Location

Location ANSS

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

2002/04/20 10:50:44 44.51 -73.66 10.0 4.97 New York

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2002/04/20 10:50:44:0  44.51  -73.66  10.0 5.0 New York
 
 Stations used:
   CN.A54 CN.A61 CN.A64 CN.GAC CN.KGNO CN.LMQ CN.MNT CN.SADO 
   CN.VLDQ IU.HRV IU.SSPA LD.ACCN LD.BRNJ LD.CONY LD.CPNY 
   LD.MVL LD.PAL LD.SDMD PO.ELGO US.BINY US.LBNH US.NCB 
 
 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 = 2.37e+23 dyne-cm
  Mw = 4.85 
  Z  = 11 km
  Plane   Strike  Dip  Rake
   NP1      187    56    97
   NP2      355    35    80
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.37e+23     78     122
    N   0.00e+00      6       3
    P  -2.37e+23     10     272

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.43e+21
       Mxy     4.21e+21
       Mxz    -2.66e+22
       Myy    -2.22e+23
       Myz     8.25e+22
       Mzz     2.19e+23
                                                     
                                                     
                                                     
                                                     
                     --------#-----                  
                 ----------######------              
              -----------##########-------           
             -----------#############------          
           ------------###############-------        
          ------------#################-------       
         ------------###################-------      
        -------------###################--------     
        ------------#####################-------     
       -   ---------#####################--------    
       - P --------#########   ###########-------    
       -   --------######### T ###########-------    
       ------------#########   ###########-------    
        -----------######################-------     
        -----------######################-------     
         -----------####################-------      
          ----------####################------       
           ---------###################------        
             --------#################-----          
              --------##############------           
                 ------############----              
                     ---########---                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.19e+23  -2.66e+22  -8.25e+22 
 -2.66e+22   2.43e+21  -4.21e+21 
 -8.25e+22  -4.21e+21  -2.22e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20020420105044/index.html
        

Preferred Solution

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

      STK = -5
      DIP = 35
     RAKE = 80
       MW = 4.85
       HS = 11.0

The NDK file is 20020420105044.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
GCMT
LD
 USGS/SLU Moment Tensor Solution
 ENS  2002/04/20 10:50:44:0  44.51  -73.66  10.0 5.0 New York
 
 Stations used:
   CN.A54 CN.A61 CN.A64 CN.GAC CN.KGNO CN.LMQ CN.MNT CN.SADO 
   CN.VLDQ IU.HRV IU.SSPA LD.ACCN LD.BRNJ LD.CONY LD.CPNY 
   LD.MVL LD.PAL LD.SDMD PO.ELGO US.BINY US.LBNH US.NCB 
 
 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 = 2.37e+23 dyne-cm
  Mw = 4.85 
  Z  = 11 km
  Plane   Strike  Dip  Rake
   NP1      187    56    97
   NP2      355    35    80
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.37e+23     78     122
    N   0.00e+00      6       3
    P  -2.37e+23     10     272

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.43e+21
       Mxy     4.21e+21
       Mxz    -2.66e+22
       Myy    -2.22e+23
       Myz     8.25e+22
       Mzz     2.19e+23
                                                     
                                                     
                                                     
                                                     
                     --------#-----                  
                 ----------######------              
              -----------##########-------           
             -----------#############------          
           ------------###############-------        
          ------------#################-------       
         ------------###################-------      
        -------------###################--------     
        ------------#####################-------     
       -   ---------#####################--------    
       - P --------#########   ###########-------    
       -   --------######### T ###########-------    
       ------------#########   ###########-------    
        -----------######################-------     
        -----------######################-------     
         -----------####################-------      
          ----------####################------       
           ---------###################------        
             --------#################-----          
              --------##############------           
                 ------############----              
                     ---########---                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.19e+23  -2.66e+22  -8.25e+22 
 -2.66e+22   2.43e+21  -4.21e+21 
 -8.25e+22  -4.21e+21  -2.22e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20020420105044/index.html
	
REVISED Quick CMT: 

04/20/2002, 10:50:44, NEW YORK, Mw=5.0

The following is a REVISED solution for the April 20, 2002,
event in New York. This CMT solution was determined by
analysis of long-period body waves (45-150 s) and
intermediate-period surface waves (40-100 s or 50-200 s).
A preliminary very-broad-band analysis of teleseismic
P waveforms gives a depth for this event of 10.4 km.

Here is the solution for the recent event.

April 20, 2002, NEW YORK, Mw=5.0

Meredith Nettles
Mike Antolik

CENTROID, MOMENT TENSOR SOLUTION
HARVARD EVENT-FILE NAME S042002A
DATA USED: GSN
L.P. BODY WAVES: 23S, 38C, T= 45
SURFACE WAVES:   27S, 58C, T= 40
CENTROID LOCATION:
ORIGIN TIME       10:50:48.2 0.2
LAT 44.69N 0.01;LON  73.80W 0.02
DEP   8.9 0.8;HALF-DURATION  1.4
MOMENT TENSOR; SCALE 10**23 D-CM
  MRR= 3.19 0.17; MTT= 0.10 0.08
  MFF=-3.29 0.10; MRT=-1.45 0.25
  MRF=-0.46 0.18; MTF=-0.13 0.04
 PRINCIPAL AXES:
 1.(T) VAL=  3.78;PLG=68;AZM=172
 2.(N)      -0.44;    21;      6
 3.(P)      -3.34;     5;    274
BEST DOUBLE COUPLE:M0=3.6*10**23
 NP1:STRIKE=343;DIP=44;SLIP=  59
 NP2:       203;    53;      116

            --#########
        ----------##-------
      -----------###---------
    -----------#######---------
   -----------##########--------
  -----------############--------
  ----------##############-------
   --------###############--------
 P -------#################-------
   ------##################-------
 --------########   #######-------
  -------######## T #######------
  -------########   #######------
   ------##################-----
    -----#################-----
      ----###############----
        --##############---
            ###########
        
The LDEO web page for this sequence has a moment tensor solution for the main event. The link is 
https://www.ldeo.columbia.edu/LCSN/NYQuake_2002/20020420_nyquake.html.
        

Magnitudes

mLg Magnitude


(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.

ML Magnitude


(a) ML computed using the IASPEI formula for Horizontal components; (b) 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.


(a) ML computed using the IASPEI formula for Vertical components (research); (b) 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.

Context

The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. 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).

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for 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 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 from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0     0    55   -90   4.72 0.5592
WVFGRD96    2.0    25    20   -50   4.81 0.5016
WVFGRD96    3.0    20    20   -55   4.76 0.5069
WVFGRD96    4.0   325    35    15   4.72 0.5232
WVFGRD96    5.0   345    30    60   4.78 0.5797
WVFGRD96    6.0   355    30    75   4.80 0.6449
WVFGRD96    7.0   355    35    75   4.81 0.7011
WVFGRD96    8.0   355    35    75   4.81 0.7397
WVFGRD96    9.0   355    35    80   4.82 0.7634
WVFGRD96   10.0   355    35    80   4.85 0.7743
WVFGRD96   11.0    -5    35    80   4.85 0.7802
WVFGRD96   12.0   355    35    80   4.85 0.7773
WVFGRD96   13.0   355    35    80   4.85 0.7671
WVFGRD96   14.0   350    35    75   4.85 0.7520
WVFGRD96   15.0   350    35    70   4.85 0.7335
WVFGRD96   16.0   350    35    70   4.86 0.7124
WVFGRD96   17.0   350    35    70   4.86 0.6887
WVFGRD96   18.0   355    45    80   4.86 0.6633
WVFGRD96   19.0    -5    45    80   4.87 0.6397
WVFGRD96   20.0     0    50    85   4.90 0.6186
WVFGRD96   21.0     0    50    85   4.90 0.5978
WVFGRD96   22.0   185    40    95   4.91 0.5770
WVFGRD96   23.0   185    35    95   4.91 0.5544
WVFGRD96   24.0     0    55    90   4.91 0.5316
WVFGRD96   25.0     0    55    90   4.92 0.5086
WVFGRD96   26.0     0    55    90   4.92 0.4841
WVFGRD96   27.0   185    35    95   4.93 0.4590
WVFGRD96   28.0   185    30    95   4.93 0.4336
WVFGRD96   29.0     0    60    90   4.93 0.4083

The best solution is

WVFGRD96   11.0    -5    35    80   4.85 0.7802

The mechanism correspond 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 and because the velocity model used in the predictions may not be perfect. 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. is relative to the first trace sample.
Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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.

Surface-Wave Focal Mechanism

The following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
Location of broadband stations used to obtain focal mechanism from surface-wave spectral amplitudes

The surface-wave determined focal mechanism is shown here.


  NODAL PLANES 

  
  STK=     179.99
  DIP=      55.00
 RAKE=      85.00
  
             OR
  
  STK=       8.66
  DIP=      35.32
 RAKE=      97.10
 
 
DEPTH = 10.0 km
 
Mw = 4.96
Best Fit 0.8355 - P-T axis plot gives solutions with FIT greater than FIT90

Surface-wave analysis

Surface wave analysis was performed using codes from Computer Programs in Seismology, specifically the multiple filter analysis program do_mft and the surface-wave radiation pattern search program srfgrd96.

Data preparation

Digital data were collected, instrument response removed and traces converted to Z, R an T components. Multiple filter analysis was applied to the Z and T traces to obtain the Rayleigh- and Love-wave spectral amplitudes, respectively. These were input to the search program which examined all depths between 1 and 25 km and all possible mechanisms.
Best mechanism fit as a function of depth. The preferred depth is given above. Lower hemisphere projection

Pressure-tension axis trends. Since the surface-wave spectra search does not distinguish between P and T axes and since there is a 180 ambiguity in strike, all possible P and T axes are plotted. First motion data and waveforms will be used to select the preferred mechanism. The purpose of this plot is to provide an idea of the possible range of solutions. The P and T-axes for all mechanisms with goodness of fit greater than 0.9 FITMAX (above) are plotted here.


Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the Love and Rayleigh wave radiation patterns. 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. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0-180 degrees are sampled.

Love-wave radiation patterns

Rayleigh-wave radiation patterns