Location

Location ANSS

2021/03/03 05:35:26 61.453 -151.924 99.9 4.5 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2021/03/03 05:35:26:0 61.45 -151.92 99.9 4.5 Alaska Stations used: AK.CAPN AK.CUT AK.GHO AK.K20K AK.L19K AK.L20K AK.L22K AK.M20K AK.N19K AK.O19K AK.PPLA AK.RC01 AK.SKN AK.SLK AK.TRF AV.RED AV.SPCP TA.O22K 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 = 9.33e+22 dyne-cm Mw = 4.58 Z = 110 km Plane Strike Dip Rake NP1 262 45 95 NP2 75 45 85 Principal Axes: Axis Value Plunge Azimuth T 9.33e+22 86 257 N 0.00e+00 4 79 P -9.33e+22 0 349 Moment Tensor: (dyne-cm) Component Value Mxx -8.96e+22 Mxy 1.83e+22 Mxz -1.49e+21 Myy -3.35e+21 Myz -5.56e+21 Mzz 9.30e+22 - P ---------- ----- -------------- ---------------------------- ------------------------------ ---------------------------------- ------------################-------- ---------########################----- -------##############################--- ----###################################- ----####################################-- --############## ####################--- -############### T ###################---- ################ ##################----- ##################################------ -###############################-------- --##########################---------- -----##################------------- ---------------------------------- ------------------------------ ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 9.30e+22 -1.49e+21 5.56e+21 -1.49e+21 -8.96e+22 -1.83e+22 5.56e+21 -1.83e+22 -3.35e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210303053526/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 = 75
      DIP = 45
     RAKE = 85
       MW = 4.58
       HS = 110.0

The NDK file is 20210303053526.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2021/03/03 05:35:26:0 61.45 -151.92 99.9 4.5 Alaska Stations used: AK.CAPN AK.CUT AK.GHO AK.K20K AK.L19K AK.L20K AK.L22K AK.M20K AK.N19K AK.O19K AK.PPLA AK.RC01 AK.SKN AK.SLK AK.TRF AV.RED AV.SPCP TA.O22K 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 = 9.33e+22 dyne-cm Mw = 4.58 Z = 110 km Plane Strike Dip Rake NP1 262 45 95 NP2 75 45 85 Principal Axes: Axis Value Plunge Azimuth T 9.33e+22 86 257 N 0.00e+00 4 79 P -9.33e+22 0 349 Moment Tensor: (dyne-cm) Component Value Mxx -8.96e+22 Mxy 1.83e+22 Mxz -1.49e+21 Myy -3.35e+21 Myz -5.56e+21 Mzz 9.30e+22 - P ---------- ----- -------------- ---------------------------- ------------------------------ ---------------------------------- ------------################-------- ---------########################----- -------##############################--- ----###################################- ----####################################-- --############## ####################--- -############### T ###################---- ################ ##################----- ##################################------ -###############################-------- --##########################---------- -----##################------------- ---------------------------------- ------------------------------ ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 9.30e+22 -1.49e+21 5.56e+21 -1.49e+21 -8.96e+22 -1.83e+22 5.56e+21 -1.83e+22 -3.35e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210303053526/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 from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    2.0   250    45   -90   3.78 0.2252
WVFGRD96    4.0   295    25   -40   3.84 0.2002
WVFGRD96    6.0   305    25   -25   3.86 0.2360
WVFGRD96    8.0   325    25     0   3.95 0.2626
WVFGRD96   10.0   325    30     0   3.97 0.2958
WVFGRD96   12.0   320    30    -5   4.00 0.3145
WVFGRD96   14.0   115    25   -30   4.02 0.3267
WVFGRD96   16.0   105    25   -45   4.05 0.3359
WVFGRD96   18.0   105    25   -45   4.07 0.3398
WVFGRD96   20.0   105    25   -45   4.09 0.3366
WVFGRD96   22.0   105    25   -45   4.12 0.3283
WVFGRD96   24.0   105    25   -45   4.14 0.3142
WVFGRD96   26.0    95    30   -55   4.14 0.2981
WVFGRD96   28.0    55    65    60   4.14 0.2928
WVFGRD96   30.0    55    65    60   4.15 0.2908
WVFGRD96   32.0    55    65    60   4.17 0.2871
WVFGRD96   34.0    50    65    55   4.18 0.2787
WVFGRD96   36.0    55    65    55   4.19 0.2700
WVFGRD96   38.0   225    75    45   4.23 0.2716
WVFGRD96   40.0   230    75    60   4.35 0.2847
WVFGRD96   42.0   230    70    60   4.35 0.2828
WVFGRD96   44.0   235    65    65   4.36 0.2853
WVFGRD96   46.0   245    60    80   4.37 0.2881
WVFGRD96   48.0    55    35    65   4.38 0.2950
WVFGRD96   50.0   245    60    75   4.40 0.3039
WVFGRD96   52.0   245    60    75   4.41 0.3137
WVFGRD96   54.0   240    60    70   4.42 0.3206
WVFGRD96   56.0   240    60    70   4.43 0.3250
WVFGRD96   58.0    65    45    70   4.43 0.3320
WVFGRD96   60.0    70    45    80   4.44 0.3545
WVFGRD96   62.0    70    50    80   4.46 0.3772
WVFGRD96   64.0    70    50    80   4.48 0.4023
WVFGRD96   66.0    70    50    80   4.49 0.4255
WVFGRD96   68.0    70    50    80   4.50 0.4473
WVFGRD96   70.0    70    50    80   4.51 0.4675
WVFGRD96   72.0    70    50    80   4.51 0.4857
WVFGRD96   74.0    70    50    80   4.52 0.5025
WVFGRD96   76.0    70    50    80   4.53 0.5175
WVFGRD96   78.0    70    50    80   4.53 0.5316
WVFGRD96   80.0    70    50    80   4.54 0.5439
WVFGRD96   82.0    70    50    80   4.54 0.5544
WVFGRD96   84.0    70    50    80   4.54 0.5637
WVFGRD96   86.0    70    50    80   4.55 0.5723
WVFGRD96   88.0    70    50    80   4.55 0.5791
WVFGRD96   90.0    70    50    80   4.55 0.5844
WVFGRD96   92.0    70    50    80   4.56 0.5894
WVFGRD96   94.0    70    50    80   4.56 0.5924
WVFGRD96   96.0    70    50    80   4.56 0.5956
WVFGRD96   98.0    70    50    80   4.56 0.5992
WVFGRD96  100.0    70    50    80   4.57 0.6013
WVFGRD96  102.0    70    50    80   4.57 0.6021
WVFGRD96  104.0    75    45    85   4.57 0.6030
WVFGRD96  106.0    70    50    80   4.57 0.6030
WVFGRD96  108.0    75    45    85   4.58 0.6039
WVFGRD96  110.0    75    45    85   4.58 0.6042
WVFGRD96  112.0    75    45    85   4.58 0.6033
WVFGRD96  114.0    75    45    85   4.58 0.6020
WVFGRD96  116.0    75    45    85   4.58 0.6013
WVFGRD96  118.0    75    45    85   4.59 0.6001
WVFGRD96  120.0    75    45    85   4.59 0.5969
WVFGRD96  122.0    75    45    85   4.59 0.5963
WVFGRD96  124.0    70    45    85   4.60 0.5937
WVFGRD96  126.0    70    45    80   4.60 0.5904
WVFGRD96  128.0    70    45    85   4.60 0.5888
WVFGRD96  130.0    70    45    85   4.60 0.5859
WVFGRD96  132.0    70    45    85   4.60 0.5826
WVFGRD96  134.0    70    45    85   4.61 0.5809
WVFGRD96  136.0    70    45    85   4.61 0.5758
WVFGRD96  138.0    70    45    85   4.61 0.5744
WVFGRD96  140.0    70    45    85   4.61 0.5700
WVFGRD96  142.0    70    45    85   4.61 0.5673
WVFGRD96  144.0    70    45    85   4.62 0.5639
WVFGRD96  146.0    70    45    85   4.62 0.5598
WVFGRD96  148.0    70    45    85   4.62 0.5554

The best solution is

WVFGRD96  110.0    75    45    85   4.58 0.6042

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 Wed Mar 3 05:42:48 CST 2021