Location

Location ANSS

2021/12/16 04:33:27 32.065 -102.239 10.1 3.7 Texas

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2021/12/16 04:33:27:0 32.06 -102.24 10.1 3.7 Texas Stations used: IM.TX31 N4.MSTX TX.ALPN TX.BRDY TX.DKNS TX.MB01 TX.MB04 TX.MB05 TX.MB06 TX.MB09 TX.MNHN TX.ODSA TX.OZNA TX.PB11 TX.PLPT TX.POST TX.SAND TX.SGCY TX.SMWD TX.SN07 US.AMTX US.JCT 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 0.08 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 2.48e+21 dyne-cm Mw = 3.53 Z = 9 km Plane Strike Dip Rake NP1 100 75 -25 NP2 197 66 -164 Principal Axes: Axis Value Plunge Azimuth T 2.48e+21 6 150 N 0.00e+00 61 251 P -2.48e+21 28 57 Moment Tensor: (dyne-cm) Component Value Mxx 1.25e+21 Mxy -1.95e+21 Mxz -7.94e+20 Myy -7.28e+20 Myz -7.31e+20 Mzz -5.25e+20 ############-- ##############-------- ###############------------- ###############--------------- ################------------------ ################------------- ---- ################-------------- P ----- ################--------------- ------ ###############------------------------- --#############--------------------------- -----##########--------------------------- ---------#####---------------------------- --------------##-------------------------- -------------#############---------##### ------------############################ -----------########################### ----------########################## ---------######################### -------####################### -------############### ### ----############### T -############# Global CMT Convention Moment Tensor: R T P -5.25e+20 -7.94e+20 7.31e+20 -7.94e+20 1.25e+21 1.95e+21 7.31e+20 1.95e+21 -7.28e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20211216043327/index.html

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is

      STK = 100
      DIP = 75
     RAKE = -25
       MW = 3.53
       HS = 9.0

The NDK file is 20211216043327.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2021/12/16 04:33:27:0 32.06 -102.24 10.1 3.7 Texas Stations used: IM.TX31 N4.MSTX TX.ALPN TX.BRDY TX.DKNS TX.MB01 TX.MB04 TX.MB05 TX.MB06 TX.MB09 TX.MNHN TX.ODSA TX.OZNA TX.PB11 TX.PLPT TX.POST TX.SAND TX.SGCY TX.SMWD TX.SN07 US.AMTX US.JCT 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 0.08 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 2.48e+21 dyne-cm Mw = 3.53 Z = 9 km Plane Strike Dip Rake NP1 100 75 -25 NP2 197 66 -164 Principal Axes: Axis Value Plunge Azimuth T 2.48e+21 6 150 N 0.00e+00 61 251 P -2.48e+21 28 57 Moment Tensor: (dyne-cm) Component Value Mxx 1.25e+21 Mxy -1.95e+21 Mxz -7.94e+20 Myy -7.28e+20 Myz -7.31e+20 Mzz -5.25e+20 ############-- ##############-------- ###############------------- ###############--------------- ################------------------ ################------------- ---- ################-------------- P ----- ################--------------- ------ ###############------------------------- --#############--------------------------- -----##########--------------------------- ---------#####---------------------------- --------------##-------------------------- -------------#############---------##### ------------############################ -----------########################### ----------########################## ---------######################### -------####################### -------############### ### ----############### T -############# Global CMT Convention Moment Tensor: R T P -5.25e+20 -7.94e+20 7.31e+20 -7.94e+20 1.25e+21 1.95e+21 7.31e+20 1.95e+21 -7.28e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20211216043327/index.html

Magnitudes

ML Magnitude

(a) ML computed using the IASPEI formula for Horizontal components; (b) 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.

(a) ML computed using the IASPEI formula for Vertical components (research); (b) 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.

Context

The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors).

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for 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 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 0.08 n 3 
br c 0.12 0.25 n 4 p 2

The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   285    85     5   3.16 0.3550
WVFGRD96    2.0   285    80    10   3.32 0.5459
WVFGRD96    3.0   285    75    15   3.38 0.6021
WVFGRD96    4.0   105    90   -10   3.40 0.6273
WVFGRD96    5.0   100    75   -20   3.44 0.6416
WVFGRD96    6.0   100    75   -20   3.46 0.6499
WVFGRD96    7.0   100    75   -20   3.48 0.6545
WVFGRD96    8.0   100    75   -25   3.51 0.6564
WVFGRD96    9.0   100    75   -25   3.53 0.6566
WVFGRD96   10.0   100    75   -25   3.54 0.6555
WVFGRD96   11.0   100    75   -25   3.55 0.6533
WVFGRD96   12.0   100    75   -25   3.57 0.6514
WVFGRD96   13.0   100    75   -25   3.58 0.6488
WVFGRD96   14.0   100    75   -25   3.59 0.6454
WVFGRD96   15.0   100    75   -20   3.59 0.6417
WVFGRD96   16.0   100    75   -20   3.60 0.6371
WVFGRD96   17.0   100    75   -20   3.61 0.6318
WVFGRD96   18.0   100    75   -20   3.62 0.6270
WVFGRD96   19.0   100    75   -20   3.63 0.6207
WVFGRD96   20.0   105    90   -25   3.63 0.6160
WVFGRD96   21.0   105    90   -25   3.64 0.6108
WVFGRD96   22.0   105    90   -25   3.65 0.6061
WVFGRD96   23.0   285    85    25   3.66 0.6008
WVFGRD96   24.0   285    85    25   3.66 0.5966
WVFGRD96   25.0   290    75    35   3.69 0.5919
WVFGRD96   26.0   290    75    35   3.70 0.5891
WVFGRD96   27.0   290    75    35   3.70 0.5861
WVFGRD96   28.0   290    75    35   3.71 0.5822
WVFGRD96   29.0   290    75    35   3.72 0.5791

The best solution is

WVFGRD96    9.0   100    75   -25   3.53 0.6566

The mechanism correspond 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 and because the velocity model used in the predictions may not be perfect. 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 0.08 n 3 
br c 0.12 0.25 n 4 p 2

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.

Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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)

Assuming only a mislocation, the time shifts are fit to a functional form:

 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.

Discussion

Acknowledgements

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureau of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Oklahoma Geological Survey, TexNet, the Iris stations, the Transportable Array of EarthScope and other networks.

Velocity Model

The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

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

Quality Control

Here we tabulate the reasons for not using certain digital data sets

The following stations did not have a valid response files:

Last Changed Thu Dec 16 04:43:20 CST 2021