Location

Location ANSS

The ANSS event ID is ak017e7b798p and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak017e7b798p/executive.

2017/11/05 17:03:02 60.225 -153.076 139.6 4.8 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2017/11/05 17:03:02:0 60.22 -153.08 139.6 4.8 Alaska Stations used: AK.BRLK AK.CAPN AK.CAST AK.CNP AK.CUT AK.FIRE AK.GHO AK.GLI AK.HOM AK.KNK AK.PPLA AK.PWL AK.RC01 AK.SAW AK.SKN AK.SSN AK.SWD AT.PMR AT.SVW2 AT.TTA AV.ILSW II.KDAK TA.K20K TA.L18K TA.L19K TA.M16K TA.M17K TA.M20K TA.M22K TA.N17K TA.N18K TA.N19K TA.O16K TA.O18K TA.O19K TA.O22K TA.P18K TA.P19K TA.Q20K Filtering commands used: cut o DIST/3.5 -40 o DIST/3.5 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 2.00e+23 dyne-cm Mw = 4.80 Z = 134 km Plane Strike Dip Rake NP1 75 75 25 NP2 338 66 164 Principal Axes: Axis Value Plunge Azimuth T 2.00e+23 28 298 N 0.00e+00 61 104 P -2.00e+23 6 205 Moment Tensor: (dyne-cm) Component Value Mxx -1.27e+23 Mxy -1.41e+23 Mxz 5.84e+22 Myy 8.45e+22 Myz -6.41e+22 Mzz 4.22e+22 -------------- ######---------------- ###########----------------- ##############---------------- #################----------------- ####################---------------- #### ###############---------------- ##### T ################---------------- ##### #################--------------# ##########################------------#### ###########################-------######## ############################--############ #########################---############## #################-----------############ ----------------------------############ ----------------------------########## ---------------------------######### --------------------------######## ------------------------###### ---- ----------------##### - P ---------------### -------------- Global CMT Convention Moment Tensor: R T P 4.22e+22 5.84e+22 6.41e+22 5.84e+22 -1.27e+23 1.41e+23 6.41e+22 1.41e+23 8.45e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171105170302/index.html

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is

      STK = 75
      DIP = 75
     RAKE = 25
       MW = 4.80
       HS = 134.0

The NDK file is 20171105170302.ndk The waveform inversion is preferred.

Magnitudes

Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and M_L by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for M_L is adjusted to remove a linear distance trend in residuals to give a regionally defined M_L. The defined M_L uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.

ML Magnitude

Left: ML computed using the IASPEI formula for Horizontal components. Center: 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. Right: Residuals from new relation as a function of distance and azimuth.

Left: ML computed using the IASPEI formula for Vertical components (research). Center: 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. Right: Residuals from new relation as a function of distance and azimuth.

Context

The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. 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). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) 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's 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/3.5 -40 o DIST/3.5 +70
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3

The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    2.0   335    90   -15   3.74 0.1679
WVFGRD96    4.0   335    70     0   3.85 0.1887
WVFGRD96    6.0   330    70   -10   3.92 0.2054
WVFGRD96    8.0   335    65     0   4.00 0.2192
WVFGRD96   10.0   335    70     5   4.04 0.2224
WVFGRD96   12.0   335    80    15   4.08 0.2223
WVFGRD96   14.0   335    80    15   4.11 0.2190
WVFGRD96   16.0   335    80    15   4.14 0.2101
WVFGRD96   18.0   335    80    15   4.16 0.1974
WVFGRD96   20.0   335    85    20   4.17 0.1822
WVFGRD96   22.0   335    85    20   4.18 0.1657
WVFGRD96   24.0   340    80    25   4.19 0.1491
WVFGRD96   26.0   340    75    25   4.19 0.1343
WVFGRD96   28.0   340    75    25   4.19 0.1208
WVFGRD96   30.0   340    75    25   4.19 0.1086
WVFGRD96   32.0   340    70    25   4.19 0.0973
WVFGRD96   34.0   340    70    25   4.19 0.0890
WVFGRD96   36.0   340    75    30   4.21 0.0857
WVFGRD96   38.0   340    75    30   4.24 0.0843
WVFGRD96   40.0   345    70    40   4.31 0.0855
WVFGRD96   42.0   345    70    40   4.34 0.0844
WVFGRD96   44.0   345    75    35   4.35 0.0825
WVFGRD96   46.0   240    55    -5   4.37 0.0824
WVFGRD96   48.0   240    55    -5   4.39 0.0847
WVFGRD96   50.0   240    55    -5   4.40 0.0873
WVFGRD96   52.0   240    55    -5   4.42 0.0902
WVFGRD96   54.0   240    55   -10   4.44 0.0932
WVFGRD96   56.0   235    55   -15   4.45 0.0965
WVFGRD96   58.0   240    60   -10   4.45 0.1005
WVFGRD96   60.0   240    60    -5   4.46 0.1056
WVFGRD96   62.0   240    60    -5   4.48 0.1124
WVFGRD96   64.0   240    60   -10   4.50 0.1212
WVFGRD96   66.0   240    60   -10   4.51 0.1313
WVFGRD96   68.0   240    65   -10   4.52 0.1414
WVFGRD96   70.0   240    65   -10   4.53 0.1504
WVFGRD96   72.0   245    70   -10   4.54 0.1590
WVFGRD96   74.0   245    70   -15   4.56 0.1759
WVFGRD96   76.0   245    75   -15   4.58 0.2021
WVFGRD96   78.0   250    80   -15   4.60 0.2342
WVFGRD96   80.0    70    90    20   4.62 0.2690
WVFGRD96   82.0    70    85    25   4.64 0.3081
WVFGRD96   84.0    75    80    30   4.67 0.3483
WVFGRD96   86.0    70    80    30   4.68 0.3882
WVFGRD96   88.0    70    80    30   4.70 0.4273
WVFGRD96   90.0    75    75    30   4.72 0.4664
WVFGRD96   92.0    75    75    35   4.74 0.5011
WVFGRD96   94.0    75    75    35   4.75 0.5252
WVFGRD96   96.0    75    75    35   4.75 0.5370
WVFGRD96   98.0    75    75    35   4.76 0.5407
WVFGRD96  100.0    75    75    35   4.76 0.5427
WVFGRD96  102.0    75    75    35   4.76 0.5449
WVFGRD96  104.0    75    75    35   4.77 0.5465
WVFGRD96  106.0    75    75    30   4.77 0.5476
WVFGRD96  108.0    75    75    30   4.77 0.5507
WVFGRD96  110.0    75    75    30   4.77 0.5528
WVFGRD96  112.0    75    75    30   4.78 0.5544
WVFGRD96  114.0    75    75    30   4.78 0.5562
WVFGRD96  116.0    75    75    30   4.78 0.5569
WVFGRD96  118.0    75    75    25   4.78 0.5581
WVFGRD96  120.0    75    75    25   4.78 0.5585
WVFGRD96  122.0    75    75    25   4.79 0.5609
WVFGRD96  124.0    75    75    25   4.79 0.5625
WVFGRD96  126.0    75    75    25   4.79 0.5648
WVFGRD96  128.0    75    75    25   4.79 0.5653
WVFGRD96  130.0    75    75    25   4.80 0.5655
WVFGRD96  132.0    75    75    25   4.80 0.5652
WVFGRD96  134.0    75    75    25   4.80 0.5667
WVFGRD96  136.0    75    75    25   4.80 0.5665
WVFGRD96  138.0    75    75    25   4.81 0.5661
WVFGRD96  140.0    75    75    25   4.81 0.5637
WVFGRD96  142.0    75    75    25   4.81 0.5643
WVFGRD96  144.0    75    75    25   4.81 0.5632
WVFGRD96  146.0    75    75    25   4.81 0.5616
WVFGRD96  148.0    75    75    25   4.82 0.5588
WVFGRD96  150.0    75    75    25   4.82 0.5581
WVFGRD96  152.0    75    75    25   4.82 0.5556
WVFGRD96  154.0    75    75    25   4.82 0.5518
WVFGRD96  156.0    75    75    25   4.82 0.5504
WVFGRD96  158.0    75    75    25   4.82 0.5477

The best solution is

WVFGRD96  134.0    75    75    25   4.80 0.5667

The mechanism corresponding 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, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. 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/3.5 -40 o DIST/3.5 +70
rtr
taper w 0.1
hp c 0.03 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. The time scale is relative to the first trace sample.

Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. 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.

Velocity Model

The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).

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

Last Changed Sat Apr 27 08:30:29 PM CDT 2024