USGS Felt map for this earthquake
USGS Felt reports page for Central 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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Strike = 90 | Strike = 299 |
Dip = 50 | Dip = 44 |
Rake = -110 | Rake = -68 |
Mw = 4.15 | Depth = 8 km |
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. The figure
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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.
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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
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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.
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.