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Intensity Lab
III

Max Est. MMI

M4.2 earthquake · shaking intensity by town 5 km W of Cobb, CA

Magnitude4.2
Depth2.0 km
Time 2026-02-21T17:22:56.030000Z UTC: 2026-02-21T17:22:56.030000Z
USGS ID nc75316827
ShakeMap View USGS ↗

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)
Cobb
CA 		760 710
Clear Lake Riviera
CA 		760 710
Middletown
CA 		760 294

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 -0.16 0.31 0.35
BSSA14 -0.24 0.36 0.43
CY14 -0.10 0.26 0.27
CB14 -0.19 0.34 0.39
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)
Geysers (NC.GDXB) 0.1252 Strong(VI) 2.691 Light(IV)
Boggs Mountain (NC.GBG) 0.0313 Light(IV) 0.994 Weak(III)
Geyser Peak (NC.GGPC) 0.0197 Weak(III) 0.990 Weak(III)
Alexander Valley (NC.GAXB) 0.0177 Weak(III) 0.454 Weak(III)
Pine Mountain #1 (NC.GPM) 0.0088 Weak(III) 0.281 Weak(III)
Round Top Mountain (NC.GRT) 0.0039 Weak(II) 0.235 Weak(II)
Lake Bordeaux (NC.NLB) 0.0033 Weak(II) 0.119 Weak(II)
Skaggs, Las Lomas (BK.SKGS) 0.0031 Weak(II) 0.084 Weak(II)
Pepperwood Preserve (BK.PWOD) 0.0025 Weak(II) 0.070 Weak(II)
Santa Rosa FS 7 (NP.1835) 0.0019 Weak(II) 0.057 Not felt(I)
Weeks Lake (NC.NWS) 0.0017 Weak(II) 0.075 Weak(II)
Healdsburg Fire Station (NP.1744) 0.0015 Weak(II) 0.059 Weak(II)
St. Helena Fire Department (NP.1764) 0.0014 Weak(II) 0.061 Weak(II)
Oakville Experimental Vineyard, Oakville, CA (BK.OAKV) 0.0013 Not felt(I) 0.061 Weak(II)
Mt.St.Helena South (NC.NHS) 0.0013 Not felt(I) 0.122 Weak(II)
Cloverdale #3 (NC.GCVB) 0.0012 Not felt(I) 0.076 Weak(II)
Glen Ellen FS Arnold (NP.1848) 0.0011 Not felt(I) 0.040 Not felt(I)
Sonoma FS 1 (NP.1829) 0.0011 Not felt(I) 0.057 Weak(II)
Santa Rosa FS 1 (NP.1767) 0.0009 Not felt(I) 0.045 Not felt(I)
Ohlone Wilderness Reg Park (NP.1684) 0.0009 Not felt(I) 0.008 Not felt(I)
Archer Taylor Preserve (BK.ATP) 0.0008 Not felt(I) 0.042 Not felt(I)
Mill Valley Golf Course (NP.1806) 0.0007 Not felt(I) 0.023 Not felt(I)
The Novato Fire Protection District "Fire Station 1" (NP.1762) 0.0007 Not felt(I) 0.032 Not felt(I)
Lloyd Lowery (NC.NLL) 0.0007 Not felt(I) 0.020 Not felt(I)
Hamilton Field (NC.NHF) 0.0006 Not felt(I) 0.022 Not felt(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.

Ground Shaking Intensities — Interpolated ShakeMap MMI

3 sites
Location MMI Description
Cobb
CA 		Weak(III)
Clear Lake Riviera
CA 		Weak(II)
Middletown
CA 		Weak(III)

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 Description
Geysers (NC.GDXB) Light(IV)
Geyser Peak (NC.GGPC) Weak(III)
Boggs Mountain (NC.GBG) Weak(III)