Moderate mag. 3.9 earthquake - 28 mi north of Ukiah, Mendocino County, California, USA, on 23 Apr 4:18 am (GMT -7) -

Moderate mag. 3.9 earthquake - 28 mi north of Ukiah, Mendocino County, California, USA, on 23 Apr 4:18 am (GMT -7) Moderate mag. 3.9 earthquake - 28 mi north of Ukiah, Mendocino County, California, USA, on 23 Apr 4:18 am (GMT -7)

Moderate magnitude 3.9 earthquake at 7 km depth

23 Apr 11:22 UTC: First to report: USGS after 4 minutes.
23 Apr 11:24: Magnitude recalculated from 3.7 to 4.0.
23 Apr 14:02: Magnitude recalculated from 4.0 to 3.9.
23 Apr 18:00: Hypocenter depth recalculated from 6.6 to 7.4 km (from 4.1 to 4.6 mi).
Update Fri, 23 Apr 2021, 11:27
Moderate magnitude 4.0 quake hits 28 miles northeast of Ukiah, California, United States early morning
4.0 quake 23 Apr 4:18 am (GMT -7)
4.0 quake 23 Apr 4:18 am (GMT -7)
The United States Geological Survey reported a magnitude 4.0 quake in the United States near Ukiah, Mendocino County, California, only 8 minutes ago. The earthquake hit early morning on Friday 23 April 2021 at 4:18 am local time at a very shallow depth of 4.1 miles. The exact magnitude, epicenter, and depth of the quake might be revised within the next few hours or minutes as seismologists review data and refine their calculations, or as other agencies issue their report.
Our monitoring service identified a second report from the European-Mediterranean Seismological Centre (EMSC) which listed the quake at magnitude 4.0 as well.
Based on the preliminary seismic data, the quake should not have caused any significant damage, but was probably felt by many people as light vibration in the area of the epicenter.
Weak shaking might have been felt in Potter Valley (pop. 650) located 15 miles from the epicenter, Willits (pop. 4,900) 19 miles away, Brooktrails (pop. 3,200) 19 miles away, and Redwood Valley (pop. 1,700) 20 miles away.
Other towns or cities near the epicenter where the quake might have been felt as very weak shaking include Covelo (pop. 1,300) located 22 miles from the epicenter, Ukiah (pop. 15,900) 28 miles away, Nice (pop. 2,700) 30 miles away, and Clearlake (pop. 15,200) 45 miles away.
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Earthquake data

Earthquake details

Date & timeApr 23, 2021 11:18:40 UTC -
Local time at epicenter23 Apr 4:18 am (GMT -7)
StatusConfirmed
Magnitude3.9
Depth7.4 km
Epicenter latitude / longitude39.52567°N / 123.0365°W (Mendocino, California, United States)
Antipode39.526°S / 56.964°E
Shaking intensityWeak shaking
Felt6 reports
Primary data sourceUSGS (United States Geological Survey)
Nearest volcanoClear Lake (66 km / 41 mi)
Nearby towns and cities24 km (15 mi) NNE of Potter Valley (pop: 646) | Show on map | Quakes nearby
30 km (19 mi) ENE of Little Lake (pop: 4,860) | Show on map | Quakes nearby
32 km (20 mi) NNE of Redwood Valley (pop: 1,730) | Show on map | Quakes nearby
35 km (22 mi) SSE of Covelo (pop: 1,260) | Show on map | Quakes nearby
42 km (26 mi) ESE of Laytonville (pop: 1,230) | Show on map | Quakes nearby
44 km (28 mi) NNE of Ukiah (pop: 15,900) | Show on map | Quakes nearby
50 km (31 mi) NNW of North Lakeport (pop: 3,310) | Show on map | Quakes nearby
67 km (41 mi) E of Fort Bragg (Mendocino County) (pop: 7,290) | Show on map | Quakes nearby
72 km (45 mi) NNW of Clearlake (Lake County) (pop: 15,200) | Show on map | Quakes nearby
170 km (105 mi) NW of Sacramento (pop: 490,700) | Show on map | Quakes nearby
Weather at epicenter at time of quakeBroken Clouds 5.9°C (43 F), humidity: 81%, wind: 2 m/s (3 kts) from NE
Estimated seismic energy released4.5 x 1010 joules (12.4 megawatt hours, equivalent to 10.7 tons of TNT) | about seismic energy
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Data for the same earthquake reported by different agencies

Info: The more agencies report about the same quake and post similar data, the more confidence you can have in the data. It takes normally up to a few hours until earthquake parameters are calculated with near-optimum precision.
Mag.DepthLocationSource
3.97.4 km15km NNW of Lake Pillsbury, CA, USAUSGS
3.45 kmNORTHERN CALIFORNIA, USAEMSC

User reports for this quake (6)

User reports estimate the perceived ground shaking intensity according to the MMI (Modified Mercalli Intensity) scale.
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San Jose, California 95148 / Light shaking (MMI IV) / single lateral shake / 2-5 s : Woke me up from sleep, a bag of items on top of another bag of items fell over. Then the wall behind me seemed to be jolted once northeasterly, creaked, rumbled and stopped. From jolt to stop was 2 seconds tops.  
Nas Lemoore (455.8 km SE of epicenter) [Map] / Very weak shaking (MMI II) / horizontal (sideways) swinging / 5-10 s
5726 Ridgewood Road Willits, CA (27.6 km SW of epicenter) [Map] / Light shaking (MMI IV) / horizontal (sideways) swinging / 2-5 s : Woke up to house shaking.  
Orange, CA / Light shaking (MMI IV) / single lateral shake / 5-10 s
San Jose / Very weak shaking (MMI II) / single vertical bump / 1-2 s
93041 / Light shaking (MMI IV) / rattling, vibrating / 2-5 s
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