons of TNT. Empirical fault-scaling laws (Wells & Coppersmith, 1994) estimate the subsurface rupture length at approximately 2.1 km — a useful intuition for the size of the slip patch on the fault.
The epicenter is located in California, a region characterized by the San Andreas transform fault system, where the Pacific and North American plates slide past each other at roughly 46 mm per year. The nearest mapped fault system is the San Andreas Fault. View all earthquakes in California.
Learn more: San Andreas Fault · Magnitude scale · Aftershock sequences
Nearest Populated Places
What the Shaking Felt Like
At MMI 4.3 (moderate), people in the most strongly affected areas would have experienced: Mild shaking, similar to a passing truck. Hanging objects swing. Glasses and dishes may rattle. Not commonly felt outdoors.
ShakeMap — predicted shaking intensity
Modeled ground-motion intensity contoured on the Modified Mercalli scale (MMI). Computed by the USGS from the moment-tensor solution and regional ground-motion prediction equations.
Source: U.S. Geological Survey ShakeMap. View interactive ShakeMap on USGS
Did You Feel It? — community-reported shaking
Aggregated felt-report intensity from 751 citizen responses to the USGS Did You Feel It? system. Each colored zone represents the average MMI from reports in that area.
Source: U.S. Geological Survey Did You Feel It?. View on USGS · submit your own report
Population exposure (USGS PAGER)
Estimated population that experienced each level of shaking. Based on the USGS PAGER rapid impact assessment, which combines ShakeMap output with global population grids.
| Shaking intensity (MMI) | Description | Population exposed |
|---|---|---|
| MMI 3 | weak | 96,223 |
| MMI 4 | light | 6,901 |
Earthquake Details
-124.4255°E
Technical Information
Nearby Earthquakes (Last 7 Days)
Common Questions
How strong was the magnitude 5.1 earthquake near Petrolia, CA?
The magnitude 5.1 earthquake that struck 12km W of Petrolia, CA on July 18, 2021 at 06:46 UTC had a depth of 30.9 km. It was felt by 751 people with maximum shaking intensity of MMI 4.3 (moderate). For context, this was a moderate earthquake that could be widely felt.
Where did the earthquake occur?
The earthquake epicenter was located at 40.3235°, -124.4255°, which is 12km W of Petrolia, CA. The nearest populated place is Redway (population 1,225). View all earthquakes in California.
Were there aftershocks?
Aftershock activity is monitored continuously by the USGS. Aftershock sequences from magnitude 5.1 earthquakes can continue for days to weeks and gradually diminish over time.
Has California had earthquakes this big before?
The largest recorded earthquake in California was the M7.9 Fort Tejon earthquake of January 9, 1857. Today's magnitude 5.1 event is significantly smaller than that historical record.
Actions
Authoritative source: USGS event page (event ID: nc73594531). All scientific values on this page are sourced verbatim from the USGS feed.
Cite this page: EarthquakeTracker.org. (2021, July 18, 2021). M5.1 Earthquake 12km W of Petrolia, CA — July 18, 2021. Retrieved December 22, 2025 from https://www.earthquaketracker.org/earthquakes/event/nc73594531/