2002/04/20 10:50:44 44.51N 73.66W 5.0 5.1M A NEW YORK Arrival time list
USGS Felt map for this earthquake
USGS Felt reports page for Northeastern US
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
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NODAL PLANES
STK= 192.15
DIP= 55.61
RAKE= 96.94
OR
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
April 20, 2002, Au Sable Forks, New York Earthquake
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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.
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.
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| 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. |
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| 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. |
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+
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
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:
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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
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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.
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.
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Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
