Location

2014/08/19 10:11:22 60.026 -153.086 134.3 4.0 Alaska

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/08/19 10:11:22:0 60.03 -153.09 134.3 4.0 Alaska Stations used: AK.BRLK AK.CNP AK.FID AK.GHO AK.GLI AK.KNK AK.SAW AK.SCM AK.SKN AK.SSN AK.SWD AT.OHAK AT.PMR AT.TTA II.KDAK Filtering commands used: cut a -30 a 110 rtr taper w 0.1 hp c 0.04 n 3 lp c 0.09 n 3 Best Fitting Double Couple Mo = 1.60e+22 dyne-cm Mw = 4.07 Z = 136 km Plane Strike Dip Rake NP1 306 60 145 NP2 55 60 35 Principal Axes: Axis Value Plunge Azimuth T 1.60e+22 45 270 N 0.00e+00 45 91 P -1.60e+22 0 0 Moment Tensor: (dyne-cm) Component Value Mxx -1.60e+22 Mxy -1.48e+20 Mxz 4.78e+14 Myy 8.07e+21 Myz -8.02e+21 Mzz 7.96e+21 ------ P ----- ---------- --------- ---------------------------- ------------------------------ #########------------------------- ###############--------------------# ####################---------------### ########################-----------##### ##########################---------##### ######## ##################-----######## ######## T ####################--######### ######## ####################--######### ##############################----######## ##########################--------###### ########################-----------##### ####################---------------### ###############--------------------# ########-------------------------- ------------------------------ ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 7.96e+21 4.78e+14 8.02e+21 4.78e+14 -1.60e+22 1.48e+20 8.02e+21 1.48e+20 8.07e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140819101122/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 = 55
      DIP = 60
     RAKE = 35
       MW = 4.07
       HS = 136.0

The NDK file is 20140819101122.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2014/08/19 10:11:22:0 60.03 -153.09 134.3 4.0 Alaska Stations used: AK.BRLK AK.CNP AK.FID AK.GHO AK.GLI AK.KNK AK.SAW AK.SCM AK.SKN AK.SSN AK.SWD AT.OHAK AT.PMR AT.TTA II.KDAK Filtering commands used: cut a -30 a 110 rtr taper w 0.1 hp c 0.04 n 3 lp c 0.09 n 3 Best Fitting Double Couple Mo = 1.60e+22 dyne-cm Mw = 4.07 Z = 136 km Plane Strike Dip Rake NP1 306 60 145 NP2 55 60 35 Principal Axes: Axis Value Plunge Azimuth T 1.60e+22 45 270 N 0.00e+00 45 91 P -1.60e+22 0 0 Moment Tensor: (dyne-cm) Component Value Mxx -1.60e+22 Mxy -1.48e+20 Mxz 4.78e+14 Myy 8.07e+21 Myz -8.02e+21 Mzz 7.96e+21 ------ P ----- ---------- --------- ---------------------------- ------------------------------ #########------------------------- ###############--------------------# ####################---------------### ########################-----------##### ##########################---------##### ######## ##################-----######## ######## T ####################--######### ######## ####################--######### ##############################----######## ##########################--------###### ########################-----------##### ####################---------------### ###############--------------------# ########-------------------------- ------------------------------ ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 7.96e+21 4.78e+14 8.02e+21 4.78e+14 -1.60e+22 1.48e+20 8.02e+21 1.48e+20 8.07e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140819101122/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 -30 a 110
rtr
taper w 0.1
hp c 0.04 n 3 
lp c 0.09 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    2.0   120    55   -40   3.16 0.1513
WVFGRD96    4.0   115    65   -40   3.23 0.1898
WVFGRD96    6.0   120    75   -25   3.26 0.2138
WVFGRD96    8.0   120    70   -25   3.34 0.2314
WVFGRD96   10.0   120    70   -20   3.38 0.2389
WVFGRD96   12.0   125    75   -20   3.42 0.2413
WVFGRD96   14.0   125    75   -20   3.44 0.2391
WVFGRD96   16.0   125    75   -15   3.47 0.2356
WVFGRD96   18.0   125    75   -15   3.49 0.2256
WVFGRD96   20.0   135    80   -15   3.52 0.2108
WVFGRD96   22.0    40    80    30   3.53 0.2122
WVFGRD96   24.0    40    80    30   3.56 0.2178
WVFGRD96   26.0   215    70    30   3.58 0.2244
WVFGRD96   28.0   215    75    30   3.61 0.2390
WVFGRD96   30.0   215    75    30   3.63 0.2496
WVFGRD96   32.0   220    70    30   3.66 0.2578
WVFGRD96   34.0   215    70    30   3.68 0.2608
WVFGRD96   36.0   220    65    35   3.71 0.2679
WVFGRD96   38.0   220    65    30   3.73 0.2746
WVFGRD96   40.0   225    65    40   3.83 0.2893
WVFGRD96   42.0   225    60    40   3.86 0.2878
WVFGRD96   44.0   225    60    40   3.88 0.2855
WVFGRD96   46.0   225    60    40   3.89 0.2822
WVFGRD96   48.0   225    60    40   3.91 0.2817
WVFGRD96   50.0   230    60    40   3.93 0.2820
WVFGRD96   52.0   230    60    40   3.94 0.2843
WVFGRD96   54.0    35    50     0   3.93 0.2952
WVFGRD96   56.0    40    55    10   3.93 0.3058
WVFGRD96   58.0    45    55     5   3.95 0.3169
WVFGRD96   60.0    50    60    10   3.97 0.3289
WVFGRD96   62.0    50    60    10   3.97 0.3429
WVFGRD96   64.0    50    60    10   3.98 0.3534
WVFGRD96   66.0    50    60    10   3.99 0.3648
WVFGRD96   68.0    55    65    15   4.01 0.3737
WVFGRD96   70.0    55    65    15   4.02 0.3847
WVFGRD96   72.0    55    65    15   4.02 0.3923
WVFGRD96   74.0    55    65    15   4.02 0.4016
WVFGRD96   76.0    55    70    20   4.03 0.4101
WVFGRD96   78.0    55    70    20   4.04 0.4172
WVFGRD96   80.0    55    70    20   4.04 0.4246
WVFGRD96   82.0    55    70    20   4.04 0.4312
WVFGRD96   84.0    55    70    20   4.04 0.4366
WVFGRD96   86.0    55    70    25   4.05 0.4408
WVFGRD96   88.0    55    70    25   4.05 0.4461
WVFGRD96   90.0    55    70    25   4.05 0.4520
WVFGRD96   92.0    55    70    25   4.05 0.4578
WVFGRD96   94.0    55    70    25   4.06 0.4628
WVFGRD96   96.0    55    70    25   4.06 0.4678
WVFGRD96   98.0    55    65    25   4.04 0.4724
WVFGRD96  100.0    55    65    25   4.05 0.4777
WVFGRD96  102.0    55    65    25   4.05 0.4818
WVFGRD96  104.0    55    65    30   4.05 0.4876
WVFGRD96  106.0    55    65    30   4.05 0.4924
WVFGRD96  108.0    55    65    30   4.06 0.4970
WVFGRD96  110.0    55    65    30   4.06 0.5014
WVFGRD96  112.0    55    65    30   4.06 0.5042
WVFGRD96  114.0    55    65    35   4.06 0.5070
WVFGRD96  116.0    55    65    35   4.06 0.5095
WVFGRD96  118.0    55    65    35   4.07 0.5113
WVFGRD96  120.0    55    65    35   4.07 0.5116
WVFGRD96  122.0    55    65    35   4.07 0.5140
WVFGRD96  124.0    55    60    35   4.06 0.5158
WVFGRD96  126.0    55    60    35   4.06 0.5166
WVFGRD96  128.0    55    60    35   4.06 0.5166
WVFGRD96  130.0    55    60    35   4.06 0.5161
WVFGRD96  132.0    55    60    35   4.06 0.5162
WVFGRD96  134.0    55    60    35   4.06 0.5169
WVFGRD96  136.0    55    60    35   4.07 0.5174
WVFGRD96  138.0    55    60    40   4.07 0.5165
WVFGRD96  140.0    55    60    40   4.07 0.5145
WVFGRD96  142.0    55    60    40   4.07 0.5146
WVFGRD96  144.0    55    60    40   4.07 0.5140
WVFGRD96  146.0    55    60    40   4.07 0.5122
WVFGRD96  148.0    55    60    40   4.07 0.5104

The best solution is

WVFGRD96  136.0    55    60    35   4.07 0.5174

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 -30 a 110
rtr
taper w 0.1
hp c 0.04 n 3 
lp c 0.09 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 Mon Dec 7 00:14:45 CST 2015