Arrival Times (from USGS)

2002/04/20 10:50:44 44.51N 73.66W 5.0 5.1M A NEW YORK Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Northeastern US

Focal Mechanism

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
Figure 1. Location of broadband stations used to obtain focal mechanism


  STK=     192.15
  DIP=      55.61
 RAKE=      96.94
  STK=     359.99
  DIP=      35.00
 RAKE=      79.99
DEPTH = 10.0 km
Mw = 4.97
Best Fit 0.8041 - P-T axis plot gives solutions with FIT greater than FIT90

Focal Mechanism


April 20, 2002, Au Sable Forks, New York Earthquake

Harvard CMT

Harvard CMT solution


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. A nearly vertical strike-slip fault striking at 75 or 165 degrees is preferred. 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

First motion data

The P-wave first motion data for focal mechanism studies are as follow:

Sta Az(deg)    Dist(km)   First motion
NCB       217      75 ee-
LBNH	  102	  141 i-
HRV	  142	  280 X
BINY	  217	  319 X
PAL	  183	  390 X
SSPA	  220	  553 X
AAM	  257	  846 e+
BLA	  217	  990 e+
WCI	  241	 1270 e+
NHSC      206    1385 i-
EYMN	  292	 1425 i+
WVT	  237	 1519 ee-
OXF	  236	 1746 ee+
ULM	  299	 1788 e+
CBKS	  263	 2252 e+
WMOK	  252	 2402 e-
ISCO	  270	 2676 e+

Broadband station distributiuon

The distribution of broadband stations with azimuth and distance is

Sta Az(deg)    Dist(km)   
LBNH	  102	  141
HRV	  142	  280
BINY	  217	  319
PAL	  183	  390
SSPA	  220	  553
CBN	  205	  766
AAM	  257	  846
BLA	  217	  990
BLO	  245	 1220
SCHQ	   21	 1249
WCI	  241	 1270
JFWS	  268	 1347
MYNC	  224	 1377
EYMN	  292	 1425
GOGA	  218	 1495
SIUC	  245	 1505
WVT	  237	 1519
SLM	  250	 1525
UTMT	  240	 1573
MPH	  239	 1734
OXF	  236	 1746
ULM	  299	 1788
DWPF	  203	 1947
MIAR	  244	 2030
FCC	  326	 2109
CBKS	  263	 2252
WMOK	  252	 2402
RSSD	  280	 2411
HKT	  238	 2532
ISCO	  270	 2676
BW06	  279	 2881
LKWY	  283	 2897
BOZ	  286	 2966
ANMO	  260	 2982
AHID	  280	 3002
HWUT	  277	 3078
WALA	  294	 3080
LTX	  248	 3133
DUG	  275	 3231
HLID	  283	 3236
MVU	  272	 3257
NEW	  293	 3326
ELK	  278	 3401
TUC	  260	 3478
RES	  349	 3526
HAWA	  290	 3546
TPNV	  272	 3651
TPH	  274	 3675
DAC	  272	 3793
ISA	  271	 3895
CMB	  276	 3927
INK	  329	 4286
COLA	  324	 4932

Waveform comparison for this mechanism

Since the analysis of the surface-wave radiation patterns uses only spectral amplitudes and because the surfave-wave radiation patterns have a 180 degree symmetry, each surface-wave solution consists of four possible focal mechanisms corresponding to the interchange of the P- and T-axes and a roation of the mechanism by 180 degrees. To select one mechanism, P-wave first motion can be used. This was not possible in this case because all the P-wave first motions were emergent ( a feature of the P-wave wave takeoff angle, the station location and the mechanism). The other way to select among the mechanisms is to compute forward synthetics and compare the observed and predicted waveforms.

The fits to the waveforms with the given mechanism are show below:

This figure shows the fit to the three components of motion (Z - vertical, R-radial and T - transverse). For each station and component, the observed traces is shown in red and the model predicted trace in blue. The traces represent filtered ground velocity in units of meters/sec (the peak value is printed adjacent to each trace; each pair of traces to plotted to the same scale to emphasize the difference in levels). Both synthetic and observed traces have been filtered using the SAC commands:

hp c 0.02 3
lp c 0.10 3


The Future

Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.


Dr. Harley Benz, USGS, provided the USGS USNSN digital data.

Appendix A

The figures below show the observed spectral amplitudes (units of cm-sec) at each station and the theoretical predictions as a function of period for the mechanism given above. The modified Utah model earth model was used to define the Green's functions. For each station, the Love and Rayleigh wave spectrail amplitudes are plotted with the same scaling so that one can get a sense fo the effects of the effects of the focal mechanism and depth on the excitation of each.

Quality Control

Here we tabulate the reasons for not using certain digital data sets

The following stations did not have a valid response files:

Last Changed Tue Jul 27 09:06:19 CDT 2004