Location

Location ANSS

The ANSS event ID is ld60024626 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ld60024626/executive.

2011/08/24 17:14:32 44.703 -74.382 5.0 3.19 New York

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2011/08/24 17:14:32:0  44.70  -74.38   5.0 3.2 New York
 
 Stations used:
   CN.GAC CN.KGNO CN.MNTQ CN.ORIO CN.PECO CN.PLVO LD.ACCN 
   LD.FRNY US.LBNH US.LONY 
 
 Filtering commands used:
   cut o DIST/3.3 -20 o DIST/3.3 +20
   rtr
   taper w 0.1
   hp c 0.10 n 3 
   lp c 0.30 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 4.12e+20 dyne-cm
  Mw = 3.01 
  Z  = 2 km
  Plane   Strike  Dip  Rake
   NP1      175    65    90
   NP2      355    25    90
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.12e+20     70      85
    N   0.00e+00     -0     175
    P  -4.12e+20     20     265

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.40e+18
       Mxy    -2.74e+19
       Mxz     2.31e+19
       Myy    -3.13e+20
       Myz     2.64e+20
       Mzz     3.16e+20
                                                     
                                                     
                                                     
                                                     
                     ----######----                  
                 -------##########-----              
              ---------##############-----           
             ----------###############-----          
           -----------##################-----        
          ------------###################-----       
         -------------####################-----      
        --------------#####################-----     
        --------------#####################-----     
       --------------###########   #########-----    
       ---   ---------########## T #########-----    
       --- P ---------##########   #########-----    
       ---   ---------######################-----    
        --------------#####################-----     
        ---------------####################-----     
         --------------###################-----      
          --------------##################----       
           -------------#################----        
             ------------##############----          
              ------------############----           
                 ----------#########---              
                     --------####--                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  3.16e+20   2.31e+19  -2.64e+20 
  2.31e+19  -2.40e+18   2.74e+19 
 -2.64e+20   2.74e+19  -3.13e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110824171432/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 = 175
      DIP = 65
     RAKE = 90
       MW = 3.01
       HS = 2.0

The NDK file is 20110824171432.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 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 -20 o DIST/3.3 +20
rtr
taper w 0.1
hp c 0.10 n 3 
lp c 0.30 n 3 
br c 0.12 0.25 n 4 p 2
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   170    85    80   3.04 0.5589
WVFGRD96    1.0   170    80    80   3.04 0.5599
WVFGRD96    2.0   175    65    90   3.01 0.6041
WVFGRD96    3.0   155    60    70   3.01 0.5813
WVFGRD96    4.0   175    55    90   3.04 0.5226
WVFGRD96    5.0   175    55    90   3.05 0.4588
WVFGRD96    6.0   295    70   -55   3.08 0.4074
WVFGRD96    7.0   180    30   -80   3.09 0.3831
WVFGRD96    8.0   -10    45   -75   3.14 0.3576
WVFGRD96    9.0   155    45   -70   3.17 0.3353
WVFGRD96   10.0   145    40   -80   3.21 0.3055
WVFGRD96   11.0   240    10   -25   3.22 0.3012
WVFGRD96   12.0   240    10   -20   3.25 0.3095
WVFGRD96   13.0   250    10   -10   3.27 0.3017
WVFGRD96   14.0   155    60    70   3.24 0.3140
WVFGRD96   15.0   340    55    65   3.31 0.3258
WVFGRD96   16.0   340    55    65   3.31 0.3331
WVFGRD96   17.0   340    60    70   3.33 0.3566
WVFGRD96   18.0   345    60    75   3.32 0.3656
WVFGRD96   19.0   340    65    70   3.34 0.3657
WVFGRD96   20.0   340    65    70   3.35 0.3650
WVFGRD96   21.0   150    70    70   3.37 0.3540
WVFGRD96   22.0   155    70    70   3.39 0.3671
WVFGRD96   23.0   155    70    70   3.41 0.3704
WVFGRD96   24.0   155    70    70   3.42 0.3613
WVFGRD96   25.0   125    15    40   3.39 0.3581
WVFGRD96   26.0   125    15    40   3.39 0.3674
WVFGRD96   27.0   135    10    55   3.41 0.3800
WVFGRD96   28.0   345    75    80   3.39 0.3968
WVFGRD96   29.0   345    75    80   3.39 0.4028

The best solution is

WVFGRD96    2.0   175    65    90   3.01 0.6041

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 -20 o DIST/3.3 +20
rtr
taper w 0.1
hp c 0.10 n 3 
lp c 0.30 n 3 
br c 0.12 0.25 n 4 p 2
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 Sat Apr 27 04:04:04 PM CDT 2024