Location

Location ANSS

2018/07/10 01:08:21 62.979 -150.664 114.6 5.0 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2018/07/10 01:08:21:0 62.98 -150.66 114.6 5.0 Alaska Stations used: AK.BPAW AK.BWN AK.CCB AK.CUT AK.DHY AK.DIV AK.FID AK.FIRE AK.GHO AK.GLI AK.HDA AK.KLU AK.KNK AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PAX AK.PPD AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AK.SWD AK.WRH AT.PMR AV.SPU IM.IL31 IU.COLA TA.G23K TA.H19K TA.H21K TA.H23K TA.H24K TA.I20K TA.J20K TA.J25K TA.J26L TA.K20K TA.L18K TA.L19K TA.M20K TA.M22K TA.M24K TA.N19K TA.N25K TA.O22K TA.POKR Filtering commands used: cut o DIST/3.5 -40 o DIST/3.5 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 2.82e+23 dyne-cm Mw = 4.90 Z = 112 km Plane Strike Dip Rake NP1 27 75 103 NP2 165 20 50 Principal Axes: Axis Value Plunge Azimuth T 2.82e+23 58 315 N 0.00e+00 13 203 P -2.82e+23 29 106 Moment Tensor: (dyne-cm) Component Value Mxx 2.17e+22 Mxy 1.90e+22 Mxz 1.22e+23 Myy -1.60e+23 Myz -2.04e+23 Mzz 1.39e+23 ############## --##################-- ---####################----- --######################------ ---#######################-------- ---#######################---------- ---######## #############----------- ----######## T ############------------- ---######### ###########-------------- ----#######################--------------- ----######################---------------- ----#####################----------------- ----####################---------- ----- ----##################----------- P ---- ----#################------------ ---- ----###############------------------- ----############-------------------- -----#########-------------------- ----#######------------------- -----###-------------------- ---##----------------- ######-------- Global CMT Convention Moment Tensor: R T P 1.39e+23 1.22e+23 2.04e+23 1.22e+23 2.17e+22 -1.90e+22 2.04e+23 -1.90e+22 -1.60e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180710010821/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 = 165
      DIP = 20
     RAKE = 50
       MW = 4.90
       HS = 112.0

The NDK file is 20180710010821.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2018/07/10 01:08:21:0 62.98 -150.66 114.6 5.0 Alaska Stations used: AK.BPAW AK.BWN AK.CCB AK.CUT AK.DHY AK.DIV AK.FID AK.FIRE AK.GHO AK.GLI AK.HDA AK.KLU AK.KNK AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PAX AK.PPD AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AK.SWD AK.WRH AT.PMR AV.SPU IM.IL31 IU.COLA TA.G23K TA.H19K TA.H21K TA.H23K TA.H24K TA.I20K TA.J20K TA.J25K TA.J26L TA.K20K TA.L18K TA.L19K TA.M20K TA.M22K TA.M24K TA.N19K TA.N25K TA.O22K TA.POKR Filtering commands used: cut o DIST/3.5 -40 o DIST/3.5 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 2.82e+23 dyne-cm Mw = 4.90 Z = 112 km Plane Strike Dip Rake NP1 27 75 103 NP2 165 20 50 Principal Axes: Axis Value Plunge Azimuth T 2.82e+23 58 315 N 0.00e+00 13 203 P -2.82e+23 29 106 Moment Tensor: (dyne-cm) Component Value Mxx 2.17e+22 Mxy 1.90e+22 Mxz 1.22e+23 Myy -1.60e+23 Myz -2.04e+23 Mzz 1.39e+23 ############## --##################-- ---####################----- --######################------ ---#######################-------- ---#######################---------- ---######## #############----------- ----######## T ############------------- ---######### ###########-------------- ----#######################--------------- ----######################---------------- ----#####################----------------- ----####################---------- ----- ----##################----------- P ---- ----#################------------ ---- ----###############------------------- ----############-------------------- -----#########-------------------- ----#######------------------- -----###-------------------- ---##----------------- ######-------- Global CMT Convention Moment Tensor: R T P 1.39e+23 1.22e+23 2.04e+23 1.22e+23 2.17e+22 -1.90e+22 2.04e+23 -1.90e+22 -1.60e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180710010821/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/3.5 -40 o DIST/3.5 +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    2.0    55    45   -90   3.95 0.1073
WVFGRD96    4.0   200    70    35   3.97 0.1070
WVFGRD96    6.0     5    75   -40   4.02 0.1190
WVFGRD96    8.0     5    70   -40   4.11 0.1303
WVFGRD96   10.0    15    60    25   4.14 0.1397
WVFGRD96   12.0   200    65    40   4.18 0.1477
WVFGRD96   14.0   200    65    40   4.21 0.1523
WVFGRD96   16.0   200    65    40   4.24 0.1529
WVFGRD96   18.0   195    65    40   4.26 0.1507
WVFGRD96   20.0   195    65    40   4.29 0.1468
WVFGRD96   22.0   195    65    40   4.31 0.1432
WVFGRD96   24.0   195    65    40   4.33 0.1397
WVFGRD96   26.0   285    45    10   4.36 0.1401
WVFGRD96   28.0   285    45    10   4.38 0.1427
WVFGRD96   30.0   285    50    10   4.40 0.1455
WVFGRD96   32.0   285    50    10   4.43 0.1529
WVFGRD96   34.0   285    55    15   4.45 0.1594
WVFGRD96   36.0   275    75   -20   4.47 0.1689
WVFGRD96   38.0   280    80   -15   4.51 0.1755
WVFGRD96   40.0    90    50   -30   4.60 0.1912
WVFGRD96   42.0    85    45   -35   4.63 0.1910
WVFGRD96   44.0    90    50   -30   4.65 0.1922
WVFGRD96   46.0    90    45   -30   4.67 0.1937
WVFGRD96   48.0    95    45   -25   4.69 0.1965
WVFGRD96   50.0   125    40    25   4.71 0.2017
WVFGRD96   52.0   125    40    25   4.73 0.2186
WVFGRD96   54.0   125    40    20   4.75 0.2348
WVFGRD96   56.0   125    40    20   4.76 0.2492
WVFGRD96   58.0   120    35    20   4.77 0.2621
WVFGRD96   60.0   120    35    20   4.78 0.2762
WVFGRD96   62.0   140    30    30   4.79 0.2899
WVFGRD96   64.0   145    30    30   4.80 0.3045
WVFGRD96   66.0   135    25    25   4.82 0.3201
WVFGRD96   68.0   135    25    25   4.83 0.3343
WVFGRD96   70.0   140    20    30   4.84 0.3498
WVFGRD96   72.0   145    20    30   4.84 0.3632
WVFGRD96   74.0   145    20    30   4.85 0.3782
WVFGRD96   76.0   145    20    30   4.86 0.3913
WVFGRD96   78.0   150    20    35   4.86 0.4025
WVFGRD96   80.0   150    20    35   4.87 0.4139
WVFGRD96   82.0   150    20    35   4.87 0.4240
WVFGRD96   84.0   150    20    35   4.87 0.4324
WVFGRD96   86.0   155    20    40   4.87 0.4398
WVFGRD96   88.0   155    20    40   4.88 0.4461
WVFGRD96   90.0   155    20    40   4.88 0.4518
WVFGRD96   92.0   155    20    40   4.88 0.4563
WVFGRD96   94.0   155    20    40   4.89 0.4597
WVFGRD96   96.0   155    20    40   4.89 0.4622
WVFGRD96   98.0   155    20    40   4.89 0.4643
WVFGRD96  100.0   160    20    45   4.89 0.4658
WVFGRD96  102.0   160    20    45   4.89 0.4665
WVFGRD96  104.0   160    20    45   4.89 0.4709
WVFGRD96  106.0   160    20    45   4.89 0.4774
WVFGRD96  108.0   165    20    50   4.89 0.4824
WVFGRD96  110.0   165    20    50   4.89 0.4855
WVFGRD96  112.0   165    20    50   4.90 0.4863
WVFGRD96  114.0   165    20    50   4.90 0.4858
WVFGRD96  116.0   165    20    50   4.90 0.4848
WVFGRD96  118.0   165    20    50   4.90 0.4844
WVFGRD96  120.0   165    20    50   4.90 0.4831
WVFGRD96  122.0   165    20    50   4.90 0.4821
WVFGRD96  124.0   165    20    50   4.90 0.4808
WVFGRD96  126.0   160    25    45   4.90 0.4791
WVFGRD96  128.0   160    25    45   4.90 0.4768
WVFGRD96  130.0   160    25    45   4.90 0.4748
WVFGRD96  132.0   160    25    45   4.90 0.4724
WVFGRD96  134.0   160    25    45   4.90 0.4697
WVFGRD96  136.0   160    25    45   4.91 0.4675
WVFGRD96  138.0   160    25    45   4.91 0.4644
WVFGRD96  140.0   160    25    45   4.91 0.4624
WVFGRD96  142.0   160    25    45   4.91 0.4595
WVFGRD96  144.0   160    25    45   4.91 0.4559
WVFGRD96  146.0   160    25    45   4.91 0.4523
WVFGRD96  148.0   160    25    45   4.91 0.4494

The best solution is

WVFGRD96  112.0   165    20    50   4.90 0.4863

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/3.5 -40 o DIST/3.5 +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 Mon Jul 9 21:20:29 CDT 2018