Location

Location ANSS

The ANSS event ID is ak024d1os5jt and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak024d1os5jt/executive.

2024/10/10 00:26:09 63.439 -150.027 3.9 4.3 NWT, Canada

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2024/10/10 00:26:09:0  63.44 -150.03   3.9 4.3 NWT, Canada
 
 Stations used:
   AK.BAE AK.BMR AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV 
   AK.DOT AK.EYAK AK.FID AK.GHO AK.GLB AK.GLI AK.H24K AK.HARP 
   AK.HDA AK.HIN AK.J19K AK.J20K AK.J25K AK.KLU AK.KNK AK.L19K 
   AK.L22K AK.L26K AK.MCK AK.MLY AK.N18K AK.NEA2 AK.O19K 
   AK.P23K AK.PAX AK.POKR AK.PPD AK.PWL AK.RAG AK.RC01 AK.RIDG 
   AK.RND AK.SAW AK.SCM AK.SCRK AK.SWD AK.VRDI AK.WAT6 AK.WRH 
   AT.PMR AT.TTA AV.RED IM.IL31 IU.COLA 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.11e+22 dyne-cm
  Mw = 4.15 
  Z  = 9 km
  Plane   Strike  Dip  Rake
   NP1      115    70   142
   NP2      220    55    25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.11e+22     41      72
    N   0.00e+00     48     271
    P  -2.11e+22      9     170

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.89e+22
       Mxy     6.86e+21
       Mxz     6.45e+21
       Myy     1.05e+22
       Myz     9.40e+21
       Mzz     8.39e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              -----------------------#####           
             -------------------###########          
           -----------------#################        
          ----------------####################       
         ---------------#######################      
        ###-----------################   #######     
        ####--------################## T #######     
       #######-----###################   ########    
       #########--###############################    
       ##########--##############################    
       #########------###########################    
        #######-----------######################     
        #######---------------##################     
         #####----------------------###########      
          ####--------------------------------       
           ###-------------------------------        
             #-----------------------------          
              #---------------------------           
                 -------------   ------              
                     --------- P --                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  8.39e+21   6.45e+21  -9.40e+21 
  6.45e+21  -1.89e+22  -6.86e+21 
 -9.40e+21  -6.86e+21   1.05e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20241010002609/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 = 220
      DIP = 55
     RAKE = 25
       MW = 4.15
       HS = 9.0

The NDK file is 20241010002609.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
USGSMWR
 USGS/SLU Moment Tensor Solution
 ENS  2024/10/10 00:26:09:0  63.44 -150.03   3.9 4.3 NWT, Canada
 
 Stations used:
   AK.BAE AK.BMR AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV 
   AK.DOT AK.EYAK AK.FID AK.GHO AK.GLB AK.GLI AK.H24K AK.HARP 
   AK.HDA AK.HIN AK.J19K AK.J20K AK.J25K AK.KLU AK.KNK AK.L19K 
   AK.L22K AK.L26K AK.MCK AK.MLY AK.N18K AK.NEA2 AK.O19K 
   AK.P23K AK.PAX AK.POKR AK.PPD AK.PWL AK.RAG AK.RC01 AK.RIDG 
   AK.RND AK.SAW AK.SCM AK.SCRK AK.SWD AK.VRDI AK.WAT6 AK.WRH 
   AT.PMR AT.TTA AV.RED IM.IL31 IU.COLA 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.11e+22 dyne-cm
  Mw = 4.15 
  Z  = 9 km
  Plane   Strike  Dip  Rake
   NP1      115    70   142
   NP2      220    55    25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.11e+22     41      72
    N   0.00e+00     48     271
    P  -2.11e+22      9     170

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.89e+22
       Mxy     6.86e+21
       Mxz     6.45e+21
       Myy     1.05e+22
       Myz     9.40e+21
       Mzz     8.39e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              -----------------------#####           
             -------------------###########          
           -----------------#################        
          ----------------####################       
         ---------------#######################      
        ###-----------################   #######     
        ####--------################## T #######     
       #######-----###################   ########    
       #########--###############################    
       ##########--##############################    
       #########------###########################    
        #######-----------######################     
        #######---------------##################     
         #####----------------------###########      
          ####--------------------------------       
           ###-------------------------------        
             #-----------------------------          
              #---------------------------           
                 -------------   ------              
                     --------- P --                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  8.39e+21   6.45e+21  -9.40e+21 
  6.45e+21  -1.89e+22  -6.86e+21 
 -9.40e+21  -6.86e+21   1.05e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20241010002609/index.html
	
Regional Moment Tensor (Mwr)
Moment
2.569e+15 N-m
Magnitude
4.21 Mwr
Depth
8.0 km
Percent DC
93%
Half Duration
-
Catalog
US
Data Source
US 2
Contributor
US 2
Nodal Planes
Plane	Strike	Dip	Rake
NP1	219	64	40
NP2	109	54	147
Principal Axes
Axis	Value	Plunge	Azimuth
T	2.616e+15	46	78
N	-0.097e+15	43	246
P	-2.520e+15	6	342

        

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.

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 +50
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    1.0   320    55    60   3.82 0.3020
WVFGRD96    2.0   320    55    60   3.95 0.3615
WVFGRD96    3.0   215    35     0   3.99 0.3777
WVFGRD96    4.0   225    40    25   4.02 0.4364
WVFGRD96    5.0   225    45    30   4.04 0.4751
WVFGRD96    6.0   220    50    25   4.05 0.4984
WVFGRD96    7.0   220    55    25   4.08 0.5128
WVFGRD96    8.0   220    50    25   4.13 0.5195
WVFGRD96    9.0   220    55    25   4.15 0.5205
WVFGRD96   10.0   220    55    25   4.16 0.5136
WVFGRD96   11.0   215    60    20   4.18 0.5027
WVFGRD96   12.0   215    60    15   4.19 0.4893
WVFGRD96   13.0   215    60    15   4.20 0.4731
WVFGRD96   14.0   210    60   -15   4.21 0.4541
WVFGRD96   15.0   210    60   -15   4.22 0.4398
WVFGRD96   16.0   210    60   -15   4.23 0.4244
WVFGRD96   17.0   205    60   -20   4.24 0.4091
WVFGRD96   18.0   205    60   -25   4.25 0.3933
WVFGRD96   19.0   205    60   -25   4.25 0.3779
WVFGRD96   20.0   205    60   -25   4.26 0.3626
WVFGRD96   21.0   205    60   -25   4.27 0.3483
WVFGRD96   22.0   205    65   -25   4.28 0.3344
WVFGRD96   23.0   205    65   -25   4.28 0.3213
WVFGRD96   24.0   205    65   -25   4.29 0.3091
WVFGRD96   25.0   205    65   -25   4.29 0.2979
WVFGRD96   26.0   115    75   -25   4.30 0.2904
WVFGRD96   27.0   120    85   -30   4.31 0.2950
WVFGRD96   28.0   120    90   -30   4.32 0.2993
WVFGRD96   29.0   300    90    30   4.33 0.3054

The best solution is

WVFGRD96    9.0   220    55    25   4.15 0.5205

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 +50
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:

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 Wed Oct 9 20:11:10 CDT 2024