M4.3 Earthquake 3km NE of Westmorland, CA — September 30, 2020
2020-09-30 23:31:51 UTC (2020-09-30) · approx. 3:31 PM UTC-8 local
Felt by 74 people across nearby locations. Maximum shaking intensity MMI 6.1 (very strong).
On September 30, 2020 at 23:31 UTC, a magnitude 4.3 shallow crustal earthquake struck 3km NE of Westmorland, CA, at a depth of 4.8 km and coordinates 33.0530°, -115.5955°. The earthquake was reported felt by 74 peopleacross nearby locations, with a maximum shaking intensity of Modified Mercalli Intensity (MMI) 6.1 (very strong). The USGS PAGER system issued a green alert level for this event, indicating no significant casualties or damage expected. This earthquake was detected by 97 seismic stations with excellent location accuracy and was assigned a USGS significance rating of 328, placing it among routine seismic activity. The nearest populated place is Westmorland (population 2,225).
Physical scale: An earthquake of magnitude 4.3 releases seismic energy equivalent to roughly 51 metric tons of TNT. Empirical fault-scaling laws (Wells & Coppersmith, 1994) estimate the subsurface rupture length at approximately 605 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
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 74 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 | 292,188 |
| MMI 4 | light | 41,312 |
| MMI 5 | moderate | 2,059 |
Earthquake Details
-115.5955°E
Technical Information
Nearby Earthquakes (Last 7 Days)
Common Questions
How strong was the magnitude 4.3 earthquake near Westmorland, CA?
The magnitude 4.3 earthquake that struck 3km NE of Westmorland, CA on September 30, 2020 at 23:31 UTC had a depth of 4.8 km. It was felt by 74 people with maximum shaking intensity of MMI 6.1 (very strong). For context, this was a light earthquake commonly felt near the epicenter.
Where did the earthquake occur?
The earthquake epicenter was located at 33.0530°, -115.5955°, which is 3km NE of Westmorland, CA. The nearest populated place is Westmorland (population 2,225). View all earthquakes in California.
Were there aftershocks?
Aftershock activity is monitored continuously by the USGS. Aftershock sequences from magnitude 4.3 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 4.3 event is significantly smaller than that historical record.
Actions
Authoritative source: USGS event page (event ID: ci39641064). All scientific values on this page are sourced verbatim from the USGS feed.
Cite this page: EarthquakeTracker.org. (2020, September 30, 2020). M4.3 Earthquake 3km NE of Westmorland, CA — September 30, 2020. Retrieved February 8, 2021 from https://www.earthquaketracker.org/earthquakes/event/ci39641064/