M4.7 Earthquake 18km NW of San Simeon, CA — October 25, 2021
2021-10-25 23:42:35 UTC (2021-10-25) · approx. 3:42 PM UTC-8 local
Felt by 619 people across surrounding communities. Maximum shaking intensity MMI 4.7 (moderate).
On October 25, 2021 at 23:42 UTC, a magnitude 4.7 shallow crustal earthquake struck 18km NW of San Simeon, CA, at a depth of 7.8 km and coordinates 35.7758°, -121.3043°. The earthquake was reported felt by 619 peopleacross surrounding communities, with a maximum shaking intensity of Modified Mercalli Intensity (MMI) 4.7 (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 67 seismic stations with good location accuracy and was assigned a USGS significance rating of 568, placing it among routine seismic activity. The nearest populated place is King City (population 12,874).
Physical scale: An earthquake of magnitude 4.7 releases seismic energy equivalent to roughly 142 metric tons of TNT. Empirical fault-scaling laws (Wells & Coppersmith, 1994) estimate the subsurface rupture length at approximately 974 m — 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 4.7 (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 619 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 | 550,094 |
| MMI 4 | light | 1,546 |
| MMI 5 | moderate | 1 |
Earthquake Details
-121.3043°E
Technical Information
Nearby Earthquakes (Last 7 Days)
Common Questions
How strong was the magnitude 4.7 earthquake near San Simeon, CA?
The magnitude 4.7 earthquake that struck 18km NW of San Simeon, CA on October 25, 2021 at 23:42 UTC had a depth of 7.8 km. It was felt by 619 people with maximum shaking intensity of MMI 4.7 (moderate). For context, this was a light earthquake commonly felt near the epicenter.
Where did the earthquake occur?
The earthquake epicenter was located at 35.7758°, -121.3043°, which is 18km NW of San Simeon, CA. The nearest populated place is King City (population 12,874). View all earthquakes in California.
Were there aftershocks?
Aftershock activity is monitored continuously by the USGS. Aftershock sequences from magnitude 4.7 earthquakes can continue for several days 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 4.7 event is significantly smaller than that historical record.
Actions
Authoritative source: USGS event page (event ID: nc73644630). All scientific values on this page are sourced verbatim from the USGS feed.
Cite this page: EarthquakeTracker.org. (2021, October 25, 2021). M4.7 Earthquake 18km NW of San Simeon, CA — October 25, 2021. Retrieved January 8, 2022 from https://www.earthquaketracker.org/earthquakes/event/nc73644630/