Location

Location ANSS

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2018/10/27 16:57:27:0 65.22 -151.66 16.6 5.3 Alaska Stations used: AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.FYU AK.GHO AK.HDA AK.KNK AK.KTH AK.MCK AK.MLY AK.NEA2 AK.PAX AK.PPD AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.TRF AK.WRH AT.PMR AT.TTA IU.COLA TA.E22K TA.E23K TA.F19K TA.F21K TA.F25K TA.F26K TA.G19K TA.G23K TA.G24K TA.H17K TA.H18K TA.H19K TA.H21K TA.H24K TA.I23K TA.J17K TA.J18K TA.J19K TA.J20K TA.J25K TA.J26L TA.K17K TA.K20K TA.L18K TA.L19K TA.M20K TA.M22K TA.M24K TA.POKR TA.TOLK Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 Best Fitting Double Couple Mo = 2.82e+23 dyne-cm Mw = 4.90 Z = 15 km Plane Strike Dip Rake NP1 175 90 -35 NP2 265 55 -180 Principal Axes: Axis Value Plunge Azimuth T 2.82e+23 24 226 N 0.00e+00 55 355 P -2.82e+23 24 124 Moment Tensor: (dyne-cm) Component Value Mxx 4.01e+22 Mxy 2.27e+23 Mxz -1.41e+22 Myy -4.01e+22 Myz -1.61e+23 Mzz 1.41e+16 -----######### ---------############# -------------############### --------------################ ----------------################## -----------------################### ---------------###----------########## ----------#########---------------###### ------##############------------------## ----#################--------------------# --###################--------------------- -####################--------------------- #####################--------------------- ####################-------------------- #####################----------- ----- ####################----------- P ---- ###### ##########----------- --- ##### T ##########---------------- ### ##########-------------- ###############------------- #############--------- #########----- Global CMT Convention Moment Tensor: R T P 1.41e+16 -1.41e+22 1.61e+23 -1.41e+22 4.01e+22 -2.27e+23 1.61e+23 -2.27e+23 -4.01e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181027165727/index.html

Preferred Solution

      STK = 175
      DIP = 90
     RAKE = -35
       MW = 4.90
       HS = 15.0

The NDK file is 20181027165727.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGSMWR

USGS/SLU Moment Tensor Solution ENS 2018/10/27 16:57:27:0 65.22 -151.66 16.6 5.3 Alaska Stations used: AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.FYU AK.GHO AK.HDA AK.KNK AK.KTH AK.MCK AK.MLY AK.NEA2 AK.PAX AK.PPD AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.TRF AK.WRH AT.PMR AT.TTA IU.COLA TA.E22K TA.E23K TA.F19K TA.F21K TA.F25K TA.F26K TA.G19K TA.G23K TA.G24K TA.H17K TA.H18K TA.H19K TA.H21K TA.H24K TA.I23K TA.J17K TA.J18K TA.J19K TA.J20K TA.J25K TA.J26L TA.K17K TA.K20K TA.L18K TA.L19K TA.M20K TA.M22K TA.M24K TA.POKR TA.TOLK Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 Best Fitting Double Couple Mo = 2.82e+23 dyne-cm Mw = 4.90 Z = 15 km Plane Strike Dip Rake NP1 175 90 -35 NP2 265 55 -180 Principal Axes: Axis Value Plunge Azimuth T 2.82e+23 24 226 N 0.00e+00 55 355 P -2.82e+23 24 124 Moment Tensor: (dyne-cm) Component Value Mxx 4.01e+22 Mxy 2.27e+23 Mxz -1.41e+22 Myy -4.01e+22 Myz -1.61e+23 Mzz 1.41e+16 -----######### ---------############# -------------############### --------------################ ----------------################## -----------------################### ---------------###----------########## ----------#########---------------###### ------##############------------------## ----#################--------------------# --###################--------------------- -####################--------------------- #####################--------------------- ####################-------------------- #####################----------- ----- ####################----------- P ---- ###### ##########----------- --- ##### T ##########---------------- ### ##########-------------- ###############------------- #############--------- #########----- Global CMT Convention Moment Tensor: R T P 1.41e+16 -1.41e+22 1.61e+23 -1.41e+22 4.01e+22 -2.27e+23 1.61e+23 -2.27e+23 -4.01e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181027165727/index.html

W-phase Moment Tensor (Mww) Moment 3.590e+16 N-m Magnitude 4.97 Mww Depth 13.5 km Percent DC 73% Half Duration 0.82 s Catalog US Data Source US 2 Contributor US 2 Nodal Planes Plane Strike Dip Rake NP1 269 56 -170 NP2 174 82 -34 Principal Axes Axis Value Plunge Azimuth T 3.302e+16 N-m 17 226 N 0.520e+16 N-m 55 342 P -3.822e+16 N-m 29 126

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

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 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 n 3 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   350    75    15   4.45 0.3771
WVFGRD96    2.0    -5    65    25   4.57 0.4671
WVFGRD96    3.0   355    70    25   4.62 0.4743
WVFGRD96    4.0   170    80   -40   4.66 0.4945
WVFGRD96    5.0   170    80   -40   4.69 0.5433
WVFGRD96    6.0   170    80   -40   4.71 0.5936
WVFGRD96    7.0   170    80   -40   4.73 0.6407
WVFGRD96    8.0   175    90   -45   4.79 0.6832
WVFGRD96    9.0   175    90   -45   4.81 0.7221
WVFGRD96   10.0   175    90   -40   4.82 0.7562
WVFGRD96   11.0   175    90   -40   4.84 0.7822
WVFGRD96   12.0    -5    90    35   4.86 0.8010
WVFGRD96   13.0    -5    90    35   4.87 0.8157
WVFGRD96   14.0    -5    90    35   4.88 0.8243
WVFGRD96   15.0   175    90   -35   4.90 0.8282
WVFGRD96   16.0   175    90   -35   4.91 0.8273
WVFGRD96   17.0   175    90   -35   4.92 0.8222
WVFGRD96   18.0   175    90   -35   4.93 0.8137
WVFGRD96   19.0   175    90   -35   4.94 0.8024
WVFGRD96   20.0   175    90   -30   4.95 0.7893
WVFGRD96   21.0   175    90   -35   4.96 0.7738
WVFGRD96   22.0   175    90   -35   4.96 0.7572
WVFGRD96   23.0   355    90    30   4.97 0.7391
WVFGRD96   24.0   355    90    30   4.98 0.7196
WVFGRD96   25.0   175    90   -30   4.99 0.6992
WVFGRD96   26.0   175    90   -30   4.99 0.6780
WVFGRD96   27.0   355    90    30   5.00 0.6561
WVFGRD96   28.0   170    85   -30   5.01 0.6337
WVFGRD96   29.0   170    80   -40   5.01 0.6130

The best solution is

WVFGRD96   15.0   175    90   -35   4.90 0.8282

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 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 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: