Location

Location ANSS

2017/07/01 16:51:51 62.237 -151.309 86.4 4.0 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2017/07/01 16:51:51:0 62.24 -151.31 86.4 4.0 Alaska Stations used: AK.BPAW AK.BWN AK.CAST AK.DHY AK.GHO AK.KNK AK.KTH AK.MLY AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AK.TRF AT.PMR AV.ILSW TA.J20K TA.K20K TA.L19K TA.M22K Filtering commands used: cut o DIST/3.5 -30 o DIST/3.5 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.06e+22 dyne-cm Mw = 3.95 Z = 96 km Plane Strike Dip Rake NP1 255 60 -40 NP2 8 56 -143 Principal Axes: Axis Value Plunge Azimuth T 1.06e+22 2 312 N 0.00e+00 42 44 P -1.06e+22 48 220 Moment Tensor: (dyne-cm) Component Value Mxx 1.99e+21 Mxy -7.56e+21 Mxz 4.34e+21 Myy 3.91e+21 Myz 3.04e+21 Mzz -5.90e+21 ###########--- ################------ ###################-------- T ####################-------- # #####################--------- ##########################---------- ######################-----####------- ###############--------------#########-- ###########------------------########### #########---------------------############ ######------------------------############ ####-------------------------############# ###--------------------------############# ----------------------------############ ------------ ------------############# ----------- P ------------############ ---------- -----------############ ----------------------############ -------------------########### -----------------########### ------------########## ------######## Global CMT Convention Moment Tensor: R T P -5.90e+21 4.34e+21 -3.04e+21 4.34e+21 1.99e+21 7.56e+21 -3.04e+21 7.56e+21 3.91e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170701165151/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 = 255
      DIP = 60
     RAKE = -40
       MW = 3.95
       HS = 96.0

The NDK file is 20170701165151.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2017/07/01 16:51:51:0 62.24 -151.31 86.4 4.0 Alaska Stations used: AK.BPAW AK.BWN AK.CAST AK.DHY AK.GHO AK.KNK AK.KTH AK.MLY AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AK.TRF AT.PMR AV.ILSW TA.J20K TA.K20K TA.L19K TA.M22K Filtering commands used: cut o DIST/3.5 -30 o DIST/3.5 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.06e+22 dyne-cm Mw = 3.95 Z = 96 km Plane Strike Dip Rake NP1 255 60 -40 NP2 8 56 -143 Principal Axes: Axis Value Plunge Azimuth T 1.06e+22 2 312 N 0.00e+00 42 44 P -1.06e+22 48 220 Moment Tensor: (dyne-cm) Component Value Mxx 1.99e+21 Mxy -7.56e+21 Mxz 4.34e+21 Myy 3.91e+21 Myz 3.04e+21 Mzz -5.90e+21 ###########--- ################------ ###################-------- T ####################-------- # #####################--------- ##########################---------- ######################-----####------- ###############--------------#########-- ###########------------------########### #########---------------------############ ######------------------------############ ####-------------------------############# ###--------------------------############# ----------------------------############ ------------ ------------############# ----------- P ------------############ ---------- -----------############ ----------------------############ -------------------########### -----------------########### ------------########## ------######## Global CMT Convention Moment Tensor: R T P -5.90e+21 4.34e+21 -3.04e+21 4.34e+21 1.99e+21 7.56e+21 -3.04e+21 7.56e+21 3.91e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170701165151/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/3.5 -30 o DIST/3.5 +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   220    50    70   3.15 0.2681
WVFGRD96    4.0   185    75   -40   3.17 0.2351
WVFGRD96    6.0    -5    50   -30   3.24 0.2734
WVFGRD96    8.0   355    50   -30   3.33 0.3002
WVFGRD96   10.0    -5    55   -35   3.37 0.3160
WVFGRD96   12.0     0    60   -30   3.39 0.3172
WVFGRD96   14.0     0    60   -30   3.42 0.3079
WVFGRD96   16.0     0    60   -30   3.45 0.2909
WVFGRD96   18.0   315    50   -10   3.44 0.2792
WVFGRD96   20.0   110    55    25   3.50 0.2876
WVFGRD96   22.0   105    60    10   3.52 0.2946
WVFGRD96   24.0   105    65    15   3.55 0.3007
WVFGRD96   26.0   105    65    15   3.57 0.3049
WVFGRD96   28.0   105    65    20   3.60 0.3163
WVFGRD96   30.0   105    65    15   3.62 0.3322
WVFGRD96   32.0   105    60    10   3.63 0.3474
WVFGRD96   34.0   105    60    10   3.65 0.3586
WVFGRD96   36.0   105    60     5   3.67 0.3658
WVFGRD96   38.0   105    65    15   3.70 0.3704
WVFGRD96   40.0   100    55   -10   3.76 0.3816
WVFGRD96   42.0   100    50   -10   3.79 0.3824
WVFGRD96   44.0   100    55   -10   3.81 0.3785
WVFGRD96   46.0   280    60     0   3.84 0.3831
WVFGRD96   48.0   280    60     0   3.86 0.3948
WVFGRD96   50.0   275    55   -10   3.87 0.4080
WVFGRD96   52.0   270    55   -20   3.88 0.4274
WVFGRD96   54.0   270    55   -25   3.88 0.4488
WVFGRD96   56.0   270    55   -25   3.89 0.4695
WVFGRD96   58.0   270    55   -25   3.90 0.4886
WVFGRD96   60.0   265    55   -30   3.91 0.5058
WVFGRD96   62.0   265    55   -30   3.91 0.5220
WVFGRD96   64.0   265    55   -30   3.92 0.5362
WVFGRD96   66.0   265    55   -30   3.92 0.5489
WVFGRD96   68.0   265    55   -30   3.92 0.5585
WVFGRD96   70.0   260    55   -35   3.93 0.5663
WVFGRD96   72.0   260    55   -35   3.93 0.5741
WVFGRD96   74.0   260    55   -35   3.93 0.5791
WVFGRD96   76.0   260    55   -35   3.93 0.5836
WVFGRD96   78.0   260    55   -35   3.93 0.5865
WVFGRD96   80.0   260    55   -35   3.93 0.5889
WVFGRD96   82.0   260    60   -35   3.94 0.5904
WVFGRD96   84.0   260    60   -35   3.94 0.5929
WVFGRD96   86.0   260    60   -35   3.94 0.5943
WVFGRD96   88.0   260    60   -35   3.94 0.5955
WVFGRD96   90.0   260    60   -35   3.94 0.5962
WVFGRD96   92.0   255    60   -40   3.95 0.5960
WVFGRD96   94.0   255    60   -40   3.95 0.5969
WVFGRD96   96.0   255    60   -40   3.95 0.5975
WVFGRD96   98.0   255    60   -40   3.95 0.5968
WVFGRD96  100.0   255    60   -40   3.95 0.5959
WVFGRD96  102.0   255    60   -40   3.95 0.5950
WVFGRD96  104.0   255    60   -40   3.95 0.5946
WVFGRD96  106.0   255    65   -40   3.97 0.5932
WVFGRD96  108.0   255    65   -40   3.97 0.5906
WVFGRD96  110.0   255    65   -40   3.97 0.5896
WVFGRD96  112.0   255    65   -40   3.97 0.5889
WVFGRD96  114.0   255    65   -40   3.97 0.5862
WVFGRD96  116.0   255    65   -40   3.97 0.5840
WVFGRD96  118.0   255    65   -40   3.97 0.5824

The best solution is

WVFGRD96   96.0   255    60   -40   3.95 0.5975

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/3.5 -30 o DIST/3.5 +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 Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Iris stations and the Transportable Array of EarthScope.

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 Sat Jul 1 12:16:54 CDT 2017