Location

Location ANSS

2018/12/01 07:57:22 61.358 -149.994 39.0 5.2 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2018/12/01 07:57:22:0 61.36 -149.99 39.0 5.2 Alaska Stations used: AK.BRLK AK.CAPN AK.CAST AK.CNP AK.CUT AK.DHY AK.FID AK.GHO AK.GLI AK.HOM AK.KNK AK.PWL AK.RND AK.SAW AK.SCM AK.SKN AK.SLK AK.SSN AK.SWD AV.ILSW AV.STLK TA.M19K TA.M20K TA.M22K TA.O19K TA.O22K TA.P19K Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 3.85e+23 dyne-cm Mw = 4.99 Z = 48 km Plane Strike Dip Rake NP1 195 55 -65 NP2 336 42 -121 Principal Axes: Axis Value Plunge Azimuth T 3.85e+23 7 267 N 0.00e+00 20 0 P -3.85e+23 69 160 Moment Tensor: (dyne-cm) Component Value Mxx -4.46e+22 Mxy 3.34e+22 Mxz 1.21e+23 Myy 3.72e+23 Myz -9.10e+22 Mzz -3.28e+23 -----------### #########---########## #############---############ ############-------########### #############----------########### #############-------------########## #############---------------########## #############-----------------########## ############-------------------######### #############--------------------######### #########---------------------######### T #########----------------------######## #########--------- ----------######## ##########---------- P ----------####### ##########---------- ----------####### #########-----------------------###### #########----------------------##### ########---------------------##### ######---------------------### ######-------------------### ####-----------------# #------------- Global CMT Convention Moment Tensor: R T P -3.28e+23 1.21e+23 9.10e+22 1.21e+23 -4.46e+22 -3.34e+22 9.10e+22 -3.34e+22 3.72e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201075722/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 = 195
      DIP = 55
     RAKE = -65
       MW = 4.99
       HS = 48.0

The NDK file is 20181201075722.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 07:57:22:0 61.36 -149.99 39.0 5.2 Alaska Stations used: AK.BRLK AK.CAPN AK.CAST AK.CNP AK.CUT AK.DHY AK.FID AK.GHO AK.GLI AK.HOM AK.KNK AK.PWL AK.RND AK.SAW AK.SCM AK.SKN AK.SLK AK.SSN AK.SWD AV.ILSW AV.STLK TA.M19K TA.M20K TA.M22K TA.O19K TA.O22K TA.P19K Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 3.85e+23 dyne-cm Mw = 4.99 Z = 48 km Plane Strike Dip Rake NP1 195 55 -65 NP2 336 42 -121 Principal Axes: Axis Value Plunge Azimuth T 3.85e+23 7 267 N 0.00e+00 20 0 P -3.85e+23 69 160 Moment Tensor: (dyne-cm) Component Value Mxx -4.46e+22 Mxy 3.34e+22 Mxz 1.21e+23 Myy 3.72e+23 Myz -9.10e+22 Mzz -3.28e+23 -----------### #########---########## #############---############ ############-------########### #############----------########### #############-------------########## #############---------------########## #############-----------------########## ############-------------------######### #############--------------------######### #########---------------------######### T #########----------------------######## #########--------- ----------######## ##########---------- P ----------####### ##########---------- ----------####### #########-----------------------###### #########----------------------##### ########---------------------##### ######---------------------### ######-------------------### ####-----------------# #------------- Global CMT Convention Moment Tensor: R T P -3.28e+23 1.21e+23 9.10e+22 1.21e+23 -4.46e+22 -3.34e+22 9.10e+22 -3.34e+22 3.72e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201075722/index.html

Magnitudes

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 +70
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   355    45    85   4.12 0.1527
WVFGRD96    2.0   355    45    85   4.27 0.2036
WVFGRD96    3.0   340    35    60   4.32 0.1933
WVFGRD96    4.0   320    35    30   4.31 0.2061
WVFGRD96    5.0   125    60   -35   4.33 0.2287
WVFGRD96    6.0   130    65   -30   4.35 0.2456
WVFGRD96    7.0   130    65   -30   4.38 0.2601
WVFGRD96    8.0   125    60   -35   4.45 0.2669
WVFGRD96    9.0   125    60   -35   4.46 0.2721
WVFGRD96   10.0   125    60   -35   4.48 0.2761
WVFGRD96   11.0   125    60   -35   4.50 0.2790
WVFGRD96   12.0   125    60   -35   4.51 0.2795
WVFGRD96   13.0   125    60   -35   4.53 0.2782
WVFGRD96   14.0   125    60   -35   4.54 0.2766
WVFGRD96   15.0   125    60   -35   4.55 0.2739
WVFGRD96   16.0   230    65    35   4.56 0.2755
WVFGRD96   17.0   230    65    35   4.58 0.2761
WVFGRD96   18.0   230    65    30   4.59 0.2777
WVFGRD96   19.0   210    90   -45   4.58 0.2802
WVFGRD96   20.0    35    80    45   4.60 0.2853
WVFGRD96   21.0    35    80    50   4.62 0.2896
WVFGRD96   22.0    40    80    50   4.64 0.2948
WVFGRD96   23.0    40    80    50   4.65 0.3006
WVFGRD96   24.0    40    80    50   4.67 0.3062
WVFGRD96   25.0    35    85    50   4.67 0.3116
WVFGRD96   26.0    35    85    50   4.69 0.3172
WVFGRD96   27.0    35    90    50   4.70 0.3245
WVFGRD96   28.0   215    90   -50   4.71 0.3325
WVFGRD96   29.0   210    80   -50   4.72 0.3430
WVFGRD96   30.0   210    75   -50   4.73 0.3550
WVFGRD96   31.0   210    75   -50   4.74 0.3702
WVFGRD96   32.0   210    75   -50   4.75 0.3857
WVFGRD96   33.0   210    65   -50   4.76 0.4027
WVFGRD96   34.0   210    65   -50   4.77 0.4191
WVFGRD96   35.0   210    65   -50   4.78 0.4343
WVFGRD96   36.0   210    65   -50   4.79 0.4470
WVFGRD96   37.0   205    60   -55   4.80 0.4587
WVFGRD96   38.0   205    60   -55   4.81 0.4684
WVFGRD96   39.0   205    60   -55   4.82 0.4789
WVFGRD96   40.0   200    60   -60   4.91 0.4791
WVFGRD96   41.0   200    60   -60   4.92 0.4864
WVFGRD96   42.0   200    55   -60   4.93 0.4937
WVFGRD96   43.0   200    55   -60   4.94 0.4995
WVFGRD96   44.0   200    55   -60   4.95 0.5043
WVFGRD96   45.0   200    55   -60   4.96 0.5068
WVFGRD96   46.0   195    55   -65   4.97 0.5105
WVFGRD96   47.0   195    55   -65   4.98 0.5108
WVFGRD96   48.0   195    55   -65   4.99 0.5126
WVFGRD96   49.0   195    55   -65   4.99 0.5117
WVFGRD96   50.0   195    55   -65   5.00 0.5112
WVFGRD96   51.0   195    55   -65   5.00 0.5097
WVFGRD96   52.0   195    55   -65   5.00 0.5071
WVFGRD96   53.0   195    55   -65   5.01 0.5050
WVFGRD96   54.0   195    55   -65   5.01 0.5010
WVFGRD96   55.0   195    50   -65   5.01 0.4988
WVFGRD96   56.0   190    50   -70   5.01 0.4948
WVFGRD96   57.0   195    50   -65   5.01 0.4929
WVFGRD96   58.0   190    50   -70   5.02 0.4887
WVFGRD96   59.0   195    50   -65   5.01 0.4844

The best solution is

WVFGRD96   48.0   195    55   -65   4.99 0.5126

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 +70
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 08:32:37 CST 2018