The ANSS event ID is tx2023ulrs and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/tx2023ulrs/executive.
2023/10/19 00:09:57 31.627 -104.507 7.5 3.5 Texas
USGS/SLU Moment Tensor Solution ENS 2023/10/19 00:09:57:0 31.63 -104.51 7.5 3.5 Texas Stations used: 4O.CV01 4O.SA02 4O.SA04 4O.SA06 4O.SA07 4O.SA09 4O.WB01 4O.WB02 4O.WB03 4O.WB04 4O.WB07 4O.WB12 TX.PB09 TX.PB12 TX.PB23 TX.PB25 TX.PB26 TX.PB29 TX.PB31 TX.PB33 TX.PB34 TX.PB38 TX.PB40 TX.PB44 TX.PECS Filtering commands used: cut o DIST/3.3 -20 o DIST/3.3 +30 rtr taper w 0.1 hp c 0.10 n 3 lp c 0.30 n 3 Best Fitting Double Couple Mo = 8.22e+20 dyne-cm Mw = 3.21 Z = 7 km Plane Strike Dip Rake NP1 55 60 -90 NP2 235 30 -90 Principal Axes: Axis Value Plunge Azimuth T 8.22e+20 15 145 N 0.00e+00 -0 235 P -8.22e+20 75 325 Moment Tensor: (dyne-cm) Component Value Mxx 4.78e+20 Mxy -3.35e+20 Mxz -3.37e+20 Myy 2.34e+20 Myz 2.36e+20 Mzz -7.12e+20 ############## ###################### ##############---------##### ##########-------------------# #########------------------------- #######----------------------------# ######-----------------------------### ######-----------------------------##### #####----------- ----------------##### #####------------ P --------------######## ####------------- -------------######### ####---------------------------########### ###---------------------------############ ##------------------------############## ##----------------------################ #-------------------################## ---------------##################### --------################## ##### ######################## T ### ####################### ## ###################### ############## Global CMT Convention Moment Tensor: R T P -7.12e+20 -3.37e+20 -2.36e+20 -3.37e+20 4.78e+20 3.35e+20 -2.36e+20 3.35e+20 2.34e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20231019000957/index.html |
STK = 55 DIP = 60 RAKE = -90 MW = 3.21 HS = 7.0
The NDK file is 20231019000957.ndk The waveform inversion is preferred.
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 ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML 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.
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.
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.
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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.
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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 -20 o DIST/3.3 +30 rtr taper w 0.1 hp c 0.10 n 3 lp c 0.30 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 1.0 230 75 -10 2.50 0.1133 WVFGRD96 2.0 230 60 -5 2.78 0.1695 WVFGRD96 3.0 235 50 25 2.88 0.2372 WVFGRD96 4.0 270 30 -60 3.00 0.3416 WVFGRD96 5.0 260 35 -70 3.08 0.4186 WVFGRD96 6.0 250 35 -80 3.15 0.4611 WVFGRD96 7.0 55 60 -90 3.21 0.4656 WVFGRD96 8.0 165 55 60 3.29 0.4392 WVFGRD96 9.0 340 35 75 3.30 0.4323 WVFGRD96 10.0 345 35 80 3.34 0.4072 WVFGRD96 11.0 350 35 90 3.36 0.3714 WVFGRD96 12.0 175 55 90 3.38 0.3403 WVFGRD96 13.0 170 55 85 3.39 0.3167 WVFGRD96 14.0 165 60 75 3.41 0.2988 WVFGRD96 15.0 160 60 70 3.42 0.2865 WVFGRD96 16.0 165 60 70 3.43 0.2751 WVFGRD96 17.0 45 45 60 3.37 0.2694 WVFGRD96 18.0 45 45 60 3.39 0.2823 WVFGRD96 19.0 45 45 60 3.40 0.2920 WVFGRD96 20.0 50 45 65 3.41 0.2974 WVFGRD96 21.0 30 45 60 3.44 0.3067 WVFGRD96 22.0 30 45 65 3.45 0.3140 WVFGRD96 23.0 50 45 65 3.45 0.3154 WVFGRD96 24.0 50 45 65 3.45 0.3198 WVFGRD96 25.0 55 45 65 3.46 0.3208 WVFGRD96 26.0 50 50 70 3.45 0.3199 WVFGRD96 27.0 50 50 70 3.45 0.3173 WVFGRD96 28.0 50 50 70 3.45 0.3152 WVFGRD96 29.0 50 50 70 3.46 0.3063
The best solution is
WVFGRD96 7.0 55 60 -90 3.21 0.4656
The mechanism corresponding to the best fit is
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The best fit as a function of depth is given in the following figure:
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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 -20 o DIST/3.3 +30 rtr taper w 0.1 hp c 0.10 n 3 lp c 0.30 n 3
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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. |
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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:
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.
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