Location

2002/12/09 22:42:50 38.86N 127.26E 8 3.65 Korea (USGS-PDE)

Arrival Times

Focal Mechanism

 SLU Moment Tensor Solution
 2002/12/09 22:42:50 38.86N 127.26E 8 3.65 Korea (USGS-PDE)
 
 Best Fitting Double Couple
    Mo = 3.35e+21 dyne-cm
    Mw = 3.65 
    Z  = 8 km
     Plane   Strike  Dip  Rake
      NP1      295    90   -15
      NP2       25    75   -180
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   3.35e+21     11     341
     N   0.00e+00     75     115
     P  -3.35e+21     11     249



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     2.48e+21
       Mxy    -2.08e+21
       Mxz     7.86e+20
       Myy    -2.48e+21
       Myz     3.66e+20
       Mzz    -2.11e+13
                                                     
                                                     
                                                     
                                                     
                       ############                  
                 ### T ##############--              
              ######   ##############-----           
             #######################-------          
           #########################---------        
          ##########################----------       
         -#########################------------      
        ------#####################-------------     
        ----------################--------------     
       ---------------###########----------------    
       -------------------#######----------------    
       -----------------------##-----------------    
       ------------------------###---------------    
        -   ------------------########----------     
        - P -----------------#############------     
            ----------------##################-      
          -----------------###################       
           --------------####################        
             -----------###################          
              ---------###################           
                 ----##################              
                     ##############                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -2.11e+13   7.86e+20  -3.66e+20 
  7.86e+20   2.48e+21   2.08e+21 
 -3.66e+20   2.08e+21  -2.48e+21 

        

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 = 295
      DIP = 90
     RAKE = -15
       MW = 3.65
       HS = 8

The waveform inversion is preferred. This solution agrees with the surface-wave solution. This solution is well determined.

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 3
lp c 0.10 3
br c 0.14 0.4 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   305    65    35   3.63 0.4638
WVFGRD96    1.0   305    60    30   3.62 0.4635
WVFGRD96    2.0   120    70    25   3.60 0.4875
WVFGRD96    3.0   120    70    25   3.61 0.5099
WVFGRD96    4.0   115    85    20   3.61 0.5320
WVFGRD96    5.0   115    90    20   3.62 0.5457
WVFGRD96    6.0   115    85    15   3.64 0.5540
WVFGRD96    7.0   115    90    15   3.65 0.5590
WVFGRD96    8.0   295    90   -15   3.65 0.5610
WVFGRD96    9.0   115    90    15   3.65 0.5575
WVFGRD96   10.0   115    90    15   3.66 0.5526
WVFGRD96   11.0   115    85    10   3.67 0.5463
WVFGRD96   12.0   115    85    10   3.68 0.5382
WVFGRD96   13.0   115    85    10   3.69 0.5292
WVFGRD96   14.0   115    85    10   3.70 0.5200
WVFGRD96   15.0   115    85    10   3.70 0.5106
WVFGRD96   16.0   115    85    10   3.71 0.5008
WVFGRD96   17.0   115    85    15   3.71 0.4900
WVFGRD96   18.0   115    85    15   3.72 0.4795
WVFGRD96   19.0   115    85    15   3.73 0.4687
WVFGRD96   20.0   115    85    15   3.74 0.4576
WVFGRD96   21.0   115    85    15   3.76 0.4469
WVFGRD96   22.0   115    85    15   3.77 0.4348
WVFGRD96   23.0   115    85    15   3.78 0.4229
WVFGRD96   24.0   115    85    15   3.79 0.4103
WVFGRD96   25.0   290    90   -15   3.81 0.3981

The best solution is

WVFGRD96    8.0   295    90   -15   3.65 0.5610

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 0.03 3
lp c 0.10 3
br c 0.14 0.4 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=     119.99
  DIP=      79.99
 RAKE=      19.99
  
             OR
  
  STK=      26.38
  DIP=      70.32
 RAKE=     169.37
 
 
DEPTH = 6.0 km
 
Mw = 3.68
Best Fit 0.8763 - 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
CHNB      187   59 iP_C
CHC       154  126 iP_C
SEO       190  148 eP_+
DGY       133  178 eP_+
CHJ       162  224 eP_+
SES       197  233 eP_+
TJN       177  269 eP_X
ULJ       139  303 eP_X
DAG       155  369 eP_X
KWJ       183  404 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.
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)   
CHNB	  187	   59
CHC	  154	  126
SEO	  190	  148
DGY	  133	  178
CHJ	  162	  224
SES	  197	  233
BRD	  248	  243
BRDG	  248	  243
HKU	  177	  243
TJN	  177	  269
ULJ	  139	  303
NPR	  186	  308
ULL	  113	  356
DAG	  155	  369
KWJ	  183	  404
BGD	  187	  518
SGP	  186	  618

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 0.03 3
lp c 0.10 3
br c 0.14 0.4 n 4 p 2

Discussion

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 Sep 12 09:31:41 CDT 2005