Location

Location ANSS

2021/08/01 16:47:28 61.583 -146.109 26.8 3.4 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2021/08/01 16:47:28:0 61.58 -146.11 26.8 3.4 Alaska Stations used: AK.BMR AK.DHY AK.DIV AK.EYAK AK.FID AK.GLB AK.GLI AK.HIN AK.KLU AK.MCAR AK.RC01 AK.SCM AK.VRDI AT.PMR Filtering commands used: cut o DIST/3.3 -30 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 = 2.75e+21 dyne-cm Mw = 3.56 Z = 42 km Plane Strike Dip Rake NP1 225 85 -55 NP2 322 35 -171 Principal Axes: Axis Value Plunge Azimuth T 2.75e+21 31 287 N 0.00e+00 35 41 P -2.75e+21 40 167 Moment Tensor: (dyne-cm) Component Value Mxx -1.38e+21 Mxy -1.96e+20 Mxz 1.67e+21 Myy 1.77e+21 Myz -1.47e+21 Mzz -3.92e+20 -------------- ---------------------- ###############------------- ####################---------# ########################----###### #################################### ##########################----######## ##### #################-------######## ##### T ###############----------####### ###### #############-------------####### ####################----------------###### ##################------------------###### ################--------------------###### ##############----------------------#### ############------------------------#### #########--------------------------### #######------------ -----------### ####-------------- P -----------## #--------------- ----------# ---------------------------# ---------------------- -------------- Global CMT Convention Moment Tensor: R T P -3.92e+20 1.67e+21 1.47e+21 1.67e+21 -1.38e+21 1.96e+20 1.47e+21 1.96e+20 1.77e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210801164728/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 = 225
      DIP = 85
     RAKE = -55
       MW = 3.56
       HS = 42.0

The NDK file is 20210801164728.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2021/08/01 16:47:28:0 61.58 -146.11 26.8 3.4 Alaska Stations used: AK.BMR AK.DHY AK.DIV AK.EYAK AK.FID AK.GLB AK.GLI AK.HIN AK.KLU AK.MCAR AK.RC01 AK.SCM AK.VRDI AT.PMR Filtering commands used: cut o DIST/3.3 -30 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 = 2.75e+21 dyne-cm Mw = 3.56 Z = 42 km Plane Strike Dip Rake NP1 225 85 -55 NP2 322 35 -171 Principal Axes: Axis Value Plunge Azimuth T 2.75e+21 31 287 N 0.00e+00 35 41 P -2.75e+21 40 167 Moment Tensor: (dyne-cm) Component Value Mxx -1.38e+21 Mxy -1.96e+20 Mxz 1.67e+21 Myy 1.77e+21 Myz -1.47e+21 Mzz -3.92e+20 -------------- ---------------------- ###############------------- ####################---------# ########################----###### #################################### ##########################----######## ##### #################-------######## ##### T ###############----------####### ###### #############-------------####### ####################----------------###### ##################------------------###### ################--------------------###### ##############----------------------#### ############------------------------#### #########--------------------------### #######------------ -----------### ####-------------- P -----------## #--------------- ----------# ---------------------------# ---------------------- -------------- Global CMT Convention Moment Tensor: R T P -3.92e+20 1.67e+21 1.47e+21 1.67e+21 -1.38e+21 1.96e+20 1.47e+21 1.96e+20 1.77e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210801164728/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 -30 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 from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0    55    40   -90   2.77 0.2363
WVFGRD96    2.0    55    40   -90   2.92 0.3259
WVFGRD96    3.0    50    35   -95   2.99 0.3286
WVFGRD96    4.0   310    40   -25   2.97 0.3552
WVFGRD96    5.0   315    40   -15   2.99 0.3831
WVFGRD96    6.0   320    40     0   3.01 0.4074
WVFGRD96    7.0   320    45     0   3.02 0.4231
WVFGRD96    8.0   325    40    10   3.09 0.4326
WVFGRD96    9.0   325    40    15   3.10 0.4368
WVFGRD96   10.0   325    45    15   3.11 0.4389
WVFGRD96   11.0   325    45    20   3.13 0.4394
WVFGRD96   12.0   325    45    20   3.14 0.4377
WVFGRD96   13.0   240    70    40   3.15 0.4348
WVFGRD96   14.0   240    65    35   3.17 0.4430
WVFGRD96   15.0   240    65    35   3.18 0.4506
WVFGRD96   16.0   240    65    35   3.19 0.4569
WVFGRD96   17.0   215    60   -40   3.23 0.4642
WVFGRD96   18.0   215    60   -40   3.24 0.4763
WVFGRD96   19.0   215    60   -40   3.26 0.4879
WVFGRD96   20.0   215    60   -40   3.27 0.4980
WVFGRD96   21.0   220    65   -40   3.28 0.5078
WVFGRD96   22.0   220    65   -40   3.29 0.5205
WVFGRD96   23.0   220    65   -40   3.31 0.5316
WVFGRD96   24.0    50    90    40   3.29 0.5409
WVFGRD96   25.0    50    90    40   3.30 0.5598
WVFGRD96   26.0    50    90    40   3.31 0.5769
WVFGRD96   27.0   225    80   -45   3.34 0.5969
WVFGRD96   28.0   225    80   -45   3.35 0.6135
WVFGRD96   29.0   225    80   -45   3.36 0.6301
WVFGRD96   30.0   225    80   -45   3.37 0.6450
WVFGRD96   31.0   225    80   -45   3.38 0.6592
WVFGRD96   32.0   225    80   -45   3.39 0.6723
WVFGRD96   33.0   225    80   -45   3.40 0.6829
WVFGRD96   34.0   225    80   -45   3.41 0.6906
WVFGRD96   35.0   225    80   -45   3.42 0.6959
WVFGRD96   36.0   225    80   -45   3.43 0.6996
WVFGRD96   37.0   225    80   -45   3.44 0.7026
WVFGRD96   38.0   225    80   -40   3.44 0.7019
WVFGRD96   39.0   225    80   -40   3.45 0.7040
WVFGRD96   40.0   225    85   -55   3.54 0.7058
WVFGRD96   41.0   225    85   -55   3.55 0.7086
WVFGRD96   42.0   225    85   -55   3.56 0.7090
WVFGRD96   43.0   225    85   -50   3.56 0.7087
WVFGRD96   44.0   225    85   -50   3.57 0.7077
WVFGRD96   45.0    50    90    50   3.57 0.7000
WVFGRD96   46.0   225    85   -50   3.58 0.7047
WVFGRD96   47.0   225    85   -50   3.59 0.7028
WVFGRD96   48.0   225    85   -50   3.59 0.7009
WVFGRD96   49.0   225    85   -50   3.60 0.6971
WVFGRD96   50.0   225    85   -50   3.61 0.6948
WVFGRD96   51.0   225    85   -50   3.61 0.6926
WVFGRD96   52.0   225    85   -50   3.62 0.6888
WVFGRD96   53.0   225    85   -50   3.62 0.6862
WVFGRD96   54.0   225    85   -50   3.63 0.6846
WVFGRD96   55.0   225    85   -50   3.63 0.6816
WVFGRD96   56.0   225    90   -50   3.63 0.6781
WVFGRD96   57.0   225    90   -50   3.63 0.6772
WVFGRD96   58.0   225    90   -50   3.64 0.6746
WVFGRD96   59.0   225    90   -50   3.64 0.6730

The best solution is

WVFGRD96   42.0   225    85   -55   3.56 0.7090

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 -30 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 Sun Aug 1 22:02:08 CDT 2021