Earthquake · reviewed by a seismologist
M 1.7md
Earthquake near Portola Valley, CA
No earthquake was likely felt in Portola Valley in the 24 hours to 1:45 am PST (09:45 UTC). The last one was M 1.9, 6 km (4 mi) N, at 4:14 pm PDT on 11 October 2026, estimated intensity II (weak) in town; 1 felt report to USGS. Every earthquake near Portola Valley
This page: a magnitude 1.7 earthquake was recorded 5 km (3 mi) south of Portola Valley, CA, at 7:22 pm PST on 2 December 2021 (03:22 UTC), 5.5 km deep. 1 person has reported feeling it to USGS.
USGS: 4km S of Portola Valley, CA
10 km (6 mi) SW of Palo Alto, CA · 37 km (23 mi) W of San Jose, CA
- When
- 7:22 pm PST on 2 December 2021 ()
- Where
- 5 km (3 mi) south of Portola Valley, CA · earthquakes near Portola Valley
- Magnitude
- M 1.7 (md)
- Depth
- 5.5 km below the surface
- Felt
- 1 report · strongest reported intensity II
- Nearest
- San Mateo County, 15 km WNW
- Nearest mapped fault
- San Andreas fault, 3 km from its surface trace — near is not the same as on
- Status
- Reviewed by a seismologist
USGS · last changed by USGS · we first saw it 1798 days ago
Earthquake alerts for Portola Valley, CA
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Alerts are on for .
Not a warning. An alert comes after USGS publishes a quake, usually minutes after the shaking. For early warning use ShakeAlert, through your phone's Wireless Emergency Alerts or the MyShake app.
This was a minor earthquake. Quakes this small are rarely felt and do not cause damage.
At this magnitude and depth, people at rest would likely notice it out to about 12 km (7 mi) from the epicentre. In Portola Valley, 5 km away, the estimate is intensity II (weak). Estimated from magnitude, depth and distance with the intensity equation USGS Did You Feel It? uses; felt reports, where there are any, are the measurement.
Likely felt in: Los Altos 12 km, intensity II. Estimated for each town centre; each link is that town's own record.
How big was it?
This earthquake released about 12 lb of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 2.7 would have released about 32 times as much.
- Mineral, Virginia 2011 M 5.8 1,200,000× the energy of this one
- Magna, Utah 2020 M 5.7 870,000× the energy of this one
- Around the size people start to feel M 3.0 78× the energy of this one
- This earthquake M 1.7 about 12 lb of TNT
Energy from the Gutenberg–Richter relation (log10E = 1.5M + 4.8), rounded to two figures; bar length is magnitude, which is already a logarithm of energy. Magnitudes of past earthquakes from the USGS catalogue.
For this area
No earthquake of M4 or more has been recorded within 50 km since our catalogue begins in 2016.
Before 2016, the USGS record lists 16 earthquakes of M5 or more within 50 km; the five largest:
- 6.9 The 1989 Loma Prieta, California Earthquake · 46 km SE of this epicentre
- 6.3 16 km SW of Davenport, California · 48 km SSW of this epicentre
- 6.2 Near San Jose, California · 39 km E of this epicentre
- 6.2 The 1984 Morgan Hill, California Earthquake · 49 km E of this epicentre
- 6.1 Near San Jose, California · 42 km ESE of this epicentre
Since 2016: the QuakeIndex catalogue (US, M1.5+), built from the USGS FDSN event service. Before 2016: USGS ComCat, M5+ from 1900. Distances are between epicentres. This is the record, not a forecast.
Where
Epicentre 37.3433, -122.2333. Map © OpenStreetMap contributors.
Nearby places
| Place | Distance | Direction from the epicentre | |
|---|---|---|---|
| San Mateo County, CA | 15 km (9 mi) | WNW | earthquakes near San Mateo County |
| Santa Cruz County, CA | 42 km (26 mi) | SSE | earthquakes near Santa Cruz County |
| Alameda County, CA | 44 km (27 mi) | NE | earthquakes near Alameda County |
| Santa Clara County, CA | 50 km (31 mi) | ESE | earthquakes near Santa Clara County |
| Contra Costa County, CA | 69 km (43 mi) | NNE | earthquakes near Contra Costa County |
| San Francisco County, CA | 82 km (51 mi) | WNW | earthquakes near San Francisco County |
Places and coordinates from the US Census Bureau Gazetteer (2024). Distances are from the epicentre.
Aftershocks so far
53 earthquakes have been recorded within 50 km since this one.
- 2.2 34km W of Bonny Doon, CA
- 2.3 8km E of Alum Rock, CA
- 2.0 4km SSE of Highlands-Baywood Park, CA
- 1.9 2km N of Castro Valley, CA
- 1.8 11km NE of Milpitas, CA
- 2.5 8km NE of Alum Rock, CA
- 1.9 11km SW of Bonny Doon, CA
- 1.6 4km SW of San Ramon, CA
- 1.6 4km SW of San Ramon, CA
- 1.7 10km ENE of Milpitas, CA
- 1.6 5km E of Fremont, CA
- 1.8 3km SW of San Francisco Zoo, CA
- 2.3 5km NNE of Alum Rock, CA
- 2.0 2km E of Hayward, CA
- 1.6 2km E of Hayward, CA
Counted from the QuakeIndex database, which ingests the USGS feed every 30 seconds · as of . QuakeIndex does not forecast aftershocks. When USGS issues an operational aftershock forecast it appears on the USGS event page.
On USGS
- The official USGS event page — the source of record for every value above
- Did you feel it? Report it to USGS — reports go to USGS, not to us
- ShakeMap for this event, when USGS has produced one
QuakeIndex does not issue warnings. For earthquake early warning see ShakeAlert.