Location

Location ANSS

The ANSS event ID is ci38443183 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ci38443183/executive.

2019/07/04 17:33:49 35.705 -117.504 10.5 6.4 California

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2019/07/04 17:33:49:0 35.71 -117.50 10.5 6.4 California Stations used: AE.113A AE.BARN AE.PRCT AE.W13A AE.X16A AE.Y14A AZ.CPE BC.SFX BC.SJX BC.TJX BC.TKX BC.VTX BK.BUCR BK.MNRC BK.MTOS BK.PACP BK.WELL BK.WINE CI.BLY CI.CCA CI.ELS2 CI.GLA CI.GRA CI.GSC CI.ISA CI.LDF CI.PASC CI.RAG CI.YUH2 II.PFO IM.NV31 LB.MVU LB.TPH NC.AFD NN.DSP NN.GMN NN.GWY NN.LHV NN.MPK NN.MZPB NN.PAH NN.PIO NN.PRN NN.Q09A NN.QSM NN.R11B NN.RYN NN.S11A NN.SHP NN.SPR3 NN.V12A NN.WAK NN.WDEM NN.WLDB SN.HEL US.ELK US.TPNV US.WUAZ UU.CCUT UU.KNB UU.LCMT UU.PSUT UU.SZCU UU.VRUT Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.05 n 3 Best Fitting Double Couple Mo = 2.60e+25 dyne-cm Mw = 6.21 Z = 14 km Plane Strike Dip Rake NP1 40 85 -20 NP2 132 70 -175 Principal Axes: Axis Value Plunge Azimuth T 2.60e+25 10 88 N 0.00e+00 69 207 P -2.60e+25 18 354 Moment Tensor: (dyne-cm) Component Value Mxx -2.33e+25 Mxy 3.47e+24 Mxz -7.26e+24 Myy 2.49e+25 Myz 5.34e+24 Mzz -1.54e+24 ---- ------- -------- P ----------- ----------- -------------# ---------------------------### ##--------------------------###### ####------------------------######## ######---------------------########### ########-------------------############# #########----------------############### ###########--------------############# # #############----------############### T # ###############-------################ # #################---###################### ######################################## ################----#################### #############--------################# ###########------------############# ########------------------######## ####-------------------------- #--------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P -1.54e+24 -7.26e+24 -5.34e+24 -7.26e+24 -2.33e+25 -3.47e+24 -5.34e+24 -3.47e+24 2.49e+25 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190704173349/index.html

Preferred Solution

      STK = 40
      DIP = 85
     RAKE = -20
       MW = 6.21
       HS = 14.0

The NDK file is 20190704173349.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

USGSW

USGS/SLU Moment Tensor Solution ENS 2019/07/04 17:33:49:0 35.71 -117.50 10.5 6.4 California Stations used: AE.113A AE.BARN AE.PRCT AE.W13A AE.X16A AE.Y14A AZ.CPE BC.SFX BC.SJX BC.TJX BC.TKX BC.VTX BK.BUCR BK.MNRC BK.MTOS BK.PACP BK.WELL BK.WINE CI.BLY CI.CCA CI.ELS2 CI.GLA CI.GRA CI.GSC CI.ISA CI.LDF CI.PASC CI.RAG CI.YUH2 II.PFO IM.NV31 LB.MVU LB.TPH NC.AFD NN.DSP NN.GMN NN.GWY NN.LHV NN.MPK NN.MZPB NN.PAH NN.PIO NN.PRN NN.Q09A NN.QSM NN.R11B NN.RYN NN.S11A NN.SHP NN.SPR3 NN.V12A NN.WAK NN.WDEM NN.WLDB SN.HEL US.ELK US.TPNV US.WUAZ UU.CCUT UU.KNB UU.LCMT UU.PSUT UU.SZCU UU.VRUT Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.05 n 3 Best Fitting Double Couple Mo = 2.60e+25 dyne-cm Mw = 6.21 Z = 14 km Plane Strike Dip Rake NP1 40 85 -20 NP2 132 70 -175 Principal Axes: Axis Value Plunge Azimuth T 2.60e+25 10 88 N 0.00e+00 69 207 P -2.60e+25 18 354 Moment Tensor: (dyne-cm) Component Value Mxx -2.33e+25 Mxy 3.47e+24 Mxz -7.26e+24 Myy 2.49e+25 Myz 5.34e+24 Mzz -1.54e+24 ---- ------- -------- P ----------- ----------- -------------# ---------------------------### ##--------------------------###### ####------------------------######## ######---------------------########### ########-------------------############# #########----------------############### ###########--------------############# # #############----------############### T # ###############-------################ # #################---###################### ######################################## ################----#################### #############--------################# ###########------------############# ########------------------######## ####-------------------------- #--------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P -1.54e+24 -7.26e+24 -5.34e+24 -7.26e+24 -2.33e+25 -3.47e+24 -5.34e+24 -3.47e+24 2.49e+25 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190704173349/index.html

W-phase Moment Tensor (Mw) Moment 6.155e+18 N-m Magnitude 6.46 Mww Depth 11.5 km Percent DC 95% Half Duration 4.44 s Catalog US Data Source US 4 Contributor US 4 Nodal Planes Plane Strike Dip Rake NP1 229Â 77Â 3Â NP2 139Â 87Â 167Â Principal Axes Axis Value Plunge Azimuth T 6.235e+18 N-m 11Â 93Â N -0.163e+18 N-m 76Â 308Â P -6.072e+18 N-m 8Â 185Â

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.

mLg Magnitude

Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.

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.

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 0.02 n 3 
lp c 0.05 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   225    90     5   5.87 0.2892
WVFGRD96    2.0    45    85   -10   5.97 0.3705
WVFGRD96    3.0   225    90     5   6.01 0.4073
WVFGRD96    4.0   225    90     5   6.04 0.4320
WVFGRD96    5.0    45    90     0   6.07 0.4496
WVFGRD96    6.0    45    90    -5   6.09 0.4639
WVFGRD96    7.0   225    90     5   6.11 0.4771
WVFGRD96    8.0    45    85   -10   6.14 0.4902
WVFGRD96    9.0    45    85   -10   6.15 0.4920
WVFGRD96   10.0    45    85   -15   6.17 0.4926
WVFGRD96   11.0    45    85   -15   6.18 0.4933
WVFGRD96   12.0    40    85   -20   6.19 0.4949
WVFGRD96   13.0   220    90    20   6.20 0.4933
WVFGRD96   14.0    40    85   -20   6.21 0.4967
WVFGRD96   15.0   220    80    20   6.22 0.4943
WVFGRD96   16.0   220    80    20   6.23 0.4944
WVFGRD96   17.0   220    80    20   6.23 0.4939
WVFGRD96   18.0   220    80    20   6.24 0.4926
WVFGRD96   19.0   220    80    20   6.25 0.4907
WVFGRD96   20.0   220    80    20   6.25 0.4883
WVFGRD96   21.0   220    80    20   6.26 0.4855
WVFGRD96   22.0   220    80    20   6.27 0.4820
WVFGRD96   23.0   220    80    20   6.27 0.4781
WVFGRD96   24.0   220    80    20   6.28 0.4739
WVFGRD96   25.0   220    80    20   6.29 0.4693
WVFGRD96   26.0   220    80    20   6.29 0.4645
WVFGRD96   27.0   220    80    20   6.30 0.4595
WVFGRD96   28.0    40    85   -20   6.30 0.4550
WVFGRD96   29.0    40    85   -20   6.31 0.4508

The best solution is

WVFGRD96   14.0    40    85   -20   6.21 0.4967

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 0.02 n 3 
lp c 0.05 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