Location

Location ANSS

2019/07/01 23:10:34 61.219 -146.898 14.6 3.7 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2019/07/01 23:10:34:0 61.22 -146.90 14.6 3.7 Alaska Stations used: AK.BARN AK.BRLK AK.CAST AK.CNP AK.CRQ AK.CUT AK.DHY AK.DIV AK.DOT AK.EYAK AK.FID AK.FIRE AK.GLB AK.GLI AK.HDA AK.HIN AK.HMT AK.KLU AK.KNK AK.KTH AK.MCAR AK.MCK AK.PAX AK.PPLA AK.RC01 AK.RIDG AK.SCM AK.SKN AK.SLK AK.SUCK AK.SWD AK.TRF AK.WAX AK.YAH AT.MENT AT.PMR AV.STLK IU.COLA TA.J25K TA.J26L TA.K24K TA.K27K TA.L26K TA.L27K TA.M22K TA.M24K TA.M27K TA.N25K TA.O22K TA.P23K 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 0.07 n 3 Best Fitting Double Couple Mo = 5.69e+21 dyne-cm Mw = 3.77 Z = 33 km Plane Strike Dip Rake NP1 260 85 -65 NP2 1 25 -168 Principal Axes: Axis Value Plunge Azimuth T 5.69e+21 35 329 N 0.00e+00 25 78 P -5.69e+21 44 195 Moment Tensor: (dyne-cm) Component Value Mxx 4.91e+19 Mxy -2.40e+21 Mxz 5.04e+21 Myy 8.46e+20 Myz -6.75e+20 Mzz -8.95e+20 ###########--- ##################---- #######################----- ###### #################---- ######## T ##################----- ######### ###################----- #################################----- ###################################----- ###################################----- ###############################------##### ###################------------------##### ##########---------------------------##### ###---------------------------------###### -----------------------------------##### -----------------------------------##### --------------- ---------------##### -------------- P --------------##### ------------- -------------##### --------------------------#### -----------------------##### ------------------#### -----------### Global CMT Convention Moment Tensor: R T P -8.95e+20 5.04e+21 6.75e+20 5.04e+21 4.91e+19 2.40e+21 6.75e+20 2.40e+21 8.46e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190701231034/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 = 260
      DIP = 85
     RAKE = -65
       MW = 3.77
       HS = 33.0

The NDK file is 20190701231034.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2019/07/01 23:10:34:0 61.22 -146.90 14.6 3.7 Alaska Stations used: AK.BARN AK.BRLK AK.CAST AK.CNP AK.CRQ AK.CUT AK.DHY AK.DIV AK.DOT AK.EYAK AK.FID AK.FIRE AK.GLB AK.GLI AK.HDA AK.HIN AK.HMT AK.KLU AK.KNK AK.KTH AK.MCAR AK.MCK AK.PAX AK.PPLA AK.RC01 AK.RIDG AK.SCM AK.SKN AK.SLK AK.SUCK AK.SWD AK.TRF AK.WAX AK.YAH AT.MENT AT.PMR AV.STLK IU.COLA TA.J25K TA.J26L TA.K24K TA.K27K TA.L26K TA.L27K TA.M22K TA.M24K TA.M27K TA.N25K TA.O22K TA.P23K 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 0.07 n 3 Best Fitting Double Couple Mo = 5.69e+21 dyne-cm Mw = 3.77 Z = 33 km Plane Strike Dip Rake NP1 260 85 -65 NP2 1 25 -168 Principal Axes: Axis Value Plunge Azimuth T 5.69e+21 35 329 N 0.00e+00 25 78 P -5.69e+21 44 195 Moment Tensor: (dyne-cm) Component Value Mxx 4.91e+19 Mxy -2.40e+21 Mxz 5.04e+21 Myy 8.46e+20 Myz -6.75e+20 Mzz -8.95e+20 ###########--- ##################---- #######################----- ###### #################---- ######## T ##################----- ######### ###################----- #################################----- ###################################----- ###################################----- ###############################------##### ###################------------------##### ##########---------------------------##### ###---------------------------------###### -----------------------------------##### -----------------------------------##### --------------- ---------------##### -------------- P --------------##### ------------- -------------##### --------------------------#### -----------------------##### ------------------#### -----------### Global CMT Convention Moment Tensor: R T P -8.95e+20 5.04e+21 6.75e+20 5.04e+21 4.91e+19 2.40e+21 6.75e+20 2.40e+21 8.46e+20 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190701231034/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 0.07 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   270    50    90   3.24 0.2429
WVFGRD96    2.0    95    40    95   3.38 0.3343
WVFGRD96    3.0    85    45    90   3.44 0.3260
WVFGRD96    4.0    40    75    15   3.38 0.2869
WVFGRD96    5.0    40    80    15   3.41 0.2869
WVFGRD96    6.0    35    70   -15   3.43 0.2956
WVFGRD96    7.0   260    90   -55   3.41 0.3219
WVFGRD96    8.0   260    90   -60   3.48 0.3467
WVFGRD96    9.0    85    85    65   3.50 0.3819
WVFGRD96   10.0    85    85    65   3.51 0.4127
WVFGRD96   11.0    90    80    65   3.53 0.4414
WVFGRD96   12.0    90    80    65   3.54 0.4680
WVFGRD96   13.0    90    80    65   3.55 0.4923
WVFGRD96   14.0    90    80    65   3.57 0.5151
WVFGRD96   15.0    90    80    65   3.58 0.5362
WVFGRD96   16.0    85    85    60   3.59 0.5557
WVFGRD96   17.0    85    85    65   3.60 0.5741
WVFGRD96   18.0   260    85   -60   3.61 0.5922
WVFGRD96   19.0    85    85    65   3.62 0.6084
WVFGRD96   20.0    80    90    60   3.64 0.6242
WVFGRD96   21.0    80    90    65   3.65 0.6393
WVFGRD96   22.0    80    90    65   3.67 0.6542
WVFGRD96   23.0   260    85   -65   3.68 0.6699
WVFGRD96   24.0    80    90    65   3.69 0.6810
WVFGRD96   25.0    80    90    65   3.70 0.6923
WVFGRD96   26.0    80    90    65   3.71 0.7023
WVFGRD96   27.0   260    85   -65   3.72 0.7156
WVFGRD96   28.0    80    90    65   3.73 0.7181
WVFGRD96   29.0   260    85   -65   3.74 0.7292
WVFGRD96   30.0    80    90    65   3.75 0.7273
WVFGRD96   31.0    80    90    65   3.76 0.7301
WVFGRD96   32.0    80    90    65   3.76 0.7308
WVFGRD96   33.0   260    85   -65   3.77 0.7355
WVFGRD96   34.0    80    90    65   3.77 0.7294
WVFGRD96   35.0    80    90    65   3.78 0.7271
WVFGRD96   36.0    80    90    65   3.78 0.7241
WVFGRD96   37.0    80    90    65   3.78 0.7202
WVFGRD96   38.0   260    85   -65   3.79 0.7185
WVFGRD96   39.0   260    85   -65   3.79 0.7141
WVFGRD96   40.0    80    90    75   3.93 0.7061
WVFGRD96   41.0    80    90    75   3.93 0.7020
WVFGRD96   42.0    80    90    75   3.94 0.6972
WVFGRD96   43.0    80    90    75   3.94 0.6923
WVFGRD96   44.0    80    90    75   3.95 0.6867
WVFGRD96   45.0    85    85    75   3.95 0.6810
WVFGRD96   46.0    85    85    75   3.95 0.6753
WVFGRD96   47.0   260    90   -75   3.96 0.6678
WVFGRD96   48.0    85    85    75   3.96 0.6627
WVFGRD96   49.0    85    85    75   3.97 0.6562
WVFGRD96   50.0   260    90   -75   3.97 0.6470
WVFGRD96   51.0    85    85    75   3.97 0.6429
WVFGRD96   52.0   260    90   -75   3.98 0.6331
WVFGRD96   53.0   260    90   -75   3.98 0.6256
WVFGRD96   54.0   265    90   -80   3.99 0.6187
WVFGRD96   55.0    85    80    75   3.99 0.6153
WVFGRD96   56.0   170   -10    -5   4.00 0.6050
WVFGRD96   57.0   160    10   -15   4.01 0.5990
WVFGRD96   58.0   160    15   -15   4.02 0.5935
WVFGRD96   59.0   165    15   -10   4.02 0.5878

The best solution is

WVFGRD96   33.0   260    85   -65   3.77 0.7355

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 0.07 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 Bureau of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Oklahoma Geological Survey, TexNet, the Iris stations, the Transportable Array of EarthScope and other networks.

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 Jul 1 21:13:00 CDT 2019