Location

SLU Location

First arrival times were read and the event was located using elocate and the WUS model. The location results are given in elocate.txt. The P-wave first motion data are comapred to the moment tensor nodal planes below.

The ANSS solution is preferred.

Location ANSS

2017/10/19 05:35:37 59.750 -153.156 100.6 4.6 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2017/10/19 05:35:37:0 59.75 -153.16 100.6 4.6 Alaska Stations used: AK.BRLK AK.CNP AK.RC01 AK.SKN AK.SSN AT.SVW2 AV.ILSW TA.M19K TA.M20K TA.M22K TA.N17K TA.N18K TA.N19K TA.O19K TA.P18K TA.P19K TA.Q16K TA.Q19K TA.Q20K Filtering commands used: cut o DIST/3.6 -30 o DIST/3.6 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.32e+23 dyne-cm Mw = 4.68 Z = 114 km Plane Strike Dip Rake NP1 300 67 136 NP2 50 50 30 Principal Axes: Axis Value Plunge Azimuth T 1.32e+23 46 257 N 0.00e+00 42 98 P -1.32e+23 11 359 Moment Tensor: (dyne-cm) Component Value Mxx -1.24e+23 Mxy 1.68e+22 Mxz -3.84e+22 Myy 5.93e+22 Myz -6.36e+22 Mzz 6.49e+22 ----- P ------ --------- ---------- ---------------------------- ------------------------------ ---------------------------------# #######---------------------------## ###############--------------------### ####################----------------#### #######################------------##### ###########################--------####### ##############################-----####### ######### ####################-######### ######### T ####################--######## ######## ###################-----##### ############################--------#### #########################-----------## #####################--------------- #################----------------- -########--------------------- ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 6.49e+22 -3.84e+22 6.36e+22 -3.84e+22 -1.24e+23 -1.68e+22 6.36e+22 -1.68e+22 5.93e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171019053537/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 = 50
      DIP = 50
     RAKE = 30
       MW = 4.68
       HS = 114.0

The NDK file is 20171019053537.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU SLUFM

USGS/SLU Moment Tensor Solution ENS 2017/10/19 05:35:37:0 59.75 -153.16 100.6 4.6 Alaska Stations used: AK.BRLK AK.CNP AK.RC01 AK.SKN AK.SSN AT.SVW2 AV.ILSW TA.M19K TA.M20K TA.M22K TA.N17K TA.N18K TA.N19K TA.O19K TA.P18K TA.P19K TA.Q16K TA.Q19K TA.Q20K Filtering commands used: cut o DIST/3.6 -30 o DIST/3.6 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.32e+23 dyne-cm Mw = 4.68 Z = 114 km Plane Strike Dip Rake NP1 300 67 136 NP2 50 50 30 Principal Axes: Axis Value Plunge Azimuth T 1.32e+23 46 257 N 0.00e+00 42 98 P -1.32e+23 11 359 Moment Tensor: (dyne-cm) Component Value Mxx -1.24e+23 Mxy 1.68e+22 Mxz -3.84e+22 Myy 5.93e+22 Myz -6.36e+22 Mzz 6.49e+22 ----- P ------ --------- ---------- ---------------------------- ------------------------------ ---------------------------------# #######---------------------------## ###############--------------------### ####################----------------#### #######################------------##### ###########################--------####### ##############################-----####### ######### ####################-######### ######### T ####################--######## ######## ###################-----##### ############################--------#### #########################-----------## #####################--------------- #################----------------- -########--------------------- ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 6.49e+22 -3.84e+22 6.36e+22 -3.84e+22 -1.24e+23 -1.68e+22 6.36e+22 -1.68e+22 5.93e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171019053537/index.html

First motions and takeoff angles from an elocate run.

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/3.6 -30 o DIST/3.6 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 
br c 0.12 0.25 n 4 p 2

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    45    50   -85   3.80 0.2421
WVFGRD96    4.0   265    25     5   3.96 0.2891
WVFGRD96    6.0   275    30    20   4.00 0.3610
WVFGRD96    8.0   280    35    30   4.04 0.3726
WVFGRD96   10.0   275    40    25   4.04 0.3755
WVFGRD96   12.0   280    45    35   4.04 0.3762
WVFGRD96   14.0   275    55    30   4.04 0.3766
WVFGRD96   16.0   275    60    30   4.05 0.3764
WVFGRD96   18.0   275    65    30   4.06 0.3733
WVFGRD96   20.0   265    70    30   4.07 0.3691
WVFGRD96   22.0   270    70    30   4.09 0.3628
WVFGRD96   24.0   260    85    35   4.11 0.3594
WVFGRD96   26.0    75    90   -30   4.13 0.3601
WVFGRD96   28.0   255    90    30   4.15 0.3620
WVFGRD96   30.0    70    85   -30   4.18 0.3632
WVFGRD96   32.0    70    85   -25   4.20 0.3650
WVFGRD96   34.0   185    65    30   4.23 0.3727
WVFGRD96   36.0   190    60    30   4.25 0.3788
WVFGRD96   38.0   190    60    30   4.28 0.3836
WVFGRD96   40.0   195    60    35   4.37 0.3812
WVFGRD96   42.0   230    50    40   4.43 0.3972
WVFGRD96   44.0   230    50    40   4.46 0.4100
WVFGRD96   46.0   235    50    45   4.48 0.4201
WVFGRD96   48.0    45    65    30   4.49 0.4308
WVFGRD96   50.0    45    60    30   4.51 0.4646
WVFGRD96   52.0    45    60    30   4.53 0.4949
WVFGRD96   54.0    45    60    30   4.55 0.5171
WVFGRD96   56.0    50    60    35   4.57 0.5364
WVFGRD96   58.0    50    60    35   4.58 0.5510
WVFGRD96   60.0    55    50    45   4.58 0.5689
WVFGRD96   62.0    55    50    45   4.60 0.5896
WVFGRD96   64.0    55    50    45   4.61 0.6092
WVFGRD96   66.0    55    50    45   4.61 0.6282
WVFGRD96   68.0    55    50    45   4.62 0.6459
WVFGRD96   70.0    55    50    45   4.63 0.6624
WVFGRD96   72.0    55    45    40   4.63 0.6774
WVFGRD96   74.0    55    45    40   4.63 0.6910
WVFGRD96   76.0    55    45    40   4.63 0.7016
WVFGRD96   78.0    55    45    40   4.64 0.7123
WVFGRD96   80.0    55    50    45   4.64 0.7215
WVFGRD96   82.0    55    50    45   4.65 0.7310
WVFGRD96   84.0    55    50    40   4.65 0.7392
WVFGRD96   86.0    55    45    40   4.65 0.7462
WVFGRD96   88.0    55    45    40   4.65 0.7536
WVFGRD96   90.0    55    45    40   4.65 0.7598
WVFGRD96   92.0    50    50    35   4.66 0.7653
WVFGRD96   94.0    50    50    35   4.66 0.7703
WVFGRD96   96.0    50    50    35   4.66 0.7744
WVFGRD96   98.0    50    50    35   4.66 0.7783
WVFGRD96  100.0    50    50    35   4.66 0.7818
WVFGRD96  102.0    50    50    35   4.66 0.7860
WVFGRD96  104.0    50    50    35   4.67 0.7887
WVFGRD96  106.0    50    50    35   4.67 0.7905
WVFGRD96  108.0    50    50    30   4.67 0.7919
WVFGRD96  110.0    50    50    30   4.68 0.7925
WVFGRD96  112.0    50    50    30   4.68 0.7936
WVFGRD96  114.0    50    50    30   4.68 0.7944
WVFGRD96  116.0    50    50    30   4.68 0.7930
WVFGRD96  118.0    50    50    30   4.68 0.7924

The best solution is

WVFGRD96  114.0    50    50    30   4.68 0.7944

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/3.6 -30 o DIST/3.6 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 
br c 0.12 0.25 n 4 p 2

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 Thu Oct 19 07:58:13 CDT 2017