Location

Location ANSS

The ANSS event ID is ak0194n7hhj6 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0194n7hhj6/executive.

2019/04/11 10:42:46 64.735 -149.189 19.9 4.5 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2019/04/11 10:42:46:0  64.74 -149.19  19.9 4.5 Alaska
 
 Stations used:
   AK.BPAW AK.BWN AK.CAST AK.CCB AK.CHUM AK.COLD AK.CUT AK.DOT 
   AK.GCSA AK.GHO AK.HDA AK.KLU AK.KNK AK.KTH AK.MCK AK.NEA2 
   AK.PAX AK.PPLA AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN 
   AK.SSN AK.TRF AK.WRH AT.MENT AT.PMR AT.TTA AV.STLK AV.WACK 
   AV.WAZA IU.COLA TA.E23K TA.E24K TA.F21K TA.F22K TA.F24K 
   TA.F25K TA.G21K TA.G22K TA.G23K TA.G24K TA.G26K TA.H19K 
   TA.H20K TA.H21K TA.H22K TA.H23K TA.H24K TA.H25L TA.H27K 
   TA.I21K TA.I23K TA.J18K TA.J19K TA.J20K TA.J25K TA.J26L 
   TA.K20K TA.K27K TA.L19K TA.L26K TA.M20K TA.M22K TA.M23K 
   TA.POKR XV.F4TN XV.F5MN XV.F8KN XV.FAPT XV.FNN1 XV.FNN2 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +60
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 2.79e+22 dyne-cm
  Mw = 4.23 
  Z  = 15 km
  Plane   Strike  Dip  Rake
   NP1      299    81   -155
   NP2      205    65   -10
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.79e+22     11      70
    N   0.00e+00     63     318
    P  -2.79e+22     24     165

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.84e+22
       Mxy     1.46e+22
       Mxz     1.18e+22
       Myy     2.21e+22
       Myz     2.08e+21
       Mzz    -3.71e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 -----------------#####              
              ------------------##########           
             -----------------#############          
           -----------------#################        
          -----------------###################       
         #######---------###################         
        ############----#################### T #     
        ###############-####################   #     
       ###############------#####################    
       ##############----------##################    
       ##############-------------###############    
       #############-----------------############    
        ############--------------------########     
        ###########-----------------------######     
         ##########-------------------------###      
          #########---------------------------       
           #######---------------------------        
             #####-------------   ---------          
              #####------------ P --------           
                 ##------------   -----              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.71e+21   1.18e+22  -2.08e+21 
  1.18e+22  -1.84e+22  -1.46e+22 
 -2.08e+21  -1.46e+22   2.21e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190411104246/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 = 205
      DIP = 65
     RAKE = -10
       MW = 4.23
       HS = 15.0

The NDK file is 20190411104246.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
SLU
USGSW
 USGS/SLU Moment Tensor Solution
 ENS  2019/04/11 10:42:46:0  64.74 -149.19  19.9 4.5 Alaska
 
 Stations used:
   AK.BPAW AK.BWN AK.CAST AK.CCB AK.CHUM AK.COLD AK.CUT AK.DOT 
   AK.GCSA AK.GHO AK.HDA AK.KLU AK.KNK AK.KTH AK.MCK AK.NEA2 
   AK.PAX AK.PPLA AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN 
   AK.SSN AK.TRF AK.WRH AT.MENT AT.PMR AT.TTA AV.STLK AV.WACK 
   AV.WAZA IU.COLA TA.E23K TA.E24K TA.F21K TA.F22K TA.F24K 
   TA.F25K TA.G21K TA.G22K TA.G23K TA.G24K TA.G26K TA.H19K 
   TA.H20K TA.H21K TA.H22K TA.H23K TA.H24K TA.H25L TA.H27K 
   TA.I21K TA.I23K TA.J18K TA.J19K TA.J20K TA.J25K TA.J26L 
   TA.K20K TA.K27K TA.L19K TA.L26K TA.M20K TA.M22K TA.M23K 
   TA.POKR XV.F4TN XV.F5MN XV.F8KN XV.FAPT XV.FNN1 XV.FNN2 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +60
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 2.79e+22 dyne-cm
  Mw = 4.23 
  Z  = 15 km
  Plane   Strike  Dip  Rake
   NP1      299    81   -155
   NP2      205    65   -10
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.79e+22     11      70
    N   0.00e+00     63     318
    P  -2.79e+22     24     165

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.84e+22
       Mxy     1.46e+22
       Mxz     1.18e+22
       Myy     2.21e+22
       Myz     2.08e+21
       Mzz    -3.71e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 -----------------#####              
              ------------------##########           
             -----------------#############          
           -----------------#################        
          -----------------###################       
         #######---------###################         
        ############----#################### T #     
        ###############-####################   #     
       ###############------#####################    
       ##############----------##################    
       ##############-------------###############    
       #############-----------------############    
        ############--------------------########     
        ###########-----------------------######     
         ##########-------------------------###      
          #########---------------------------       
           #######---------------------------        
             #####-------------   ---------          
              #####------------ P --------           
                 ##------------   -----              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.71e+21   1.18e+22  -2.08e+21 
  1.18e+22  -1.84e+22  -1.46e+22 
 -2.08e+21  -1.46e+22   2.21e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190411104246/index.html
	
W-phase Moment Tensor (Mww)
Moment 5.792e+15 N-m
Magnitude 4.44 Mww
Depth 15.5 km
Percent DC 27%
Half Duration 0.38 s
Catalog US
Data Source US 2
Contributor US 2
Nodal Planes
Plane Strike Dip Rake
NP1 184 54 -15
NP2 283 78 -143

Principal Axes
Axis Value Plunge Azimuth
T 4.184e+15 N-m 16 49
N 2.426e+15 N-m 51 298
P -6.610e+15 N-m 34 150

        

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 ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML 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.

mLg Magnitude


Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.

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/3.3 -40 o DIST/3.3 +60
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 n 3 
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   220    70    10   3.75 0.2412
WVFGRD96    2.0   220    65    10   3.88 0.3144
WVFGRD96    3.0   210    50    -5   3.96 0.3548
WVFGRD96    4.0   205    50   -10   4.00 0.4006
WVFGRD96    5.0   205    55   -10   4.02 0.4375
WVFGRD96    6.0   205    55   -10   4.04 0.4680
WVFGRD96    7.0   205    60   -10   4.06 0.4934
WVFGRD96    8.0   205    55   -10   4.11 0.5151
WVFGRD96    9.0   200    60   -20   4.14 0.5356
WVFGRD96   10.0   200    60   -20   4.16 0.5514
WVFGRD96   11.0   200    60   -15   4.17 0.5642
WVFGRD96   12.0   205    65   -10   4.19 0.5738
WVFGRD96   13.0   205    65   -10   4.20 0.5810
WVFGRD96   14.0   205    65   -10   4.22 0.5844
WVFGRD96   15.0   205    65   -10   4.23 0.5866
WVFGRD96   16.0   205    65   -10   4.24 0.5858
WVFGRD96   17.0   205    65   -10   4.25 0.5834
WVFGRD96   18.0   205    65    -5   4.27 0.5802
WVFGRD96   19.0   205    65    -5   4.28 0.5744
WVFGRD96   20.0   205    65    -5   4.29 0.5674
WVFGRD96   21.0   205    65    -5   4.30 0.5587
WVFGRD96   22.0   205    65    -5   4.31 0.5489
WVFGRD96   23.0   205    65    -5   4.32 0.5380
WVFGRD96   24.0   205    65    -5   4.32 0.5262
WVFGRD96   25.0   205    65    -5   4.33 0.5138
WVFGRD96   26.0   205    60    -5   4.33 0.5015
WVFGRD96   27.0   205    60    -5   4.34 0.4891
WVFGRD96   28.0   205    65    -5   4.35 0.4769
WVFGRD96   29.0   205    65    -5   4.36 0.4644

The best solution is

WVFGRD96   15.0   205    65   -10   4.23 0.5866

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/3.3 -40 o DIST/3.3 +60
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 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:

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 Thu Apr 25 11:06:38 AM CDT 2024