Location

2006/06/20 20:11:18 41.84N 81.17W 5 3.8 Ohio

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Central and Southeastern US

Focal Mechanism

 SLU Moment Tensor Solution
 2006/06/20 20:11:18 41.84N 81.17W 5 3.8 Ohio
 
 Best Fitting Double Couple
    Mo = 2.37e+21 dyne-cm
    Mw = 3.55 
    Z  = 4 km
     Plane   Strike  Dip  Rake
      NP1        5    90   -165
      NP2      275    75     0
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   2.37e+21     11     139
     N   0.00e+00     75       5
     P  -2.37e+21     11     231



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     3.98e+20
       Mxy    -2.26e+21
       Mxz    -5.35e+19
       Myy    -3.98e+20
       Myz     6.11e+20
       Mzz     0.00e+00
                                                     
                                                     
                                                     
                                                     
                     #########-----                  
                 #############---------              
              ###############-------------           
             ################--------------          
           ##################----------------        
          ###################-----------------       
         ####################------------------      
        #####################-------------------     
        ####################--------------------     
       ###------------------########-------------    
       ---------------------###############------    
       ---------------------###################--    
       ---------------------#####################    
        --------------------####################     
        -------------------#####################     
         ------------------####################      
          --   ------------###################       
           - P ------------############   ###        
               ------------############ T #          
              -------------############              
                 ---------#############              
                     -----#########                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  0.00e+00  -5.35e+19  -6.11e+20 
 -5.35e+19   3.98e+20   2.26e+21 
 -6.11e+20   2.26e+21  -3.98e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20060620201118/index.html
        

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 = 275
      DIP = 75
     RAKE = 0
       MW = 3.55
       HS = 4

The surface-wave was not attempted because of the small size. The use of the data from the three closest solutions made this solution possible. In addition the shallow depth permitted the signal to rise above the noise. However, the inversion required the use of higher frequencies than usual in order to see the signal.

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 0.06 n 3
lp c 0.30 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    10    65    25   3.42 0.5593
WVFGRD96    1.0   360    75    15   3.45 0.5802
WVFGRD96    2.0    90    80    10   3.50 0.6091
WVFGRD96    3.0   275    75     0   3.52 0.6263
WVFGRD96    4.0   275    75     0   3.55 0.6278
WVFGRD96    5.0   280    75    -5   3.58 0.6180
WVFGRD96    6.0   280    75    -5   3.60 0.5943
WVFGRD96    7.0   280    70   -10   3.63 0.5567
WVFGRD96    8.0   290    70    15   3.67 0.5110
WVFGRD96    9.0   290    70    15   3.69 0.4510
WVFGRD96   10.0    15    90    25   3.71 0.3860
WVFGRD96   11.0   190    85   -25   3.73 0.3573
WVFGRD96   12.0   180    60   -20   3.76 0.3220
WVFGRD96   13.0   185    85   -25   3.80 0.3633
WVFGRD96   14.0   185    85   -25   3.82 0.3970
WVFGRD96   15.0   360    85   -15   3.85 0.4191
WVFGRD96   16.0   360    85   -15   3.86 0.4536
WVFGRD96   17.0   360    85   -20   3.85 0.4503
WVFGRD96   18.0   360    75    -5   3.85 0.4460
WVFGRD96   19.0    90    75    -5   3.84 0.4776

The best solution is

WVFGRD96    4.0   275    75     0   3.55 0.6278

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

hp c 0.06 n 3
lp c 0.30 n 3
br c 0.12 0.25 n 4 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.

First motion data

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

Sta Az(deg)    Dist(km)   First motion
ALLY      103   93 iP_C
ELFO      357  151 iP_C
TYNO       38  179 iP_D

Surface-Wave Focal Mechanism


  NODAL PLANES 

  
  STK=     174.99
  DIP=      85.00
 RAKE=    -170.00
  
             OR
  
  STK=      84.11
  DIP=      80.04
 RAKE=      -5.08
 
 
DEPTH = 2.0 km
 
Mw = 3.39
Best Fit 0.9376 - 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
ALLY      103   93 iP_C
ELFO      357  151 iP_C
TYNO       38  179 iP_D

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, 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.
Location of broadband stations used to obtain focal mechanism from suraface-wave spectral amplitudes

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

The distribution of broadband stations with azimuth and distance is

Sta Az(deg)    Dist(km)   
ELFO	  357	  151
TYNO	   38	  179
ACTO	   26	  219
BRCO	  356	  268
MEDO	   56	  272
SADO	   27	  367
MVL	  115	  459
KGNO	   54	  468
BRNJ	  101	  573
PAL	   96	  619
CPNY	   98	  620
FOR	   98	  624
ACCN	   72	  644
FRNY	   59	  703

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 0.06 n 3
lp c 0.30 n 3

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

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 CUS 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 Mon Jun 26 08:04:27 CDT 2006