Location

Location ANSS

2020/09/11 13:58:32 61.630 -152.186 120.3 4.3 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2020/09/11 13:58:32:0 61.63 -152.19 120.3 4.3 Alaska Stations used: AK.BRLK AK.BWN AK.CAPN AK.CAST AK.CNP AK.CUT AK.DHY AK.DIV AK.FID AK.GHO AK.GLI AK.HIN AK.J19K AK.J20K AK.K20K AK.KTH AK.L18K AK.L19K AK.L20K AK.M20K AK.MCK AK.N18K AK.N19K AK.NEA2 AK.O18K AK.O19K AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SLK AK.TRF AK.WRH AT.PMR AV.ILSW AV.STLK TA.J18K TA.K17K TA.M22K TA.N17K TA.O22K Filtering commands used: cut o DIST/3.4 -40 o DIST/3.4 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 4.52e+22 dyne-cm Mw = 4.37 Z = 128 km Plane Strike Dip Rake NP1 309 63 127 NP2 70 45 40 Principal Axes: Axis Value Plunge Azimuth T 4.52e+22 55 268 N 0.00e+00 33 110 P -4.52e+22 10 13 Moment Tensor: (dyne-cm) Component Value Mxx -4.14e+22 Mxy -9.41e+21 Mxz -8.37e+21 Myy 1.23e+22 Myz -2.30e+22 Mzz 2.90e+22 ---------- - -------------- P ----- ----------------- -------- ------------------------------ ##########------------------------ ###############--------------------- ###################------------------- #######################----------------# #########################--------------# ############################-----------### ########### ################---------### ########### T ##################------#### ########### ###################---###### ######################################## ###############################---###### -###########################------#### ---####################-----------## --------#######------------------# ------------------------------ ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 2.90e+22 -8.37e+21 2.30e+22 -8.37e+21 -4.14e+22 9.41e+21 2.30e+22 9.41e+21 1.23e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200911135832/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 = 70
      DIP = 45
     RAKE = 40
       MW = 4.37
       HS = 128.0

The NDK file is 20200911135832.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2020/09/11 13:58:32:0 61.63 -152.19 120.3 4.3 Alaska Stations used: AK.BRLK AK.BWN AK.CAPN AK.CAST AK.CNP AK.CUT AK.DHY AK.DIV AK.FID AK.GHO AK.GLI AK.HIN AK.J19K AK.J20K AK.K20K AK.KTH AK.L18K AK.L19K AK.L20K AK.M20K AK.MCK AK.N18K AK.N19K AK.NEA2 AK.O18K AK.O19K AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SLK AK.TRF AK.WRH AT.PMR AV.ILSW AV.STLK TA.J18K TA.K17K TA.M22K TA.N17K TA.O22K Filtering commands used: cut o DIST/3.4 -40 o DIST/3.4 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 4.52e+22 dyne-cm Mw = 4.37 Z = 128 km Plane Strike Dip Rake NP1 309 63 127 NP2 70 45 40 Principal Axes: Axis Value Plunge Azimuth T 4.52e+22 55 268 N 0.00e+00 33 110 P -4.52e+22 10 13 Moment Tensor: (dyne-cm) Component Value Mxx -4.14e+22 Mxy -9.41e+21 Mxz -8.37e+21 Myy 1.23e+22 Myz -2.30e+22 Mzz 2.90e+22 ---------- - -------------- P ----- ----------------- -------- ------------------------------ ##########------------------------ ###############--------------------- ###################------------------- #######################----------------# #########################--------------# ############################-----------### ########### ################---------### ########### T ##################------#### ########### ###################---###### ######################################## ###############################---###### -###########################------#### ---####################-----------## --------#######------------------# ------------------------------ ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 2.90e+22 -8.37e+21 2.30e+22 -8.37e+21 -4.14e+22 9.41e+21 2.30e+22 9.41e+21 1.23e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200911135832/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/3.4 -40 o DIST/3.4 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3

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

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    2.0   100    45  -100   3.44 0.1097
WVFGRD96    4.0   325    60   -35   3.45 0.1158
WVFGRD96    6.0   330    80   -40   3.49 0.1254
WVFGRD96    8.0   330    80   -40   3.57 0.1324
WVFGRD96   10.0   165    65    40   3.62 0.1380
WVFGRD96   12.0   340    50    30   3.65 0.1408
WVFGRD96   14.0   340    50    30   3.68 0.1410
WVFGRD96   16.0   250    55    35   3.70 0.1411
WVFGRD96   18.0   250    50    35   3.72 0.1393
WVFGRD96   20.0   250    50    35   3.75 0.1370
WVFGRD96   22.0   250    50    35   3.77 0.1367
WVFGRD96   24.0   250    50    35   3.79 0.1365
WVFGRD96   26.0   250    55    30   3.82 0.1375
WVFGRD96   28.0   250    60    30   3.84 0.1400
WVFGRD96   30.0   250    60    30   3.86 0.1436
WVFGRD96   32.0   250    65    25   3.88 0.1451
WVFGRD96   34.0   250    65    25   3.89 0.1446
WVFGRD96   36.0   250    65    25   3.91 0.1423
WVFGRD96   38.0   250    70    20   3.94 0.1410
WVFGRD96   40.0   250    70    25   4.01 0.1408
WVFGRD96   42.0   250    70    25   4.03 0.1401
WVFGRD96   44.0   250    70    25   4.06 0.1407
WVFGRD96   46.0   250    70    25   4.08 0.1415
WVFGRD96   48.0   250    70    25   4.09 0.1431
WVFGRD96   50.0   245    75    25   4.10 0.1458
WVFGRD96   52.0   245    75    25   4.12 0.1512
WVFGRD96   54.0   245    80    25   4.14 0.1611
WVFGRD96   56.0   245    80    20   4.15 0.1734
WVFGRD96   58.0    60    90   -25   4.17 0.1883
WVFGRD96   60.0    60    85   -20   4.19 0.2079
WVFGRD96   62.0    60    85   -20   4.21 0.2283
WVFGRD96   64.0    60    85   -20   4.22 0.2480
WVFGRD96   66.0    55    75   -15   4.23 0.2670
WVFGRD96   68.0    70    45    40   4.22 0.2799
WVFGRD96   70.0    70    45    40   4.23 0.2944
WVFGRD96   72.0    65    45    40   4.24 0.3095
WVFGRD96   74.0    65    45    40   4.25 0.3220
WVFGRD96   76.0    65    45    35   4.26 0.3315
WVFGRD96   78.0    65    45    35   4.27 0.3409
WVFGRD96   80.0    65    45    35   4.27 0.3501
WVFGRD96   82.0    65    45    35   4.28 0.3592
WVFGRD96   84.0    65    45    35   4.29 0.3670
WVFGRD96   86.0    65    45    35   4.29 0.3742
WVFGRD96   88.0    65    45    35   4.30 0.3810
WVFGRD96   90.0    65    45    35   4.31 0.3872
WVFGRD96   92.0    65    45    35   4.31 0.3930
WVFGRD96   94.0    65    45    35   4.32 0.3994
WVFGRD96   96.0    65    45    35   4.32 0.4046
WVFGRD96   98.0    65    45    35   4.33 0.4101
WVFGRD96  100.0    65    45    35   4.33 0.4146
WVFGRD96  102.0    65    45    35   4.33 0.4195
WVFGRD96  104.0    70    45    40   4.34 0.4235
WVFGRD96  106.0    70    45    40   4.34 0.4276
WVFGRD96  108.0    70    45    40   4.34 0.4309
WVFGRD96  110.0    70    45    40   4.35 0.4341
WVFGRD96  112.0    70    45    40   4.35 0.4364
WVFGRD96  114.0    70    45    40   4.35 0.4383
WVFGRD96  116.0    70    45    40   4.36 0.4405
WVFGRD96  118.0    70    45    40   4.36 0.4426
WVFGRD96  120.0    70    45    40   4.36 0.4442
WVFGRD96  122.0    70    45    40   4.37 0.4449
WVFGRD96  124.0    70    45    40   4.37 0.4449
WVFGRD96  126.0    70    45    40   4.37 0.4447
WVFGRD96  128.0    70    45    40   4.37 0.4457
WVFGRD96  130.0    70    45    40   4.38 0.4453
WVFGRD96  132.0    70    45    40   4.38 0.4445
WVFGRD96  134.0    70    45    40   4.38 0.4431
WVFGRD96  136.0    70    45    40   4.38 0.4428
WVFGRD96  138.0    70    45    40   4.38 0.4413
WVFGRD96  140.0    70    45    40   4.38 0.4384
WVFGRD96  142.0    70    45    40   4.39 0.4374
WVFGRD96  144.0    70    45    40   4.39 0.4355
WVFGRD96  146.0    70    50    40   4.39 0.4335
WVFGRD96  148.0    70    50    40   4.39 0.4320

The best solution is

WVFGRD96  128.0    70    45    40   4.37 0.4457

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/3.4 -40 o DIST/3.4 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 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 Fri Sep 11 10:38:42 CDT 2020