1995/01/28 06:26:21 44.51N 114.83W 5 4.6 IDAHO

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Intermountain Western US

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

Waveform Inversion

The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.

The observed and predicted traces are filtered using the following gsac commands:

hp c 0.02 3
lp c 0.06 3
The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   170    75    15   3.95 0.3820
WVFGRD96    1.0   170    80    15   3.96 0.3955
WVFGRD96    2.0   170    75    15   4.01 0.4335
WVFGRD96    3.0   170    60    10   4.06 0.4539
WVFGRD96    4.0   170    60    10   4.09 0.4777
WVFGRD96    5.0   170    65    15   4.10 0.4952
WVFGRD96    6.0   170    65    20   4.12 0.5120
WVFGRD96    7.0   170    65    20   4.14 0.5331
WVFGRD96    8.0   170    65    20   4.16 0.5516
WVFGRD96    9.0   170    65    25   4.18 0.5602
WVFGRD96   10.0   170    65    25   4.19 0.5725
WVFGRD96   11.0   170    70    25   4.20 0.5743
WVFGRD96   12.0   155    70   -30   4.22 0.5874
WVFGRD96   13.0   155    70   -30   4.23 0.5984
WVFGRD96   14.0   160    75   -25   4.23 0.5917
WVFGRD96   15.0   160    75   -25   4.24 0.5974
WVFGRD96   16.0   160    75   -20   4.24 0.5863
WVFGRD96   17.0   160    75   -20   4.25 0.5883
WVFGRD96   18.0   160    75   -20   4.26 0.5880
WVFGRD96   19.0   155    80    20   4.26 0.5734

The mechanism correspond to the best fit is
Figure 1. Wavefrom inversion focal mechanism

The best fit as a function of depth is given in the following figure:

Figure 2. Depth sensitivity for waveform mechanism

The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted. Each observed-predicted componnet is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. The number in black at the rightr of each predicted traces it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed and because the velocity model used in the predictions may not be perfect. A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The bandpass filter used in the processing and for the display was

hp c 0.02 3
lp c 0.06 3
Figure 3. Waveform comparison for depth of 8 km

Surface-Wave Focal Mechanism


  STK=     160.55
  DIP=      76.00
 RAKE=     -46.78
  STK=     264.98
  DIP=      45.00
 RAKE=    -159.99
DEPTH = 13.0 km
Mw = 4.31
Best Fit 0.8929 - P-T axis plot gives solutions with FIT greater than FIT90

Surface-Wave 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
WVOR      234  385 eP_X
WCP       173  446 iP_C
NEW       338  453 eP_X
BMN       204  494 eP_X
WMT       161  516 eP_X

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)   
WVOR	  234	  385
WCP	  173	  446
NEW	  338	  453
BMN	  204	  494
WMT	  161	  516
RCC	  140	  565
GAR	  174	  628
MLC	  195	  634
MDW	  161	  657
SRS	  149	  714
MNV	  203	  730
LMP	  156	  787
MLAC	  205	  835
CYF	  150	  878
GOL	  121	  947
ISA	  199	 1030
GSC	  190	 1036
NEE	  179	 1076
VTV	  192	 1125
SVD	  190	 1172
PAS	  195	 1186
SBC	  202	 1194
CALB	  197	 1196
USC	  196	 1202
PFO	  187	 1218
DGR	  190	 1220
RPV	  196	 1234
SNCC	  200	 1314
BAR	  188	 1323

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 0.06 3


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.


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 Sat Jun 4 13:38:26 CDT 2005