USGS Felt map for this earthquake
USGS Felt reports page for Central and Southeastern US
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
|
|
|
|
NODAL PLANES
STK= 20.00
DIP= 90.00
RAKE= -165.00
OR
STK= 290.00
DIP= 75.00
RAKE= -0.01
DEPTH = 7.0 km
Mw = 4.15
Best Fit 0.8280 - P-T axis plot gives solutions with FIT greater than FIT90
http://www.ldeo.columbia.edu/LCSN/Eq/20040628_Illinois/20040628_0610.html
This is the solution used in the source parameter tabulation
 |
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, intreument 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.
|
|
|
|
| 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. |
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion JFWS 326 187 e- BLO 141 334 i+ SLM 199 340 e- USIN 164 411 e+ FVM 198 414 X CCM 208 433 i-
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) PPMSH 58 102 JFWS 326 187 PPNVW 145 270 BLO 141 334 PPEGH 145 335 SLM 199 340 PPFAY 86 386 USIN 164 411 FVM 198 414 WCI 147 432 CCM 208 433 PPNAF 143 439 AAM 77 447 ACSO 104 523 PVMO 187 572 UTMT 179 577 MPH 187 717 PLAL 174 731 MCWV 102 798 UALR 203 807 SADO 62 874 LRAL 169 959 CBKS 255 966 KAPO 28 1013 CBN 107 1056 KGNO 70 1060 ULM 333 1106 WMOK 232 1144 NCB 72 1234 MUMO 355 1236 PAL 88 1260 RSSD 288 1264 HNH 74 1386 ISCO 268 1420 HKT 208 1427 HRV 80 1442 BRYW 82 1447 ANMO 250 1694 BW06 282 1704 FCC 351 1949 LMN 68 1996 SCHQ 40 2194 EDM 314 2236 DRLN 60 2585
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
|
|
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.
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
