Location

Location ANSS

2019/07/10 06:39:58 60.516 -150.150 40.8 3.8 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2019/07/10 06:39:58:0 60.52 -150.15 40.8 3.8 Alaska Stations used: AK.BRLK AK.CNP AK.FIRE AK.HOM AK.KNK AK.PWL AK.RC01 AK.SKN AK.SWD AT.PMR AV.SPU TA.O22K Filtering commands used: cut o DIST/3.3 -30 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 = 1.14e+22 dyne-cm Mw = 3.97 Z = 46 km Plane Strike Dip Rake NP1 180 80 -65 NP2 290 27 -157 Principal Axes: Axis Value Plunge Azimuth T 1.14e+22 31 250 N 0.00e+00 25 355 P -1.14e+22 49 117 Moment Tensor: (dyne-cm) Component Value Mxx -8.26e+14 Mxy 4.72e+21 Mxz 8.33e+20 Myy 3.52e+21 Myz -9.67e+21 Mzz -3.52e+21 -------####### ----------############ ---------####-############## ---##########---------######## --#############------------####### -###############--------------###### #################----------------##### ##################------------------#### ##################-------------------### ##################---------------------### ##################----------------------## ##################----------------------## ###### #########----------- --------## ##### T ##########---------- P --------- ##### ##########---------- --------- #################--------------------- ################-------------------- ###############------------------- #############----------------- #############--------------- ##########------------ #######------- Global CMT Convention Moment Tensor: R T P -3.52e+21 8.33e+20 9.67e+21 8.33e+20 -8.26e+14 -4.72e+21 9.67e+21 -4.72e+21 3.52e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190710063958/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 = 180
      DIP = 80
     RAKE = -65
       MW = 3.97
       HS = 46.0

The NDK file is 20190710063958.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2019/07/10 06:39:58:0 60.52 -150.15 40.8 3.8 Alaska Stations used: AK.BRLK AK.CNP AK.FIRE AK.HOM AK.KNK AK.PWL AK.RC01 AK.SKN AK.SWD AT.PMR AV.SPU TA.O22K Filtering commands used: cut o DIST/3.3 -30 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 = 1.14e+22 dyne-cm Mw = 3.97 Z = 46 km Plane Strike Dip Rake NP1 180 80 -65 NP2 290 27 -157 Principal Axes: Axis Value Plunge Azimuth T 1.14e+22 31 250 N 0.00e+00 25 355 P -1.14e+22 49 117 Moment Tensor: (dyne-cm) Component Value Mxx -8.26e+14 Mxy 4.72e+21 Mxz 8.33e+20 Myy 3.52e+21 Myz -9.67e+21 Mzz -3.52e+21 -------####### ----------############ ---------####-############## ---##########---------######## --#############------------####### -###############--------------###### #################----------------##### ##################------------------#### ##################-------------------### ##################---------------------### ##################----------------------## ##################----------------------## ###### #########----------- --------## ##### T ##########---------- P --------- ##### ##########---------- --------- #################--------------------- ################-------------------- ###############------------------- #############----------------- #############--------------- ##########------------ #######------- Global CMT Convention Moment Tensor: R T P -3.52e+21 8.33e+20 9.67e+21 8.33e+20 -8.26e+14 -4.72e+21 9.67e+21 -4.72e+21 3.52e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190710063958/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 +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   260    55   -50   3.05 0.1831
WVFGRD96    2.0   255    55   -60   3.23 0.2537
WVFGRD96    3.0   270    70   -35   3.23 0.2747
WVFGRD96    4.0   280    60    40   3.32 0.3108
WVFGRD96    5.0   275    65    30   3.34 0.3362
WVFGRD96    6.0   275    60    25   3.37 0.3579
WVFGRD96    7.0   270    55    10   3.40 0.3788
WVFGRD96    8.0   275    50    20   3.47 0.3949
WVFGRD96    9.0   275    50    20   3.49 0.4043
WVFGRD96   10.0   275    55    25   3.52 0.4090
WVFGRD96   11.0   270    55    10   3.53 0.4111
WVFGRD96   12.0   270    55    10   3.55 0.4111
WVFGRD96   13.0   270    60    10   3.56 0.4089
WVFGRD96   14.0   175    90    30   3.58 0.4173
WVFGRD96   15.0   175    90    30   3.59 0.4200
WVFGRD96   16.0   175    90    30   3.60 0.4211
WVFGRD96   17.0   175    90    30   3.62 0.4208
WVFGRD96   18.0   175    90    30   3.63 0.4193
WVFGRD96   19.0    -5    90   -30   3.64 0.4169
WVFGRD96   20.0   185    75   -40   3.63 0.4183
WVFGRD96   21.0   185    75   -40   3.65 0.4342
WVFGRD96   22.0   185    75   -40   3.66 0.4495
WVFGRD96   23.0   190    85   -40   3.67 0.4649
WVFGRD96   24.0   190    85   -40   3.68 0.4815
WVFGRD96   25.0   190    85   -40   3.69 0.4963
WVFGRD96   26.0    10    90    45   3.71 0.5115
WVFGRD96   27.0    10    90    45   3.72 0.5282
WVFGRD96   28.0   190    90   -45   3.73 0.5432
WVFGRD96   29.0    10    90    45   3.73 0.5573
WVFGRD96   30.0   190    90   -50   3.75 0.5692
WVFGRD96   31.0    10    90    50   3.75 0.5790
WVFGRD96   32.0   190    90   -50   3.76 0.5898
WVFGRD96   33.0    10    85    45   3.76 0.6009
WVFGRD96   34.0    10    85    45   3.77 0.6073
WVFGRD96   35.0    10    85    45   3.77 0.6135
WVFGRD96   36.0    10    85    45   3.78 0.6196
WVFGRD96   37.0    10    90    45   3.78 0.6235
WVFGRD96   38.0   180    80   -55   3.81 0.6303
WVFGRD96   39.0   180    80   -55   3.82 0.6368
WVFGRD96   40.0   185    85   -65   3.92 0.6432
WVFGRD96   41.0   180    80   -65   3.93 0.6492
WVFGRD96   42.0   180    80   -65   3.94 0.6539
WVFGRD96   43.0   180    80   -65   3.95 0.6571
WVFGRD96   44.0   180    80   -65   3.96 0.6598
WVFGRD96   45.0   180    80   -65   3.96 0.6614
WVFGRD96   46.0   180    80   -65   3.97 0.6615
WVFGRD96   47.0   180    80   -65   3.98 0.6612
WVFGRD96   48.0   180    80   -65   3.98 0.6611
WVFGRD96   49.0   180    80   -65   3.98 0.6612
WVFGRD96   50.0   180    80   -65   3.99 0.6583
WVFGRD96   51.0   185    80   -60   3.97 0.6552
WVFGRD96   52.0   185    80   -60   3.97 0.6533
WVFGRD96   53.0   185    80   -60   3.98 0.6502
WVFGRD96   54.0   185    85   -60   3.98 0.6467
WVFGRD96   55.0   185    85   -60   3.98 0.6428
WVFGRD96   56.0   185    85   -60   3.98 0.6383
WVFGRD96   57.0    10    90    55   3.96 0.6327
WVFGRD96   58.0    10    90    55   3.97 0.6280
WVFGRD96   59.0   190    90   -55   3.97 0.6258

The best solution is

WVFGRD96   46.0   180    80   -65   3.97 0.6615

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 +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 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 Wed Jul 10 07:25:07 CDT 2019