Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Intermountain Western US

Waveform inversion focal mechanism

This event was recorded on scale at short distances. The waveform inversion code was used to invert the REDR and AHID waveforms using the modified Utah model velocity model.
Figure 1. Waveform inversion focal mechanism
Figure 2. Depth sensitivity for waveform mechanism
Figure 3. Mechanism sensitivity for waveform mechanism for depth of 8 km

The waveform inversion solution has the following parameters:


Depth  8.0    
Strike  20    
Dip     75   
Rake   -60   
Mw    3.63

The next figure compares the observed and predicted waveforms for these two stations. The traces are filtered using the SAC command
hp c 0.02 3
lp c 0.20 3
Figure 4. Waveform comparison for depth of 8 km

Surface-Wave Focal Mechanism

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 5. The results of the search are shown in the following figures:
Figure 5. Location of broadband stations used to obtain focal mechanism


  NODAL PLANES 

  
  STK=      26.77
  DIP=      68.36
 RAKE=     -55.50
  
             OR
  
  STK=     144.99
  DIP=      40.00
 RAKE=    -144.99
 
 
DEPTH = 11.0 km
 
Mw = 3.66
Best Fit 0.8980 - P-T axis plot gives solutions with FIT greater than FIT90

Focal Mechanism

First motion data

The P-wave first motion data for focal mechanism studies are as follow:

Sta Az(deg)    Dist(km)   First motion
REDW	   31	   25 i+
AHID	  189	   46 i-
BW06	  110	  127 i+
HWUT	  195	  180 i+
BOZ	  350	  280 e-

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.

The velocity model used for the search is a modified Utah model .

Data preparation

Digital data were collected, instrument 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.
Best mechanism fit as a function of depth. The preferred depth is given above. Lower hemisphere projection

Pressure-tension axis trends. Since the surface-wave spectra search does not distinguish between P and T axes and since there is a 180 ambiguity in strike, all possible P and T axes are plotted. First motion data and waveforms will be used to select the preferred mechanism. The purpose of this plot is to provide an idea of the possible range of solutions. The P and T-axes for all mechanisms with goodness of fit greater than 0.9 FITMAX (above) are plotted here.


Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the Love and Rayleigh wave radiation patterns. Each solution is plotted as a vector at a given value of strike and dip with the angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure. A nearly vertical strike-slip fault striking at 75 or 165 degrees is preferred. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0-180 degrees are sampled.

Love-wave radiation patterns

Rayleigh-wave radiation patterns

Broadband station distributiuon

Sta Az(deg)    Dist(km)   
REDW	   31	   25
AHID	  189	   46
BW06	  110	  127
HWUT	  195	  180
HLID	  280	  279
BOZ	  350	  280
DUG	  205	  363
MSO	  331	  468
LAO	   42	  544
ISCO	  128	  586
BMN	  242	  599
WVOR	  265	  629
NEW	  322	  739
SDCO	  140	  762
HAWA	  301	  764
TPH	  225	  772
MNV	  231	  800
CMB	  237	  976
COR	  283	 1000
CBKS	  113	 1064
SAO	  235	 1139
KSU1	  106	 1289
WMOK	  128	 1412

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 observed and predicted waveforms.

The velocity model used for the waveform fit is a modified Utah model .

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 3
lp c 0.10 3

Discussion

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.

Appendix A

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 modified Utah model 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.

Quality Control

Here we tabulate the reasons for not using certain digital data sets

The following stations did not have a valid response files:

Last Changed Sun Aug 22 16:58:42 CDT 2004