Location

2013/07/03 19:34:01 62.163 -149.506 49.6 3.8 Alaska

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2013/07/03 19:34:01:0 62.16 -149.51 49.6 3.8 Alaska Stations used: AK.CAST AK.DHY AK.DOT AK.GLI AK.HIN AK.KNK AK.KTH AK.MCK AK.PPLA AK.RC01 AK.RIDG AK.SCM AK.TRF AK.WAT1 AK.WAT2 AK.WAT3 AK.WAT4 AT.PMR IU.COLA Filtering commands used: cut a -10 a 110 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 7.24e+21 dyne-cm Mw = 3.84 Z = 60 km Plane Strike Dip Rake NP1 205 65 -45 NP2 318 50 -147 Principal Axes: Axis Value Plunge Azimuth T 7.24e+21 9 265 N 0.00e+00 40 2 P -7.24e+21 49 164 Moment Tensor: (dyne-cm) Component Value Mxx -2.86e+21 Mxy 1.48e+21 Mxz 3.35e+21 Myy 6.78e+21 Myz -2.07e+21 Mzz -3.92e+21 -------------- -----------------##### ------------------########## #############----############# #################-################ #################-----############## ################---------############# ################------------############ ###############--------------########### ###############----------------########### ###########-------------------######### T ###########-------------------######### ##########---------------------######## ############----------------------###### ###########-----------------------###### ##########---------- -----------#### #########---------- P -----------### #######----------- -----------## #####------------------------# #####----------------------- ##-------------------- -------------- Global CMT Convention Moment Tensor: R T P -3.92e+21 3.35e+21 2.07e+21 3.35e+21 -2.86e+21 -1.48e+21 2.07e+21 -1.48e+21 6.78e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130703193401/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 = 205
      DIP = 65
     RAKE = -45
       MW = 3.84
       HS = 60.0

The NDK file is 20130703193401.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2013/07/03 19:34:01:0 62.16 -149.51 49.6 3.8 Alaska Stations used: AK.CAST AK.DHY AK.DOT AK.GLI AK.HIN AK.KNK AK.KTH AK.MCK AK.PPLA AK.RC01 AK.RIDG AK.SCM AK.TRF AK.WAT1 AK.WAT2 AK.WAT3 AK.WAT4 AT.PMR IU.COLA Filtering commands used: cut a -10 a 110 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 7.24e+21 dyne-cm Mw = 3.84 Z = 60 km Plane Strike Dip Rake NP1 205 65 -45 NP2 318 50 -147 Principal Axes: Axis Value Plunge Azimuth T 7.24e+21 9 265 N 0.00e+00 40 2 P -7.24e+21 49 164 Moment Tensor: (dyne-cm) Component Value Mxx -2.86e+21 Mxy 1.48e+21 Mxz 3.35e+21 Myy 6.78e+21 Myz -2.07e+21 Mzz -3.92e+21 -------------- -----------------##### ------------------########## #############----############# #################-################ #################-----############## ################---------############# ################------------############ ###############--------------########### ###############----------------########### ###########-------------------######### T ###########-------------------######### ##########---------------------######## ############----------------------###### ###########-----------------------###### ##########---------- -----------#### #########---------- P -----------### #######----------- -----------## #####------------------------# #####----------------------- ##-------------------- -------------- Global CMT Convention Moment Tensor: R T P -3.92e+21 3.35e+21 2.07e+21 3.35e+21 -2.86e+21 -1.48e+21 2.07e+21 -1.48e+21 6.78e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130703193401/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:

cut a -10 a 110
rtr
taper w 0.1
hp c 0.02 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    0.5    90    45   -85   2.81 0.1772
WVFGRD96    1.0   105    45   -65   2.84 0.1481
WVFGRD96    2.0    95    45   -80   3.00 0.1988
WVFGRD96    3.0   285    70     5   3.00 0.1760
WVFGRD96    4.0   320    65    30   3.09 0.2024
WVFGRD96    5.0   320    65    30   3.12 0.2390
WVFGRD96    6.0   320    65    30   3.15 0.2685
WVFGRD96    7.0   320    65    30   3.17 0.2904
WVFGRD96    8.0   320    65    30   3.24 0.3092
WVFGRD96    9.0   320    65    30   3.26 0.3214
WVFGRD96   10.0   320    65    30   3.27 0.3274
WVFGRD96   11.0   320    60    30   3.28 0.3298
WVFGRD96   12.0   320    60    30   3.30 0.3307
WVFGRD96   13.0   320    60    30   3.31 0.3334
WVFGRD96   14.0   320    60    30   3.32 0.3339
WVFGRD96   15.0   320    60    30   3.34 0.3323
WVFGRD96   16.0   320    60    30   3.35 0.3287
WVFGRD96   17.0   320    60    30   3.36 0.3236
WVFGRD96   18.0   320    60    30   3.37 0.3158
WVFGRD96   19.0   320    60    30   3.38 0.3080
WVFGRD96   20.0   320    60    30   3.39 0.2981
WVFGRD96   21.0   325    60    35   3.40 0.2844
WVFGRD96   22.0   325    55    35   3.40 0.2723
WVFGRD96   23.0    90    40    15   3.39 0.2626
WVFGRD96   24.0    95    35    20   3.40 0.2599
WVFGRD96   25.0    50    65    20   3.43 0.2558
WVFGRD96   26.0    50    65    20   3.44 0.2524
WVFGRD96   27.0   220    75   -25   3.48 0.2534
WVFGRD96   28.0   220    75   -25   3.49 0.2576
WVFGRD96   29.0   220    70   -30   3.49 0.2594
WVFGRD96   30.0   220    65   -30   3.49 0.2696
WVFGRD96   31.0   220    65   -30   3.51 0.2902
WVFGRD96   32.0   220    70   -30   3.53 0.3086
WVFGRD96   33.0   215    65   -35   3.54 0.3260
WVFGRD96   34.0   215    65   -35   3.55 0.3432
WVFGRD96   35.0   215    70   -35   3.57 0.3574
WVFGRD96   36.0   215    70   -35   3.58 0.3679
WVFGRD96   37.0   215    70   -35   3.59 0.3779
WVFGRD96   38.0   215    70   -35   3.60 0.3855
WVFGRD96   39.0   215    65   -35   3.61 0.3914
WVFGRD96   40.0   210    65   -40   3.71 0.3977
WVFGRD96   41.0   210    65   -45   3.72 0.4106
WVFGRD96   42.0   210    65   -45   3.73 0.4217
WVFGRD96   43.0   210    65   -45   3.74 0.4302
WVFGRD96   44.0   210    65   -45   3.75 0.4368
WVFGRD96   45.0   210    65   -45   3.76 0.4431
WVFGRD96   46.0   210    65   -45   3.76 0.4488
WVFGRD96   47.0   210    65   -45   3.77 0.4536
WVFGRD96   48.0   210    65   -45   3.78 0.4573
WVFGRD96   49.0   210    65   -45   3.78 0.4614
WVFGRD96   50.0   205    65   -45   3.80 0.4640
WVFGRD96   51.0   205    65   -45   3.80 0.4667
WVFGRD96   52.0   205    65   -45   3.81 0.4696
WVFGRD96   53.0   205    65   -45   3.81 0.4722
WVFGRD96   54.0   205    65   -45   3.82 0.4745
WVFGRD96   55.0   205    65   -45   3.82 0.4764
WVFGRD96   56.0   205    65   -45   3.83 0.4771
WVFGRD96   57.0   205    65   -45   3.83 0.4787
WVFGRD96   58.0   205    65   -45   3.84 0.4789
WVFGRD96   59.0   205    65   -45   3.84 0.4785
WVFGRD96   60.0   205    65   -45   3.84 0.4792
WVFGRD96   61.0   205    65   -45   3.85 0.4778
WVFGRD96   62.0   205    65   -45   3.85 0.4772
WVFGRD96   63.0   205    70   -45   3.86 0.4761
WVFGRD96   64.0   205    70   -45   3.86 0.4761
WVFGRD96   65.0   205    70   -45   3.87 0.4765
WVFGRD96   66.0   205    70   -45   3.87 0.4760
WVFGRD96   67.0   205    70   -40   3.88 0.4738
WVFGRD96   68.0   205    70   -40   3.88 0.4738
WVFGRD96   69.0   205    70   -40   3.88 0.4732
WVFGRD96   70.0   205    70   -40   3.89 0.4713
WVFGRD96   71.0   205    70   -40   3.89 0.4692
WVFGRD96   72.0   205    70   -40   3.89 0.4680
WVFGRD96   73.0   205    70   -40   3.90 0.4660
WVFGRD96   74.0   205    70   -40   3.90 0.4635
WVFGRD96   75.0   205    70   -40   3.90 0.4609
WVFGRD96   76.0   205    70   -40   3.91 0.4583
WVFGRD96   77.0   205    70   -40   3.91 0.4556
WVFGRD96   78.0   205    70   -40   3.91 0.4524
WVFGRD96   79.0   205    70   -40   3.91 0.4491

The best solution is

WVFGRD96   60.0   205    65   -45   3.84 0.4792

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 a -10 a 110
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.10 n 3

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 00:21:41 CST 2015