Location

SLU Location

Phase times were manually read and the program elocate was run with the WUS model defoned below. The output of the program is elocate.txt. The first motions are compared to the RMT nodal planes in the focal mechanism comparison below.

ANSS Location

This was use for the epicenter and origin time. The RMT depth is greater.
2016/02/08 01:42:50 61.551 -146.396 10.9 3.9 Alaska

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports main page

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2016/02/08 01:42:50:0 61.55 -146.40 10.9 3.9 Alaska Stations used: AK.CUT AK.DIV AK.EYAK AK.FID AK.GLB AK.GLI AK.KLU AK.KNK AK.PWL AK.SCM AK.VRDI TA.M24K 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 0.5 n 4 p 2 Best Fitting Double Couple Mo = 6.76e+21 dyne-cm Mw = 3.82 Z = 35 km Plane Strike Dip Rake NP1 235 65 -40 NP2 345 54 -149 Principal Axes: Axis Value Plunge Azimuth T 6.76e+21 6 292 N 0.00e+00 44 28 P -6.76e+21 45 195 Moment Tensor: (dyne-cm) Component Value Mxx -2.18e+21 Mxy -3.17e+21 Mxz 3.54e+21 Myy 5.51e+21 Myz 1.91e+20 Mzz -3.33e+21 ###----------- ##########------------ ###############------------- ##################------------ #####################----------##- #######################-############ ##################-----############# T ###############---------############# #############------------############ ##############----------------############ ############------------------############ ##########--------------------############ #########----------------------########### ######------------------------########## #####-------------------------########## ###------------ -----------######### #------------- P -----------######## ------------- ----------######## ------------------------###### ----------------------###### ------------------#### -------------# Global CMT Convention Moment Tensor: R T P -3.33e+21 3.54e+21 -1.91e+20 3.54e+21 -2.18e+21 3.17e+21 -1.91e+20 3.17e+21 5.51e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160208014250/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 = 235
      DIP = 65
     RAKE = -40
       MW = 3.82
       HS = 35.0

The NDK file is 20160208014250.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU SLUFM

USGS/SLU Moment Tensor Solution ENS 2016/02/08 01:42:50:0 61.55 -146.40 10.9 3.9 Alaska Stations used: AK.CUT AK.DIV AK.EYAK AK.FID AK.GLB AK.GLI AK.KLU AK.KNK AK.PWL AK.SCM AK.VRDI TA.M24K 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 0.5 n 4 p 2 Best Fitting Double Couple Mo = 6.76e+21 dyne-cm Mw = 3.82 Z = 35 km Plane Strike Dip Rake NP1 235 65 -40 NP2 345 54 -149 Principal Axes: Axis Value Plunge Azimuth T 6.76e+21 6 292 N 0.00e+00 44 28 P -6.76e+21 45 195 Moment Tensor: (dyne-cm) Component Value Mxx -2.18e+21 Mxy -3.17e+21 Mxz 3.54e+21 Myy 5.51e+21 Myz 1.91e+20 Mzz -3.33e+21 ###----------- ##########------------ ###############------------- ##################------------ #####################----------##- #######################-############ ##################-----############# T ###############---------############# #############------------############ ##############----------------############ ############------------------############ ##########--------------------############ #########----------------------########### ######------------------------########## #####-------------------------########## ###------------ -----------######### #------------- P -----------######## ------------- ----------######## ------------------------###### ----------------------###### ------------------#### -------------# Global CMT Convention Moment Tensor: R T P -3.33e+21 3.54e+21 -1.91e+20 3.54e+21 -2.18e+21 3.17e+21 -1.91e+20 3.17e+21 5.51e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160208014250/index.html

First motions and takeoff angles from an elocate run.

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 0.5 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   245    85     0   3.33 0.4556
WVFGRD96    2.0   245    55     0   3.45 0.4745
WVFGRD96    3.0   245    45     0   3.52 0.4690
WVFGRD96    4.0   155    85   -70   3.64 0.4680
WVFGRD96    5.0   150    80   -70   3.61 0.4714
WVFGRD96    6.0   155    85   -60   3.57 0.4714
WVFGRD96    7.0   155    85   -55   3.56 0.4721
WVFGRD96    8.0   150    80   -60   3.62 0.4754
WVFGRD96    9.0   150    80   -60   3.61 0.4766
WVFGRD96   10.0   150    80   -55   3.61 0.4773
WVFGRD96   11.0   105    55    55   3.58 0.4804
WVFGRD96   12.0   105    55    55   3.59 0.4913
WVFGRD96   13.0   105    55    55   3.59 0.4995
WVFGRD96   14.0   105    55    55   3.60 0.5060
WVFGRD96   15.0   105    55    55   3.60 0.5113
WVFGRD96   16.0   105    60    55   3.61 0.5162
WVFGRD96   17.0   105    60    55   3.62 0.5211
WVFGRD96   18.0   240    65   -35   3.63 0.5253
WVFGRD96   19.0   240    70   -35   3.64 0.5349
WVFGRD96   20.0   240    70   -35   3.65 0.5457
WVFGRD96   21.0   240    70   -40   3.66 0.5544
WVFGRD96   22.0   240    70   -40   3.68 0.5650
WVFGRD96   23.0   240    75   -40   3.68 0.5754
WVFGRD96   24.0   240    75   -40   3.70 0.5869
WVFGRD96   25.0   240    75   -40   3.71 0.5970
WVFGRD96   26.0   240    75   -40   3.72 0.6061
WVFGRD96   27.0   240    75   -40   3.74 0.6143
WVFGRD96   28.0   240    75   -40   3.75 0.6208
WVFGRD96   29.0   240    75   -40   3.76 0.6257
WVFGRD96   30.0   235    70   -40   3.78 0.6305
WVFGRD96   31.0   235    65   -40   3.79 0.6383
WVFGRD96   32.0   235    65   -40   3.80 0.6455
WVFGRD96   33.0   235    65   -40   3.81 0.6511
WVFGRD96   34.0   235    65   -40   3.82 0.6557
WVFGRD96   35.0   235    65   -40   3.82 0.6576
WVFGRD96   36.0   235    65   -40   3.83 0.6574
WVFGRD96   37.0   235    65   -40   3.84 0.6561
WVFGRD96   38.0   235    65   -40   3.85 0.6527
WVFGRD96   39.0   235    65   -35   3.86 0.6475
WVFGRD96   40.0   230    65   -50   3.94 0.6440
WVFGRD96   41.0   230    65   -45   3.94 0.6420
WVFGRD96   42.0   230    65   -50   3.95 0.6427
WVFGRD96   43.0   230    65   -50   3.96 0.6423
WVFGRD96   44.0   230    65   -50   3.96 0.6417
WVFGRD96   45.0   230    65   -50   3.97 0.6404
WVFGRD96   46.0   230    65   -50   3.97 0.6374
WVFGRD96   47.0   230    65   -45   3.98 0.6353
WVFGRD96   48.0   230    65   -45   3.98 0.6325
WVFGRD96   49.0   230    65   -45   3.99 0.6292

The best solution is

WVFGRD96   35.0   235    65   -40   3.82 0.6576

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 0.5 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 Sun Feb 7 22:09:58 CST 2016