Location

SLU Location

In order to test the moment tensor solution, P-wave first motions were compared to the prediction of the moment tensor solution. The results are not overwhelming. Thus the depth and moment of this solution may be believed. The results of running ,b>elocate are given in elocate.txt.

Location ANSS

2021/09/13 09:27:14 37.427 -102.931 5.0 3.8 Colorado

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2021/09/13 09:27:14:0 37.43 -102.93 5.0 3.8 Colorado Stations used: C0.LAMA C0.Q24A C0.T25A GS.ALQ1 N4.KSCO TX.DRZT TX.PH02 TX.SMWD US.CBKS US.ISCO US.SDCO YX.UNM2 YX.UNM3 YX.UNM5 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 0.06 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 3.16e+21 dyne-cm Mw = 3.60 Z = 17 km Plane Strike Dip Rake NP1 320 65 40 NP2 210 54 149 Principal Axes: Axis Value Plunge Azimuth T 3.16e+21 45 180 N 0.00e+00 44 347 P -3.16e+21 6 83 Moment Tensor: (dyne-cm) Component Value Mxx 1.52e+21 Mxy -3.85e+20 Mxz -1.62e+21 Myy -3.08e+21 Myz -3.43e+20 Mzz 1.56e+21 ############## ###################--- --###############----------- --------########-------------- -------------##------------------- --------------###------------------- --------------######------------------ -------------##########----------------- ------------############-------------- ------------###############------------ P -----------#################----------- -----------###################------------ ----------#####################----------- ---------######################--------- --------#######################--------- -------########################------- ------########### ###########----- -----########### T ###########---- ---########### ###########-- ---########################- ###################### ############## Global CMT Convention Moment Tensor: R T P 1.56e+21 -1.62e+21 3.43e+20 -1.62e+21 1.52e+21 3.85e+20 3.43e+20 3.85e+20 -3.08e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210913092714/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 = 320
      DIP = 65
     RAKE = 40
       MW = 3.60
       HS = 17.0

The NDK file is 20210913092714.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU SLUFM

USGS/SLU Moment Tensor Solution ENS 2021/09/13 09:27:14:0 37.43 -102.93 5.0 3.8 Colorado Stations used: C0.LAMA C0.Q24A C0.T25A GS.ALQ1 N4.KSCO TX.DRZT TX.PH02 TX.SMWD US.CBKS US.ISCO US.SDCO YX.UNM2 YX.UNM3 YX.UNM5 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 0.06 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 3.16e+21 dyne-cm Mw = 3.60 Z = 17 km Plane Strike Dip Rake NP1 320 65 40 NP2 210 54 149 Principal Axes: Axis Value Plunge Azimuth T 3.16e+21 45 180 N 0.00e+00 44 347 P -3.16e+21 6 83 Moment Tensor: (dyne-cm) Component Value Mxx 1.52e+21 Mxy -3.85e+20 Mxz -1.62e+21 Myy -3.08e+21 Myz -3.43e+20 Mzz 1.56e+21 ############## ###################--- --###############----------- --------########-------------- -------------##------------------- --------------###------------------- --------------######------------------ -------------##########----------------- ------------############-------------- ------------###############------------ P -----------#################----------- -----------###################------------ ----------#####################----------- ---------######################--------- --------#######################--------- -------########################------- ------########### ###########----- -----########### T ###########---- ---########### ###########-- ---########################- ###################### ############## Global CMT Convention Moment Tensor: R T P 1.56e+21 -1.62e+21 3.43e+20 -1.62e+21 1.52e+21 3.85e+20 3.43e+20 3.85e+20 -3.08e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210913092714/index.html

First motions and takeoff angles from an elocate run.

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 0.06 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   310    85   -10   3.30 0.3843
WVFGRD96    2.0   310    85   -15   3.37 0.4196
WVFGRD96    3.0   310    85   -20   3.41 0.4160
WVFGRD96    4.0   310    85   -35   3.46 0.4108
WVFGRD96    5.0   315    85    60   3.53 0.4286
WVFGRD96    6.0   320    80    60   3.53 0.4482
WVFGRD96    7.0   315    80    55   3.51 0.4621
WVFGRD96    8.0   320    80    60   3.57 0.4741
WVFGRD96    9.0   325    75    60   3.57 0.4859
WVFGRD96   10.0   325    70    55   3.58 0.4996
WVFGRD96   11.0   325    65    55   3.58 0.5104
WVFGRD96   12.0   330    60    55   3.60 0.5208
WVFGRD96   13.0   325    60    50   3.60 0.5267
WVFGRD96   14.0   325    60    45   3.60 0.5316
WVFGRD96   15.0   325    60    45   3.60 0.5339
WVFGRD96   16.0   320    65    40   3.60 0.5350
WVFGRD96   17.0   320    65    40   3.60 0.5352
WVFGRD96   18.0   320    65    35   3.61 0.5342
WVFGRD96   19.0   125    70   -35   3.61 0.5338
WVFGRD96   20.0   125    70   -35   3.61 0.5348
WVFGRD96   21.0   125    70   -35   3.62 0.5344
WVFGRD96   22.0   125    70   -35   3.63 0.5330
WVFGRD96   23.0   125    70   -35   3.63 0.5305
WVFGRD96   24.0   125    70   -35   3.64 0.5270
WVFGRD96   25.0   125    75   -35   3.64 0.5227
WVFGRD96   26.0   125    75   -35   3.65 0.5180
WVFGRD96   27.0   125    75   -35   3.65 0.5124
WVFGRD96   28.0   125    75   -35   3.66 0.5061
WVFGRD96   29.0   125    75   -35   3.66 0.4992

The best solution is

WVFGRD96   17.0   320    65    40   3.60 0.5352

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 0.06 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 Mon Sep 13 07:33:49 CDT 2021