Location

Location ANSS

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2023/03/19 11:10:10:0 37.28 -104.44 4.2 3.8 Colorado Stations used: C0.LAMA C0.MCSU C0.Q24A C0.SPVA C0.T25A IU.ANMO TX.PH02 TX.RTBA US.ISCO US.MVCO US.SDCO YX.UNM2 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 Best Fitting Double Couple Mo = 4.79e+21 dyne-cm Mw = 3.72 Z = 6 km Plane Strike Dip Rake NP1 112 50 -113 NP2 325 45 -65 Principal Axes: Axis Value Plunge Azimuth T 4.79e+21 3 218 N 0.00e+00 17 127 P -4.79e+21 72 316 Moment Tensor: (dyne-cm) Component Value Mxx 2.77e+21 Mxy 2.53e+21 Mxz -1.17e+21 Myy 1.57e+21 Myz 8.20e+20 Mzz -4.34e+21 ############## ###################### ------------################ ----------------############## ---------------------############# ------------------------############ ---------------------------########### #----------------------------########### #--------------- -----------########## ###-------------- P ------------########## ####------------- -------------######### ######----------------------------######## #######---------------------------######## ########--------------------------###### ###########-----------------------###### #############--------------------####- #################---------------#--- ###############################--- # ########################-- T ########################- ###################### ############## Global CMT Convention Moment Tensor: R T P -4.34e+21 -1.17e+21 -8.20e+20 -1.17e+21 2.77e+21 -2.53e+21 -8.20e+20 -2.53e+21 1.57e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230319111010/index.html

Preferred Solution

      STK = 325
      DIP = 45
     RAKE = -65
       MW = 3.72
       HS = 6.0

The NDK file is 20230319111010.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGSMWR

USGS/SLU Moment Tensor Solution ENS 2023/03/19 11:10:10:0 37.28 -104.44 4.2 3.8 Colorado Stations used: C0.LAMA C0.MCSU C0.Q24A C0.SPVA C0.T25A IU.ANMO TX.PH02 TX.RTBA US.ISCO US.MVCO US.SDCO YX.UNM2 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 Best Fitting Double Couple Mo = 4.79e+21 dyne-cm Mw = 3.72 Z = 6 km Plane Strike Dip Rake NP1 112 50 -113 NP2 325 45 -65 Principal Axes: Axis Value Plunge Azimuth T 4.79e+21 3 218 N 0.00e+00 17 127 P -4.79e+21 72 316 Moment Tensor: (dyne-cm) Component Value Mxx 2.77e+21 Mxy 2.53e+21 Mxz -1.17e+21 Myy 1.57e+21 Myz 8.20e+20 Mzz -4.34e+21 ############## ###################### ------------################ ----------------############## ---------------------############# ------------------------############ ---------------------------########### #----------------------------########### #--------------- -----------########## ###-------------- P ------------########## ####------------- -------------######### ######----------------------------######## #######---------------------------######## ########--------------------------###### ###########-----------------------###### #############--------------------####- #################---------------#--- ###############################--- # ########################-- T ########################- ###################### ############## Global CMT Convention Moment Tensor: R T P -4.34e+21 -1.17e+21 -8.20e+20 -1.17e+21 2.77e+21 -2.53e+21 -8.20e+20 -2.53e+21 1.57e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230319111010/index.html

Regional Moment Tensor (Mwr) Moment 5.957e+14 N-m Magnitude 3.78 Mwr Depth 5.0 km Percent DC 71% Half Duration - Catalog US Data Source US 1 Contributor US 1 Nodal Planes Plane Strike Dip Rake NP1 127 50 -97 NP2 318 40 -82 Principal Axes Axis Value Plunge Azimuth T 6.369e+14 N-m 5 222 N -0.933e+14 N-m 5 132 P -5.436e+14 N-m 83 356

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

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 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   335    55   -50   3.44 0.3814
WVFGRD96    2.0   335    50   -50   3.56 0.4488
WVFGRD96    3.0   335    40   -45   3.64 0.5027
WVFGRD96    4.0   330    40   -55   3.68 0.5598
WVFGRD96    5.0   325    40   -65   3.71 0.6025
WVFGRD96    6.0   325    45   -65   3.72 0.6048
WVFGRD96    7.0   115    50  -105   3.73 0.5750
WVFGRD96    8.0   320    45   -75   3.79 0.6033
WVFGRD96    9.0   135    50   -85   3.79 0.5416
WVFGRD96   10.0   170    80   -25   3.75 0.4982
WVFGRD96   11.0   355    85    20   3.76 0.4948
WVFGRD96   12.0   355    80    20   3.77 0.4886
WVFGRD96   13.0   355    80    20   3.78 0.4785
WVFGRD96   14.0    -5    80    15   3.78 0.4684
WVFGRD96   15.0   355    75    15   3.78 0.4574
WVFGRD96   16.0   355    75    15   3.79 0.4491
WVFGRD96   17.0    -5    75    15   3.80 0.4411
WVFGRD96   18.0    -5    75    15   3.81 0.4332
WVFGRD96   19.0    -5    70    15   3.81 0.4277
WVFGRD96   20.0    -5    70    15   3.82 0.4235
WVFGRD96   21.0     0    70    15   3.82 0.4201
WVFGRD96   22.0     0    70    15   3.82 0.4150
WVFGRD96   23.0    -5    70    10   3.84 0.4096
WVFGRD96   24.0     0    70    10   3.83 0.4084
WVFGRD96   25.0     0    70    10   3.84 0.4074
WVFGRD96   26.0    -5    70     5   3.86 0.4064
WVFGRD96   27.0    -5    70     5   3.87 0.4050
WVFGRD96   28.0    -5    70     5   3.88 0.4027
WVFGRD96   29.0    -5    70     5   3.89 0.4006

The best solution is

WVFGRD96    6.0   325    45   -65   3.72 0.6048

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 

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 based on the last trace plotted. It is not used to represent travel time or absolute time, but rather to indicate the number of seconds plotted. If there is not a trace directly above the scale, then a default, meaningless scale is plotted.

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)

 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: