Location

Location ANSS

2018/02/25 11:32:53 63.212 -150.595 131.9 4.0 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2018/02/25 11:32:53:0 63.21 -150.60 131.9 4.0 Alaska Stations used: AK.BPAW AK.CAST AK.CUT AK.GHO AK.KNK AK.KTH AK.NEA2 AK.PAX AK.RND AK.SAW AK.SCM AK.SSN AK.TRF AT.PMR TA.M19K Filtering commands used: cut o DIST/3.1 -30 o DIST/3.1 +60 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 = 2.19e+22 dyne-cm Mw = 4.16 Z = 138 km Plane Strike Dip Rake NP1 35 84 98 NP2 165 10 40 Principal Axes: Axis Value Plunge Azimuth T 2.19e+22 51 314 N 0.00e+00 8 215 P -2.19e+22 38 119 Moment Tensor: (dyne-cm) Component Value Mxx 1.13e+21 Mxy 1.32e+21 Mxz 1.25e+22 Myy -5.94e+21 Myz -1.70e+22 Mzz 4.81e+21 ############## -##################### --########################-- -#########################---- -##########################------- -##########################--------- --######## #############------------ --######### T ############-------------- -########## ###########--------------- --#######################----------------- --######################------------------ --####################-------------------- --###################--------------------- --#################----------- ------- --###############------------- P ------- --#############-------------- ------ --##########------------------------ --########------------------------ --####------------------------ ---------------------------- ##-------------------- ###----------- Global CMT Convention Moment Tensor: R T P 4.81e+21 1.25e+22 1.70e+22 1.25e+22 1.13e+21 -1.32e+21 1.70e+22 -1.32e+21 -5.94e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180225113253/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 = 165
      DIP = 10
     RAKE = 40
       MW = 4.16
       HS = 138.0

The NDK file is 20180225113253.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2018/02/25 11:32:53:0 63.21 -150.60 131.9 4.0 Alaska Stations used: AK.BPAW AK.CAST AK.CUT AK.GHO AK.KNK AK.KTH AK.NEA2 AK.PAX AK.RND AK.SAW AK.SCM AK.SSN AK.TRF AT.PMR TA.M19K Filtering commands used: cut o DIST/3.1 -30 o DIST/3.1 +60 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 = 2.19e+22 dyne-cm Mw = 4.16 Z = 138 km Plane Strike Dip Rake NP1 35 84 98 NP2 165 10 40 Principal Axes: Axis Value Plunge Azimuth T 2.19e+22 51 314 N 0.00e+00 8 215 P -2.19e+22 38 119 Moment Tensor: (dyne-cm) Component Value Mxx 1.13e+21 Mxy 1.32e+21 Mxz 1.25e+22 Myy -5.94e+21 Myz -1.70e+22 Mzz 4.81e+21 ############## -##################### --########################-- -#########################---- -##########################------- -##########################--------- --######## #############------------ --######### T ############-------------- -########## ###########--------------- --#######################----------------- --######################------------------ --####################-------------------- --###################--------------------- --#################----------- ------- --###############------------- P ------- --#############-------------- ------ --##########------------------------ --########------------------------ --####------------------------ ---------------------------- ##-------------------- ###----------- Global CMT Convention Moment Tensor: R T P 4.81e+21 1.25e+22 1.70e+22 1.25e+22 1.13e+21 -1.32e+21 1.70e+22 -1.32e+21 -5.94e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180225113253/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/3.1 -30 o DIST/3.1 +60
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    30    40    80   3.35 0.2053
WVFGRD96    4.0    40    20   -80   3.52 0.2545
WVFGRD96    6.0    40    25   -80   3.53 0.2781
WVFGRD96    8.0    40    20   -80   3.57 0.2755
WVFGRD96   10.0   185    75    65   3.54 0.2583
WVFGRD96   12.0   180    80    60   3.54 0.2572
WVFGRD96   14.0   180    80    55   3.55 0.2561
WVFGRD96   16.0   180    80    55   3.56 0.2538
WVFGRD96   18.0   175    85    50   3.58 0.2506
WVFGRD96   20.0   175    75    50   3.59 0.2497
WVFGRD96   22.0   180    70    55   3.61 0.2506
WVFGRD96   24.0   180    65    55   3.63 0.2531
WVFGRD96   26.0   185    60    55   3.64 0.2581
WVFGRD96   28.0   110    50    65   3.71 0.2620
WVFGRD96   30.0   115    55    70   3.74 0.2628
WVFGRD96   32.0   170    60    50   3.71 0.2554
WVFGRD96   34.0   170    60    50   3.73 0.2496
WVFGRD96   36.0   165    60    45   3.75 0.2436
WVFGRD96   38.0   165    60    45   3.78 0.2395
WVFGRD96   40.0    45    50   -45   3.85 0.2537
WVFGRD96   42.0    40    50   -45   3.90 0.2619
WVFGRD96   44.0    45    55   -45   3.90 0.2684
WVFGRD96   46.0    45    55   -45   3.92 0.2800
WVFGRD96   48.0    55    65   -55   3.91 0.2941
WVFGRD96   50.0    50    65   -55   3.94 0.3099
WVFGRD96   52.0    50    65   -55   3.96 0.3238
WVFGRD96   54.0    55    70   -50   3.97 0.3366
WVFGRD96   56.0    55    70   -50   3.98 0.3453
WVFGRD96   58.0    55    70   -50   4.00 0.3515
WVFGRD96   60.0    50    70   -50   4.02 0.3564
WVFGRD96   62.0    50    70   -45   4.04 0.3603
WVFGRD96   64.0    50    70   -45   4.05 0.3637
WVFGRD96   66.0    50    70   -45   4.05 0.3646
WVFGRD96   68.0    55    75   -40   4.05 0.3642
WVFGRD96   70.0    55    75   -40   4.05 0.3636
WVFGRD96   72.0    55    75   -40   4.06 0.3617
WVFGRD96   74.0    55    75   -40   4.06 0.3592
WVFGRD96   76.0    55    75    15   4.04 0.3797
WVFGRD96   78.0   140    35    40   4.12 0.4221
WVFGRD96   80.0   140    35    40   4.13 0.4733
WVFGRD96   82.0   140    30    40   4.13 0.5162
WVFGRD96   84.0   140    30    35   4.14 0.5476
WVFGRD96   86.0   140    30    35   4.15 0.5726
WVFGRD96   88.0   140    30    35   4.15 0.5867
WVFGRD96   90.0   135    35    30   4.17 0.5967
WVFGRD96   92.0   135    35    30   4.18 0.6059
WVFGRD96   94.0   135    35    30   4.18 0.6150
WVFGRD96   96.0   135    35    30   4.18 0.6221
WVFGRD96   98.0   135    35    30   4.18 0.6291
WVFGRD96  100.0   135    35    30   4.18 0.6341
WVFGRD96  102.0   135    35    30   4.19 0.6393
WVFGRD96  104.0   135    35    30   4.19 0.6455
WVFGRD96  106.0   135    35    30   4.19 0.6508
WVFGRD96  108.0   135    35    30   4.19 0.6549
WVFGRD96  110.0   135    35    30   4.19 0.6571
WVFGRD96  112.0   135    35    30   4.19 0.6594
WVFGRD96  114.0   175    10    50   4.14 0.6650
WVFGRD96  116.0   170    10    45   4.14 0.6688
WVFGRD96  118.0   170    10    45   4.14 0.6701
WVFGRD96  120.0   170    10    45   4.15 0.6749
WVFGRD96  122.0   170    10    45   4.15 0.6770
WVFGRD96  124.0   170    10    45   4.15 0.6790
WVFGRD96  126.0   165    10    40   4.15 0.6806
WVFGRD96  128.0   170    10    45   4.15 0.6823
WVFGRD96  130.0   165    10    40   4.16 0.6831
WVFGRD96  132.0   165    10    40   4.16 0.6834
WVFGRD96  134.0   165    10    40   4.16 0.6840
WVFGRD96  136.0   165    10    40   4.16 0.6847
WVFGRD96  138.0   165    10    40   4.16 0.6853
WVFGRD96  140.0   165    10    40   4.16 0.6850
WVFGRD96  142.0   165    10    40   4.17 0.6848
WVFGRD96  144.0   165    10    40   4.17 0.6837
WVFGRD96  146.0   165    10    40   4.17 0.6824
WVFGRD96  148.0   165    10    40   4.17 0.6817

The best solution is

WVFGRD96  138.0   165    10    40   4.16 0.6853

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/3.1 -30 o DIST/3.1 +60
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 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 Sun Feb 25 09:49:57 CST 2018