M3.7 Earthquake 14km WNW of Pinnacles, CA — October 4, 2021
2021-10-04 23:07:48 UTC (2021-10-04) · approx. 3:07 PM UTC-8 local
Felt by 531 people across surrounding communities. Maximum shaking intensity MMI 4.1 (moderate).
On October 4, 2021 at 23:07 UTC, a magnitude 3.7 shallow crustal earthquake struck 14km WNW of Pinnacles, CA, at a depth of 6.4 km and coordinates 36.5892°, -121.2887°. The earthquake was reported felt by 531 peopleacross surrounding communities, with a maximum shaking intensity of Modified Mercalli Intensity (MMI) 4.1 (moderate). This earthquake was detected by 82 seismic stations with excellent location accuracy and was assigned a USGS significance rating of 424, placing it among routine seismic activity.
Physical scale: An earthquake of magnitude 3.7 releases seismic energy equivalent to roughly 4.7 metric tons of TNT. Empirical fault-scaling laws (Wells & Coppersmith, 1994) estimate the subsurface rupture length at approximately 202 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
What the Shaking Felt Like
At MMI 4.1 (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 531 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
Earthquake Details
-121.2887°E
Technical Information
Nearby Earthquakes (Last 7 Days)
Common Questions
How strong was the magnitude 3.7 earthquake near Pinnacles, CA?
The magnitude 3.7 earthquake that struck 14km WNW of Pinnacles, CA on October 4, 2021 at 23:07 UTC had a depth of 6.4 km. It was felt by 531 people with maximum shaking intensity of MMI 4.1 (moderate). For context, this was a minor earthquake typically detected only by instruments.
Where did the earthquake occur?
The earthquake epicenter was located at 36.5892°, -121.2887°, which is 14km WNW of Pinnacles, CA. View all earthquakes in California.
Were there aftershocks?
Aftershock activity is monitored continuously by the USGS. Aftershock sequences from magnitude 3.7 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 3.7 event is significantly smaller than that historical record.
Actions
Authoritative source: USGS event page (event ID: nc73633396). All scientific values on this page are sourced verbatim from the USGS feed.
Cite this page: EarthquakeTracker.org. (2021, October 4, 2021). M3.7 Earthquake 14km WNW of Pinnacles, CA — October 4, 2021. Retrieved December 10, 2021 from https://www.earthquaketracker.org/earthquakes/event/nc73633396/