Location

Location ANSS

2018/11/13 15:26:42 64.789 -150.962 16.6 4.1 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2018/11/13 15:26:42:0 64.79 -150.96 16.6 4.1 Alaska Stations used: AK.BPAW AK.BWN AK.CAST AK.CCB AK.COLD AK.CUT AK.DHY AK.HDA AK.KNK AK.KTH AK.MCK AK.MLY AK.NEA2 AK.PAX AK.PPD AK.PPLA AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.TRF AK.WRH AT.MENT AT.PMR AV.SPU IM.IL31 IU.COLA TA.E19K TA.E22K TA.E23K TA.E24K TA.F19K TA.F20K TA.F21K TA.F24K TA.F25K TA.G18K TA.G19K TA.G21K TA.G23K TA.G24K TA.H17K TA.H18K TA.H19K TA.H21K TA.H24K TA.I20K TA.I21K TA.I23K TA.J17K TA.J18K TA.J19K TA.J20K TA.J25K TA.J26L TA.K17K TA.K27K TA.L18K TA.L19K TA.L26K TA.M19K TA.M20K TA.M24K TA.POKR TA.TOLK Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 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 = 1.02e+22 dyne-cm Mw = 3.94 Z = 15 km Plane Strike Dip Rake NP1 215 75 -20 NP2 310 71 -164 Principal Axes: Axis Value Plunge Azimuth T 1.02e+22 3 263 N 0.00e+00 65 360 P -1.02e+22 25 172 Moment Tensor: (dyne-cm) Component Value Mxx -8.15e+21 Mxy 2.35e+21 Mxz 3.78e+21 Myy 9.90e+21 Myz -1.06e+21 Mzz -1.75e+21 -------------- ---------------------- ------------------------#### ----------------------######## #######---------------############ ############---------############### ################-----################# ######################################## ###################---################## ##################-------################# #################----------############### ##############-------------############# T #############----------------########### ############------------------######### ############--------------------######## ##########-----------------------##### ########-------------------------### ######--------------------------## ####------------ ----------- ##------------- P ---------- ------------ ------- -------------- Global CMT Convention Moment Tensor: R T P -1.75e+21 3.78e+21 1.06e+21 3.78e+21 -8.15e+21 -2.35e+21 1.06e+21 -2.35e+21 9.90e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181113152642/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 = 215
      DIP = 75
     RAKE = -20
       MW = 3.94
       HS = 15.0

The NDK file is 20181113152642.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2018/11/13 15:26:42:0 64.79 -150.96 16.6 4.1 Alaska Stations used: AK.BPAW AK.BWN AK.CAST AK.CCB AK.COLD AK.CUT AK.DHY AK.HDA AK.KNK AK.KTH AK.MCK AK.MLY AK.NEA2 AK.PAX AK.PPD AK.PPLA AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.TRF AK.WRH AT.MENT AT.PMR AV.SPU IM.IL31 IU.COLA TA.E19K TA.E22K TA.E23K TA.E24K TA.F19K TA.F20K TA.F21K TA.F24K TA.F25K TA.G18K TA.G19K TA.G21K TA.G23K TA.G24K TA.H17K TA.H18K TA.H19K TA.H21K TA.H24K TA.I20K TA.I21K TA.I23K TA.J17K TA.J18K TA.J19K TA.J20K TA.J25K TA.J26L TA.K17K TA.K27K TA.L18K TA.L19K TA.L26K TA.M19K TA.M20K TA.M24K TA.POKR TA.TOLK Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 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 = 1.02e+22 dyne-cm Mw = 3.94 Z = 15 km Plane Strike Dip Rake NP1 215 75 -20 NP2 310 71 -164 Principal Axes: Axis Value Plunge Azimuth T 1.02e+22 3 263 N 0.00e+00 65 360 P -1.02e+22 25 172 Moment Tensor: (dyne-cm) Component Value Mxx -8.15e+21 Mxy 2.35e+21 Mxz 3.78e+21 Myy 9.90e+21 Myz -1.06e+21 Mzz -1.75e+21 -------------- ---------------------- ------------------------#### ----------------------######## #######---------------############ ############---------############### ################-----################# ######################################## ###################---################## ##################-------################# #################----------############### ##############-------------############# T #############----------------########### ############------------------######### ############--------------------######## ##########-----------------------##### ########-------------------------### ######--------------------------## ####------------ ----------- ##------------- P ---------- ------------ ------- -------------- Global CMT Convention Moment Tensor: R T P -1.75e+21 3.78e+21 1.06e+21 3.78e+21 -8.15e+21 -2.35e+21 1.06e+21 -2.35e+21 9.90e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181113152642/index.html

Magnitudes

mLg Magnitude

(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.

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 +70
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    1.0    35    90     0   3.44 0.3189
WVFGRD96    2.0   215    85     0   3.62 0.5295
WVFGRD96    3.0   215    85   -10   3.69 0.5997
WVFGRD96    4.0    40    90    25   3.74 0.6488
WVFGRD96    5.0   220    90   -35   3.79 0.6983
WVFGRD96    6.0    40    90    35   3.81 0.7381
WVFGRD96    7.0    40    90    30   3.82 0.7662
WVFGRD96    8.0    40    90    35   3.86 0.7851
WVFGRD96    9.0   215    80   -35   3.87 0.8043
WVFGRD96   10.0   215    80   -30   3.88 0.8193
WVFGRD96   11.0   215    75   -25   3.90 0.8312
WVFGRD96   12.0   215    75   -25   3.91 0.8407
WVFGRD96   13.0   215    75   -25   3.92 0.8451
WVFGRD96   14.0   215    75   -25   3.93 0.8466
WVFGRD96   15.0   215    75   -20   3.94 0.8466
WVFGRD96   16.0   215    75   -20   3.95 0.8436
WVFGRD96   17.0   215    75   -20   3.96 0.8399
WVFGRD96   18.0   215    80   -20   3.97 0.8334
WVFGRD96   19.0   215    80   -20   3.98 0.8255
WVFGRD96   20.0   215    80   -20   3.99 0.8160
WVFGRD96   21.0   215    75   -20   4.00 0.8048
WVFGRD96   22.0   215    75   -20   4.01 0.7914
WVFGRD96   23.0   215    80   -25   4.01 0.7773
WVFGRD96   24.0   215    80   -25   4.02 0.7620
WVFGRD96   25.0   215    80   -25   4.02 0.7466
WVFGRD96   26.0   215    80   -25   4.03 0.7298
WVFGRD96   27.0   215    80   -25   4.03 0.7114
WVFGRD96   28.0   215    80   -25   4.04 0.6938
WVFGRD96   29.0   215    80   -25   4.04 0.6748

The best solution is

WVFGRD96   15.0   215    75   -20   3.94 0.8466

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 +70
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 Tue Nov 13 13:03:44 CST 2018