Location

Location ANSS

2018/06/03 23:21:41 62.848 -148.374 63.4 3.9 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2018/06/03 23:21:41:0 62.85 -148.37 63.4 3.9 Alaska Stations used: AK.BPAW AK.BWN AK.CAST AK.CUT AK.DHY AK.DIV AK.FID AK.GHO AK.GLI AK.HDA AK.KLU AK.KTH AK.MCK AK.NEA2 AK.PAX AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.TRF AT.PMR TA.J25K TA.M22K TA.M24K TA.N25K Filtering commands used: cut o DIST/3.5 -40 o DIST/3.5 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.06e+22 dyne-cm Mw = 3.95 Z = 74 km Plane Strike Dip Rake NP1 232 74 -143 NP2 130 55 -20 Principal Axes: Axis Value Plunge Azimuth T 1.06e+22 12 357 N 0.00e+00 50 252 P -1.06e+22 37 96 Moment Tensor: (dyne-cm) Component Value Mxx 1.00e+22 Mxy 2.60e+20 Mxz 2.72e+21 Myy -6.62e+21 Myz -5.17e+21 Mzz -3.40e+21 ##### ###### ######### T ########## ############ ############# ############################## -#############################---- --########################---------- ---#####################-------------- -----#################------------------ ------#############--------------------- --------##########------------------------ ---------#######----------------- ------ ----------####------------------- P ------ -----------#--------------------- ------ ---------###---------------------------- --------######-------------------------- -----##########----------------------- ---##############------------------- -###################-------------- ############################## ############################ ###################### ############## Global CMT Convention Moment Tensor: R T P -3.40e+21 2.72e+21 5.17e+21 2.72e+21 1.00e+22 -2.60e+20 5.17e+21 -2.60e+20 -6.62e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180603232141/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 = 130
      DIP = 55
     RAKE = -20
       MW = 3.95
       HS = 74.0

The NDK file is 20180603232141.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2018/06/03 23:21:41:0 62.85 -148.37 63.4 3.9 Alaska Stations used: AK.BPAW AK.BWN AK.CAST AK.CUT AK.DHY AK.DIV AK.FID AK.GHO AK.GLI AK.HDA AK.KLU AK.KTH AK.MCK AK.NEA2 AK.PAX AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.TRF AT.PMR TA.J25K TA.M22K TA.M24K TA.N25K Filtering commands used: cut o DIST/3.5 -40 o DIST/3.5 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.06e+22 dyne-cm Mw = 3.95 Z = 74 km Plane Strike Dip Rake NP1 232 74 -143 NP2 130 55 -20 Principal Axes: Axis Value Plunge Azimuth T 1.06e+22 12 357 N 0.00e+00 50 252 P -1.06e+22 37 96 Moment Tensor: (dyne-cm) Component Value Mxx 1.00e+22 Mxy 2.60e+20 Mxz 2.72e+21 Myy -6.62e+21 Myz -5.17e+21 Mzz -3.40e+21 ##### ###### ######### T ########## ############ ############# ############################## -#############################---- --########################---------- ---#####################-------------- -----#################------------------ ------#############--------------------- --------##########------------------------ ---------#######----------------- ------ ----------####------------------- P ------ -----------#--------------------- ------ ---------###---------------------------- --------######-------------------------- -----##########----------------------- ---##############------------------- -###################-------------- ############################## ############################ ###################### ############## Global CMT Convention Moment Tensor: R T P -3.40e+21 2.72e+21 5.17e+21 2.72e+21 1.00e+22 -2.60e+20 5.17e+21 -2.60e+20 -6.62e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180603232141/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 +50
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    50    45   3.06 0.1826
WVFGRD96    4.0    45    70    20   3.10 0.2046
WVFGRD96    6.0    40    75   -30   3.18 0.2314
WVFGRD96    8.0    40    75   -35   3.26 0.2486
WVFGRD96   10.0   225    85    35   3.31 0.2551
WVFGRD96   12.0   225    85    35   3.35 0.2559
WVFGRD96   14.0   315    65   -20   3.37 0.2527
WVFGRD96   16.0   315    65   -15   3.41 0.2695
WVFGRD96   18.0   310    65   -20   3.45 0.2860
WVFGRD96   20.0   310    65   -15   3.48 0.3024
WVFGRD96   22.0   310    65   -15   3.51 0.3183
WVFGRD96   24.0   310    70   -10   3.54 0.3313
WVFGRD96   26.0   130    80   -20   3.56 0.3435
WVFGRD96   28.0   130    80   -20   3.58 0.3505
WVFGRD96   30.0   130    70   -10   3.60 0.3538
WVFGRD96   32.0   135    60     5   3.63 0.3656
WVFGRD96   34.0   125    70   -15   3.65 0.3835
WVFGRD96   36.0   125    70   -15   3.67 0.3997
WVFGRD96   38.0   125    65   -10   3.71 0.4182
WVFGRD96   40.0   125    60   -10   3.78 0.4328
WVFGRD96   42.0   125    55   -10   3.81 0.4378
WVFGRD96   44.0   125    60   -10   3.83 0.4430
WVFGRD96   46.0   120    60   -20   3.85 0.4567
WVFGRD96   48.0   120    60   -20   3.87 0.4729
WVFGRD96   50.0   120    60   -25   3.87 0.4896
WVFGRD96   52.0   120    55   -25   3.89 0.5039
WVFGRD96   54.0   125    60   -20   3.89 0.5169
WVFGRD96   56.0   125    60   -20   3.90 0.5277
WVFGRD96   58.0   125    60   -20   3.91 0.5360
WVFGRD96   60.0   125    55   -20   3.92 0.5436
WVFGRD96   62.0   125    55   -20   3.92 0.5486
WVFGRD96   64.0   125    55   -20   3.93 0.5527
WVFGRD96   66.0   125    55   -20   3.93 0.5555
WVFGRD96   68.0   125    55   -20   3.93 0.5566
WVFGRD96   70.0   130    55   -20   3.94 0.5583
WVFGRD96   72.0   130    55   -20   3.94 0.5590
WVFGRD96   74.0   130    55   -20   3.95 0.5590
WVFGRD96   76.0   130    55   -20   3.95 0.5579
WVFGRD96   78.0   130    55   -20   3.95 0.5568
WVFGRD96   80.0   130    55   -20   3.95 0.5556
WVFGRD96   82.0   130    55   -20   3.96 0.5553
WVFGRD96   84.0   130    55   -20   3.96 0.5545
WVFGRD96   86.0   130    55   -20   3.96 0.5511
WVFGRD96   88.0   130    55   -20   3.96 0.5487
WVFGRD96   90.0   130    50   -20   3.98 0.5475
WVFGRD96   92.0   130    50   -20   3.98 0.5448
WVFGRD96   94.0   130    50   -20   3.98 0.5418
WVFGRD96   96.0   130    50   -20   3.98 0.5403
WVFGRD96   98.0   130    50   -20   3.98 0.5373

The best solution is

WVFGRD96   74.0   130    55   -20   3.95 0.5590

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 +50
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 Jun 4 00:29:31 CDT 2018