Location

Location ANSS

2021/04/30 17:19:24 64.830 -152.396 19.4 4.3 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2021/04/30 17:19:24:0 64.83 -152.40 19.4 4.3 Alaska Stations used: AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.DOT AK.E24K AK.F20K AK.F21K AK.FYU AK.G19K AK.G23K AK.G24K AK.H17K AK.H21K AK.H22K AK.H24K AK.HDA AK.I23K AK.J17K AK.J19K AK.J20K AK.J25K AK.K20K AK.K24K AK.KNK AK.L18K AK.L19K AK.L20K AK.M20K AK.MCK AK.MLY AK.NEA2 AK.PAX AK.POKR AK.PPD AK.PPLA AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.SSN AK.TRF AK.WRH AT.PMR AV.SPCP IM.IL31 IU.COLA TA.E23K TA.F19K TA.F24K TA.F25K TA.G18K TA.H19K TA.I17K TA.I20K TA.J16K TA.J18K TA.K17K TA.M22K 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 Best Fitting Double Couple Mo = 1.60e+22 dyne-cm Mw = 4.07 Z = 17 km Plane Strike Dip Rake NP1 291 64 -146 NP2 185 60 -30 Principal Axes: Axis Value Plunge Azimuth T 1.60e+22 3 57 N 0.00e+00 49 324 P -1.60e+22 41 150 Moment Tensor: (dyne-cm) Component Value Mxx -2.04e+21 Mxy 1.12e+22 Mxz 7.27e+21 Myy 8.98e+21 Myz -3.39e+21 Mzz -6.94e+21 -------####### ----------############ -----------################# -----------################### ------------##################### -------------##################### T --################################# #############--------################### #############-------------############## ##############----------------############ #############--------------------######### #############----------------------####### #############------------------------##### ############--------------------------## ############---------------------------# ############------------ ----------- ###########------------ P ---------- ##########------------ --------- #########--------------------- #########------------------- #######--------------- ####---------- Global CMT Convention Moment Tensor: R T P -6.94e+21 7.27e+21 3.39e+21 7.27e+21 -2.04e+21 -1.12e+22 3.39e+21 -1.12e+22 8.98e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210430171924/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 = 60
     RAKE = -30
       MW = 4.07
       HS = 17.0

The NDK file is 20210430171924.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2021/04/30 17:19:24:0 64.83 -152.40 19.4 4.3 Alaska Stations used: AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.DOT AK.E24K AK.F20K AK.F21K AK.FYU AK.G19K AK.G23K AK.G24K AK.H17K AK.H21K AK.H22K AK.H24K AK.HDA AK.I23K AK.J17K AK.J19K AK.J20K AK.J25K AK.K20K AK.K24K AK.KNK AK.L18K AK.L19K AK.L20K AK.M20K AK.MCK AK.MLY AK.NEA2 AK.PAX AK.POKR AK.PPD AK.PPLA AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.SSN AK.TRF AK.WRH AT.PMR AV.SPCP IM.IL31 IU.COLA TA.E23K TA.F19K TA.F24K TA.F25K TA.G18K TA.H19K TA.I17K TA.I20K TA.J16K TA.J18K TA.K17K TA.M22K 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 Best Fitting Double Couple Mo = 1.60e+22 dyne-cm Mw = 4.07 Z = 17 km Plane Strike Dip Rake NP1 291 64 -146 NP2 185 60 -30 Principal Axes: Axis Value Plunge Azimuth T 1.60e+22 3 57 N 0.00e+00 49 324 P -1.60e+22 41 150 Moment Tensor: (dyne-cm) Component Value Mxx -2.04e+21 Mxy 1.12e+22 Mxz 7.27e+21 Myy 8.98e+21 Myz -3.39e+21 Mzz -6.94e+21 -------####### ----------############ -----------################# -----------################### ------------##################### -------------##################### T --################################# #############--------################### #############-------------############## ##############----------------############ #############--------------------######### #############----------------------####### #############------------------------##### ############--------------------------## ############---------------------------# ############------------ ----------- ###########------------ P ---------- ##########------------ --------- #########--------------------- #########------------------- #######--------------- ####---------- Global CMT Convention Moment Tensor: R T P -6.94e+21 7.27e+21 3.39e+21 7.27e+21 -2.04e+21 -1.12e+22 3.39e+21 -1.12e+22 8.98e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210430171924/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

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

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0    15    85    -5   3.48 0.2450
WVFGRD96    2.0    55    45    85   3.72 0.3095
WVFGRD96    3.0    20    65    10   3.67 0.3058
WVFGRD96    4.0    20    55    10   3.72 0.3198
WVFGRD96    5.0   200    80    50   3.76 0.3476
WVFGRD96    6.0   200    80    45   3.78 0.3852
WVFGRD96    7.0   200    45    15   3.81 0.4257
WVFGRD96    8.0   200    45    15   3.88 0.4605
WVFGRD96    9.0   205    45    30   3.91 0.5000
WVFGRD96   10.0   180    50   -35   3.94 0.5462
WVFGRD96   11.0   185    55   -30   3.96 0.5858
WVFGRD96   12.0   185    60   -30   3.98 0.6169
WVFGRD96   13.0   185    60   -30   4.00 0.6417
WVFGRD96   14.0   185    60   -30   4.02 0.6597
WVFGRD96   15.0   185    60   -30   4.04 0.6714
WVFGRD96   16.0   185    60   -30   4.05 0.6777
WVFGRD96   17.0   185    60   -30   4.07 0.6787
WVFGRD96   18.0   185    60   -30   4.08 0.6752
WVFGRD96   19.0   185    65   -30   4.09 0.6681
WVFGRD96   20.0   185    65   -30   4.10 0.6578
WVFGRD96   21.0   185    60   -30   4.11 0.6443
WVFGRD96   22.0   185    60   -25   4.12 0.6291
WVFGRD96   23.0   185    60   -25   4.13 0.6119
WVFGRD96   24.0   185    60   -25   4.14 0.5929
WVFGRD96   25.0   185    60   -25   4.14 0.5730
WVFGRD96   26.0   185    60   -25   4.15 0.5526
WVFGRD96   27.0   185    60   -25   4.15 0.5314
WVFGRD96   28.0   185    60   -30   4.15 0.5094
WVFGRD96   29.0   185    60   -30   4.16 0.4871

The best solution is

WVFGRD96   17.0   185    60   -30   4.07 0.6787

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

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 Apr 30 13:29:51 CDT 2021