Location

Location ANSS

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2017/05/30 02:18:45:0 60.84 -151.83 78.4 5.3 Alaska Stations used: AK.BRLK AK.CAPN AK.CAST AK.CNP AK.CUT AK.FIRE AK.GHO AK.HOM AK.RC01 AK.SAW AK.SSN AK.SWD AT.PMR AT.SVW2 AV.ILSW TA.L19K TA.M19K TA.M20K TA.M22K TA.N18K TA.N19K TA.O18K TA.O22K TA.P18K TA.Q19K Filtering commands used: cut o DIST/3.3 -50 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 = 9.44e+23 dyne-cm Mw = 5.25 Z = 96 km Plane Strike Dip Rake NP1 185 75 -85 NP2 346 16 -108 Principal Axes: Axis Value Plunge Azimuth T 9.44e+23 30 271 N 0.00e+00 5 4 P -9.44e+23 60 102 Moment Tensor: (dyne-cm) Component Value Mxx -1.02e+22 Mxy 3.74e+22 Mxz 9.22e+22 Myy 4.80e+23 Myz -8.10e+23 Mzz -4.70e+23 ########--#### ###########-------#### #############-----------#### ##############-------------### ###############---------------#### ################-----------------### ################-------------------### #################--------------------### #################--------------------### ##### #########----------------------### ##### T #########---------- ---------### ##### #########---------- P ---------### #################---------- ---------### ################---------------------### ################---------------------### ###############--------------------### ##############--------------------## #############-------------------## ###########-----------------## ###########---------------## ########-------------# #####--------- Global CMT Convention Moment Tensor: R T P -4.70e+23 9.22e+22 8.10e+23 9.22e+22 -1.02e+22 -3.74e+22 8.10e+23 -3.74e+22 4.80e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170530021845/index.html

Preferred Solution

      STK = 185
      DIP = 75
     RAKE = -85
       MW = 5.25
       HS = 96.0

The NDK file is 20170530021845.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGSMT

USGS/SLU Moment Tensor Solution ENS 2017/05/30 02:18:45:0 60.84 -151.83 78.4 5.3 Alaska Stations used: AK.BRLK AK.CAPN AK.CAST AK.CNP AK.CUT AK.FIRE AK.GHO AK.HOM AK.RC01 AK.SAW AK.SSN AK.SWD AT.PMR AT.SVW2 AV.ILSW TA.L19K TA.M19K TA.M20K TA.M22K TA.N18K TA.N19K TA.O18K TA.O22K TA.P18K TA.Q19K Filtering commands used: cut o DIST/3.3 -50 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 = 9.44e+23 dyne-cm Mw = 5.25 Z = 96 km Plane Strike Dip Rake NP1 185 75 -85 NP2 346 16 -108 Principal Axes: Axis Value Plunge Azimuth T 9.44e+23 30 271 N 0.00e+00 5 4 P -9.44e+23 60 102 Moment Tensor: (dyne-cm) Component Value Mxx -1.02e+22 Mxy 3.74e+22 Mxz 9.22e+22 Myy 4.80e+23 Myz -8.10e+23 Mzz -4.70e+23 ########--#### ###########-------#### #############-----------#### ##############-------------### ###############---------------#### ################-----------------### ################-------------------### #################--------------------### #################--------------------### ##### #########----------------------### ##### T #########---------- ---------### ##### #########---------- P ---------### #################---------- ---------### ################---------------------### ################---------------------### ###############--------------------### ##############--------------------## #############-------------------## ###########-----------------## ###########---------------## ########-------------# #####--------- Global CMT Convention Moment Tensor: R T P -4.70e+23 9.22e+22 8.10e+23 9.22e+22 -1.02e+22 -3.74e+22 8.10e+23 -3.74e+22 4.80e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170530021845/index.html

Regional Moment Tensor (Mwr) Moment 8.549e+16 N-m Magnitude 5.2 Mwr Depth 91.0 km Percent DC 80 % Half Duration – Catalog US Data Source US3 Contributor US3 Nodal Planes Plane Strike Dip Rake NP1 342 21 -112 NP2 186 70 -82 Principal Axes Axis Value Plunge Azimuth T 8.071e+16 N-m 25 269 N 0.888e+16 N-m 8 3 P -8.959e+16 N-m 64 109

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

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 -50 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    2.0     5    55    95   4.33 0.1208
WVFGRD96    4.0   300    55   -50   4.36 0.1269
WVFGRD96    6.0   265    35   -40   4.44 0.1631
WVFGRD96    8.0   265    35   -40   4.53 0.1810
WVFGRD96   10.0   300    50   -45   4.55 0.1983
WVFGRD96   12.0   300    50   -40   4.58 0.2049
WVFGRD96   14.0   305    55   -40   4.60 0.2058
WVFGRD96   16.0   310    60   -40   4.62 0.2037
WVFGRD96   18.0   310    60   -40   4.65 0.1986
WVFGRD96   20.0   315    65   -35   4.67 0.1975
WVFGRD96   22.0   320    70   -35   4.69 0.1952
WVFGRD96   24.0   325    75   -40   4.71 0.1943
WVFGRD96   26.0   335    85   -45   4.73 0.1954
WVFGRD96   28.0   190    85    60   4.78 0.2011
WVFGRD96   30.0   190    85    60   4.81 0.2025
WVFGRD96   32.0     5    90   -60   4.82 0.1987
WVFGRD96   34.0    25    80   -70   4.83 0.1995
WVFGRD96   36.0    30    75   -65   4.83 0.2049
WVFGRD96   38.0    30    70   -65   4.84 0.2171
WVFGRD96   40.0    30    65   -65   4.97 0.2580
WVFGRD96   42.0    25    55   -70   5.00 0.3001
WVFGRD96   44.0    25    50   -70   5.04 0.3404
WVFGRD96   46.0    20    45   -75   5.07 0.3704
WVFGRD96   48.0    25    45   -70   5.09 0.3840
WVFGRD96   50.0    20    40   -75   5.11 0.3967
WVFGRD96   52.0    15    35   -80   5.13 0.4096
WVFGRD96   54.0   200    55   -80   5.13 0.4270
WVFGRD96   56.0   185    55   -90   5.14 0.4420
WVFGRD96   58.0   185    55   -90   5.15 0.4573
WVFGRD96   60.0   185    60   -90   5.17 0.4747
WVFGRD96   62.0   185    60   -90   5.17 0.4892
WVFGRD96   64.0   185    60   -90   5.18 0.5010
WVFGRD96   66.0     5    30   -90   5.18 0.5139
WVFGRD96   68.0   185    65   -90   5.19 0.5216
WVFGRD96   70.0     0    25   -95   5.20 0.5319
WVFGRD96   72.0   185    65   -90   5.20 0.5394
WVFGRD96   74.0     0    25   -95   5.20 0.5452
WVFGRD96   76.0   185    65   -90   5.20 0.5507
WVFGRD96   78.0   185    65   -90   5.20 0.5536
WVFGRD96   80.0   185    65   -90   5.20 0.5575
WVFGRD96   82.0   185    70   -85   5.22 0.5592
WVFGRD96   84.0    -5    20  -100   5.22 0.5660
WVFGRD96   86.0    -5    20  -100   5.22 0.5669
WVFGRD96   88.0   -10    20  -105   5.23 0.5711
WVFGRD96   90.0   -10    20  -105   5.23 0.5711
WVFGRD96   92.0   185    70   -85   5.23 0.5730
WVFGRD96   94.0   -15    20  -110   5.23 0.5743
WVFGRD96   96.0   185    75   -85   5.25 0.5751
WVFGRD96   98.0   -15    20  -110   5.23 0.5749
WVFGRD96  100.0   -10    15  -100   5.25 0.5732
WVFGRD96  102.0   -10    15  -100   5.25 0.5736
WVFGRD96  104.0   -15    15  -105   5.26 0.5706
WVFGRD96  106.0   -15    15  -105   5.26 0.5717
WVFGRD96  108.0   -15    15  -105   5.26 0.5695
WVFGRD96  110.0   185    75   -80   5.25 0.5672
WVFGRD96  112.0   185    75   -80   5.25 0.5651
WVFGRD96  114.0   185    75   -80   5.25 0.5607
WVFGRD96  116.0   185    75   -80   5.25 0.5573
WVFGRD96  118.0   185    75   -80   5.25 0.5531

The best solution is

WVFGRD96   96.0   185    75   -85   5.25 0.5751

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 -50 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)

 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: