Location

2011/08/23 07:17:58 37.099 -104.610 5 3.80 Colorado

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports main page

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2011/08/23 07:17:58:7  37.10 -104.61   5.0 3.8 Colorado
 
 Stations used:
   IW.SMCO TA.MSTX TA.O20A TA.Q24A TA.S22A TA.T25A TA.U33A 
   TA.X32A US.AMTX US.CBKS US.ISCO US.MVCO US.SDCO UU.RDMU 
 
 Filtering commands used:
   hp c 0.03 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 2.40e+21 dyne-cm
  Mw = 3.52 
  Z  = 4 km
  Plane   Strike  Dip  Rake
   NP1       30    70   -92
   NP2      215    20   -85
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.40e+21     25     121
    N   0.00e+00      2      30
    P  -2.40e+21     65     297

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.38e+20
       Mxy    -6.97e+20
       Mxz    -8.89e+20
       Myy     1.10e+21
       Myz     1.61e+21
       Mzz    -1.54e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 #######--------------#              
              #######-----------------####           
             #####--------------------#####          
           #####----------------------#######        
          #####-----------------------########       
         ####------------------------##########      
        ####-------------------------###########     
        ####---------   ------------############     
       ####---------- P -----------##############    
       ####----------   ----------###############    
       ###-----------------------################    
       ###----------------------#################    
        ##---------------------#################     
        ###-------------------##########   #####     
         ##------------------########### T ####      
          ##---------------#############   ###       
           #--------------###################        
             #----------###################          
              #-------####################           
                 --####################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.54e+21  -8.89e+20  -1.61e+21 
 -8.89e+20   4.38e+20   6.97e+20 
 -1.61e+21   6.97e+20   1.10e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110823071758/index.html
        

Preferred Solution

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

      STK = 215
      DIP = 20
     RAKE = -85
       MW = 3.52
       HS = 4.0

The NDK file is 20110823071758.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2011/08/23 07:17:58:7  37.10 -104.61   5.0 3.8 Colorado
 
 Stations used:
   IW.SMCO TA.MSTX TA.O20A TA.Q24A TA.S22A TA.T25A TA.U33A 
   TA.X32A US.AMTX US.CBKS US.ISCO US.MVCO US.SDCO UU.RDMU 
 
 Filtering commands used:
   hp c 0.03 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 2.40e+21 dyne-cm
  Mw = 3.52 
  Z  = 4 km
  Plane   Strike  Dip  Rake
   NP1       30    70   -92
   NP2      215    20   -85
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.40e+21     25     121
    N   0.00e+00      2      30
    P  -2.40e+21     65     297

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.38e+20
       Mxy    -6.97e+20
       Mxz    -8.89e+20
       Myy     1.10e+21
       Myz     1.61e+21
       Mzz    -1.54e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 #######--------------#              
              #######-----------------####           
             #####--------------------#####          
           #####----------------------#######        
          #####-----------------------########       
         ####------------------------##########      
        ####-------------------------###########     
        ####---------   ------------############     
       ####---------- P -----------##############    
       ####----------   ----------###############    
       ###-----------------------################    
       ###----------------------#################    
        ##---------------------#################     
        ###-------------------##########   #####     
         ##------------------########### T ####      
          ##---------------#############   ###       
           #--------------###################        
             #----------###################          
              #-------####################           
                 --####################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.54e+21  -8.89e+20  -1.61e+21 
 -8.89e+20   4.38e+20   6.97e+20 
 -1.61e+21   6.97e+20   1.10e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110823071758/index.html
	

Magnitudes

mLg Magnitude


(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.

ML Magnitude


(a) ML computed using the IASPEI formula for Horizontal components; (b) ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.


(a) ML computed using the IASPEI formula for Vertical components (research); (b) ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.

Context

The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors).

Waveform Inversion

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.
Location of broadband stations used for 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 0.03 n 3
lp c 0.06 n 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   245    10   -25   3.74 0.4882
WVFGRD96    1.0   250    15   -20   3.69 0.4948
WVFGRD96    2.0   235    15   -50   3.58 0.5153
WVFGRD96    3.0   220    15   -75   3.53 0.5354
WVFGRD96    4.0   215    20   -85   3.52 0.5483
WVFGRD96    5.0   215    25   -90   3.52 0.5445
WVFGRD96    6.0    40    65   -85   3.51 0.5379
WVFGRD96    7.0    35    65   -85   3.48 0.5147
WVFGRD96    8.0    40    65   -80   3.47 0.4876
WVFGRD96    9.0    40    70   -75   3.43 0.4597
WVFGRD96   10.0    40    70   -75   3.45 0.4446
WVFGRD96   11.0   275    55    55   3.56 0.4278
WVFGRD96   12.0   275    50    55   3.56 0.4241
WVFGRD96   13.0   275    50    55   3.57 0.4189
WVFGRD96   14.0   275    50    50   3.56 0.4108
WVFGRD96   15.0   275    50    50   3.56 0.4025
WVFGRD96   16.0   270    50    45   3.56 0.3942
WVFGRD96   17.0   275    45    50   3.56 0.3865
WVFGRD96   18.0   275    45    50   3.57 0.3788
WVFGRD96   19.0   110    70    55   3.59 0.3740
WVFGRD96   20.0   110    70    55   3.62 0.3702
WVFGRD96   21.0   110    70    55   3.63 0.3661
WVFGRD96   22.0   105    75    55   3.64 0.3622
WVFGRD96   23.0   105    75    50   3.65 0.3582
WVFGRD96   24.0   105    75    50   3.65 0.3529
WVFGRD96   25.0   105    75    50   3.66 0.3460
WVFGRD96   26.0   105    75    50   3.67 0.3379
WVFGRD96   27.0   100    80    45   3.68 0.3330
WVFGRD96   28.0   215    65   -65   3.53 0.3311
WVFGRD96   29.0   210    65   -70   3.53 0.3331

The best solution is

WVFGRD96    4.0   215    20   -85   3.52 0.5483

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 component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number 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 lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).

The bandpass filter used in the processing and for the display was

hp c 0.03 n 3
lp c 0.06 n 3
Figure 3. Waveform comparison for selected depth
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.

A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:

Assuming only a mislocation, the time shifts are fit to a functional form:

 Time_shift = A + B cos Azimuth + C Sin Azimuth

The time shifts for this inversion lead to the next figure:

The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.

Discussion

Acknowledgements

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Iris stations and the Transportable Array of EarthScope.

Velocity Model

The CUS model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

MODEL.01
CUS Model with Q from simple gamma values
ISOTROPIC
KGS
FLAT EARTH
1-D
CONSTANT VELOCITY
LINE08
LINE09
LINE10
LINE11
  H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC)   QP   QS  ETAP  ETAS  FREFP  FREFS
  1.0000  5.0000  2.8900  2.5000 0.172E-02 0.387E-02 0.00  0.00  1.00  1.00 
  9.0000  6.1000  3.5200  2.7300 0.160E-02 0.363E-02 0.00  0.00  1.00  1.00 
 10.0000  6.4000  3.7000  2.8200 0.149E-02 0.336E-02 0.00  0.00  1.00  1.00 
 20.0000  6.7000  3.8700  2.9020 0.000E-04 0.000E-04 0.00  0.00  1.00  1.00 
  0.0000  8.1500  4.7000  3.3640 0.194E-02 0.431E-02 0.00  0.00  1.00  1.00 

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 Dec 6 21:39:58 CST 2015