Location

Location ANSS

2019/01/01 03:03:30 61.296 -149.947 43.4 5.7 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2019/01/01 03:03:30:0 61.30 -149.95 43.4 5.7 Alaska Stations used: AK.CUT AK.DHY AK.GHO AK.GLI AK.KLU AK.KNK AK.KTH AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN AK.SLK AK.TRF AT.PMR AV.ILSW AV.SPU GM.AD13 TA.L19K TA.M20K TA.M22K TA.O22K 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 Best Fitting Double Couple Mo = 2.63e+23 dyne-cm Mw = 4.88 Z = 48 km Plane Strike Dip Rake NP1 175 53 -106 NP2 20 40 -70 Principal Axes: Axis Value Plunge Azimuth T 2.63e+23 7 276 N 0.00e+00 13 184 P -2.63e+23 76 33 Moment Tensor: (dyne-cm) Component Value Mxx -8.70e+21 Mxy -3.39e+22 Mxz -5.01e+22 Myy 2.52e+23 Myz -6.39e+22 Mzz -2.43e+23 ####---------- ######--------------## ########----------------#### ########------------------#### #########--------------------##### ##########---------------------##### ##########----------------------###### ###########----------------------####### #########--------- ----------####### T #########--------- P ----------######## #########--------- ----------######## ############----------------------######## ############---------------------######### ###########---------------------######## ############-------------------######### ###########------------------######### ###########----------------######### ###########-------------########## ##########----------########## ##########-------########### #########--########### #------####### Global CMT Convention Moment Tensor: R T P -2.43e+23 -5.01e+22 6.39e+22 -5.01e+22 -8.70e+21 3.39e+22 6.39e+22 3.39e+22 2.52e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190101030330/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 = 20
      DIP = 40
     RAKE = -70
       MW = 4.88
       HS = 48.0

The NDK file is 20190101030330.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU USGSW

USGS/SLU Moment Tensor Solution ENS 2019/01/01 03:03:30:0 61.30 -149.95 43.4 5.7 Alaska Stations used: AK.CUT AK.DHY AK.GHO AK.GLI AK.KLU AK.KNK AK.KTH AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN AK.SLK AK.TRF AT.PMR AV.ILSW AV.SPU GM.AD13 TA.L19K TA.M20K TA.M22K TA.O22K 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 Best Fitting Double Couple Mo = 2.63e+23 dyne-cm Mw = 4.88 Z = 48 km Plane Strike Dip Rake NP1 175 53 -106 NP2 20 40 -70 Principal Axes: Axis Value Plunge Azimuth T 2.63e+23 7 276 N 0.00e+00 13 184 P -2.63e+23 76 33 Moment Tensor: (dyne-cm) Component Value Mxx -8.70e+21 Mxy -3.39e+22 Mxz -5.01e+22 Myy 2.52e+23 Myz -6.39e+22 Mzz -2.43e+23 ####---------- ######--------------## ########----------------#### ########------------------#### #########--------------------##### ##########---------------------##### ##########----------------------###### ###########----------------------####### #########--------- ----------####### T #########--------- P ----------######## #########--------- ----------######## ############----------------------######## ############---------------------######### ###########---------------------######## ############-------------------######### ###########------------------######### ###########----------------######### ###########-------------########## ##########----------########## ##########-------########### #########--########### #------####### Global CMT Convention Moment Tensor: R T P -2.43e+23 -5.01e+22 6.39e+22 -5.01e+22 -8.70e+21 3.39e+22 6.39e+22 3.39e+22 2.52e+23 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190101030330/index.html

W-phase Moment Tensor (Mww) Moment 2.902e+16 N-m Magnitude 4.91 Mww Depth 45.5 km Percent DC 85% Half Duration 0.73 s Catalog US Data Source US 3 Contributor US 3 Nodal Planes Plane Strike Dip Rake NP1 183 55 -99 NP2 19 36 -78 Principal Axes Axis Value Plunge Azimuth T 2.781e+16 N-m 9 280 N 0.230e+16 N-m 7 188 P -3.010e+16 N-m 78 61

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/3.3 -40 o DIST/3.3 +60
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3

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   180    50    85   3.98 0.1475
WVFGRD96    2.0   180    45    80   4.14 0.2038
WVFGRD96    3.0   160    50    55   4.17 0.2040
WVFGRD96    4.0   310    60   -35   4.16 0.2177
WVFGRD96    5.0   305    55   -30   4.20 0.2350
WVFGRD96    6.0   310    60   -25   4.22 0.2501
WVFGRD96    7.0   235    70    35   4.26 0.2705
WVFGRD96    8.0   235    70    40   4.33 0.2863
WVFGRD96    9.0   240    65    35   4.35 0.3013
WVFGRD96   10.0   240    65    35   4.37 0.3132
WVFGRD96   11.0   240    65    35   4.39 0.3203
WVFGRD96   12.0   240    65    35   4.41 0.3239
WVFGRD96   13.0   235    70    30   4.43 0.3265
WVFGRD96   14.0   235    70    30   4.44 0.3271
WVFGRD96   15.0    50    60   -15   4.44 0.3265
WVFGRD96   16.0    50    60   -15   4.45 0.3280
WVFGRD96   17.0    55    65    20   4.47 0.3294
WVFGRD96   18.0    55    65    20   4.48 0.3300
WVFGRD96   19.0    55    65    20   4.49 0.3296
WVFGRD96   20.0    55    65    20   4.50 0.3296
WVFGRD96   21.0   225    70   -35   4.53 0.3284
WVFGRD96   22.0   225    70   -35   4.54 0.3321
WVFGRD96   23.0   225    70   -35   4.55 0.3345
WVFGRD96   24.0    50    60   -15   4.54 0.3356
WVFGRD96   25.0    50    65   -10   4.54 0.3392
WVFGRD96   26.0    50    65   -10   4.55 0.3423
WVFGRD96   27.0    50    65   -10   4.56 0.3458
WVFGRD96   28.0    45    65   -25   4.58 0.3532
WVFGRD96   29.0    45    60   -30   4.59 0.3593
WVFGRD96   30.0   220    65   -40   4.62 0.3656
WVFGRD96   31.0   220    65   -40   4.62 0.3751
WVFGRD96   32.0   215    60   -50   4.64 0.3825
WVFGRD96   33.0   215    60   -50   4.65 0.3913
WVFGRD96   34.0   205    55   -65   4.66 0.3997
WVFGRD96   35.0   205    55   -65   4.67 0.4065
WVFGRD96   36.0    -5    40  -110   4.68 0.4099
WVFGRD96   37.0    -5    40  -110   4.69 0.4115
WVFGRD96   38.0     0    40  -100   4.70 0.4133
WVFGRD96   39.0    15    40   -80   4.72 0.4175
WVFGRD96   40.0    20    40   -70   4.80 0.4351
WVFGRD96   41.0    20    40   -70   4.81 0.4406
WVFGRD96   42.0    15    40   -75   4.83 0.4453
WVFGRD96   43.0    15    40   -75   4.84 0.4495
WVFGRD96   44.0    15    40   -75   4.85 0.4525
WVFGRD96   45.0    20    40   -70   4.86 0.4546
WVFGRD96   46.0    20    40   -70   4.87 0.4567
WVFGRD96   47.0    20    40   -70   4.88 0.4570
WVFGRD96   48.0    20    40   -70   4.88 0.4585
WVFGRD96   49.0    20    40   -70   4.89 0.4583
WVFGRD96   50.0    20    40   -70   4.90 0.4574
WVFGRD96   51.0    20    40   -70   4.90 0.4565
WVFGRD96   52.0    25    45   -65   4.91 0.4554
WVFGRD96   53.0    25    45   -65   4.91 0.4552
WVFGRD96   54.0    25    45   -65   4.91 0.4544
WVFGRD96   55.0    25    45   -60   4.91 0.4538
WVFGRD96   56.0    25    45   -60   4.92 0.4540
WVFGRD96   57.0    25    45   -60   4.92 0.4528
WVFGRD96   58.0    25    45   -60   4.92 0.4522
WVFGRD96   59.0    30    45   -55   4.92 0.4502

The best solution is

WVFGRD96   48.0    20    40   -70   4.88 0.4585

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

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 Jan 1 07:11:20 CST 2019