Arrival time list
USGS Felt map for this earthquake
USGS Felt reports page for Intermountain Western US
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
The focal mechanism was determined from surface-wave spectral amplitudes using the CUS model. This model has an upper crust that is too fast fort the source and propagation region but should provide a good estimate of the focal mechanism, source depth and seismic moment. However the fit to the surface-wave waveforms will not be perfect because of the timing of the surface-wave arrivals.
NODAL PLANES STK= 25.00 DIP= 50.00 RAKE= -90.00 OR STK= 204.99 DIP= 40.00 RAKE= -90.01 DEPTH = 10.0 km MW = 4.19
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
The P-wave first motion data for focal mechanism studies are as follow:
Station Azimuth(deg) Distance(km) P-first motion AHID 54.8 405.5900 ANMO 127.3 997.4000 BEKR 259.5 453.0200 e+ BMN 260.0 178.5100 e+ BOZ 26.5 616.5236 BW06 62.2 517.0494 CMB 238.1 541.2372 CTU 89.7 287.2894 DAC 204.1 538.2498 DUG 105.9 206.8900 e+ ELK 282 7.7 i- HAWA 332.1 721.6469 HLID 10.7 320.4500 i- HVU 58.9 230.6600 X HWUT 70.9 316.1800 ISA 208.4 633.2300 ISCO 94.2 817.3474 MNV 226.3 362.7468 MOD 288.5 450.6000 MPU 103.7 308.9976 MSO 7.8 684.6000 NEW 350.1 851.5769 PAHR 253.9 377.8679 PFO 188.7 798.7094 RSSD 63.9 988.1000 SDCO 108.6 895.8799 TPNV 193.2 430.4600 e+ WCN 249.6 423.5700 e+ WDC 270.9 625.1262 WUAZ 149.0 666.5076 WVOR 304.2 346.8790 i+ YBH 282.4 643.3400 YMR 36.7 555.7300
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.
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 np 2 lp c 0.10 np 2The fit to ELK and especially the FOR to the radial component ande the PnL at DUG show that this is an acceptable foral mechanism.
The focal mechanism nodal planes are well developed. However since the P-wave first motion data are of poor quality and the local velocity model is poorly known, we cannot resolve whther this is a thrust or normal faulting event.
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 CUS 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.