Earthquake · reviewed by a seismologist
M 2.4md
Earthquake near Portola Valley, CA
No earthquake was likely felt in Portola Valley in the 24 hours to 11:53 pm PST (07:53 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 2.4 earthquake was recorded 5 km (3 mi) southeast of Portola Valley, CA, at 9:07 am PDT on 8 August 2021 (16:07 UTC), 10 km deep. 12 people have reported feeling it to USGS.
USGS: 5km SE of Portola Valley, CA
7 km (5 mi) SW of Palo Alto, CA · 35 km (22 mi) W of San Jose, CA
- When
- 9:07 am PDT on 8 August 2021 ()
- Where
- 5 km (3 mi) southeast of Portola Valley, CA · earthquakes near Portola Valley
- Magnitude
- M 2.4 (md)
- Depth
- 10 km below the surface
- Felt
- 12 reports · strongest reported intensity III
- Nearest
- San Mateo County, 16 km WNW
- Nearest mapped fault
- San Andreas fault, under 1 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 1914 days ago
Earthquake alerts for Portola Valley, CA
Get a notification in this browser when USGS records an earthquake near Portola Valley, CA. No account or email needed.
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 18 km (11 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 9 km, intensity II · Mountain View 11 km, intensity II · Palo Alto 11 km, intensity II · Menlo Park 12 km, intensity II · East Palo Alto 14 km, intensity II · Sunnyvale 15 km, intensity II · Redwood City 15 km, intensity II · Cupertino 15 km, intensity II. Estimated for each town centre; each link is that town's own record.
How big was it?
This earthquake released about 110 lb of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 3.4 would have released about 32 times as much.
- Mineral, Virginia 2011 M 5.8 130,000× the energy of this one
- Magna, Utah 2020 M 5.7 95,000× the energy of this one
- Around the size people start to feel M 3.0 8.5× the energy of this one
- This earthquake M 2.4 about 110 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
The largest earthquake within 50 km since 2016 was M 4.1 on 2017-10-10 — about 370 times the energy of this one.
Before 2016, the USGS record lists 17 earthquakes of M5 or more within 50 km; the five largest:
- 6.9 The 1989 Loma Prieta, California Earthquake · 45 km SE of this epicentre
- 6.3 16 km SW of Davenport, California · 49 km SSW of this epicentre
- 6.2 Near San Jose, California · 36 km E of this epicentre
- 6.2 The 1984 Morgan Hill, California Earthquake · 47 km E of this epicentre
- 6.1 Near San Jose, California · 39 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 37.3510, -122.2030. Map © OpenStreetMap contributors.
Nearby places
| Place | Distance | Direction from the epicentre | |
|---|---|---|---|
| San Mateo County, CA | 16 km (10 mi) | WNW | earthquakes near San Mateo County |
| Santa Cruz County, CA | 41 km (26 mi) | SSE | earthquakes near Santa Cruz County |
| Alameda County, CA | 42 km (26 mi) | NE | earthquakes near Alameda County |
| Santa Clara County, CA | 48 km (30 mi) | ESE | earthquakes near Santa Clara County |
| Contra Costa County, CA | 67 km (42 mi) | NNE | earthquakes near Contra Costa County |
| San Francisco County, CA | 84 km (52 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.
- 2.0 5km E of Fremont, CA
- 1.6 6km N of San Leandro, CA
- 1.5 2km SW of San Francisco Zoo, CA
- 2.6 8km ENE of Alum Rock, CA
- 1.6 4km ENE of Burlingame, CA
- 1.6 10km NE of Milpitas, CA
- 1.6 9km SE of Sunol, CA
- 2.8 16km SSE of Livermore, CA
- 1.6 3km W of Fruitdale, CA
- 1.5 11km WSW of Saratoga, CA
- 1.6 11km WSW of Saratoga, CA
- 1.7 11km WSW of Saratoga, CA
- 2.5 16km SSW of Bonny Doon, CA
- 1.6 12km W of Saratoga, CA
- 1.6 4km SSW of San Francisco Zoo, 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.