Location

Location ANSS

2021/02/03 19:06:36 62.090 -149.539 49.3 3.7 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2021/02/03 19:06:36:0 62.09 -149.54 49.3 3.7 Alaska Stations used: AK.BPAW AK.CAST AK.CUT AK.EYAK AK.GHO AK.HIN AK.KNK AK.MCAR AK.MCK AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN AK.SSN AK.TRF AT.PMR 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 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 7.50e+21 dyne-cm Mw = 3.85 Z = 60 km Plane Strike Dip Rake NP1 9 64 -114 NP2 235 35 -50 Principal Axes: Axis Value Plunge Azimuth T 7.50e+21 16 117 N 0.00e+00 22 20 P -7.50e+21 63 240 Moment Tensor: (dyne-cm) Component Value Mxx 1.02e+21 Mxy -3.48e+21 Mxz 6.55e+20 Myy 4.37e+21 Myz 4.36e+21 Mzz -5.40e+21 ###########--- ################------ ####################-------- ############---------########- ###########-------------########## #########----------------########### ########------------------############ #######--------------------############# ######---------------------############# ######----------------------############## #####-----------------------############## ####---------- -----------############## ####---------- P ----------############### ##----------- ----------############## ##-----------------------######### ### #-----------------------######### T ## #---------------------########## # --------------------############## -----------------############# ---------------############# -----------########### -----######### Global CMT Convention Moment Tensor: R T P -5.40e+21 6.55e+20 -4.36e+21 6.55e+20 1.02e+21 3.48e+21 -4.36e+21 3.48e+21 4.37e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210203190636/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 = 235
      DIP = 35
     RAKE = -50
       MW = 3.85
       HS = 60.0

The NDK file is 20210203190636.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2021/02/03 19:06:36:0 62.09 -149.54 49.3 3.7 Alaska Stations used: AK.BPAW AK.CAST AK.CUT AK.EYAK AK.GHO AK.HIN AK.KNK AK.MCAR AK.MCK AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN AK.SSN AK.TRF AT.PMR 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 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 7.50e+21 dyne-cm Mw = 3.85 Z = 60 km Plane Strike Dip Rake NP1 9 64 -114 NP2 235 35 -50 Principal Axes: Axis Value Plunge Azimuth T 7.50e+21 16 117 N 0.00e+00 22 20 P -7.50e+21 63 240 Moment Tensor: (dyne-cm) Component Value Mxx 1.02e+21 Mxy -3.48e+21 Mxz 6.55e+20 Myy 4.37e+21 Myz 4.36e+21 Mzz -5.40e+21 ###########--- ################------ ####################-------- ############---------########- ###########-------------########## #########----------------########### ########------------------############ #######--------------------############# ######---------------------############# ######----------------------############## #####-----------------------############## ####---------- -----------############## ####---------- P ----------############### ##----------- ----------############## ##-----------------------######### ### #-----------------------######### T ## #---------------------########## # --------------------############## -----------------############# ---------------############# -----------########### -----######### Global CMT Convention Moment Tensor: R T P -5.40e+21 6.55e+20 -4.36e+21 6.55e+20 1.02e+21 3.48e+21 -4.36e+21 3.48e+21 4.37e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210203190636/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 0.10 n 3 
br c 0.12 0.25 n 4 p 2

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    30    45    90   3.19 0.4344
WVFGRD96    4.0   345    25    15   3.26 0.3397
WVFGRD96    6.0   345    20    15   3.30 0.4881
WVFGRD96    8.0   345    20    20   3.36 0.5284
WVFGRD96   10.0   350    25    30   3.35 0.5429
WVFGRD96   12.0   340    30    10   3.35 0.5490
WVFGRD96   14.0   340    30    10   3.36 0.5511
WVFGRD96   16.0   330    35     0   3.38 0.5506
WVFGRD96   18.0   330    30     0   3.39 0.5495
WVFGRD96   20.0   325    35    -5   3.42 0.5458
WVFGRD96   22.0   325    30     0   3.44 0.5388
WVFGRD96   24.0   330    25    20   3.46 0.5298
WVFGRD96   26.0   325    30    15   3.48 0.5191
WVFGRD96   28.0    65    90   -55   3.54 0.5076
WVFGRD96   30.0    65    90   -55   3.56 0.4923
WVFGRD96   32.0   305    25    25   3.59 0.4805
WVFGRD96   34.0   300    25    20   3.60 0.4699
WVFGRD96   36.0   295    25    15   3.61 0.4549
WVFGRD96   38.0   250    85    45   3.63 0.4507
WVFGRD96   40.0   220    30   -65   3.74 0.4886
WVFGRD96   42.0   230    25   -50   3.78 0.5221
WVFGRD96   44.0   230    25   -50   3.80 0.5608
WVFGRD96   46.0   235    30   -45   3.82 0.5878
WVFGRD96   48.0   235    30   -45   3.83 0.6081
WVFGRD96   50.0   235    30   -45   3.84 0.6233
WVFGRD96   52.0   230    30   -55   3.83 0.6310
WVFGRD96   54.0   230    30   -55   3.84 0.6374
WVFGRD96   56.0   235    35   -45   3.86 0.6405
WVFGRD96   58.0   235    35   -45   3.86 0.6426
WVFGRD96   60.0   235    35   -50   3.85 0.6426
WVFGRD96   62.0   235    35   -50   3.85 0.6415
WVFGRD96   64.0   230    35   -55   3.85 0.6398
WVFGRD96   66.0   230    35   -55   3.85 0.6356
WVFGRD96   68.0   230    35   -55   3.85 0.6298
WVFGRD96   70.0   235    40   -50   3.86 0.6247
WVFGRD96   72.0   230    40   -55   3.86 0.6196
WVFGRD96   74.0   230    40   -55   3.86 0.6125
WVFGRD96   76.0   230    40   -55   3.86 0.6067
WVFGRD96   78.0   230    40   -55   3.86 0.6028
WVFGRD96   80.0   230    40   -55   3.86 0.5982
WVFGRD96   82.0   230    40   -55   3.87 0.5921
WVFGRD96   84.0   225    40   -65   3.85 0.5872
WVFGRD96   86.0   230    40   -60   3.86 0.5807
WVFGRD96   88.0   225    40   -65   3.86 0.5776
WVFGRD96   90.0   225    40   -65   3.86 0.5734
WVFGRD96   92.0   225    40   -65   3.86 0.5685
WVFGRD96   94.0   215    40   -80   3.86 0.5630
WVFGRD96   96.0    25    50   -95   3.86 0.5601
WVFGRD96   98.0    25    50   -95   3.86 0.5549

The best solution is

WVFGRD96   60.0   235    35   -50   3.85 0.6426

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 0.10 n 3 
br c 0.12 0.25 n 4 p 2

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 Feb 3 13:55:27 CST 2021