Location

Location ANSS

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2019/02/26 04:09:41:0 66.27 -157.25 9.2 4.1 Alaska Stations used: AK.ANM AK.BPAW AK.CAST AK.COLD AK.KTH AK.NEA2 AK.RDOG AT.TTA TA.B20K TA.B21K TA.C16K TA.C18K TA.D19K TA.D22K TA.D23K TA.E18K TA.E19K TA.E21K TA.E22K TA.E23K TA.E24K TA.F15K TA.F17K TA.F19K TA.F20K TA.F21K TA.F24K TA.G16K TA.G18K TA.G19K TA.G21K TA.G23K TA.G24K TA.H17K TA.H18K TA.H19K TA.H21K TA.H24K TA.I17K TA.I20K TA.I21K TA.I23K TA.J16K TA.J17K TA.J18K TA.J19K TA.J20K TA.K17K TA.K20K TA.TOLK Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +60 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 = 1.14e+22 dyne-cm Mw = 3.97 Z = 8 km Plane Strike Dip Rake NP1 350 85 15 NP2 259 75 175 Principal Axes: Axis Value Plunge Azimuth T 1.14e+22 14 215 N 0.00e+00 74 8 P -1.14e+22 7 123 Moment Tensor: (dyne-cm) Component Value Mxx 3.72e+21 Mxy 1.02e+22 Mxz -1.44e+21 Myy -4.23e+21 Myz -2.68e+21 Mzz 5.10e+20 ----########## --------############## ------------################ -------------################# ----------------################## -----------------################### -------------------################### --------------------#################### --------------------#####-------------## ----------------######-------------------- ----------############-------------------- ------################-------------------- --#####################------------------- ######################------------------ ######################------------------ ######################------------ - #####################------------ P #### #############------------ ## T #############------------ # #############----------- ###############------- ###########--- Global CMT Convention Moment Tensor: R T P 5.10e+20 -1.44e+21 2.68e+21 -1.44e+21 3.72e+21 -1.02e+22 2.68e+21 -1.02e+22 -4.23e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190226040941/index.html

Preferred Solution

      STK = 350
      DIP = 85
     RAKE = 15
       MW = 3.97
       HS = 8.0

The NDK file is 20190226040941.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2019/02/26 04:09:41:0 66.27 -157.25 9.2 4.1 Alaska Stations used: AK.ANM AK.BPAW AK.CAST AK.COLD AK.KTH AK.NEA2 AK.RDOG AT.TTA TA.B20K TA.B21K TA.C16K TA.C18K TA.D19K TA.D22K TA.D23K TA.E18K TA.E19K TA.E21K TA.E22K TA.E23K TA.E24K TA.F15K TA.F17K TA.F19K TA.F20K TA.F21K TA.F24K TA.G16K TA.G18K TA.G19K TA.G21K TA.G23K TA.G24K TA.H17K TA.H18K TA.H19K TA.H21K TA.H24K TA.I17K TA.I20K TA.I21K TA.I23K TA.J16K TA.J17K TA.J18K TA.J19K TA.J20K TA.K17K TA.K20K TA.TOLK Filtering commands used: cut o DIST/3.3 -40 o DIST/3.3 +60 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 = 1.14e+22 dyne-cm Mw = 3.97 Z = 8 km Plane Strike Dip Rake NP1 350 85 15 NP2 259 75 175 Principal Axes: Axis Value Plunge Azimuth T 1.14e+22 14 215 N 0.00e+00 74 8 P -1.14e+22 7 123 Moment Tensor: (dyne-cm) Component Value Mxx 3.72e+21 Mxy 1.02e+22 Mxz -1.44e+21 Myy -4.23e+21 Myz -2.68e+21 Mzz 5.10e+20 ----########## --------############## ------------################ -------------################# ----------------################## -----------------################### -------------------################### --------------------#################### --------------------#####-------------## ----------------######-------------------- ----------############-------------------- ------################-------------------- --#####################------------------- ######################------------------ ######################------------------ ######################------------ - #####################------------ P #### #############------------ ## T #############------------ # #############----------- ###############------- ###########--- Global CMT Convention Moment Tensor: R T P 5.10e+20 -1.44e+21 2.68e+21 -1.44e+21 3.72e+21 -1.02e+22 2.68e+21 -1.02e+22 -4.23e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190226040941/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

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 -40 o DIST/3.3 +60
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

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   355    90    -5   3.56 0.3565
WVFGRD96    2.0   350    90     0   3.74 0.6077
WVFGRD96    3.0   170    90     0   3.81 0.7183
WVFGRD96    4.0   170    90     0   3.85 0.7817
WVFGRD96    5.0   350    85    -5   3.89 0.8155
WVFGRD96    6.0   170    90    -5   3.91 0.8299
WVFGRD96    7.0   170    90   -15   3.94 0.8395
WVFGRD96    8.0   350    85    15   3.97 0.8462
WVFGRD96    9.0   170    90   -15   3.99 0.8437
WVFGRD96   10.0   350    85    15   4.00 0.8405
WVFGRD96   11.0   350    85    15   4.01 0.8367
WVFGRD96   12.0   170    90   -15   4.03 0.8294
WVFGRD96   13.0   350    90    15   4.04 0.8235
WVFGRD96   14.0   350    90    15   4.05 0.8152
WVFGRD96   15.0   350    90    10   4.06 0.8075
WVFGRD96   16.0   350    90    10   4.07 0.7995
WVFGRD96   17.0   350    90    10   4.08 0.7896
WVFGRD96   18.0   350    90    10   4.09 0.7795
WVFGRD96   19.0   170    85     5   4.09 0.7726
WVFGRD96   20.0   170    85     5   4.10 0.7622
WVFGRD96   21.0   170    85     5   4.11 0.7504
WVFGRD96   22.0   170    85     5   4.12 0.7382
WVFGRD96   23.0   170    80    10   4.12 0.7264
WVFGRD96   24.0   170    80    10   4.13 0.7145
WVFGRD96   25.0   170    80    10   4.13 0.7022
WVFGRD96   26.0   170    80    10   4.14 0.6897
WVFGRD96   27.0   170    80    10   4.15 0.6776
WVFGRD96   28.0   170    80    10   4.15 0.6666
WVFGRD96   29.0   170    80    10   4.16 0.6564

The best solution is

WVFGRD96    8.0   350    85    15   3.97 0.8462

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 -40 o DIST/3.3 +60
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)

 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: