Location

Location ANSS

2022/07/09 10:56:01 60.852 -146.857 24.1 4.1 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2022/07/09 10:56:01:0 60.85 -146.86 24.1 4.1 Alaska Stations used: AK.BARN AK.BMR AK.BPAW AK.BRLK AK.CAST AK.CCB AK.CNP AK.CUT AK.DHY AK.DIV AK.EYAK AK.FID AK.GHO AK.GLB AK.GLI AK.HIN AK.HOM AK.J26L AK.KLU AK.KNK AK.L26K AK.LOGN AK.M26K AK.MCAR AK.MCK AK.NEA2 AK.P23K AK.PAX AK.RC01 AK.RND AK.SAW AK.SCM AK.SWD AK.VRDI AT.PMR AV.SPCP 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 Best Fitting Double Couple Mo = 3.43e+22 dyne-cm Mw = 4.29 Z = 33 km Plane Strike Dip Rake NP1 331 67 -153 NP2 230 65 -25 Principal Axes: Axis Value Plunge Azimuth T 3.43e+22 2 100 N 0.00e+00 55 8 P -3.43e+22 35 191 Moment Tensor: (dyne-cm) Component Value Mxx -2.12e+22 Mxy -1.04e+22 Mxz 1.56e+22 Myy 3.23e+22 Myz 4.07e+21 Mzz -1.11e+22 -------------- ##-------------------- ########-------------------- ############------------------ ###############----------######### ##################----############## ###################################### ##################-----################# ################--------################ ###############-----------################ #############--------------############### ###########-----------------############ ##########-------------------########### T #######---------------------########### ######-----------------------########### #####-----------------------########## ###-------------------------######## #------------ -----------####### ----------- P -----------##### ---------- -----------#### ---------------------# -------------- Global CMT Convention Moment Tensor: R T P -1.11e+22 1.56e+22 -4.07e+21 1.56e+22 -2.12e+22 1.04e+22 -4.07e+21 1.04e+22 3.23e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20220709105601/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 = 230
      DIP = 65
     RAKE = -25
       MW = 4.29
       HS = 33.0

The NDK file is 20220709105601.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2022/07/09 10:56:01:0 60.85 -146.86 24.1 4.1 Alaska Stations used: AK.BARN AK.BMR AK.BPAW AK.BRLK AK.CAST AK.CCB AK.CNP AK.CUT AK.DHY AK.DIV AK.EYAK AK.FID AK.GHO AK.GLB AK.GLI AK.HIN AK.HOM AK.J26L AK.KLU AK.KNK AK.L26K AK.LOGN AK.M26K AK.MCAR AK.MCK AK.NEA2 AK.P23K AK.PAX AK.RC01 AK.RND AK.SAW AK.SCM AK.SWD AK.VRDI AT.PMR AV.SPCP 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 Best Fitting Double Couple Mo = 3.43e+22 dyne-cm Mw = 4.29 Z = 33 km Plane Strike Dip Rake NP1 331 67 -153 NP2 230 65 -25 Principal Axes: Axis Value Plunge Azimuth T 3.43e+22 2 100 N 0.00e+00 55 8 P -3.43e+22 35 191 Moment Tensor: (dyne-cm) Component Value Mxx -2.12e+22 Mxy -1.04e+22 Mxz 1.56e+22 Myy 3.23e+22 Myz 4.07e+21 Mzz -1.11e+22 -------------- ##-------------------- ########-------------------- ############------------------ ###############----------######### ##################----############## ###################################### ##################-----################# ################--------################ ###############-----------################ #############--------------############### ###########-----------------############ ##########-------------------########### T #######---------------------########### ######-----------------------########### #####-----------------------########## ###-------------------------######## #------------ -----------####### ----------- P -----------##### ---------- -----------#### ---------------------# -------------- Global CMT Convention Moment Tensor: R T P -1.11e+22 1.56e+22 -4.07e+21 1.56e+22 -2.12e+22 1.04e+22 -4.07e+21 1.04e+22 3.23e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20220709105601/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

The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0    90    90     0   3.58 0.2238
WVFGRD96    2.0   280    45    85   3.81 0.3030
WVFGRD96    3.0    60    55     0   3.74 0.3239
WVFGRD96    4.0    60    55     5   3.79 0.3529
WVFGRD96    5.0    60    55     5   3.82 0.3799
WVFGRD96    6.0    60    60    10   3.85 0.4048
WVFGRD96    7.0    60    60    10   3.88 0.4260
WVFGRD96    8.0    60    55    15   3.94 0.4487
WVFGRD96    9.0    60    55    15   3.96 0.4638
WVFGRD96   10.0    60    55    15   3.98 0.4768
WVFGRD96   11.0    60    55    15   3.99 0.4888
WVFGRD96   12.0    60    60    20   4.01 0.4989
WVFGRD96   13.0    60    60    15   4.02 0.5072
WVFGRD96   14.0    50    65   -25   4.04 0.5256
WVFGRD96   15.0    50    65   -25   4.06 0.5406
WVFGRD96   16.0    50    65   -25   4.07 0.5546
WVFGRD96   17.0   235    70   -15   4.09 0.5697
WVFGRD96   18.0   235    70   -15   4.11 0.5880
WVFGRD96   19.0   235    70   -15   4.13 0.6056
WVFGRD96   20.0   235    70   -20   4.14 0.6234
WVFGRD96   21.0   230    65   -20   4.16 0.6401
WVFGRD96   22.0   230    65   -20   4.18 0.6562
WVFGRD96   23.0   230    65   -20   4.19 0.6713
WVFGRD96   24.0   230    65   -20   4.20 0.6857
WVFGRD96   25.0   230    65   -20   4.21 0.7006
WVFGRD96   26.0   230    65   -25   4.22 0.7147
WVFGRD96   27.0   230    65   -25   4.23 0.7271
WVFGRD96   28.0   230    65   -25   4.25 0.7382
WVFGRD96   29.0   230    65   -25   4.25 0.7470
WVFGRD96   30.0   230    65   -25   4.26 0.7538
WVFGRD96   31.0   230    65   -25   4.27 0.7576
WVFGRD96   32.0   230    65   -25   4.28 0.7593
WVFGRD96   33.0   230    65   -25   4.29 0.7593
WVFGRD96   34.0   230    65   -25   4.30 0.7571
WVFGRD96   35.0   230    65   -25   4.30 0.7532
WVFGRD96   36.0   230    65   -25   4.31 0.7479
WVFGRD96   37.0   230    65   -25   4.32 0.7412
WVFGRD96   38.0   235    70   -20   4.33 0.7369
WVFGRD96   39.0   235    70   -20   4.35 0.7338
WVFGRD96   40.0   230    65   -25   4.39 0.7380
WVFGRD96   41.0   230    65   -25   4.41 0.7418
WVFGRD96   42.0   230    65   -25   4.42 0.7429
WVFGRD96   43.0   230    65   -25   4.42 0.7402
WVFGRD96   44.0   230    65   -25   4.43 0.7373
WVFGRD96   45.0   230    65   -25   4.44 0.7325
WVFGRD96   46.0   230    65   -25   4.44 0.7269
WVFGRD96   47.0   235    70   -20   4.45 0.7227
WVFGRD96   48.0   235    70   -20   4.45 0.7171
WVFGRD96   49.0   235    70   -20   4.46 0.7117
WVFGRD96   50.0   235    70   -20   4.46 0.7058
WVFGRD96   51.0   235    70   -20   4.46 0.7003
WVFGRD96   52.0   235    70   -20   4.47 0.6928
WVFGRD96   53.0   235    70   -20   4.47 0.6872
WVFGRD96   54.0   235    70   -20   4.47 0.6805
WVFGRD96   55.0   235    75   -20   4.47 0.6745
WVFGRD96   56.0   235    75   -20   4.47 0.6688
WVFGRD96   57.0   235    75   -20   4.48 0.6638
WVFGRD96   58.0   235    75   -20   4.48 0.6580
WVFGRD96   59.0   235    75   -20   4.48 0.6523

The best solution is

WVFGRD96   33.0   230    65   -25   4.29 0.7593

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

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 Sat Jul 9 06:33:03 CDT 2022