Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Central 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

Focal Mechanism

Strike = 90 Strike = 299
Dip = 50 Dip = 44
Rake = -110 Rake = -68
Mw = 4.15 Depth = 8 km

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, 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. The figure
Best mechanism fit as a function of depth. the preferred depth is 8 km

Love-wave radiation patterns

Rayleigh-wave radiation patterns

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 osberved and predicted waveforms.

To make the synthetics, the CUS model was used to generate surface-wave synthetics (this is much faster than the compelte wavenumber integration synthetics; the filtered data only exhibited surface-wave pulses). This model is adequate in predicting the arrival times at CCM (distance=837km) but did not generate the short period surface waves seen at RSSD (439 km) or ISCO (653 km). The comparison below presents predicted and observed ground velocities (units of /sec) which have been passed through a 3-pole highpass Butterworth filter at 0.01 Hz and a 3-pole lowpass Butterworth filter at 0.06 Hz.

The peak amplitudes well matched. The trace comaprison at CCM is very good - this is the path for which the CUS model is more appropriate. The Rayleigh and Love-wave phases are matched.

The RSSD comparison is OK - the transverse motion is very sensitive to the location of the Love-wave node in the radiation pattern (RSSD is at an azimuth of 291 degrees).

The ISCO fit is not as good - indicating a very different shallow crustal structure to this station.
Comparison of observed and predicted waveforms at CCM
Comparison of observed and predicted waveforms at RSSD
Comparison of observed and predicted waveforms at ISCO

Waveform grid search

Since the station RSSD is the closest station and since this station has good Rayleigh and Love wave signals, the waveform grid search program wvfgrd96 was used together with the CUS earth model and compelte wavenumber intergration synthetic Green's functions ( P, S and surface waves are all included) to find the mechanism and depth that best fit the RSSD waveforms. For compatibility with the previously displayed traces, the Green's functions and observed ground velocities were filtered as follows: 3-pole Butterworth highpass at 0.01 Hz and a 3-pole Butterworth lowpass at 0.06 Hz. The grid search looked at all possible forcal mechanisms for depths between 0.5 and 29 km. The wvfgrd96 results for the best fit at each depth are

         Depth    Stk   Dip  Rake    Mw    Fit
WVFGRD96    0.5   320    65   -60   4.53 0.4203
WVFGRD96    1.0   320    65   -60   4.50 0.4155
WVFGRD96    2.0   325    55   -50   4.27 0.4255
WVFGRD96    3.0    90    50  -135   4.19 0.4310
WVFGRD96    4.0    90    55  -135   4.13 0.4355
WVFGRD96    5.0   325    50   -55   4.14 0.4429
WVFGRD96    6.0   320    50   -65   4.16 0.4571
WVFGRD96    7.0   110    45  -110   4.16 0.4670
WVFGRD96    8.0   110    45  -110   4.14 0.4732
WVFGRD96    9.0   110    50  -110   4.13 0.4761
WVFGRD96   10.0   110    50  -110   4.15 0.4792 
WVFGRD96   11.0   320    45   -70   4.13 0.4745
WVFGRD96   12.0   320    45   -70   4.12 0.4721
WVFGRD96   13.0   115    50  -110   4.10 0.4667
WVFGRD96   14.0   160    45  -120   4.03 0.4581
WVFGRD96   15.0   160    45  -120   4.03 0.4564
WVFGRD96   16.0   155    45  -120   4.01 0.4520
WVFGRD96   17.0   155    50  -125   4.02 0.4393
WVFGRD96   18.0    30    55   -45   4.03 0.4331
WVFGRD96   19.0    35    55   -40   4.05 0.4165
WVFGRD96   20.0   150    60  -145   4.08 0.4092
WVFGRD96   21.0   150    60  -145   4.08 0.4039
WVFGRD96   22.0   150    65  -150   4.10 0.3865
WVFGRD96   23.0   150    65  -150   4.10 0.3806
WVFGRD96   24.0   150    65  -150   4.10 0.3741
WVFGRD96   25.0   150    70  -155   4.12 0.3559
WVFGRD96   26.0    50    70   -20   4.12 0.3507
WVFGRD96   27.0    50    70   -20   4.12 0.3465
WVFGRD96   28.0    45    60   -25   4.09 0.3280
WVFGRD96   29.0    45    60   -25   4.09 0.3229                 

Selecting the 10 km depth (highlighted in blue) because it had the best fit, the
Comparison of observed and predicted waveforms at RSSD for a source depth of 10km, strike of 110, dip of 50 and rake of -110 degrees
 

This single station mechanism and depth are very similar to that determined from the surface wave technique. Note that the expected P-wave amplitudes at RSSD are expected to be small for this mechanism, which is evident in the emeergent P-waves in the original broadband data.

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.

Acknowledgements

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

Last Changed 02/11/05