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Intensity Lab
IV

Max Est. MMI

M4.4 earthquake · shaking intensity by town 9 km ESE of Willits, CA

Magnitude4.4
Depth8.5 km
Time 2026-01-13T21:10:54.210000Z UTC: 2026-01-13T21:10:54.210000Z
USGS ID nc75295231
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)
Redwood Valley
CA 		760 369
Willits
CA 		760 352
Brooktrails
CA 		760 468

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 -0.18 0.35 0.40
BSSA14 -0.17 0.33 0.37
CY14 -0.13 0.31 0.34
CB14 -0.14 0.32 0.34
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)
Van Arsdale (NC.GVA) 0.0834 Moderate(V) 2.199 Light(IV)
Barra Winery, Redwood Valley, CA (BK.BARR) 0.0733 Moderate(V) 2.747 Light(IV)
DeCamp, Mendocino County, CA (BK.DCMP) 0.0432 Light(IV) 0.444 Weak(III)
Bevans Ridge (BK.BEVN) 0.0380 Light(IV) 1.430 Light(IV)
Wonder Ranch (NC.GWR) 0.0129 Weak(III) 0.483 Weak(III)
Orr Springs (BK.ORRS) 0.0094 Weak(III) 0.453 Weak(III)
Cow Mountain (NC.GCWB) 0.0078 Weak(III) 0.190 Weak(II)
Beacon Light (NC.GBL) 0.0067 Weak(III) 0.257 Weak(II)
Spanish Creek (BK.SPAN) 0.0061 Weak(III) 0.292 Weak(III)
Bradford Ranch, Boonville, CA (BK.BONV) 0.0052 Weak(II) 0.227 Weak(II)
Navarro Ridge (NC.GNR) 0.0046 Weak(II) 0.199 Weak(II)
Hull Mountain (BK.HULL) 0.0043 Weak(II) 0.254 Weak(II)
Hamilton Opening (NC.GHO) 0.0035 Weak(II) 0.222 Weak(II)
Sherwood Rancheria area, Willits (BK.SHWD) 0.0034 Weak(II) 0.109 Weak(II)
Spanish Ridge (NC.GSR) 0.0032 Weak(II) 0.212 Weak(II)
Woodlands Fire Station, Mendocino, CA (BK.MNDO) 0.0027 Weak(II) 0.148 Weak(II)
Big Meadow Rd Philo (NC.GMR) 0.0027 Weak(II) 0.115 Weak(II)
Cloverdale #3 (NC.GCVB) 0.0027 Weak(II) 0.130 Weak(II)
Wildwood Ranch (NC.GWW) 0.0024 Weak(II) 0.206 Weak(II)
Hogback Ridge (NC.GHGB) 0.0020 Weak(II) 0.121 Weak(II)
Skaggs, Las Lomas (BK.SKGS) 0.0018 Weak(II) 0.107 Weak(II)
Round Top Mountain (NC.GRT) 0.0015 Weak(II) 0.151 Weak(II)
Snow Mountain (NC.GSN) 0.0015 Weak(II) 0.122 Weak(II)
Hopland Research and Extension Center, Hopland, CA, USA (BK.HOPS) 0.0013 Weak(II) 0.076 Weak(II)
Rackout Springs (NC.KRK) 0.0013 Not felt(I) 0.135 Weak(II)
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
Redwood Valley
CA 		Light(IV)
Willits
CA 		Weak(III)
Brooktrails
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
Barra Winery, Redwood Valley, CA (BK.BARR) Light(IV)
Van Arsdale (NC.GVA) Light(IV)
Bevans Ridge (BK.BEVN) Light(IV)