Location

2006/07/14 09:34:46 47.00N 68.79W 5. 3.9 Maine

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Eastern US

Focal Mechanism

 SLU Moment Tensor Solution
 2006/07/14 09:34:46 47.00N  68.79W   5. 3.9 Maine
 
 Best Fitting Double Couple
    Mo = 1.86e+21 dyne-cm
    Mw = 3.48 
    Z  = 17 km
     Plane   Strike  Dip  Rake
      NP1       38    73   108
      NP2      170    25    45
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   1.86e+21     58     333
     N   0.00e+00     17     212
     P  -1.86e+21     25     114



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     1.60e+20
       Mxy     3.50e+20
       Mxz     1.03e+21
       Myy    -1.17e+21
       Myz    -1.04e+21
       Mzz     1.01e+21
                                                     
                                                     
                                                     
                                                     
                     -#############                  
                 ---###################              
              ----#######################-           
             ---########################---          
           ----#########################-----        
          ----#########################-------       
         ----##########   ############---------      
        -----########## T ###########-----------     
        ----###########   ##########------------     
       -----#######################--------------    
       -----######################---------------    
       -----#####################----------------    
       -----###################------------------    
        -----#################-----------   ----     
        -----###############------------- P ----     
         -----#############--------------   ---      
          -----##########---------------------       
           -----#######----------------------        
             -----##-----------------------          
              ---###----------------------           
                 #####-----------------              
                     #####---------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  1.01e+21   1.03e+21   1.04e+21 
  1.03e+21   1.60e+20  -3.50e+20 
  1.04e+21  -3.50e+20  -1.17e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20060714093446/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 = 170
      DIP = 25
     RAKE = 45
       MW = 3.48
       HS = 17

This is a small event that is not well recorded. The waveforms solution is preferred because of the many waveforms available from the Canadian National Seismograph Network Station. The surface-wave spectral amplitude solution is affected by the small number of observations, but is reasonably compatible with the direct waveform 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 location of the event and the station distribution used are given in the following figure.
Location of broadband stations used for the waveform inversion

The observed and predicted traces are filtered using the following gsac commands:

hp c 0.025 n 4
lp c 0.10 n 4
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    40    50   -75   3.31 0.3739
WVFGRD96    1.0    40    80   -85   3.63 0.3719
WVFGRD96    2.0   125    10    -5   3.65 0.3821
WVFGRD96    3.0   130    15     5   3.55 0.4045
WVFGRD96    4.0   140    15    10   3.50 0.4204
WVFGRD96    5.0   140    20    15   3.47 0.4335
WVFGRD96    6.0   145    20    20   3.45 0.4444
WVFGRD96    7.0   160    20    40   3.44 0.4526
WVFGRD96    8.0   155    25    35   3.44 0.4613
WVFGRD96    9.0   160    25    45   3.44 0.4677
WVFGRD96   10.0   160    25    40   3.46 0.4731
WVFGRD96   11.0   160    25    40   3.46 0.4787
WVFGRD96   12.0   170    25    50   3.46 0.4830
WVFGRD96   13.0   170    25    50   3.46 0.4864
WVFGRD96   14.0   170    25    45   3.46 0.4888
WVFGRD96   15.0   170    25    45   3.47 0.4908
WVFGRD96   16.0   170    25    45   3.47 0.4917
WVFGRD96   17.0   170    25    45   3.48 0.4919
WVFGRD96   18.0   170    25    45   3.48 0.4915
WVFGRD96   19.0   170    25    45   3.49 0.4906
WVFGRD96   20.0   180    25    55   3.53 0.4892
WVFGRD96   21.0   180    25    55   3.53 0.4870
WVFGRD96   22.0   180    25    55   3.54 0.4840
WVFGRD96   23.0   180    25    50   3.55 0.4804
WVFGRD96   24.0   185    30    55   3.57 0.4761
WVFGRD96   25.0   185    30    55   3.58 0.4712
WVFGRD96   26.0   180    30    50   3.59 0.4654
WVFGRD96   27.0   190    30    60   3.60 0.4593
WVFGRD96   28.0   190    30    65   3.60 0.4524
WVFGRD96   29.0   190    30    60   3.62 0.4453

The best solution is

WVFGRD96   17.0   170    25    45   3.48 0.4919

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 0.025 n 4
lp c 0.10 n 4
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.

Surface-Wave Focal Mechanism


  NODAL PLANES 

  
  STK=     169.99
  DIP=      55.00
 RAKE=      65.00
  
             OR
  
  STK=      29.10
  DIP=      42.07
 RAKE=     121.11
 
 
DEPTH = 20.0 km
 
Mw = 3.61
Best Fit 0.9173 - 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
A21       320  104 iP_D
A16       300  106 eP_X
A11       285  110 iP_D
LMQ       298  131 iP_D
A54       293  133 eP_X
GGN       143  259 iP_D
ICQ        21  303 eP_X
LMN       111  332 iP_D
MNT       248  408 iP_C
NCB       234  542 eP_X
HRV       205  545 eP_X
KGNO      246  675 eP_X
SADO      257  840 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.

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.

The location of the event and the station distribution used for the surface-wave spectral amplitude technique are given in the following figure.
Location of broadband stations used to obtain focal mechanism

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)   
A21	  320	  104
A16	  300	  106
A11	  285	  110
LMQ	  298	  131
GGN	  143	  259
ICQ	   21	  303
LMN	  111	  332
MNT	  248	  408
NCB	  234	  542
OTT	  254	  564
KGNO	  246	  675
VLDQ	  284	  687
PECO	  245	  726
BUKO	  262	  836
SADO	  257	  840
MALO	  297	  877
SCHQ	    8	  883
ACTO	  251	  960
TOBO	  263	 1003
SSPA	  229	 1017
KAPO	  290	 1054
SDMD	  221	 1066
ELFO	  251	 1072
EYMN	  282	 1710
WCI	  242	 1730
USIN	  244	 1841

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 0.025 n 4
lp c 0.10 n 4
br c 0.12 0.25 n 4 p 2

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 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

Last Changed Mon Jul 17 11:20:52 CDT 2006