Location

2015/11/06 14:26:49 61.969 -149.916 42.6 4.7 Alaska

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2015/11/06 14:26:49:0 61.97 -149.92 42.6 4.7 Alaska Stations used: AK.CUT AK.GHO AK.KLU AK.KNK AK.KTH AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.TRF AT.PMR TA.M24K TA.O22K Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 5.19e+22 dyne-cm Mw = 4.41 Z = 56 km Plane Strike Dip Rake NP1 230 50 -60 NP2 8 48 -121 Principal Axes: Axis Value Plunge Azimuth T 5.19e+22 1 299 N 0.00e+00 23 30 P -5.19e+22 67 207 Moment Tensor: (dyne-cm) Component Value Mxx 6.40e+21 Mxy -2.52e+22 Mxz 1.67e+22 Myy 3.79e+22 Myz 7.76e+21 Mzz -4.42e+22 ###########--- #################----- #####################------- #####################--######- ################---------######## T #############-------------######## ##########----------------######### ############------------------########## ##########---------------------######### ##########----------------------########## #########-----------------------########## #######------------------------########### #######---------- -----------########### #####----------- P -----------########## ####------------ ----------########### ###------------------------########### ##------------------------########## #-----------------------########## --------------------########## ------------------########## -------------######### ------######## Global CMT Convention Moment Tensor: R T P -4.42e+22 1.67e+22 -7.76e+21 1.67e+22 6.40e+21 2.52e+22 -7.76e+21 2.52e+22 3.79e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20151106142649/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 = 230
      DIP = 50
     RAKE = -60
       MW = 4.41
       HS = 56.0

The NDK file is 20151106142649.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2015/11/06 14:26:49:0 61.97 -149.92 42.6 4.7 Alaska Stations used: AK.CUT AK.GHO AK.KLU AK.KNK AK.KTH AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.TRF AT.PMR TA.M24K TA.O22K Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.06 n 3 Best Fitting Double Couple Mo = 5.19e+22 dyne-cm Mw = 4.41 Z = 56 km Plane Strike Dip Rake NP1 230 50 -60 NP2 8 48 -121 Principal Axes: Axis Value Plunge Azimuth T 5.19e+22 1 299 N 0.00e+00 23 30 P -5.19e+22 67 207 Moment Tensor: (dyne-cm) Component Value Mxx 6.40e+21 Mxy -2.52e+22 Mxz 1.67e+22 Myy 3.79e+22 Myz 7.76e+21 Mzz -4.42e+22 ###########--- #################----- #####################------- #####################--######- ################---------######## T #############-------------######## ##########----------------######### ############------------------########## ##########---------------------######### ##########----------------------########## #########-----------------------########## #######------------------------########### #######---------- -----------########### #####----------- P -----------########## ####------------ ----------########### ###------------------------########### ##------------------------########## #-----------------------########## --------------------########## ------------------########## -------------######### ------######## Global CMT Convention Moment Tensor: R T P -4.42e+22 1.67e+22 -7.76e+21 1.67e+22 6.40e+21 2.52e+22 -7.76e+21 2.52e+22 3.79e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20151106142649/index.html

Magnitudes

mLg Magnitude

(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.

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

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 0.02 n 3 
lp c 0.06 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   210    45    85   3.78 0.4173
WVFGRD96    4.0    45    50   -80   3.84 0.3431
WVFGRD96    6.0   205    70    75   3.85 0.3383
WVFGRD96    8.0   180    20    30   3.95 0.3884
WVFGRD96   10.0   175    25    25   3.97 0.4308
WVFGRD96   12.0   180    25    30   3.98 0.4561
WVFGRD96   14.0   120    35    40   3.97 0.4705
WVFGRD96   16.0   180    25    30   4.01 0.4800
WVFGRD96   18.0   180    25    25   4.03 0.4822
WVFGRD96   20.0   175    30    20   4.04 0.4804
WVFGRD96   22.0   130    35    80   4.10 0.4782
WVFGRD96   24.0   260    40   -30   4.04 0.4779
WVFGRD96   26.0    70    70   -50   4.08 0.4940
WVFGRD96   28.0   255    45   -30   4.08 0.5087
WVFGRD96   30.0   250    45   -35   4.10 0.5292
WVFGRD96   32.0   250    50   -35   4.12 0.5515
WVFGRD96   34.0   245    50   -40   4.15 0.5732
WVFGRD96   36.0   245    50   -40   4.17 0.5908
WVFGRD96   38.0   240    50   -45   4.21 0.6080
WVFGRD96   40.0   235    45   -55   4.30 0.6589
WVFGRD96   42.0   235    45   -55   4.32 0.6733
WVFGRD96   44.0   230    45   -60   4.34 0.6868
WVFGRD96   46.0   230    45   -60   4.36 0.7006
WVFGRD96   48.0   230    45   -60   4.37 0.7117
WVFGRD96   50.0   225    45   -70   4.38 0.7183
WVFGRD96   52.0   235    50   -55   4.39 0.7243
WVFGRD96   54.0   230    50   -60   4.41 0.7298
WVFGRD96   56.0   230    50   -60   4.41 0.7313
WVFGRD96   58.0   225    50   -70   4.42 0.7300
WVFGRD96   60.0   220    50   -80   4.42 0.7261
WVFGRD96   62.0   220    50   -80   4.43 0.7198
WVFGRD96   64.0   215    50   -90   4.43 0.7112
WVFGRD96   66.0    35    35   -90   4.44 0.7046
WVFGRD96   68.0    40    35   -85   4.44 0.6996
WVFGRD96   70.0    40    35   -85   4.44 0.6921
WVFGRD96   72.0    45    35   -80   4.44 0.6832
WVFGRD96   74.0    50    35   -75   4.45 0.6740
WVFGRD96   76.0    55    35   -70   4.45 0.6642
WVFGRD96   78.0    55    35   -70   4.45 0.6540
WVFGRD96   80.0    60    35   -65   4.45 0.6438
WVFGRD96   82.0    65    35   -60   4.45 0.6331
WVFGRD96   84.0    65    35   -60   4.45 0.6228
WVFGRD96   86.0    70    35   -55   4.46 0.6121
WVFGRD96   88.0    70    35   -55   4.45 0.6021
WVFGRD96   90.0    70    35   -50   4.45 0.5918
WVFGRD96   92.0    70    35   -50   4.45 0.5819
WVFGRD96   94.0    75    35   -45   4.46 0.5718
WVFGRD96   96.0    75    35   -45   4.46 0.5632
WVFGRD96   98.0    75    35   -45   4.45 0.5540

The best solution is

WVFGRD96   56.0   230    50   -60   4.41 0.7313

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 0.02 n 3 
lp c 0.06 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 Mon Dec 7 00:08:50 CST 2015