Location

Location ANSS

2019/12/19 14:09:53 59.331 -153.597 103.2 3.5 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2019/12/19 14:09:53:0 59.33 -153.60 103.2 3.5 Alaska Stations used: AK.BRLK AK.CNP AK.HOM AK.N18K AK.O18K AK.O19K AK.Q19K II.KDAK TA.P18K TA.P19K Filtering commands used: cut o DIST/3.4 -40 o DIST/3.4 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.97e+22 dyne-cm Mw = 4.13 Z = 104 km Plane Strike Dip Rake NP1 230 80 -65 NP2 340 27 -157 Principal Axes: Axis Value Plunge Azimuth T 1.97e+22 31 300 N 0.00e+00 25 45 P -1.97e+22 49 167 Moment Tensor: (dyne-cm) Component Value Mxx -4.50e+21 Mxy -4.44e+21 Mxz 1.38e+22 Myy 1.06e+22 Myz -9.69e+21 Mzz -6.11e+21 #####--------- ###############------- #####################------- ########################------ ############################--#### ############################--###### ##### ##################------###### ###### T ###############----------###### ###### #############-------------##### ####################----------------###### ##################-------------------##### ################---------------------##### ##############-----------------------##### ###########-------------------------#### #########--------------------------##### #######------------- -----------#### ####--------------- P ----------#### ##---------------- ----------### ----------------------------## -------------------------### --------------------## -------------- Global CMT Convention Moment Tensor: R T P -6.11e+21 1.38e+22 9.69e+21 1.38e+22 -4.50e+21 4.44e+21 9.69e+21 4.44e+21 1.06e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191219140953/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 = 230
      DIP = 80
     RAKE = -65
       MW = 4.13
       HS = 104.0

The NDK file is 20191219140953.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2019/12/19 14:09:53:0 59.33 -153.60 103.2 3.5 Alaska Stations used: AK.BRLK AK.CNP AK.HOM AK.N18K AK.O18K AK.O19K AK.Q19K II.KDAK TA.P18K TA.P19K Filtering commands used: cut o DIST/3.4 -40 o DIST/3.4 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.97e+22 dyne-cm Mw = 4.13 Z = 104 km Plane Strike Dip Rake NP1 230 80 -65 NP2 340 27 -157 Principal Axes: Axis Value Plunge Azimuth T 1.97e+22 31 300 N 0.00e+00 25 45 P -1.97e+22 49 167 Moment Tensor: (dyne-cm) Component Value Mxx -4.50e+21 Mxy -4.44e+21 Mxz 1.38e+22 Myy 1.06e+22 Myz -9.69e+21 Mzz -6.11e+21 #####--------- ###############------- #####################------- ########################------ ############################--#### ############################--###### ##### ##################------###### ###### T ###############----------###### ###### #############-------------##### ####################----------------###### ##################-------------------##### ################---------------------##### ##############-----------------------##### ###########-------------------------#### #########--------------------------##### #######------------- -----------#### ####--------------- P ----------#### ##---------------- ----------### ----------------------------## -------------------------### --------------------## -------------- Global CMT Convention Moment Tensor: R T P -6.11e+21 1.38e+22 9.69e+21 1.38e+22 -4.50e+21 4.44e+21 9.69e+21 4.44e+21 1.06e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191219140953/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/3.4 -40 o DIST/3.4 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 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    2.0   350    50    50   3.42 0.2947
WVFGRD96    4.0   180    50    65   3.52 0.3238
WVFGRD96    6.0    40    65    90   3.60 0.3391
WVFGRD96    8.0    40    60    90   3.63 0.3415
WVFGRD96   10.0   315    55   -50   3.59 0.3448
WVFGRD96   12.0   260    35   -25   3.62 0.3517
WVFGRD96   14.0   260    40   -25   3.63 0.3579
WVFGRD96   16.0   265    40   -15   3.64 0.3623
WVFGRD96   18.0   265    40   -15   3.66 0.3659
WVFGRD96   20.0   265    40   -15   3.68 0.3694
WVFGRD96   22.0   260    35   -20   3.72 0.3749
WVFGRD96   24.0   260    35   -20   3.74 0.3809
WVFGRD96   26.0   260    35   -15   3.77 0.3869
WVFGRD96   28.0   255    35   -20   3.79 0.3910
WVFGRD96   30.0   255    35   -20   3.81 0.3954
WVFGRD96   32.0   255    35   -20   3.82 0.3983
WVFGRD96   34.0   250    35   -25   3.85 0.4005
WVFGRD96   36.0   245    35   -30   3.87 0.4007
WVFGRD96   38.0   240    35   -35   3.90 0.4006
WVFGRD96   40.0   250    45   -40   3.93 0.4087
WVFGRD96   42.0   250    45   -40   3.95 0.4180
WVFGRD96   44.0   255    50   -35   3.96 0.4249
WVFGRD96   46.0   255    50   -40   3.98 0.4306
WVFGRD96   48.0   255    50   -40   4.00 0.4332
WVFGRD96   50.0   245    80   -55   4.01 0.4604
WVFGRD96   52.0   245    80   -55   4.02 0.4833
WVFGRD96   54.0   245    85   -50   4.01 0.5075
WVFGRD96   56.0   245    85   -50   4.03 0.5301
WVFGRD96   58.0   245    85   -55   4.05 0.5487
WVFGRD96   60.0    55    90    55   4.05 0.5687
WVFGRD96   62.0    55    90    55   4.05 0.5832
WVFGRD96   64.0    55    90    55   4.06 0.5956
WVFGRD96   66.0    65    85    65   4.09 0.6082
WVFGRD96   68.0    65    85    65   4.09 0.6184
WVFGRD96   70.0    65    85    65   4.10 0.6301
WVFGRD96   72.0   240    90   -65   4.09 0.6362
WVFGRD96   74.0   240    90   -65   4.10 0.6448
WVFGRD96   76.0    65    85    65   4.11 0.6533
WVFGRD96   78.0    65    85    65   4.11 0.6580
WVFGRD96   80.0    60    90    65   4.11 0.6634
WVFGRD96   82.0    60    90    65   4.11 0.6680
WVFGRD96   84.0    60    90    65   4.11 0.6721
WVFGRD96   86.0    60    90    65   4.11 0.6741
WVFGRD96   88.0    60    90    65   4.11 0.6764
WVFGRD96   90.0    60    90    65   4.12 0.6786
WVFGRD96   92.0    60    90    65   4.12 0.6800
WVFGRD96   94.0   235    85   -65   4.12 0.6831
WVFGRD96   96.0   235    85   -65   4.12 0.6842
WVFGRD96   98.0   230    80   -65   4.13 0.6856
WVFGRD96  100.0   230    80   -65   4.13 0.6866
WVFGRD96  102.0   230    80   -65   4.13 0.6872
WVFGRD96  104.0   230    80   -65   4.13 0.6876
WVFGRD96  106.0   230    80   -65   4.13 0.6869
WVFGRD96  108.0   230    80   -65   4.14 0.6872
WVFGRD96  110.0   225    75   -65   4.15 0.6854
WVFGRD96  112.0   225    75   -65   4.15 0.6856
WVFGRD96  114.0   215    70   -65   4.17 0.6839
WVFGRD96  116.0   215    70   -65   4.17 0.6845
WVFGRD96  118.0   215    70   -65   4.18 0.6838
WVFGRD96  120.0   215    70   -65   4.18 0.6828
WVFGRD96  122.0   215    70   -65   4.18 0.6822
WVFGRD96  124.0   215    70   -65   4.18 0.6796
WVFGRD96  126.0   215    70   -65   4.19 0.6785
WVFGRD96  128.0   215    70   -65   4.19 0.6766
WVFGRD96  130.0   215    70   -65   4.19 0.6750
WVFGRD96  132.0   215    70   -65   4.19 0.6736
WVFGRD96  134.0   215    70   -65   4.19 0.6703
WVFGRD96  136.0   215    70   -65   4.20 0.6682
WVFGRD96  138.0   210    65   -65   4.22 0.6672

The best solution is

WVFGRD96  104.0   230    80   -65   4.13 0.6876

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/3.4 -40 o DIST/3.4 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 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 Bureau of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Oklahoma Geological Survey, TexNet, the Iris stations, the Transportable Array of EarthScope and other networks.

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 Thu Dec 19 09:07:17 CST 2019