Location

The initial location was difficult. However there were two IRIS school seismographs in the area:
http://www.iris.edu/hq/ssn/schools/view/DAIL
http://www.iris.edu/hq/ssn/schools/view/WCIL
http://www.iris.edu/hq/sis/resources/seismometers

that provided two additional P times.

SLU Location - preferred

This fixed depth solution uses the program elocate and secondary phases such as Pg and Lg. The depth is fixed at 10 km to be compatible with the waveform inversion solution. The modified VEL.MOD and the output elocate.txt of the command
elocate -M 4 -D 10 -F -BATCH > elocate.txt
are available here.

The SLU location is

Error Ellipse  X=   1.0596 km  Y= 1.7505 km  Theta = 135.0333 deg

 RMS Error        :               0.387              sec
 Travel_Time_Table:          CUS     
 Latitude         :             41.9720 +-    0.0130 N         1.4465 km
 Longitude        :            -88.4917 +-    0.0176 E         1.4473 km
 Depth            :               10.00 +-      1.50 km
 Epoch Time       :      1265795975.160 +-      0.14 sec
 Event Time       :  20100210095935.160 +-      0.14 sec
 Event (OCAL)     :  2010 02 10 09 59 35 160
 HYPO71 Quality   :                  DB
 Gap              :                  96              deg

This is not the best solution but the epicenter is not in the center of the felt area.

NEIC Location 2010 02 11 00:00UT

  DATE     TIME(UT)     LAT        LON      H   M       Location
2010/02/10 09:59:35    42.029    -88.429   13.8 3.8Mw   ILLINOIS

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  2010/02/10 09:59:35:1  41.97  -88.49  10.0 3.8 Illinois
 
 Stations used:
   TA.BGNE TA.SFIN TA.SPMN US.AAM US.ECSD US.EYMN US.JFWS 
   US.KSU1 US.SCIA 
 
 Filtering commands used:
   taper w 0.1
   hp c 0.02 n 3
   lp c 0.10 n 3
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 6.31e+21 dyne-cm
  Mw = 3.80 
  Z  = 11 km
  Plane   Strike  Dip  Rake
   NP1        9    85   170
   NP2      100    80     5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.31e+21     11     324
    N   0.00e+00     79     163
    P  -6.31e+21      4      55

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.93e+21
       Mxy    -5.85e+21
       Mxz     6.98e+20
       Myy    -2.12e+21
       Myz    -9.85e+20
       Mzz     1.88e+20
                                                     
                                                     
                                                     
                                                     
                     ##########----                  
                   ############--------              
              ## T ############-----------           
             ###   ############------------          
           ####################------------ P        
          #####################------------          
         #####################-----------------      
        ######################------------------     
        ######################------------------     
       --#####################-------------------    
       -------###############--------------------    
       -------------#########--------------------    
       ------------------------------------------    
        --------------------####################     
        --------------------####################     
         -------------------###################      
          -----------------###################       
           ----------------##################        
             --------------################          
              ------------################           
                 ---------#############              
                     ----##########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.88e+20   6.98e+20   9.85e+20 
  6.98e+20   1.93e+21   5.85e+21 
  9.85e+20   5.85e+21  -2.12e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100210095935/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 = 100
      DIP = 80
     RAKE = 5
       MW = 3.80
       HS = 11.0

This is a marginal solution because of the high microseism noise level. The large SH pulses on the transverse component and the small Rauleigh wave are indicative of a strike-slip mechanism in the upper crust.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2010/02/10 09:59:35:1  41.97  -88.49  10.0 3.8 Illinois
 
 Stations used:
   TA.BGNE TA.SFIN TA.SPMN US.AAM US.ECSD US.EYMN US.JFWS 
   US.KSU1 US.SCIA 
 
 Filtering commands used:
   taper w 0.1
   hp c 0.02 n 3
   lp c 0.10 n 3
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 6.31e+21 dyne-cm
  Mw = 3.80 
  Z  = 11 km
  Plane   Strike  Dip  Rake
   NP1        9    85   170
   NP2      100    80     5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.31e+21     11     324
    N   0.00e+00     79     163
    P  -6.31e+21      4      55

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.93e+21
       Mxy    -5.85e+21
       Mxz     6.98e+20
       Myy    -2.12e+21
       Myz    -9.85e+20
       Mzz     1.88e+20
                                                     
                                                     
                                                     
                                                     
                     ##########----                  
                   ############--------              
              ## T ############-----------           
             ###   ############------------          
           ####################------------ P        
          #####################------------          
         #####################-----------------      
        ######################------------------     
        ######################------------------     
       --#####################-------------------    
       -------###############--------------------    
       -------------#########--------------------    
       ------------------------------------------    
        --------------------####################     
        --------------------####################     
         -------------------###################      
          -----------------###################       
           ----------------##################        
             --------------################          
              ------------################           
                 ---------#############              
                     ----##########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.88e+20   6.98e+20   9.85e+20 
  6.98e+20   1.93e+21   5.85e+21 
  9.85e+20   5.85e+21  -2.12e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100210095935/index.html
	

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:

taper w 0.1
hp c 0.02 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 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   105    65    35   3.71 0.2946
WVFGRD96    1.0   105    70    40   3.74 0.3056
WVFGRD96    2.0   105    75    30   3.70 0.3135
WVFGRD96    3.0   105    75    25   3.70 0.3232
WVFGRD96    4.0   100    80    10   3.71 0.3349
WVFGRD96    5.0   100    75     5   3.72 0.3443
WVFGRD96    6.0   100    75     5   3.74 0.3535
WVFGRD96    7.0   100    75     5   3.75 0.3615
WVFGRD96    8.0   100    75     0   3.76 0.3698
WVFGRD96    9.0   100    80     5   3.78 0.3744
WVFGRD96   10.0   100    75     0   3.79 0.3769
WVFGRD96   11.0   100    80     5   3.80 0.3785
WVFGRD96   12.0   100    80     5   3.81 0.3773
WVFGRD96   13.0   100    80     5   3.82 0.3738
WVFGRD96   14.0   100    85     5   3.83 0.3680
WVFGRD96   15.0   100    85     5   3.84 0.3611
WVFGRD96   16.0   280    90   -15   3.84 0.3527
WVFGRD96   17.0   100    85    10   3.85 0.3450
WVFGRD96   18.0   280    90   -15   3.86 0.3363
WVFGRD96   19.0   105    85    15   3.86 0.3297
WVFGRD96   20.0   105    85    25   3.87 0.3219
WVFGRD96   21.0   105    85    25   3.88 0.3138
WVFGRD96   22.0   110    80    30   3.89 0.3046
WVFGRD96   23.0   110    80    35   3.90 0.2982
WVFGRD96   24.0   110    80    35   3.91 0.2919
WVFGRD96   25.0   110    80    35   3.91 0.2847
WVFGRD96   26.0   110    80    40   3.92 0.2783
WVFGRD96   27.0   115    80    45   3.94 0.2720
WVFGRD96   28.0   115    80    45   3.95 0.2666
WVFGRD96   29.0   115    80    50   3.96 0.2606

The best solution is

WVFGRD96   11.0   100    80     5   3.80 0.3785

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 0.02 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2
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.

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. The digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface.

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

Velocity Model

The CUS 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:

DATE=Thu Feb 11 07:48:06 CST 2010

Last Changed 2010/02/10