The ANSS event ID is tx2021tiop and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/tx2021tiop/executive.
2021/10/03 01:39:18 31.706 -104.024 8.1 4 Texas
USGS/SLU Moment Tensor Solution ENS 2021/10/03 01:39:18:0 31.71 -104.02 8.1 4.0 Texas Stations used: GM.NMP02 GM.NMP25 GM.NMP41 GM.NMP44 GM.NMP45 IM.TX31 IU.ANMO N4.MSTX SC.121A SC.Y22A TX.ALPN TX.APMT TX.DKNS TX.MB01 TX.MB04 TX.MB05 TX.MB06 TX.MB09 TX.MNHN TX.ODSA TX.OZNA TX.PB01 TX.PB05 TX.PB11 TX.PB28 TX.PECS TX.POST TX.SAND TX.SN07 TX.SN08 TX.VHRN US.AMTX US.JCT Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 Best Fitting Double Couple Mo = 5.89e+21 dyne-cm Mw = 3.78 Z = 8 km Plane Strike Dip Rake NP1 64 66 -116 NP2 295 35 -45 Principal Axes: Axis Value Plunge Azimuth T 5.89e+21 17 173 N 0.00e+00 24 76 P -5.89e+21 60 295 Moment Tensor: (dyne-cm) Component Value Mxx 5.04e+21 Mxy -3.65e+19 Mxz -2.73e+21 Myy -1.13e+21 Myz 2.49e+21 Mzz -3.91e+21 ############## ###################### ############################ #####-----------############## ##---------------------########### #--------------------------######### -------------------------------######- ----------------------------------###--- ------------ --------------------#---- ------------- P -------------------##----- ------------- -----------------#####---- -------------------------------########--- -----------------------------##########--- ------------------------###############- --------------------###################- --------------######################## #################################### ################################## ############################## ############## ########### ########### T ######## ####### #### Global CMT Convention Moment Tensor: R T P -3.91e+21 -2.73e+21 -2.49e+21 -2.73e+21 5.04e+21 3.65e+19 -2.49e+21 3.65e+19 -1.13e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20211003013918/index.html |
STK = 295 DIP = 35 RAKE = -45 MW = 3.78 HS = 8.0
The NDK file is 20211003013918.ndk The waveform inversion is preferred.
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.
USGS/SLU Moment Tensor Solution ENS 2021/10/03 01:39:18:0 31.71 -104.02 8.1 4.0 Texas Stations used: GM.NMP02 GM.NMP25 GM.NMP41 GM.NMP44 GM.NMP45 IM.TX31 IU.ANMO N4.MSTX SC.121A SC.Y22A TX.ALPN TX.APMT TX.DKNS TX.MB01 TX.MB04 TX.MB05 TX.MB06 TX.MB09 TX.MNHN TX.ODSA TX.OZNA TX.PB01 TX.PB05 TX.PB11 TX.PB28 TX.PECS TX.POST TX.SAND TX.SN07 TX.SN08 TX.VHRN US.AMTX US.JCT Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 Best Fitting Double Couple Mo = 5.89e+21 dyne-cm Mw = 3.78 Z = 8 km Plane Strike Dip Rake NP1 64 66 -116 NP2 295 35 -45 Principal Axes: Axis Value Plunge Azimuth T 5.89e+21 17 173 N 0.00e+00 24 76 P -5.89e+21 60 295 Moment Tensor: (dyne-cm) Component Value Mxx 5.04e+21 Mxy -3.65e+19 Mxz -2.73e+21 Myy -1.13e+21 Myz 2.49e+21 Mzz -3.91e+21 ############## ###################### ############################ #####-----------############## ##---------------------########### #--------------------------######### -------------------------------######- ----------------------------------###--- ------------ --------------------#---- ------------- P -------------------##----- ------------- -----------------#####---- -------------------------------########--- -----------------------------##########--- ------------------------###############- --------------------###################- --------------######################## #################################### ################################## ############################## ############## ########### ########### T ######## ####### #### Global CMT Convention Moment Tensor: R T P -3.91e+21 -2.73e+21 -2.49e+21 -2.73e+21 5.04e+21 3.65e+19 -2.49e+21 3.65e+19 -1.13e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20211003013918/index.html |
Regional Moment Tensor (Mwr) Moment 7.426e+14 N-m Magnitude 3.85 Mwr Depth 8.0 km Percent DC 89% Half Duration - Catalog US Data Source US 2 Contributor US 2 Nodal Planes Plane Strike Dip Rake NP1 78° 59° -106° NP2 287° 34° -66° Principal Axes Axis Value Plunge Azimuth T 7.632e+14 N-m 13° 180° N -0.432e+14 N-m 13° 87° P -7.201e+14 N-m 71° 312° |
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 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 1.0 305 55 -25 3.41 0.3046 WVFGRD96 2.0 120 70 -45 3.56 0.3881 WVFGRD96 3.0 120 80 -55 3.65 0.4466 WVFGRD96 4.0 300 30 -35 3.69 0.5152 WVFGRD96 5.0 300 35 -35 3.70 0.5649 WVFGRD96 6.0 295 35 -45 3.71 0.5941 WVFGRD96 7.0 300 40 -40 3.72 0.6050 WVFGRD96 8.0 295 35 -45 3.78 0.6285 WVFGRD96 9.0 300 40 -40 3.79 0.6207 WVFGRD96 10.0 305 45 -30 3.79 0.6047 WVFGRD96 11.0 310 50 -15 3.79 0.5907 WVFGRD96 12.0 310 50 -15 3.80 0.5757 WVFGRD96 13.0 315 55 5 3.80 0.5616 WVFGRD96 14.0 315 55 5 3.81 0.5465 WVFGRD96 15.0 315 55 5 3.81 0.5299 WVFGRD96 16.0 315 55 5 3.82 0.5144 WVFGRD96 17.0 315 55 5 3.83 0.4988 WVFGRD96 18.0 315 55 10 3.83 0.4826 WVFGRD96 19.0 320 55 15 3.83 0.4685 WVFGRD96 20.0 320 55 15 3.83 0.4542 WVFGRD96 21.0 320 50 15 3.85 0.4402 WVFGRD96 22.0 320 50 15 3.85 0.4275 WVFGRD96 23.0 320 50 15 3.86 0.4172 WVFGRD96 24.0 320 50 15 3.86 0.4066 WVFGRD96 25.0 320 50 15 3.87 0.3968 WVFGRD96 26.0 325 50 20 3.87 0.3871 WVFGRD96 27.0 325 50 20 3.87 0.3778 WVFGRD96 28.0 325 50 20 3.88 0.3687 WVFGRD96 29.0 325 50 25 3.88 0.3593
The best solution is
WVFGRD96 8.0 295 35 -45 3.78 0.6285
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 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 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