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.
The ANSS event ID is tx2023zosh and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/tx2023zosh/executive.
2023/12/30 19:32:33 31.616 -103.975 7.7 3.9 Texas
USGS/SLU Moment Tensor Solution
ENS 2023/12/30 19:32:33:0 31.62 -103.97 7.7 3.9 Texas
Stations used:
4O.AT01 4O.BP01 4O.CV01 4O.DB03 4O.DB04 4O.GV02 4O.LWM1
4O.LWM2 4O.MBBB2 4O.MID01 4O.MID02 4O.MID03 4O.SA02 4O.SA04
4O.SA06 4O.SA07 4O.SA09 4O.SM02 4O.SM03 4O.WB02 4O.WB06
4O.WB07 4O.WB08 4O.WB09 4O.WB10 4O.WB11 4O.WB12 TX.ALPN
TX.MB02 TX.MB07 TX.MB09 TX.MB11 TX.MB17 TX.MB18 TX.MB23
TX.MB25 TX.MNHN TX.ODSA TX.PB01 TX.PB03 TX.PB04 TX.PB07
TX.PB09 TX.PB10 TX.PB11 TX.PB12 TX.PB13 TX.PB14 TX.PB16
TX.PB17 TX.PB18 TX.PB19 TX.PB21 TX.PB22 TX.PB26 TX.PB28
TX.PB29 TX.PB30 TX.PB31 TX.PB35 TX.PB36 TX.PB37 TX.PB39
TX.PB42 TX.PB43 TX.PB44 TX.PB47 TX.PB51 TX.PCOS TX.PECS
TX.POST TX.VHRN
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 = 1.55e+22 dyne-cm
Mw = 4.06
Z = 10 km
Plane Strike Dip Rake
NP1 82 80 -129
NP2 340 40 -15
Principal Axes:
Axis Value Plunge Azimuth
T 1.55e+22 25 201
N 0.00e+00 38 89
P -1.55e+22 41 315
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.64e+21
Mxy 8.64e+21
Mxz -1.10e+22
Myy -2.70e+21
Myz 3.27e+21
Mzz -3.95e+21
-#############
-----------###########
-----------------###########
--------------------##########
------------------------##########
-------- ----------------#########
--------- P -----------------#########
---------- ------------------#########
--------------------------------########
---------------------------------########-
----------------------------------##------
-----------------------------#####--------
-------------------###############--------
#################################-------
#################################-------
################################------
###############################-----
########## ################-----
######## T ###############----
####### ##############----
####################--
##############
Global CMT Convention Moment Tensor:
R T P
-3.95e+21 -1.10e+22 -3.27e+21
-1.10e+22 6.64e+21 -8.64e+21
-3.27e+21 -8.64e+21 -2.70e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20231230193233/index.html
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STK = 340
DIP = 40
RAKE = -15
MW = 4.06
HS = 10.0
The NDK file is 20231230193233.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 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 345 80 5 3.57 0.2315
WVFGRD96 2.0 350 65 25 3.75 0.3069
WVFGRD96 3.0 330 35 -20 3.87 0.3493
WVFGRD96 4.0 335 35 -15 3.90 0.4290
WVFGRD96 5.0 330 35 -25 3.93 0.4863
WVFGRD96 6.0 335 40 -20 3.94 0.5248
WVFGRD96 7.0 335 40 -25 3.96 0.5486
WVFGRD96 8.0 335 40 -25 4.04 0.5677
WVFGRD96 9.0 335 40 -25 4.05 0.5771
WVFGRD96 10.0 340 40 -15 4.06 0.5796
WVFGRD96 11.0 340 45 -15 4.08 0.5764
WVFGRD96 12.0 345 45 -5 4.09 0.5690
WVFGRD96 13.0 345 45 -5 4.10 0.5590
WVFGRD96 14.0 345 45 -5 4.11 0.5458
WVFGRD96 15.0 345 50 -5 4.13 0.5308
WVFGRD96 16.0 345 50 0 4.13 0.5151
WVFGRD96 17.0 350 50 5 4.14 0.4982
WVFGRD96 18.0 350 50 10 4.15 0.4823
WVFGRD96 19.0 350 50 10 4.15 0.4664
WVFGRD96 20.0 350 50 10 4.16 0.4505
WVFGRD96 21.0 350 45 10 4.17 0.4351
WVFGRD96 22.0 350 45 10 4.17 0.4203
WVFGRD96 23.0 350 45 10 4.18 0.4062
WVFGRD96 24.0 350 45 10 4.18 0.3929
WVFGRD96 25.0 350 45 10 4.19 0.3804
WVFGRD96 26.0 350 45 10 4.20 0.3685
WVFGRD96 27.0 350 45 10 4.20 0.3576
WVFGRD96 28.0 350 45 10 4.21 0.3470
WVFGRD96 29.0 350 45 10 4.21 0.3369
The best solution is
WVFGRD96 10.0 340 40 -15 4.06 0.5796
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 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 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