Location

2012/10/31 02:57:43 62.045 -146.545 40.7 4.00 Alaska

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2012/10/31 02:57:43:0 62.04 -146.54 40.7 4.0 Alaska Stations used: AK.BMR AK.BPAW AK.CCB AK.DOT AK.EYAK AK.FID AK.GLI AK.HDA AK.KLU AK.KNK AK.RC01 AK.RIDG AK.SAW AK.SCM AK.SWD AK.WRH IU.COLA US.EGAK Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.66e+22 dyne-cm Mw = 4.08 Z = 58 km Plane Strike Dip Rake NP1 148 71 159 NP2 245 70 20 Principal Axes: Axis Value Plunge Azimuth T 1.66e+22 28 106 N 0.00e+00 62 288 P -1.66e+22 1 197 Moment Tensor: (dyne-cm) Component Value Mxx -1.42e+22 Mxy -8.02e+21 Mxz -1.69e+21 Myy 1.06e+22 Myz 6.67e+21 Mzz 3.65e+21 -------------- ---------------------- ###------------------------- ####-------------------------- #######--------------------------- ########---------------------------- ##########------------------########## ###########-----------################## ############------###################### ##############-########################### ############---########################### ##########------########################## #######----------################# ##### ####-------------################ T #### ###----------------############## #### -------------------################### --------------------################ --------------------############## --------------------########## ----------------------###### --- ---------------- P ------------ Global CMT Convention Moment Tensor: R T P 3.65e+21 -1.69e+21 -6.67e+21 -1.69e+21 -1.42e+22 8.02e+21 -6.67e+21 8.02e+21 1.06e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20121031025743/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 = 245
      DIP = 70
     RAKE = 20
       MW = 4.08
       HS = 58.0

The NDK file is 20121031025743.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2012/10/31 02:57:43:0 62.04 -146.54 40.7 4.0 Alaska Stations used: AK.BMR AK.BPAW AK.CCB AK.DOT AK.EYAK AK.FID AK.GLI AK.HDA AK.KLU AK.KNK AK.RC01 AK.RIDG AK.SAW AK.SCM AK.SWD AK.WRH IU.COLA US.EGAK Filtering commands used: hp c 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 1.66e+22 dyne-cm Mw = 4.08 Z = 58 km Plane Strike Dip Rake NP1 148 71 159 NP2 245 70 20 Principal Axes: Axis Value Plunge Azimuth T 1.66e+22 28 106 N 0.00e+00 62 288 P -1.66e+22 1 197 Moment Tensor: (dyne-cm) Component Value Mxx -1.42e+22 Mxy -8.02e+21 Mxz -1.69e+21 Myy 1.06e+22 Myz 6.67e+21 Mzz 3.65e+21 -------------- ---------------------- ###------------------------- ####-------------------------- #######--------------------------- ########---------------------------- ##########------------------########## ###########-----------################## ############------###################### ##############-########################### ############---########################### ##########------########################## #######----------################# ##### ####-------------################ T #### ###----------------############## #### -------------------################### --------------------################ --------------------############## --------------------########## ----------------------###### --- ---------------- P ------------ Global CMT Convention Moment Tensor: R T P 3.65e+21 -1.69e+21 -6.67e+21 -1.69e+21 -1.42e+22 8.02e+21 -6.67e+21 8.02e+21 1.06e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20121031025743/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

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:

hp c 0.02 n 3
lp c 0.10 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    0.5    80    60    50   3.14 0.1757
WVFGRD96    1.0    55    80   -15   3.13 0.1966
WVFGRD96    2.0    55    70   -25   3.35 0.3499
WVFGRD96    3.0   240    75     5   3.39 0.4006
WVFGRD96    4.0   240    70    15   3.45 0.4464
WVFGRD96    5.0   240    70    20   3.49 0.4805
WVFGRD96    6.0   240    70    20   3.51 0.4990
WVFGRD96    7.0   235    75    20   3.54 0.5077
WVFGRD96    8.0   240    70    20   3.56 0.5127
WVFGRD96    9.0   240    70    20   3.58 0.5112
WVFGRD96   10.0   235    75    20   3.60 0.5102
WVFGRD96   11.0   235    75    20   3.61 0.5127
WVFGRD96   12.0   240    75    20   3.61 0.5178
WVFGRD96   13.0   240    75    20   3.62 0.5223
WVFGRD96   14.0   240    75    15   3.63 0.5259
WVFGRD96   15.0   240    75    15   3.64 0.5281
WVFGRD96   16.0   235    80    15   3.66 0.5302
WVFGRD96   17.0   240    75    15   3.67 0.5318
WVFGRD96   18.0   240    75    10   3.67 0.5330
WVFGRD96   19.0   240    75    10   3.68 0.5351
WVFGRD96   20.0   240    75    10   3.69 0.5360
WVFGRD96   21.0   235    75    10   3.71 0.5381
WVFGRD96   22.0   235    75    10   3.72 0.5388
WVFGRD96   23.0   235    75    10   3.73 0.5414
WVFGRD96   24.0   235    75    10   3.74 0.5436
WVFGRD96   25.0   235    75     5   3.74 0.5473
WVFGRD96   26.0   235    75     5   3.75 0.5505
WVFGRD96   27.0   235    75     5   3.76 0.5542
WVFGRD96   28.0   235    75     5   3.77 0.5577
WVFGRD96   29.0   240    70     0   3.78 0.5604
WVFGRD96   30.0   240    70     0   3.79 0.5668
WVFGRD96   31.0   240    70     0   3.80 0.5726
WVFGRD96   32.0   240    70     0   3.81 0.5787
WVFGRD96   33.0   240    70     0   3.82 0.5824
WVFGRD96   34.0   240    70     0   3.83 0.5842
WVFGRD96   35.0   240    70     0   3.84 0.5855
WVFGRD96   36.0   240    75     5   3.85 0.5870
WVFGRD96   37.0   240    75     5   3.87 0.5883
WVFGRD96   38.0   240    75     5   3.88 0.5887
WVFGRD96   39.0   240    75    10   3.90 0.5902
WVFGRD96   40.0   245    65    15   3.94 0.5946
WVFGRD96   41.0   245    65    15   3.95 0.5990
WVFGRD96   42.0   245    65    15   3.96 0.6030
WVFGRD96   43.0   245    65    15   3.97 0.6059
WVFGRD96   44.0   245    65    15   3.98 0.6073
WVFGRD96   45.0   245    65    15   3.99 0.6073
WVFGRD96   46.0   245    65    15   4.00 0.6087
WVFGRD96   47.0   245    65    15   4.01 0.6106
WVFGRD96   48.0   245    65    15   4.02 0.6117
WVFGRD96   49.0   245    70    20   4.02 0.6123
WVFGRD96   50.0   245    70    20   4.03 0.6146
WVFGRD96   51.0   245    70    20   4.04 0.6152
WVFGRD96   52.0   245    70    20   4.04 0.6155
WVFGRD96   53.0   245    70    20   4.05 0.6178
WVFGRD96   54.0   245    70    20   4.06 0.6187
WVFGRD96   55.0   245    70    20   4.06 0.6192
WVFGRD96   56.0   245    70    20   4.07 0.6210
WVFGRD96   57.0   245    70    20   4.07 0.6204
WVFGRD96   58.0   245    70    20   4.08 0.6217
WVFGRD96   59.0   245    70    20   4.08 0.6214
WVFGRD96   60.0   245    70    20   4.09 0.6212
WVFGRD96   61.0   245    70    20   4.09 0.6207
WVFGRD96   62.0   245    70    20   4.10 0.6184
WVFGRD96   63.0   245    70    20   4.10 0.6174
WVFGRD96   64.0   245    70    20   4.10 0.6141
WVFGRD96   65.0   245    70    20   4.11 0.6142
WVFGRD96   66.0   245    70    20   4.11 0.6114
WVFGRD96   67.0   245    70    25   4.12 0.6099
WVFGRD96   68.0   245    70    25   4.12 0.6058
WVFGRD96   69.0   245    70    25   4.12 0.6043

The best solution is

WVFGRD96   58.0   245    70    20   4.08 0.6217

The mechanism corresponding 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

hp c 0.02 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

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 Mon Dec 7 00:25:58 CST 2015