Intensity Lab
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II

Max Est. MMI

23 km W of Petrolia, CA

Magnitude3.6
Depth9.1 km
Time 2026-05-01T01:58:41.550000Z UTC: 2026-05-01T01:58:41.550000Z
USGS ID nc75354667
ShakeMap View USGS ↗

Ground Shaking Intensities — Interpolated ShakeMap MMI

3 sites
Location MMI
Ferndale
CA 		II
Rio Dell
CA 		I
Fortuna
CA 		II

Seismic Station Observed MMI

MMI values are derived directly from observed ground motion measurements at seismic stations and are not spatially interpolated.

3 stations
Station MMI
Punta Gorda, Petrolia, CA (BK.PETL) III
Walker Ranch; near Petrolia CA (BK.WLKR) II
Cooskie Peak (NC.KCO) III

Engineering Ground Motion Parameters

GMPE predictions provide an independent check on the ShakeMap interpolated intensities above. ShakeMap relies on seismic station recordings; in areas with sparse station coverage — or for large teleseismic events and earthquakes outside dense West Coast networks — ShakeMap MMI can be inaccurate. A significant difference between ShakeMap and GMPE estimates is a signal that the ShakeMap result should be treated as preliminary. Also includes site Vs30 values and raw ground motion parameters (PGA/PGV).

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Est. Vs30 Values

Estimated shear-wave velocity in the upper 30 m used in GMPE calculations. Ref. column shows the 760 m/s rock-site reference for comparison.

Location Ref. (760 m/s) Est. Vs30 (m/s)
Ferndale
CA 		760 273
Rio Dell
CA 		760 468
Fortuna
CA 		760 418

MMI vs Distance — Default Vs30 (760 m/s)

All methods using the 760 m/s rock-site reference. ShakeMap values are independent of Vs30.

MMI vs distance — default Vs30

MMI vs Distance — Est. Vs30

GMPEs use the Est. Vs30 values shown in the table above. ShakeMap values are independent of Vs30.

MMI vs distance — Est. Vs30

GMPE Residual — Observed − Predicted MMI

Signed residual at each seismic station. Positive values mean the GMPE underestimated shaking; negative values mean it overestimated. The dashed line at zero indicates perfect agreement.

GMPE residual plot

GMPE Predicted vs. Observed MMI at Seismic Stations

Each point is one seismic station. Points on the dashed 1:1 line indicate perfect agreement; above the line = GMPE underestimated shaking; below = overestimated.

GMPE predicted vs observed scatter

GMPE Performance — Station Observations

Statistics computed by comparing each GMPE's predicted MMI against ShakeMap observed MMI at seismic stations. Bias = mean(observed − predicted). Lower RMSE indicates better overall agreement with instrumental measurements.

GMPE Bias Sigma RMSE
ASK14 +0.08 0.26 0.27
BSSA14 +0.08 0.26 0.27
CY14 +0.08 0.26 0.27
CB14 +0.08 0.26 0.27
Bias > 0: GMPE underestimated shaking at stations. Bias < 0: GMPE overestimated shaking.

Peak Ground Motion — Seismic Station Observations

Recorded PGA and PGV at seismic stations, sorted by highest PGA. Top 25 stations shown. MMI (PGA) and MMI (PGV) are independent conversions via Worden et al. (2012) GMICE.

Station PGA (g) MMI (PGA) PGV (cm/s) MMI (PGV)
Punta Gorda, Petrolia, CA (BK.PETL) 0.0029 I 0.148 III
Cooskie Peak (NC.KCO) 0.0020 I 0.102 II
Slide Mountain (NC.KSM) 0.0008 I 0.044 II
Fire Station (NP.1023) 0.0006 I 0.029 II
Needle Rock (NC.KNR) 0.0006 I 0.026 I
Loleta Fire Station (NP.1586) 0.0006 I 0.026 II
Usal Creek (BK.USAL) 0.0006 I 0.023 I
Sproul Creek (BK.SPRL) 0.0005 I 0.037 II
Thorn Junction (NC.KTJ) 0.0005 I 0.012 I
Cape Town (NC.KCT) 0.0005 I 0.053 II
Hales Grove, Mendocino, CA (BK.HALS) 0.0005 I 0.025 I
Lost Coast Ranch (NP.1725) 0.0004 I 0.023 I
College of the Redwoods (NP.1582) 0.0004 I 0.031 II
Walker Ranch; near Petrolia CA (BK.WLKR) 0.0004 I 0.033 II
Bunker Hill, NOAA Radar (NP.1584B) 0.0004 I 0.015 I
Mount Pierce (NC.KMPB) 0.0004 I 0.023 I
Garberville CALFire station (NP.1591) 0.0004 I 0.022 I
Paradise Ridge (BK.PRDS) 0.0003 I 0.014 I
Briceland Vineyard, Redway, CA (BK.BRIC) 0.0003 I 0.015 I
South Bay School (NP.1581) 0.0003 I 0.019 I
Jewett Creek (BK.JEWT) 0.0002 I 0.015 I
Rainbow Ridge, Humboldt, CA (BK.RBOW) 0.0002 I 0.011 I
Bald Jesse, Humboldt County, CA (BK.BJES) 0.0002 I 0.016 I
Healdsburg Fire Station (NP.1744) 0.0001 I 0.003 I
Boyer?s Landing, near Robbins (BK.BOYR) 0.0001 I 0.002 I
Source: USGS ShakeMap station recordings (view ShakeMap). PGA in g (fraction of gravitational acceleration); PGV in cm/s. The observed MMI shown on the map uses ShakeMap's reported instrumental intensity where available, or max(MMI from PGA, MMI from PGV) otherwise.