Location

Location ANSS

2022/10/22 05:02:35 61.817 -151.082 63.3 4.4 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2022/10/22 05:02:35:0 61.82 -151.08 63.3 4.4 Alaska Stations used: AK.CAST AK.CUT AK.DHY AK.DIV AK.FID AK.GHO AK.GLI AK.K20K AK.KNK AK.KTH AK.L19K AK.L20K AK.L22K AK.N19K AK.O19K AK.RC01 AK.SAW AK.SCM AK.SLK AK.SSN AT.PMR AV.RED AV.SPCP AV.STLK Filtering commands used: cut o DIST/3.3 -50 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 Best Fitting Double Couple Mo = 4.84e+22 dyne-cm Mw = 4.39 Z = 74 km Plane Strike Dip Rake NP1 185 70 -80 NP2 338 22 -116 Principal Axes: Axis Value Plunge Azimuth T 4.84e+22 24 267 N 0.00e+00 9 2 P -4.84e+22 64 111 Moment Tensor: (dyne-cm) Component Value Mxx -1.14e+21 Mxy 5.12e+21 Mxz 6.05e+21 Myy 3.18e+22 Myz -3.61e+22 Mzz -3.06e+22 #####---###### ###########----####### #############---------###### #############------------##### ##############---------------##### ###############----------------##### ###############------------------##### ################-------------------##### ###############---------------------#### ################---------------------##### #### #########----------------------#### #### T #########---------- ---------#### #### #########---------- P ---------#### ###############---------- ---------### ###############---------------------#### ##############---------------------### #############--------------------### ############-------------------### ###########-----------------## ##########----------------## ########-------------# #####--------- Global CMT Convention Moment Tensor: R T P -3.06e+22 6.05e+21 3.61e+22 6.05e+21 -1.14e+21 -5.12e+21 3.61e+22 -5.12e+21 3.18e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20221022050235/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 = 185
      DIP = 70
     RAKE = -80
       MW = 4.39
       HS = 74.0

The NDK file is 20221022050235.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 2022/10/22 05:02:35:0 61.82 -151.08 63.3 4.4 Alaska Stations used: AK.CAST AK.CUT AK.DHY AK.DIV AK.FID AK.GHO AK.GLI AK.K20K AK.KNK AK.KTH AK.L19K AK.L20K AK.L22K AK.N19K AK.O19K AK.RC01 AK.SAW AK.SCM AK.SLK AK.SSN AT.PMR AV.RED AV.SPCP AV.STLK Filtering commands used: cut o DIST/3.3 -50 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 Best Fitting Double Couple Mo = 4.84e+22 dyne-cm Mw = 4.39 Z = 74 km Plane Strike Dip Rake NP1 185 70 -80 NP2 338 22 -116 Principal Axes: Axis Value Plunge Azimuth T 4.84e+22 24 267 N 0.00e+00 9 2 P -4.84e+22 64 111 Moment Tensor: (dyne-cm) Component Value Mxx -1.14e+21 Mxy 5.12e+21 Mxz 6.05e+21 Myy 3.18e+22 Myz -3.61e+22 Mzz -3.06e+22 #####---###### ###########----####### #############---------###### #############------------##### ##############---------------##### ###############----------------##### ###############------------------##### ################-------------------##### ###############---------------------#### ################---------------------##### #### #########----------------------#### #### T #########---------- ---------#### #### #########---------- P ---------#### ###############---------- ---------### ###############---------------------#### ##############---------------------### #############--------------------### ############-------------------### ###########-----------------## ##########----------------## ########-------------# #####--------- Global CMT Convention Moment Tensor: R T P -3.06e+22 6.05e+21 3.61e+22 6.05e+21 -1.14e+21 -5.12e+21 3.61e+22 -5.12e+21 3.18e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20221022050235/index.html

Regional Moment Tensor (Mwr) Moment 5.377e+15 N-m Magnitude 4.42 Mwr Depth 76.0 km Percent DC 92% Half Duration - Catalog US Data Source US 3 Contributor US 3 Nodal Planes Plane Strike Dip Rake NP1 347 21 -98 NP2 175 70 -87 Principal Axes Axis Value Plunge Azimuth T 5.270e+15 N-m 25 263 N 0.209e+15 N-m 3 354 P -5.479e+15 N-m 65 91

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 -50 o DIST/3.3 +40
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    2.0   335    45    90   3.59 0.1931
WVFGRD96    4.0     5    55   -55   3.65 0.2073
WVFGRD96    6.0    35    40     5   3.67 0.2462
WVFGRD96    8.0    25    35   -15   3.76 0.2790
WVFGRD96   10.0    25    40   -20   3.79 0.3020
WVFGRD96   12.0    25    45   -15   3.81 0.3121
WVFGRD96   14.0    25    45   -15   3.83 0.3152
WVFGRD96   16.0    25    45   -15   3.85 0.3147
WVFGRD96   18.0   215    60    40   3.87 0.3148
WVFGRD96   20.0   210    60    35   3.89 0.3206
WVFGRD96   22.0   215    60    40   3.91 0.3253
WVFGRD96   24.0   210    65    35   3.93 0.3294
WVFGRD96   26.0   210    65    35   3.95 0.3322
WVFGRD96   28.0    40    50    35   3.99 0.3358
WVFGRD96   30.0    35    50    30   4.01 0.3391
WVFGRD96   32.0    35    50    25   4.02 0.3398
WVFGRD96   34.0    35    50    20   4.04 0.3435
WVFGRD96   36.0    30    55    20   4.05 0.3500
WVFGRD96   38.0    30    55    20   4.07 0.3565
WVFGRD96   40.0   350    30   -80   4.18 0.3798
WVFGRD96   42.0   160    60   -90   4.21 0.3980
WVFGRD96   44.0   340    30   -95   4.23 0.4136
WVFGRD96   46.0   165    60   -85   4.25 0.4255
WVFGRD96   48.0   175    60   -75   4.27 0.4382
WVFGRD96   50.0   170    65   -85   4.28 0.4574
WVFGRD96   52.0   170    65   -85   4.30 0.4759
WVFGRD96   54.0   175    65   -80   4.31 0.4949
WVFGRD96   56.0   180    65   -75   4.32 0.5129
WVFGRD96   58.0   180    65   -75   4.33 0.5306
WVFGRD96   60.0   180    65   -75   4.34 0.5443
WVFGRD96   62.0   180    65   -75   4.35 0.5552
WVFGRD96   64.0   180    65   -75   4.35 0.5643
WVFGRD96   66.0   180    65   -75   4.36 0.5710
WVFGRD96   68.0   185    65   -75   4.37 0.5758
WVFGRD96   70.0   185    65   -75   4.37 0.5778
WVFGRD96   72.0   185    70   -80   4.39 0.5798
WVFGRD96   74.0   185    70   -80   4.39 0.5816
WVFGRD96   76.0   185    70   -80   4.39 0.5815
WVFGRD96   78.0   185    70   -80   4.40 0.5803
WVFGRD96   80.0   185    70   -80   4.40 0.5778
WVFGRD96   82.0   180    70   -80   4.40 0.5738
WVFGRD96   84.0   185    70   -75   4.40 0.5691
WVFGRD96   86.0   185    70   -75   4.40 0.5627
WVFGRD96   88.0   185    70   -75   4.40 0.5572
WVFGRD96   90.0   185    70   -75   4.40 0.5498
WVFGRD96   92.0   185    70   -75   4.41 0.5432
WVFGRD96   94.0   180    65   -75   4.40 0.5375
WVFGRD96   96.0   180    65   -75   4.40 0.5319
WVFGRD96   98.0   180    65   -70   4.40 0.5272

The best solution is

WVFGRD96   74.0   185    70   -80   4.39 0.5816

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 -50 o DIST/3.3 +40
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 Oct 22 06:59:04 CDT 2022