2002/01/31 18:17:45 40.28N 107.70W 5 4.3 Colorado

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

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is

      STK = 290
      DIP = 70
     RAKE = 40
       MW = 3.76
       HS = 12

The waveform inversion solution is preferred. It is compatible with the sparse surface-wave spectral amplitude inversion.

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:

taper w 0.1
hp c 0.04 3
lp c 0.10 3
br c 0.12 0.20 n 8 p 2
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   265    55   -45   3.36 0.2274
WVFGRD96    1.0   250    45   -70   3.43 0.2372
WVFGRD96    2.0   260    50   -50   3.52 0.3095
WVFGRD96    3.0   295    80    15   3.49 0.3308
WVFGRD96    4.0   285    65    15   3.53 0.3529
WVFGRD96    5.0   285    65    30   3.59 0.3860
WVFGRD96    6.0   280    75    50   3.67 0.4091
WVFGRD96    7.0   285    70    45   3.68 0.4361
WVFGRD96    8.0   285    70    45   3.71 0.4503
WVFGRD96    9.0   290    65    45   3.73 0.4660
WVFGRD96   10.0   290    70    45   3.75 0.4750
WVFGRD96   11.0   290    70    45   3.76 0.4792
WVFGRD96   12.0   290    70    40   3.76 0.4806
WVFGRD96   13.0   285    75    40   3.78 0.4805
WVFGRD96   14.0   285    75    40   3.79 0.4784
WVFGRD96   15.0   285    75    35   3.79 0.4746
WVFGRD96   16.0   285    75    35   3.80 0.4686
WVFGRD96   17.0   285    75    40   3.82 0.4611
WVFGRD96   18.0   275    85    70   3.88 0.4551
WVFGRD96   19.0   275    85    70   3.88 0.4551

The best solution is

WVFGRD96   12.0   290    70    40   3.76 0.4806

The mechanism correspond to the best fit is
Figure 1. Waveform 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

taper w 0.1
hp c 0.04 3
lp c 0.10 3
br c 0.12 0.20 n 8 p 2
Figure 3. Waveform comparison for depth of 8 km
Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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.

Surface-Wave Focal Mechanism


  STK=     299.98
  DIP=      69.99
 RAKE=      64.99
  STK=     173.73
  DIP=      31.62
 RAKE=     139.26
DEPTH = 11.0 km
Mw = 3.87
Best Fit 0.8081 - P-T axis plot gives solutions with FIT greater than FIT90

First motion data

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

Sta Az(deg)    Dist(km)   First motion
ISCO      106  186 iP_C
SRU       243  275 eP_+
BW06      331  318 eP_X
JLU       278  320 eP_X
MPU       266  337 eP_X
CTU       279  347 eP_-
NLU       266  375 eP_X
DUG       270  435 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. 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)   
ISCO	  106	  186
BW06	  331	  318
JLU	  278	  320
MPU	  266	  336
CTU	  279	  347
HWUT	  296	  357
NLU	  266	  375
DUG	  270	  435
MVU	  244	  436
HVU	  293	  458
RSSD	   34	  523
KNB	  232	  574
HLID	  305	  666
BOZ	  333	  677
CBKS	  101	  703
TUC	  198	  927
WALA	  335	 1093

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:

taper w 0.1
hp c 0.04 3
lp c 0.10 3
br c 0.12 0.20 n 8 p 2


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 Wed Jun 8 19:34:04 CDT 2005