Location

Location ANSS

The ANSS event ID is ci41533416 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ci41533416/executive.

2026/08/23 07:59:55 36.070 -117.841 4.3 3.5 California

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2026/08/23 07:59:55.0 36.07 -117.84 4.3 3.5 California Stations used: BK.BITR BK.CGRV BK.KARE BK.LEGD BK.LIND BK.MMI BK.OVRO BK.PATT BK.PORG BK.TOLH CI.ADO CI.ARV CI.BBR CI.BFS CI.CCC CI.CGO CI.CHF CI.CRG CI.CWC CI.DAW CI.DEC CI.DJJ CI.DTP CI.EDW2 CI.FUR CI.GMR CI.GRA CI.GSC CI.ISA CI.KRV1 CI.LDR CI.LMR2 CI.LMY CI.MLAC CI.MOP CI.MPM CI.MWC CI.OSI CI.PASC CI.RRX CI.RVR CI.RYS CI.SHO CI.SLA CI.SUN CI.SVD CI.TEJ CI.TIN CI.TJR CI.VES CI.VOG CI.VTV CI.WAS2 CI.WBP CI.WLH2 LB.TPH NC.MBM NC.MCC NC.MCY NC.MDR NC.MDY NC.MED NC.MEM NC.MFBB NC.MINS NC.MLI NC.MLMB NC.MSD NC.MWE NN.DSP NN.GWY NN.S11A NN.SHP NN.SLVR QN.SCFL QN.TCPC WR.PYR Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 6.76e+21 dyne-cm Mw = 3.82 Z = 9 km Plane Strike Dip Rake NP1 342 71 -159 NP2 245 70 -20 Principal Axes: Axis Value Plunge Azimuth T 6.76e+21 1 113 N 0.00e+00 62 22 P -6.76e+21 28 204 Moment Tensor: (dyne-cm) Component Value Mxx -3.35e+21 Mxy -4.41e+21 Mxz 2.52e+21 Myy 4.84e+21 Myz 1.22e+21 Mzz -1.49e+21 #------------- #######--------------- ###########----------------- ##############---------------- #################----------------- ###################----------------- #####################-------########## ######################-################# ##################-----################# ###############----------################# ############--------------################ ##########----------------################ #######--------------------############### ####----------------------############## ###------------------------########## --------------------------########## T -------------------------########## --------- ------------########## ------- P ------------######## ------ ------------####### ------------------#### -------------# Global CMT Convention Moment Tensor: R T P -1.49e+21 2.52e+21 -1.22e+21 2.52e+21 -3.35e+21 4.41e+21 -1.22e+21 4.41e+21 4.84e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20260823075955/index.html

Preferred Solution

      STK = 245
      DIP = 70
     RAKE = -20
       MW = 3.82
       HS = 9.0

The NDK file is 20260823075955.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.

SLU

SCAL

USGS/SLU Moment Tensor Solution ENS 2026/08/23 07:59:55.0 36.07 -117.84 4.3 3.5 California Stations used: BK.BITR BK.CGRV BK.KARE BK.LEGD BK.LIND BK.MMI BK.OVRO BK.PATT BK.PORG BK.TOLH CI.ADO CI.ARV CI.BBR CI.BFS CI.CCC CI.CGO CI.CHF CI.CRG CI.CWC CI.DAW CI.DEC CI.DJJ CI.DTP CI.EDW2 CI.FUR CI.GMR CI.GRA CI.GSC CI.ISA CI.KRV1 CI.LDR CI.LMR2 CI.LMY CI.MLAC CI.MOP CI.MPM CI.MWC CI.OSI CI.PASC CI.RRX CI.RVR CI.RYS CI.SHO CI.SLA CI.SUN CI.SVD CI.TEJ CI.TIN CI.TJR CI.VES CI.VOG CI.VTV CI.WAS2 CI.WBP CI.WLH2 LB.TPH NC.MBM NC.MCC NC.MCY NC.MDR NC.MDY NC.MED NC.MEM NC.MFBB NC.MINS NC.MLI NC.MLMB NC.MSD NC.MWE NN.DSP NN.GWY NN.S11A NN.SHP NN.SLVR QN.SCFL QN.TCPC WR.PYR Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 6.76e+21 dyne-cm Mw = 3.82 Z = 9 km Plane Strike Dip Rake NP1 342 71 -159 NP2 245 70 -20 Principal Axes: Axis Value Plunge Azimuth T 6.76e+21 1 113 N 0.00e+00 62 22 P -6.76e+21 28 204 Moment Tensor: (dyne-cm) Component Value Mxx -3.35e+21 Mxy -4.41e+21 Mxz 2.52e+21 Myy 4.84e+21 Myz 1.22e+21 Mzz -1.49e+21 #------------- #######--------------- ###########----------------- ##############---------------- #################----------------- ###################----------------- #####################-------########## ######################-################# ##################-----################# ###############----------################# ############--------------################ ##########----------------################ #######--------------------############### ####----------------------############## ###------------------------########## --------------------------########## T -------------------------########## --------- ------------########## ------- P ------------######## ------ ------------####### ------------------#### -------------# Global CMT Convention Moment Tensor: R T P -1.49e+21 2.52e+21 -1.22e+21 2.52e+21 -3.35e+21 4.41e+21 -1.22e+21 4.41e+21 4.84e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20260823075955/index.html

Moment Tensor (Mw) Moment 9.756e+14 N-m Magnitude 3.93 Mw Depth 14.0 km Percent DC 96% Half Duration - Catalog CI Data Source CI Contributor CI Nodal Planes Plane Strike Dip Rake NP1 246 75 -15 NP2 340 75 -164 Principal Axes Axis Value Plunge Azimuth T 9.861e+14 0 113 N -0.215e+14 69 22 P -9.647e+14 21 203

Magnitudes

Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and M_L by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for M_L is adjusted to remove a linear distance trend in residuals to give a regionally defined M_L. The defined M_L uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.

ML Magnitude

Left: ML computed using the IASPEI formula for Horizontal components. Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot. Right: Residuals from new relation as a function of distance and azimuth.

Left: ML computed using the IASPEI formula for Vertical components (research). Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot. Right: Residuals from new relation as a function of distance and azimuth.

Map showing station locations used for computing the ML's. No distinction is made whether the vertical (Z) or horizontal (H) components were used.

Context

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) the waveform inversion are shown in the next figure.

Location of broadband stations used for waveform inversion

The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green's functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.

The observed and predicted traces are filtered using the following gsac commands:

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   340    80   -10   3.39 0.3017
WVFGRD96    2.0    75    75    25   3.55 0.3807
WVFGRD96    3.0   245    70   -30   3.62 0.4238
WVFGRD96    4.0   245    70   -30   3.66 0.4659
WVFGRD96    5.0   245    70   -25   3.69 0.4977
WVFGRD96    6.0   245    70   -25   3.72 0.5179
WVFGRD96    7.0   245    75   -20   3.75 0.5318
WVFGRD96    8.0   245    70   -25   3.80 0.5426
WVFGRD96    9.0   245    70   -20   3.82 0.5452
WVFGRD96   10.0   245    75   -20   3.84 0.5420
WVFGRD96   11.0   245    75   -20   3.86 0.5353
WVFGRD96   12.0   245    75   -15   3.87 0.5256
WVFGRD96   13.0   245    75   -15   3.88 0.5140
WVFGRD96   14.0   245    75   -15   3.90 0.5015
WVFGRD96   15.0   245    75   -15   3.91 0.4884
WVFGRD96   16.0   245    75   -15   3.92 0.4741
WVFGRD96   17.0   245    75   -15   3.92 0.4592
WVFGRD96   18.0   245    75   -15   3.93 0.4446
WVFGRD96   19.0   245    75   -15   3.94 0.4303
WVFGRD96   20.0   245    75   -15   3.95 0.4171
WVFGRD96   21.0   245    75   -15   3.95 0.4048
WVFGRD96   22.0   245    75   -20   3.96 0.3935
WVFGRD96   23.0   245    75   -20   3.97 0.3840
WVFGRD96   24.0   245    75   -15   3.97 0.3760
WVFGRD96   25.0   245    75   -15   3.98 0.3698
WVFGRD96   26.0   245    75   -15   3.98 0.3645
WVFGRD96   27.0   245    75   -15   3.99 0.3598
WVFGRD96   28.0   245    75   -15   3.99 0.3569
WVFGRD96   29.0   245    75   -15   4.00 0.3545

The best solution is

WVFGRD96    9.0   245    70   -20   3.82 0.5452

The mechanism corresponding to the best fit is

Figure 1. Waveform inversion focal mechanism

The best fit as a function of depth is given in the following figure:

Figure 2. Depth sensitivity for waveform mechanism

The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted. Each observed-predicted component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).

The bandpass filter used in the processing and for the display was

cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 

Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample.

Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. Each solution is plotted as a vector at a given value of strike and dip with the angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure.

A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:

• The origin time and epicentral distance are incorrect
• The velocity model used for the inversion is incorrect
• The velocity model used to define the P-arrival time is not the same as the velocity model used for the waveform inversion (assuming that the initial trace alignment is based on the P arrival time)

 Time_shift = A + B cos Azimuth + C Sin Azimuth

The time shifts for this inversion lead to the next figure:

The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.

Velocity Model

The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).

MODEL.01
Model after     8 iterations
ISOTROPIC
KGS
FLAT EARTH
1-D
CONSTANT VELOCITY
LINE08
LINE09
LINE10
LINE11
      H(KM)   VP(KM/S)   VS(KM/S) RHO(GM/CC)         QP         QS       ETAP       ETAS      FREFP      FREFS
     1.9000     3.4065     2.0089     2.2150  0.302E-02  0.679E-02   0.00       0.00       1.00       1.00    
     6.1000     5.5445     3.2953     2.6089  0.349E-02  0.784E-02   0.00       0.00       1.00       1.00    
    13.0000     6.2708     3.7396     2.7812  0.212E-02  0.476E-02   0.00       0.00       1.00       1.00    
    19.0000     6.4075     3.7680     2.8223  0.111E-02  0.249E-02   0.00       0.00       1.00       1.00    
     0.0000     7.9000     4.6200     3.2760  0.164E-10  0.370E-10   0.00       0.00       1.00       1.00