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.
NODAL PLANES STK= 274.13 DIP= 85.07 RAKE= -10.04 OR STK= 5.00 DIP= 80.00 RAKE= -174.99 DEPTH = 7.0 km MW = 4.14
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 BMN 12.6 251.9100 X BOZ 30.1 973.2200 BW06 51.7 866.2262 CMB 265.5 221.5900 e- DUG 61.8 488.6500 ELK 38.1 360.3618 HLID 25.0 660.8784 HOPS 282.4 461.1600 HVU 46.1 587.6804 HWUT 53.1 657.0565 ISA 190.9 288.7377 e- MNV 313.9 34.2940 i+ MOD 333.9 458.6000 MPU 67.8 574.9000 MVU 84.6 495.4100 PFO 165.6 527.1752 TPNV 134.1 200.8900 TUC 133.4 918.9400 WDC 304.5 480.0956 WUAZ 115.4 652.0100 WVOR 352.3 472.5400 YBH 314.9 568.2575
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 3 lp c 0.05 np 3The fit to the SH pulse on the T component is good. Note that this is the dominant signal at stations ISA, CMB and BMN. The Rayleigh pulse is larger at TPNV but the waveform fit is affected by the microseism level. The fit to the nearest station, MNV is very good and would not change much if the focal mechanism is modified by +-5 degrees.
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.