Location

Location ANSS

2018/12/01 12:44:53 61.468 -149.903 29.5 4.5 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2018/12/01 12:44:53:0 61.47 -149.90 29.5 4.5 Alaska Stations used: AK.CAPN AK.CUT AK.FID AK.GHO AK.GLI AK.KLU AK.KNK AK.KTH AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AV.ILSW AV.STLK TA.M19K TA.M20K TA.M22K TA.O22K Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 5.96e+22 dyne-cm Mw = 4.45 Z = 48 km Plane Strike Dip Rake NP1 185 55 -80 NP2 348 36 -104 Principal Axes: Axis Value Plunge Azimuth T 5.96e+22 9 268 N 0.00e+00 8 359 P -5.96e+22 77 129 Moment Tensor: (dyne-cm) Component Value Mxx -1.05e+21 Mxy 3.56e+21 Mxz 7.66e+21 Myy 5.62e+22 Myz -1.95e+22 Mzz -5.51e+22 ######-####### #########-----######## ##########----------######## ##########------------######## ###########---------------######## ###########-----------------######## ############------------------######## ############--------------------######## ############---------------------####### ############----------------------######## # ########----------------------######## # T ########----------- --------######## # ########----------- P ---------####### ###########----------- --------####### ###########----------------------####### ###########---------------------###### ##########--------------------###### ##########------------------###### ########------------------#### ########---------------##### ######-------------### ####---------# Global CMT Convention Moment Tensor: R T P -5.51e+22 7.66e+21 1.95e+22 7.66e+21 -1.05e+21 -3.56e+21 1.95e+22 -3.56e+21 5.62e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201124453/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 = 55
     RAKE = -80
       MW = 4.45
       HS = 48.0

The NDK file is 20181201124453.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2018/12/01 12:44:53:0 61.47 -149.90 29.5 4.5 Alaska Stations used: AK.CAPN AK.CUT AK.FID AK.GHO AK.GLI AK.KLU AK.KNK AK.KTH AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AV.ILSW AV.STLK TA.M19K TA.M20K TA.M22K TA.O22K Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 5.96e+22 dyne-cm Mw = 4.45 Z = 48 km Plane Strike Dip Rake NP1 185 55 -80 NP2 348 36 -104 Principal Axes: Axis Value Plunge Azimuth T 5.96e+22 9 268 N 0.00e+00 8 359 P -5.96e+22 77 129 Moment Tensor: (dyne-cm) Component Value Mxx -1.05e+21 Mxy 3.56e+21 Mxz 7.66e+21 Myy 5.62e+22 Myz -1.95e+22 Mzz -5.51e+22 ######-####### #########-----######## ##########----------######## ##########------------######## ###########---------------######## ###########-----------------######## ############------------------######## ############--------------------######## ############---------------------####### ############----------------------######## # ########----------------------######## # T ########----------- --------######## # ########----------- P ---------####### ###########----------- --------####### ###########----------------------####### ###########---------------------###### ##########--------------------###### ##########------------------###### ########------------------#### ########---------------##### ######-------------### ####---------# Global CMT Convention Moment Tensor: R T P -5.51e+22 7.66e+21 1.95e+22 7.66e+21 -1.05e+21 -3.56e+21 1.95e+22 -3.56e+21 5.62e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201124453/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 +40
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    -5    45    80   3.59 0.2105
WVFGRD96    2.0     0    40    80   3.75 0.2727
WVFGRD96    3.0    -5    45    75   3.80 0.2533
WVFGRD96    4.0   340    30    55   3.79 0.2438
WVFGRD96    5.0   135    75   -40   3.80 0.2707
WVFGRD96    6.0   135    75   -40   3.83 0.2933
WVFGRD96    7.0   130    70   -40   3.86 0.3120
WVFGRD96    8.0   130    70   -45   3.92 0.3213
WVFGRD96    9.0   130    70   -45   3.94 0.3326
WVFGRD96   10.0   130    70   -45   3.96 0.3411
WVFGRD96   11.0   130    70   -45   3.98 0.3471
WVFGRD96   12.0   130    70   -45   4.00 0.3500
WVFGRD96   13.0   130    70   -45   4.01 0.3508
WVFGRD96   14.0    70    60    50   4.03 0.3497
WVFGRD96   15.0    70    60    50   4.05 0.3545
WVFGRD96   16.0    65    60    45   4.06 0.3576
WVFGRD96   17.0    70    55    45   4.08 0.3605
WVFGRD96   18.0    70    55    45   4.09 0.3628
WVFGRD96   19.0    65    55    40   4.11 0.3661
WVFGRD96   20.0    65    55    40   4.12 0.3694
WVFGRD96   21.0    65    55    45   4.14 0.3703
WVFGRD96   22.0   210    70   -55   4.10 0.3783
WVFGRD96   23.0   210    70   -55   4.11 0.3875
WVFGRD96   24.0   210    70   -55   4.12 0.3959
WVFGRD96   25.0   210    65   -55   4.14 0.4032
WVFGRD96   26.0   210    65   -55   4.15 0.4122
WVFGRD96   27.0   210    70   -55   4.16 0.4195
WVFGRD96   28.0   210    70   -55   4.17 0.4264
WVFGRD96   29.0   210    70   -55   4.18 0.4313
WVFGRD96   30.0   210    75   -65   4.19 0.4479
WVFGRD96   31.0   210    75   -65   4.20 0.4650
WVFGRD96   32.0   205    70   -65   4.21 0.4845
WVFGRD96   33.0   205    65   -65   4.22 0.5031
WVFGRD96   34.0   205    65   -65   4.23 0.5241
WVFGRD96   35.0   200    60   -70   4.24 0.5434
WVFGRD96   36.0   195    60   -70   4.25 0.5631
WVFGRD96   37.0   195    60   -70   4.26 0.5774
WVFGRD96   38.0   195    60   -70   4.27 0.5906
WVFGRD96   39.0   190    55   -75   4.28 0.6022
WVFGRD96   40.0   195    60   -75   4.37 0.6041
WVFGRD96   41.0   195    60   -75   4.38 0.6115
WVFGRD96   42.0   190    55   -75   4.40 0.6196
WVFGRD96   43.0   185    55   -80   4.41 0.6262
WVFGRD96   44.0   185    55   -80   4.42 0.6316
WVFGRD96   45.0   185    55   -80   4.43 0.6362
WVFGRD96   46.0   185    55   -80   4.44 0.6387
WVFGRD96   47.0   185    55   -80   4.44 0.6400
WVFGRD96   48.0   185    55   -80   4.45 0.6404
WVFGRD96   49.0   185    55   -80   4.45 0.6402
WVFGRD96   50.0   185    55   -80   4.46 0.6376
WVFGRD96   51.0   185    55   -80   4.46 0.6354
WVFGRD96   52.0   180    55   -85   4.47 0.6305
WVFGRD96   53.0   -10    35  -100   4.47 0.6275
WVFGRD96   54.0    -5    35   -95   4.47 0.6204
WVFGRD96   55.0    -5    35   -95   4.47 0.6162
WVFGRD96   56.0    -5    35   -95   4.48 0.6093
WVFGRD96   57.0    -5    35   -95   4.48 0.6043
WVFGRD96   58.0    -5    35   -95   4.48 0.5982
WVFGRD96   59.0     5    35   -80   4.49 0.5909

The best solution is

WVFGRD96   48.0   185    55   -80   4.45 0.6404

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 +40
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 Dec 1 08:51:52 CST 2018