Location

Location ANSS

The ANSS event ID is us6000k6kx and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us6000k6kx/executive.

2023/04/23 18:10:08 43.864 -76.020 8.7 3.6 New York

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2023/04/23 18:10:08:0  43.86  -76.02   8.7 3.6 New York
 
 Stations used:
   CN.MNTQ CN.SADO CN.TRQ CN.WBO IU.HRV IU.SSPA LD.KSCT LD.NCB 
   LD.NPNY N4.H62A N4.I62A N4.I63A N4.J57A N4.J59A N4.J61A 
   N4.K57A N4.K62A N4.L56A N4.L59A N4.L61B N4.M57A N4.N58A 
   NE.HNH NE.TRY NE.WES PE.PALB PE.PAOC PE.PSUB US.BINY 
   US.ERPA US.LBNH US.LONY WU.BUKO WU.DELO WU.MEDO WU.PECO 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +30
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.20 n 3 
 
 Best Fitting Double Couple
  Mo = 3.63e+21 dyne-cm
  Mw = 3.64 
  Z  = 8 km
  Plane   Strike  Dip  Rake
   NP1      166    65    95
   NP2      335    25    80
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.63e+21     69      85
    N   0.00e+00      4     344
    P  -3.63e+21     20     253

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.85e+20
       Mxy    -8.78e+20
       Mxz     4.53e+20
       Myy    -2.45e+21
       Myz     2.32e+21
       Mzz     2.74e+21
                                                     
                                                     
                                                     
                                                     
                     ##------------                  
                 -----#########--------              
              -------##############-------           
             --------################------          
           ----------##################------        
          -----------###################------       
         ------------####################------      
        -------------#####################------     
        -------------######################-----     
       --------------###########   #########-----    
       --------------########### T #########-----    
       ---------------##########   #########-----    
       ---   ---------######################-----    
        -- P ----------#####################----     
        --   ----------#####################----     
         ---------------####################---      
          ---------------##################---       
           ---------------################---        
             --------------##############--          
              --------------############--           
                 -------------########-              
                     -----------###                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.74e+21   4.53e+20  -2.32e+21 
  4.53e+20  -2.85e+20   8.78e+20 
 -2.32e+21   8.78e+20  -2.45e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230423181008/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 = 335
      DIP = 25
     RAKE = 80
       MW = 3.64
       HS = 8.0

The NDK file is 20230423181008.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  2023/04/23 18:10:08:0  43.86  -76.02   8.7 3.6 New York
 
 Stations used:
   CN.MNTQ CN.SADO CN.TRQ CN.WBO IU.HRV IU.SSPA LD.KSCT LD.NCB 
   LD.NPNY N4.H62A N4.I62A N4.I63A N4.J57A N4.J59A N4.J61A 
   N4.K57A N4.K62A N4.L56A N4.L59A N4.L61B N4.M57A N4.N58A 
   NE.HNH NE.TRY NE.WES PE.PALB PE.PAOC PE.PSUB US.BINY 
   US.ERPA US.LBNH US.LONY WU.BUKO WU.DELO WU.MEDO WU.PECO 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +30
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.20 n 3 
 
 Best Fitting Double Couple
  Mo = 3.63e+21 dyne-cm
  Mw = 3.64 
  Z  = 8 km
  Plane   Strike  Dip  Rake
   NP1      166    65    95
   NP2      335    25    80
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.63e+21     69      85
    N   0.00e+00      4     344
    P  -3.63e+21     20     253

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.85e+20
       Mxy    -8.78e+20
       Mxz     4.53e+20
       Myy    -2.45e+21
       Myz     2.32e+21
       Mzz     2.74e+21
                                                     
                                                     
                                                     
                                                     
                     ##------------                  
                 -----#########--------              
              -------##############-------           
             --------################------          
           ----------##################------        
          -----------###################------       
         ------------####################------      
        -------------#####################------     
        -------------######################-----     
       --------------###########   #########-----    
       --------------########### T #########-----    
       ---------------##########   #########-----    
       ---   ---------######################-----    
        -- P ----------#####################----     
        --   ----------#####################----     
         ---------------####################---      
          ---------------##################---       
           ---------------################---        
             --------------##############--          
              --------------############--           
                 -------------########-              
                     -----------###                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.74e+21   4.53e+20  -2.32e+21 
  4.53e+20  -2.85e+20   8.78e+20 
 -2.32e+21   8.78e+20  -2.45e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230423181008/index.html
	
Regional Moment Tensor (Mwr)
Moment 3.716e+14 N-m
Magnitude 3.65 Mwr
Depth 12.0 km
Percent DC 79%
Half Duration -
Catalog US
Data Source US 1
Contributor US 1

Nodal Planes
Plane Strike Dip Rake
NP1 353 27 94
NP2 169 63 88

Principal Axes
Axis Value Plunge Azimuth
T 3.498e+14 N-m 72 73
N 0.404e+14 N-m 2 170
P -3.901e+14 N-m 18 260

        

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 -30 o DIST/3.3 +30
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.20 n 3 
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   345    30   -90   3.58 0.4703
WVFGRD96    2.0   255    25    -5   3.56 0.4784
WVFGRD96    3.0   270    25    15   3.53 0.5306
WVFGRD96    4.0   340    20    85   3.56 0.5772
WVFGRD96    5.0   335    25    80   3.59 0.6140
WVFGRD96    6.0   165    65    95   3.60 0.6355
WVFGRD96    7.0   165    65    95   3.62 0.6466
WVFGRD96    8.0   335    25    80   3.64 0.6493
WVFGRD96    9.0   335    25    80   3.66 0.6424
WVFGRD96   10.0   340    20    85   3.70 0.6261
WVFGRD96   11.0   170    70    85   3.72 0.6046
WVFGRD96   12.0   345    20    90   3.73 0.5778
WVFGRD96   13.0   170    70    85   3.75 0.5465
WVFGRD96   14.0   165    75    80   3.76 0.5136
WVFGRD96   15.0   165    75    75   3.77 0.4797
WVFGRD96   16.0   165    75    75   3.78 0.4434
WVFGRD96   17.0   160    75    70   3.79 0.4061
WVFGRD96   18.0   235    30   -25   3.78 0.3682
WVFGRD96   19.0   230    30   -30   3.79 0.3454
WVFGRD96   20.0   230    30   -30   3.82 0.3260
WVFGRD96   21.0   225    30   -35   3.82 0.3089
WVFGRD96   22.0   225    30   -35   3.83 0.2955
WVFGRD96   23.0   225    30   -35   3.84 0.2852
WVFGRD96   24.0   270    15    20   3.84 0.2890
WVFGRD96   25.0   270    15    20   3.85 0.2946
WVFGRD96   26.0   265    15    15   3.86 0.3025
WVFGRD96   27.0   270    15    20   3.87 0.3120
WVFGRD96   28.0   265    15    15   3.88 0.3208
WVFGRD96   29.0   270    10    20   3.90 0.3300

The best solution is

WVFGRD96    8.0   335    25    80   3.64 0.6493

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 -30 o DIST/3.3 +30
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.20 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 CUS.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
CUS Model with Q from simple gamma values
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
  1.0000  5.0000  2.8900  2.5000 0.172E-02 0.387E-02 0.00  0.00  1.00  1.00 
  9.0000  6.1000  3.5200  2.7300 0.160E-02 0.363E-02 0.00  0.00  1.00  1.00 
 10.0000  6.4000  3.7000  2.8200 0.149E-02 0.336E-02 0.00  0.00  1.00  1.00 
 20.0000  6.7000  3.8700  2.9020 0.000E-04 0.000E-04 0.00  0.00  1.00  1.00 
  0.0000  8.1500  4.7000  3.3640 0.194E-02 0.431E-02 0.00  0.00  1.00  1.00 
Last Changed Mon Apr 22 11:17:10 PM CDT 2024