Location

Location ANSS

2018/12/01 11:55:08 61.356 -150.003 39.3 4.1 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2018/12/01 11:55:08:0 61.36 -150.00 39.3 4.1 Alaska Stations used: AK.FID AK.GHO AK.GLI AK.KNK AK.PWL AK.RC01 AK.SAW AK.SKN AK.SSN AV.ILSW AV.STLK TA.M19K TA.M20K TA.M22K Filtering commands used: cut o DIST/3.3 -30 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 = 2.19e+22 dyne-cm Mw = 4.16 Z = 46 km Plane Strike Dip Rake NP1 190 60 -70 NP2 334 36 -121 Principal Axes: Axis Value Plunge Azimuth T 2.19e+22 13 266 N 0.00e+00 17 360 P -2.19e+22 68 141 Moment Tensor: (dyne-cm) Component Value Mxx -1.68e+21 Mxy 3.04e+21 Mxz 5.47e+21 Myy 1.95e+22 Myz -9.47e+21 Mzz -1.78e+22 ---------##### ##########-########### ############-----########### ############---------######### #############------------######### #############--------------######### #############-----------------######## ##############------------------######## #############--------------------####### ##############--------------------######## # #########----------------------####### # T #########----------------------####### # #########---------- ---------####### ############---------- P ----------##### ############---------- ---------###### ###########----------------------##### ##########----------------------#### ##########--------------------#### ########-------------------### ########-----------------### ######---------------# ###----------- Global CMT Convention Moment Tensor: R T P -1.78e+22 5.47e+21 9.47e+21 5.47e+21 -1.68e+21 -3.04e+21 9.47e+21 -3.04e+21 1.95e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201115508/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 = 190
      DIP = 60
     RAKE = -70
       MW = 4.16
       HS = 46.0

The NDK file is 20181201115508.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 11:55:08:0 61.36 -150.00 39.3 4.1 Alaska Stations used: AK.FID AK.GHO AK.GLI AK.KNK AK.PWL AK.RC01 AK.SAW AK.SKN AK.SSN AV.ILSW AV.STLK TA.M19K TA.M20K TA.M22K Filtering commands used: cut o DIST/3.3 -30 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 = 2.19e+22 dyne-cm Mw = 4.16 Z = 46 km Plane Strike Dip Rake NP1 190 60 -70 NP2 334 36 -121 Principal Axes: Axis Value Plunge Azimuth T 2.19e+22 13 266 N 0.00e+00 17 360 P -2.19e+22 68 141 Moment Tensor: (dyne-cm) Component Value Mxx -1.68e+21 Mxy 3.04e+21 Mxz 5.47e+21 Myy 1.95e+22 Myz -9.47e+21 Mzz -1.78e+22 ---------##### ##########-########### ############-----########### ############---------######### #############------------######### #############--------------######### #############-----------------######## ##############------------------######## #############--------------------####### ##############--------------------######## # #########----------------------####### # T #########----------------------####### # #########---------- ---------####### ############---------- P ----------##### ############---------- ---------###### ###########----------------------##### ##########----------------------#### ##########--------------------#### ########-------------------### ########-----------------### ######---------------# ###----------- Global CMT Convention Moment Tensor: R T P -1.78e+22 5.47e+21 9.47e+21 5.47e+21 -1.68e+21 -3.04e+21 9.47e+21 -3.04e+21 1.95e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201115508/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 +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   180    45    85   3.31 0.2003
WVFGRD96    2.0     0    45    85   3.47 0.2690
WVFGRD96    3.0   335    30    55   3.52 0.2618
WVFGRD96    4.0   325    30    40   3.53 0.2814
WVFGRD96    5.0   315    30    20   3.53 0.3066
WVFGRD96    6.0   310    35    10   3.55 0.3287
WVFGRD96    7.0   305    45   -30   3.59 0.3459
WVFGRD96    8.0   305    35     5   3.63 0.3553
WVFGRD96    9.0   330    45    30   3.67 0.3684
WVFGRD96   10.0   330    50    30   3.69 0.3830
WVFGRD96   11.0   330    50    30   3.71 0.3912
WVFGRD96   12.0   330    50    30   3.72 0.3956
WVFGRD96   13.0   330    55    30   3.74 0.3963
WVFGRD96   14.0   330    55    30   3.75 0.3934
WVFGRD96   15.0   330    60    35   3.77 0.3913
WVFGRD96   16.0   240    55    30   3.78 0.3933
WVFGRD96   17.0   240    55    30   3.79 0.3959
WVFGRD96   18.0    35    75    45   3.79 0.3969
WVFGRD96   19.0    35    80    50   3.80 0.4035
WVFGRD96   20.0    35    80    50   3.81 0.4108
WVFGRD96   21.0    35    80    50   3.83 0.4179
WVFGRD96   22.0    35    80    50   3.84 0.4259
WVFGRD96   23.0    35    80    50   3.86 0.4347
WVFGRD96   24.0    35    80    50   3.87 0.4446
WVFGRD96   25.0    30    85    50   3.88 0.4541
WVFGRD96   26.0    30    85    50   3.89 0.4633
WVFGRD96   27.0   210    90   -50   3.90 0.4738
WVFGRD96   28.0    30    90    50   3.91 0.4855
WVFGRD96   29.0    30    90    50   3.92 0.4971
WVFGRD96   30.0    30    90    50   3.93 0.5076
WVFGRD96   31.0   205    80   -55   3.94 0.5314
WVFGRD96   32.0   205    80   -55   3.95 0.5461
WVFGRD96   33.0   205    75   -60   3.97 0.5647
WVFGRD96   34.0   205    75   -60   3.97 0.5816
WVFGRD96   35.0   200    70   -60   3.98 0.5962
WVFGRD96   36.0   200    70   -60   3.99 0.6124
WVFGRD96   37.0   200    70   -60   3.99 0.6243
WVFGRD96   38.0   200    65   -60   4.00 0.6365
WVFGRD96   39.0   200    65   -60   4.02 0.6495
WVFGRD96   40.0   190    65   -70   4.11 0.6467
WVFGRD96   41.0   195    65   -65   4.12 0.6597
WVFGRD96   42.0   195    65   -70   4.13 0.6700
WVFGRD96   43.0   195    65   -65   4.14 0.6767
WVFGRD96   44.0   195    65   -65   4.15 0.6824
WVFGRD96   45.0   190    60   -70   4.16 0.6857
WVFGRD96   46.0   190    60   -70   4.16 0.6885
WVFGRD96   47.0   190    60   -70   4.17 0.6879
WVFGRD96   48.0   190    60   -70   4.18 0.6877
WVFGRD96   49.0   190    60   -70   4.18 0.6853
WVFGRD96   50.0   190    60   -70   4.18 0.6822
WVFGRD96   51.0   190    60   -70   4.19 0.6796
WVFGRD96   52.0   190    60   -70   4.19 0.6749
WVFGRD96   53.0   190    60   -70   4.20 0.6704
WVFGRD96   54.0   190    60   -70   4.20 0.6646
WVFGRD96   55.0   190    60   -70   4.20 0.6605
WVFGRD96   56.0   190    60   -70   4.20 0.6540
WVFGRD96   57.0   190    60   -70   4.20 0.6491
WVFGRD96   58.0   190    60   -70   4.21 0.6438
WVFGRD96   59.0   185    60   -75   4.21 0.6367

The best solution is

WVFGRD96   46.0   190    60   -70   4.16 0.6885

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 +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 Dec 1 08:38:11 CST 2018