Location

2016/01/15 23:12:31 61.105 -146.873 20.1 3.7 Alaska

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2016/01/15 23:12:31:0 61.10 -146.87 20.1 3.7 Alaska Stations used: AK.EYAK AK.FID AK.GHO AK.GLI AK.KLU AK.KNK AK.PWL AK.RC01 AK.SAW AK.SCM AT.PMR TA.N25K Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 4.79e+21 dyne-cm Mw = 3.72 Z = 31 km Plane Strike Dip Rake NP1 20 53 -106 NP2 225 40 -70 Principal Axes: Axis Value Plunge Azimuth T 4.79e+21 7 121 N 0.00e+00 13 29 P -4.79e+21 76 238 Moment Tensor: (dyne-cm) Component Value Mxx 1.16e+21 Mxy -2.21e+21 Mxz 3.34e+20 Myy 3.27e+21 Myz 1.44e+21 Mzz -4.43e+21 ############## ###################--- ##################-----##--- #############------------##### ############---------------####### ###########-----------------######## ##########--------------------######## #########----------------------######### ########-----------------------######### ########-----------------------########### #######------------------------########### ######--------- -------------########### ######--------- P ------------############ ####---------- -----------############ ####-----------------------############# ###----------------------######### # ##---------------------########## T #--------------------########### -----------------############# --------------############## ---------############# -############# Global CMT Convention Moment Tensor: R T P -4.43e+21 3.34e+20 -1.44e+21 3.34e+20 1.16e+21 2.21e+21 -1.44e+21 2.21e+21 3.27e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160115231231/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 = 225
      DIP = 40
     RAKE = -70
       MW = 3.72
       HS = 31.0

The NDK file is 20160115231231.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2016/01/15 23:12:31:0 61.10 -146.87 20.1 3.7 Alaska Stations used: AK.EYAK AK.FID AK.GHO AK.GLI AK.KLU AK.KNK AK.PWL AK.RC01 AK.SAW AK.SCM AT.PMR TA.N25K Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2 Best Fitting Double Couple Mo = 4.79e+21 dyne-cm Mw = 3.72 Z = 31 km Plane Strike Dip Rake NP1 20 53 -106 NP2 225 40 -70 Principal Axes: Axis Value Plunge Azimuth T 4.79e+21 7 121 N 0.00e+00 13 29 P -4.79e+21 76 238 Moment Tensor: (dyne-cm) Component Value Mxx 1.16e+21 Mxy -2.21e+21 Mxz 3.34e+20 Myy 3.27e+21 Myz 1.44e+21 Mzz -4.43e+21 ############## ###################--- ##################-----##--- #############------------##### ############---------------####### ###########-----------------######## ##########--------------------######## #########----------------------######### ########-----------------------######### ########-----------------------########### #######------------------------########### ######--------- -------------########### ######--------- P ------------############ ####---------- -----------############ ####-----------------------############# ###----------------------######### # ##---------------------########## T #--------------------########### -----------------############# --------------############## ---------############# -############# Global CMT Convention Moment Tensor: R T P -4.43e+21 3.34e+20 -1.44e+21 3.34e+20 1.16e+21 2.21e+21 -1.44e+21 2.21e+21 3.27e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160115231231/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 o DIST/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 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    1.0   195    85     5   3.21 0.2447
WVFGRD96    2.0    30    45    90   3.33 0.3861
WVFGRD96    3.0   100    80     0   3.41 0.3766
WVFGRD96    4.0   100    80     0   3.45 0.3709
WVFGRD96    5.0   290    50    20   3.50 0.4057
WVFGRD96    6.0   290    50    20   3.52 0.4493
WVFGRD96    7.0   290    50    25   3.52 0.4760
WVFGRD96    8.0   290    50    20   3.59 0.4936
WVFGRD96    9.0   290    50    20   3.59 0.5013
WVFGRD96   10.0   290    50    25   3.58 0.5066
WVFGRD96   11.0   290    55    20   3.60 0.5103
WVFGRD96   12.0   285    55    15   3.59 0.5133
WVFGRD96   13.0   285    55    15   3.60 0.5179
WVFGRD96   14.0   285    55    15   3.60 0.5208
WVFGRD96   15.0   285    55    15   3.61 0.5229
WVFGRD96   16.0   285    60    10   3.64 0.5238
WVFGRD96   17.0   285    60    10   3.65 0.5251
WVFGRD96   18.0    40    55   -75   3.56 0.5274
WVFGRD96   19.0    40    55   -75   3.58 0.5391
WVFGRD96   20.0    35    55   -80   3.59 0.5500
WVFGRD96   21.0    35    55   -85   3.61 0.5604
WVFGRD96   22.0    35    55   -85   3.62 0.5716
WVFGRD96   23.0   210    35   -90   3.64 0.5806
WVFGRD96   24.0    30    55   -90   3.65 0.5881
WVFGRD96   25.0    30    50   -95   3.66 0.5943
WVFGRD96   26.0   215    40   -85   3.67 0.6003
WVFGRD96   27.0   220    40   -80   3.68 0.6046
WVFGRD96   28.0   225    40   -70   3.70 0.6084
WVFGRD96   29.0   225    40   -70   3.71 0.6122
WVFGRD96   30.0   225    40   -70   3.71 0.6157
WVFGRD96   31.0   225    40   -70   3.72 0.6160
WVFGRD96   32.0   225    40   -70   3.73 0.6126
WVFGRD96   33.0   225    40   -70   3.73 0.6062
WVFGRD96   34.0   225    40   -70   3.74 0.5963
WVFGRD96   35.0   220    40   -75   3.74 0.5868
WVFGRD96   36.0   220    40   -75   3.75 0.5754
WVFGRD96   37.0   215    40   -75   3.75 0.5620
WVFGRD96   38.0   215    40   -75   3.76 0.5472
WVFGRD96   39.0   215    40   -75   3.77 0.5340
WVFGRD96   40.0   220    40   -75   3.85 0.5257
WVFGRD96   41.0   220    40   -75   3.86 0.5233
WVFGRD96   42.0   215    40   -80   3.86 0.5155
WVFGRD96   43.0   220    40   -70   3.87 0.5066
WVFGRD96   44.0   220    40   -70   3.88 0.4987
WVFGRD96   45.0   220    40   -70   3.88 0.4901
WVFGRD96   46.0   220    40   -70   3.88 0.4816
WVFGRD96   47.0   210    35   -80   3.88 0.4730
WVFGRD96   48.0   210    35   -80   3.89 0.4681
WVFGRD96   49.0    25    55   -90   3.89 0.4634
WVFGRD96   50.0    25    55   -85   3.89 0.4588
WVFGRD96   51.0    25    55   -85   3.89 0.4537
WVFGRD96   52.0    25    55   -85   3.90 0.4488
WVFGRD96   53.0    30    60   -75   3.90 0.4446
WVFGRD96   54.0    30    60   -75   3.91 0.4402
WVFGRD96   55.0    30    60   -75   3.91 0.4361
WVFGRD96   56.0    35    60   -70   3.91 0.4311
WVFGRD96   57.0   230    20   -65   3.92 0.4320
WVFGRD96   58.0   230    20   -65   3.93 0.4323
WVFGRD96   59.0   230    20   -65   3.93 0.4319

The best solution is

WVFGRD96   31.0   225    40   -70   3.72 0.6160

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 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 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. 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 Fri Jan 15 18:25:35 CST 2016