## 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

Because of the small size of this earthquake, an attempt to determine the focal mechanism was made by applying a grid search to determine the best waveform fit to the stations at the nearest distances. The stations were CBN (79 km), BLA (216 km) and MCWV (261 km). In addition P-wave first motions were obtained at only a few stations: BLA ( 216 km, az=255, e-), CBN (79 km, az=51, e-), SSPA (320 km, az-3, e+).

After some experimenting with filters, a grid search was applied to the waveforms at CBN, BLA and MCWV. To enhance different features in the seismograms, the gridsearch was applied to two filtered traces for each station and component - one consisted of the SAC command bp c 0.05 0.10 np 3, and the other bp c 0.05 0.20 np 3

The grid search program wvfgrd96 searched for the best fitting double-couple focal mechanism in the depth rand of 0.5 - 29 km using the CUS model. Figure 2 gives the search fit as a function of depth. There is a preference for a shallow depth.
 Figure 2. Goodness of fit function for best fitting mechanism as a function of depth

Because of the small size of the earthquake, there are many possible focal mechanisms. The best fitting mechanism for a depth 2.0 km is
 Strike = 3 Strike = 100 Dip = 71 Dip = 70 Rake = 158 Rake = 20 Mw = 3.55 Depth = 2 km

This solution agrees with the 3 P-wave first motion estimates.

The fits to the waveforms are show below. The fit to CBN Z anr R is good, and the fits to BLA are adequate. MCWV was at too large a distance for the earthquake signal to be above the noise.

 Fig. 3. Comparison of the observed data (top trace in each doublet) and predicted filtered using bp c 0.05 0.10 np 3
 Fig. 4. Comparison of the observed data (top trace in each doublet) and predicted filtered using bp c 0.05 0.20 np 3

The signal to noise ratio for this small event precluded the use of the surface-wave spectral amplitude technique to provide additional constraints on the focal mechanism. To further investigate the possible focal mehcnaisms, the grid search was performed again using only the data from the stations BLA and CBN. The grid research results were:

```
H    STK   DIP  RAKE    MW   FIT
WVFGRD96    0.5   340    45   130   3.32 0.3028
WVFGRD96    1.0   340    45   125   3.39 0.3067
WVFGRD96    2.0   100    70    20   3.56 0.2685
WVFGRD96    3.0    95    85     0   3.53 0.1231
WVFGRD96    4.0    55    40   -85   3.56 0.1590
WVFGRD96    5.0    55    40   -85   3.56 0.1899

```
The jump in moment magnitude at 1.0 km is related to the fact that the CUS earth model has a slightly lower velocity 1km thick surface layer. The mechanisms at 1.0 and 2.0 km share a common nodal plane strikeing SE. These are shown in Figure 5.
 Fig. 5. Search focal mechanisms. Left for 1.0 km source depth, right for 2.0 km source depth Fig. 6. Mechanism variability in range of best fit to three comonent data. The axes give the strike and dip of the nodal plane, and the vector indicates the rake angle. The colors vary from blue (worst ) to red (best) for source depth of 1.0 km. Fig. 7. Plot of orientation of P- (green) and T-axes (blue) for the mechanisms shown in Figure 6 for source depth of 1.0 km. A P-axis is rwquired in the first quadrant for all of the better solutions.

### Discussion

Until we can obtain better Green's functions at shorter periods, the only to enhance our ability to obtain focal mechanisms for events as small as this is to have more broadband observations within distances less than 100 km.

## 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 2003 May 12