Location (from USGS)

2001/09/05 10:52:07 37.14N 104.47W 5.0 4.3M A COLORADO

Aftershock Survey (USGS)

Investigation of an Earthquake Swarm near Trinidad, Colorado, August-October 2001, By Mark E. Meremonte, John C. Lahr, Arthur D. Frankel, James W. Dewey, Anthony J. Crone, Dee E. Overturf, David L. Carver, and W. Thomas Bice, U.S. Geological Survey Open-File Report 02-0073

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Central and Southeastern 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


  STK=      52.77
  DIP=      71.11
 RAKE=     -73.32
  STK=     189.99
  DIP=      25.00
 RAKE=    -129.99
DEPTH = 2.0 km
Mw = 4.44
Best Fit 0.8772 - P-T axis plot gives solutions with FIT greater than FIT90

Focal Mechanism

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.

Data preparation

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.
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

First motion data

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

Sta Az(deg)    Dist(km)   First motion
ANMO	  217	  302	ee-
ISCO	  342	  312	e-
CBKS	   64	  455	X

Broadband station distributiuon

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

Sta Az(deg)    Dist(km)   
ANMO	  217	  302
ISCO	  342	  312
CBKS	   64	  455
WMOK	  116	  578
WUAZ	  256	  645
BW06	  327	  762
RSSD	    3	  776
HWUT	  311	  787
TUC	  229	  788
DUG	  298	  801
AHID	  320	  843
LTX	  175	  869
LKWY	  331	  964
ELK	  296	 1015
MIAR	  103	 1025
HLID	  313	 1104
BOZ	  330	 1118
HKT	  132	 1130
TPH	  279	 1130
BMN	  292	 1166
CCM	   81	 1172
DAC	  269	 1176
MNV	  281	 1213
JFWS	   58	 1371
MOD	  296	 1458
WVT	   89	 1491
BLO	   76	 1587
WCI	   80	 1600
BLA	   82	 2130
MCWV	   75	 2163

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 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 Tue Jul 20 13:55:18 CDT 2004