Earthquake · reviewed by a seismologist
M 1.8md
Earthquake near Portola Valley, CA
No earthquake was likely felt in Portola Valley in the 24 hours to 4:44 am PST (12:44 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 1.8 earthquake was recorded 7 km (5 mi) south-southeast of Portola Valley, CA, at 1:00 am PDT on 23 October 2018 (08:00 UTC), 9.4 km deep. 1 person has reported feeling it to USGS.
USGS: 7km SSE of Portola Valley, CA
9 km (5 mi) SSW of Palo Alto, CA · 34 km (21 mi) W of San Jose, CA
- When
- 1:00 am PDT on 23 October 2018 ()
- Where
- 7 km (5 mi) south-southeast of Portola Valley, CA · earthquakes near Portola Valley
- Magnitude
- M 1.8 (md)
- Depth
- 9.4 km below the surface
- Felt
- 1 report · strongest reported intensity II
- Nearest
- San Mateo County, 19 km WNW
- Nearest mapped fault
- San Andreas fault, 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 2942 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 10 km (6 mi) from the epicentre. In Portola Valley, 7 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. Estimated for each town centre; each link is that town's own record.
How big was it?
This earthquake released about 15 lb of TNT. Magnitude is logarithmic: each whole step up releases about 32 times more energy, so a magnitude 2.8 would have released about 32 times as much.
- Mineral, Virginia 2011 M 5.8 1,000,000× the energy of this one
- Magna, Utah 2020 M 5.7 730,000× the energy of this one
- Around the size people start to feel M 3.0 65× the energy of this one
- This earthquake M 1.8 about 15 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 2,800 times the energy of this one.
Before 2016, the USGS record lists 18 earthquakes of M5 or more within 50 km; the five largest:
- 6.9 The 1989 Loma Prieta, California Earthquake · 43 km SE of this epicentre
- 6.3 16 km SW of Davenport, California · 47 km SSW of this epicentre
- 6.2 Near San Jose, California · 35 km E of this epicentre
- 6.2 The 1984 Morgan Hill, California Earthquake · 45 km E of this epicentre
- 6.1 Near San Jose, California · 37 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 37.3278, -122.1920. Map © OpenStreetMap contributors.
Nearby places
| Place | Distance | Direction from the epicentre | |
|---|---|---|---|
| San Mateo County, CA | 19 km (12 mi) | WNW | earthquakes near San Mateo County |
| Santa Cruz County, CA | 39 km (24 mi) | SSE | earthquakes near Santa Cruz County |
| Alameda County, CA | 43 km (27 mi) | NE | earthquakes near Alameda County |
| Santa Clara County, CA | 46 km (29 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 | 86 km (54 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
55 earthquakes have been recorded within 50 km since this one.
- 2.2 10km ENE of Milpitas, CA
- 2.6 9km ENE of Milpitas, CA
- 1.9 4km WSW of Daly City, CA
- 2.0 15km E of Seven Trees, CA
- 1.8 9km NE of Milpitas, CA
- 2.0 4km WSW of Daly City, CA
- 2.8 4km WSW of Daly City, CA
- 1.5 3km S of Loyola, CA
- 2.2 10km ESE of Alum Rock, CA
- 1.6 4km S of Loyola, CA
- 1.9 2km NNE of Union City, CA
- 1.6 6km SSE of Sunol, CA
- 2.3 4km SSW of Los Altos Hills, CA
- 1.7 3km S of Loyola, CA
- 2.0 3km NNE of Fairview, 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.