Location

Location ANSS

2016/09/07 11:47:04 59.738 -153.146 101 4.2 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2016/09/07 11:47:04:0 59.74 -153.15 101.0 4.2 Alaska Stations used: AK.BRLK AK.CAPN AK.CNP AK.CUT AK.HOM AK.PWL AK.RC01 AK.SKN AT.OHAK AT.PMR AT.SVW2 II.KDAK TA.L19K TA.M22K TA.N18K TA.N19K TA.O18K TA.O19K TA.O22K TA.P19K TA.Q19K Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 2.34e+22 dyne-cm Mw = 4.18 Z = 114 km Plane Strike Dip Rake NP1 314 64 146 NP2 60 60 30 Principal Axes: Axis Value Plunge Azimuth T 2.34e+22 41 275 N 0.00e+00 49 101 P -2.34e+22 3 8 Moment Tensor: (dyne-cm) Component Value Mxx -2.28e+22 Mxy -4.40e+21 Mxz 1.75e+15 Myy 1.27e+22 Myz -1.17e+22 Mzz 1.02e+22 -------- P --- ------------ ------- ---------------------------- ####-------------------------- ###########----------------------- ################-------------------- ###################-----------------## #######################--------------### #########################-----------#### ####### #################---------###### ####### T ###################-----######## ####### ####################--########## ##############################--########## ###########################-----######## ########################---------####### ###################--------------##### #############-------------------#### --------------------------------## ------------------------------ ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 1.02e+22 1.75e+15 1.17e+22 1.75e+15 -2.28e+22 4.40e+21 1.17e+22 4.40e+21 1.27e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160907114704/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 = 60
      DIP = 60
     RAKE = 30
       MW = 4.18
       HS = 114.0

The NDK file is 20160907114704.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2016/09/07 11:47:04:0 59.74 -153.15 101.0 4.2 Alaska Stations used: AK.BRLK AK.CAPN AK.CNP AK.CUT AK.HOM AK.PWL AK.RC01 AK.SKN AT.OHAK AT.PMR AT.SVW2 II.KDAK TA.L19K TA.M22K TA.N18K TA.N19K TA.O18K TA.O19K TA.O22K TA.P19K TA.Q19K Filtering commands used: cut o DIST/3.3 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 2.34e+22 dyne-cm Mw = 4.18 Z = 114 km Plane Strike Dip Rake NP1 314 64 146 NP2 60 60 30 Principal Axes: Axis Value Plunge Azimuth T 2.34e+22 41 275 N 0.00e+00 49 101 P -2.34e+22 3 8 Moment Tensor: (dyne-cm) Component Value Mxx -2.28e+22 Mxy -4.40e+21 Mxz 1.75e+15 Myy 1.27e+22 Myz -1.17e+22 Mzz 1.02e+22 -------- P --- ------------ ------- ---------------------------- ####-------------------------- ###########----------------------- ################-------------------- ###################-----------------## #######################--------------### #########################-----------#### ####### #################---------###### ####### T ###################-----######## ####### ####################--########## ##############################--########## ###########################-----######## ########################---------####### ###################--------------##### #############-------------------#### --------------------------------## ------------------------------ ---------------------------- ---------------------- -------------- Global CMT Convention Moment Tensor: R T P 1.02e+22 1.75e+15 1.17e+22 1.75e+15 -2.28e+22 4.40e+21 1.17e+22 4.40e+21 1.27e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160907114704/index.html

Magnitudes

ML Magnitude

(a) ML computed using the IASPEI formula; (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 +70
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    2.0   330    55    25   3.04 0.0896
WVFGRD96    4.0   330    55    25   3.16 0.1207
WVFGRD96    6.0   325    60    20   3.23 0.1486
WVFGRD96    8.0   325    55    20   3.32 0.1703
WVFGRD96   10.0   325    60    20   3.38 0.1861
WVFGRD96   12.0   325    60    15   3.42 0.1910
WVFGRD96   14.0   325    60    15   3.45 0.1863
WVFGRD96   16.0   325    60    10   3.47 0.1764
WVFGRD96   18.0   325    55     5   3.49 0.1618
WVFGRD96   20.0   240    80     0   3.54 0.1681
WVFGRD96   22.0   240    85    -5   3.57 0.1812
WVFGRD96   24.0    55    90     5   3.58 0.1905
WVFGRD96   26.0   235    85    -5   3.61 0.1968
WVFGRD96   28.0   235    90     0   3.62 0.2010
WVFGRD96   30.0    55    90     0   3.64 0.2026
WVFGRD96   32.0    50    85     0   3.64 0.2030
WVFGRD96   34.0    50    85    -5   3.66 0.2013
WVFGRD96   36.0    50    80    -5   3.67 0.1966
WVFGRD96   38.0   235    90     5   3.71 0.1936
WVFGRD96   40.0    50    80   -15   3.75 0.1971
WVFGRD96   42.0    45    70   -25   3.79 0.1983
WVFGRD96   44.0    45    65   -20   3.81 0.1991
WVFGRD96   46.0    45    65   -20   3.84 0.2014
WVFGRD96   48.0    50    65   -15   3.86 0.2047
WVFGRD96   50.0    50    65   -15   3.88 0.2105
WVFGRD96   52.0    60    80    15   3.90 0.2245
WVFGRD96   54.0    55    70    25   3.92 0.2453
WVFGRD96   56.0    60    65    30   3.95 0.2722
WVFGRD96   58.0    60    65    30   3.98 0.2949
WVFGRD96   60.0    60    65    30   3.99 0.3115
WVFGRD96   62.0    60    65    30   4.01 0.3264
WVFGRD96   64.0    60    65    30   4.02 0.3406
WVFGRD96   66.0    60    65    30   4.03 0.3550
WVFGRD96   68.0    60    65    30   4.04 0.3695
WVFGRD96   70.0    60    60    30   4.06 0.3829
WVFGRD96   72.0    60    60    30   4.08 0.3971
WVFGRD96   74.0    60    60    30   4.08 0.4098
WVFGRD96   76.0    60    60    30   4.09 0.4210
WVFGRD96   78.0    60    60    30   4.10 0.4326
WVFGRD96   80.0    60    60    35   4.11 0.4422
WVFGRD96   82.0    60    60    35   4.11 0.4552
WVFGRD96   84.0    60    60    35   4.12 0.4641
WVFGRD96   86.0    60    60    35   4.12 0.4743
WVFGRD96   88.0    60    60    35   4.13 0.4826
WVFGRD96   90.0    60    60    35   4.13 0.4893
WVFGRD96   92.0    60    60    35   4.14 0.4974
WVFGRD96   94.0    60    60    35   4.14 0.5033
WVFGRD96   96.0    60    60    35   4.15 0.5081
WVFGRD96   98.0    60    60    35   4.15 0.5140
WVFGRD96  100.0    60    60    30   4.16 0.5180
WVFGRD96  102.0    60    60    30   4.16 0.5218
WVFGRD96  104.0    60    60    30   4.16 0.5239
WVFGRD96  106.0    60    60    30   4.17 0.5276
WVFGRD96  108.0    60    60    30   4.17 0.5306
WVFGRD96  110.0    60    60    30   4.17 0.5328
WVFGRD96  112.0    60    60    30   4.18 0.5342
WVFGRD96  114.0    60    60    30   4.18 0.5348
WVFGRD96  116.0    60    60    30   4.18 0.5344
WVFGRD96  118.0    60    60    30   4.18 0.5340

The best solution is

WVFGRD96  114.0    60    60    30   4.18 0.5348

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 +70
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.10 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 Wed Sep 7 07:46:25 CDT 2016