Intensity Lab
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II

Max Est. MMI

26 km WNW of Ludlow, CA

Magnitude3.9
Depth1.6 km
Time 2026-04-01T16:30:47.910000Z UTC: 2026-04-01T16:30:47.910000Z
USGS ID ci41430336
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Ground Shaking Intensities — Interpolated ShakeMap MMI

3 sites
Location MMI
Fort Irwin
CA 		II
Barstow
CA 		II
Barstow Heights
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
Hector (CI.HEC) IV
Newberry Springs (CI.NBS) III
Rodman Mountain (CI.RMM) 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)
Fort Irwin
CA 		760 352
Barstow
CA 		760 352
Barstow Heights
CA 		760 352

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 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 +0.06 0.23 0.24
BSSA14 +0.06 0.22 0.22
CY14 +0.06 0.23 0.23
CB14 +0.06 0.21 0.21
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)
Hector (CI.HEC) 0.0251 IV 0.867 IV
Newberry Springs (CI.NBS) 0.0073 II 0.225 III
Rodman Mountain (CI.RMM) 0.0030 I 0.108 II
Goat Mountain Astronomical Research Station (CI.GMA) 0.0017 I 0.088 II
Victorville (CI.VTV) 0.0014 I 0.043 II
Alvord Mountain (CI.AVM) 0.0013 I 0.034 II
Granite Mountains Research Center (CI.GMR) 0.0012 I 0.021 I
Piute (CI.PUT) 0.0011 I 0.016 I
Mission Creek (CI.MSC) 0.0010 I 0.036 II
Central Fire Station (CI.CFS) 0.0010 I 0.014 I
Yucca Valley (CI.YUC) 0.0009 I 0.039 II
Adelanto Receiving (CI.ADO) 0.0009 I 0.037 II
Turquoise Mountain (CI.TUQ) 0.0009 I 0.029 II
Ivy Glen (CI.IVY) 0.0008 I 0.013 I
Barstow Service Center (CI.RRX) 0.0008 I 0.039 II
Joshua Tree National Park Headquarters (CI.JTH) 0.0008 I 0.049 II
Cajon Mountain (CI.CJM) 0.0007 I 0.021 I
Johnson Valley (CI.JVA) 0.0007 I 0.031 II
Wilsona 2 (CI.WLS2) 0.0007 I 0.030 II
Hole In The Wall (CI.HIW) 0.0007 I 0.035 II
Mountain Pass (CI.MTP) 0.0007 I 0.012 I
Crystal Lake 2 (CI.XTL2) 0.0007 I 0.022 I
Black Rock Canyon Campground 2 (CI.BLA2) 0.0006 I 0.017 I
Sunset Peak (CI.SUN) 0.0006 I 0.021 I
Ocotillo Community Park (CI.OCP) 0.0006 I 0.011 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.