Left: mb_Lg computed using the IASPEI formula. Center: mb_Lg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.
The ANSS event ID is tx2026rmntsl and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/tx2026rmntsl/executive.
2026/09/05 14:46:57 31.715 -101.703 6.8 3.6 Texas
USGS/SLU Moment Tensor Solution
ENS 2026/09/05 14:46:57.0 31.72 -101.70 6.8 3.6 Texas
Stations used:
4O.AT01 4O.CW01 4O.DBL01 4O.DRO01 4O.EE01 4O.EE04 4O.GV01
4O.MBBB2 4O.MID01 4O.MID03 4O.NGL02 4O.OE02 4O.SE01 4O.SM05
4O.VW02 TX.MB02 TX.MB08 TX.MB09 TX.MB10 TX.MB11 TX.MB12
TX.MB13 TX.MB15 TX.MB18 TX.MB19 TX.MB21 TX.MB22 TX.MB23
TX.ODSA TX.OZNA TX.PB08 TX.SGCY
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +30
rtr
taper w 0.1
hp c 0.05 n 3
lp c 0.12 n 3
Best Fitting Double Couple
Mo = 1.53e+21 dyne-cm
Mw = 3.39
Z = 4 km
Plane Strike Dip Rake
NP1 48 57 -130
NP2 285 50 -45
Principal Axes:
Axis Value Plunge Azimuth
T 1.53e+21 4 165
N 0.00e+00 33 72
P -1.53e+21 57 261
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.41e+21
Mxy -4.52e+20
Mxz 1.48e+18
Myy -3.43e+20
Myz 7.21e+20
Mzz -1.07e+21
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#######------------##############---
###-----------------------########----
#-----------------------------####------
---------------------------------#------
----------------------------------##------
----------- -------------------#####----
----------- P -----------------########---
----------- ----------------##########--
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########### T
Global CMT Convention Moment Tensor:
R T P
-1.07e+21 1.48e+18 -7.21e+20
1.48e+18 1.41e+21 4.52e+20
-7.21e+20 4.52e+20 -3.43e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20260905144657/index.html
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STK = 285
DIP = 50
RAKE = -45
MW = 3.39
HS = 4.0
The NDK file is 20260905144657.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: mb_Lg computed using the IASPEI formula. Center: mb_Lg 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.
Map showing station locations used for computing the ML's. No distinction is made whether the vertical (Z) or horizontal (H) components were used.
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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 +30 rtr taper w 0.1 hp c 0.05 n 3 lp c 0.12 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 110 75 -45 3.09 0.3313
WVFGRD96 2.0 110 65 -40 3.25 0.4029
WVFGRD96 3.0 100 50 -55 3.37 0.4517
WVFGRD96 4.0 285 50 -45 3.39 0.4868
WVFGRD96 5.0 110 60 -40 3.41 0.4862
WVFGRD96 6.0 110 65 -40 3.43 0.4767
WVFGRD96 7.0 300 60 -10 3.44 0.4592
WVFGRD96 8.0 300 55 -10 3.49 0.4378
WVFGRD96 9.0 305 55 10 3.50 0.4155
WVFGRD96 10.0 305 60 20 3.52 0.3932
WVFGRD96 11.0 305 75 30 3.54 0.3722
WVFGRD96 12.0 305 75 30 3.55 0.3498
WVFGRD96 13.0 310 70 40 3.57 0.3286
WVFGRD96 14.0 310 70 40 3.58 0.3066
WVFGRD96 15.0 310 70 40 3.59 0.2855
WVFGRD96 16.0 315 65 45 3.61 0.2655
WVFGRD96 17.0 315 65 50 3.62 0.2469
WVFGRD96 18.0 320 60 55 3.63 0.2322
WVFGRD96 19.0 330 55 70 3.66 0.2201
WVFGRD96 20.0 45 45 30 3.65 0.2139
WVFGRD96 21.0 45 50 35 3.65 0.2128
WVFGRD96 22.0 45 50 35 3.66 0.2156
WVFGRD96 23.0 45 50 35 3.68 0.2185
WVFGRD96 24.0 45 55 35 3.68 0.2230
WVFGRD96 25.0 45 55 35 3.69 0.2293
WVFGRD96 26.0 45 60 40 3.70 0.2370
WVFGRD96 27.0 45 60 40 3.71 0.2461
WVFGRD96 28.0 45 60 40 3.73 0.2561
WVFGRD96 29.0 45 60 40 3.74 0.2649
The best solution is
WVFGRD96 4.0 285 50 -45 3.39 0.4868
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 +30 rtr taper w 0.1 hp c 0.05 n 3 lp c 0.12 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