2004/01/05 16:49:41 38.7N 125.1E 10 3.33 Korea
SLU Moment Tensor Solution
2004/01/05 16:49:41 38.7N 125.1E 10 3.33 Korea
Best Fitting Double Couple
Mo = 1.11e+21 dyne-cm
Mw = 3.33
Z = 10 km
Plane Strike Dip Rake
NP1 130 65 65
NP2 358 35 132
Principal Axes:
Axis Value Plunge Azimuth
T 1.11e+21 62 1
N 0.00e+00 23 141
P -1.11e+21 16 238
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.35e+19
Mxy -4.53e+20
Mxz 6.22e+20
Myy -7.37e+20
Myz 2.64e+20
Mzz 7.70e+20
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---##########################-------
-----############# ##########-------
------############# T ###########-------
-------############ ###########-------
----------########################--------
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--- ------------###############-------
-- P ---------------###########-------
- ------------------########------
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Harvard Convention
Moment Tensor:
R T F
7.70e+20 6.22e+20 -2.64e+20
6.22e+20 -3.35e+19 4.53e+20
-2.64e+20 4.53e+20 -7.37e+20
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The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
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STK = 130
DIP = 65
RAKE = 65
MW = 3.33
HS = 10
The waveform inversion with and the surface-wave solution give similar solutions. However, the surface-wave solution may have better depth control because of the low Rayleigh wave amplitudes on the Z and R waveforms. This solution is reasonably determined.
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 0.05 3 lp c 0.20 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 335 45 90 3.13 0.3177
WVFGRD96 1.0 325 45 90 3.21 0.3245
WVFGRD96 2.0 270 80 -30 3.17 0.2972
WVFGRD96 3.0 280 85 -40 3.16 0.3372
WVFGRD96 4.0 130 70 45 3.18 0.3804
WVFGRD96 5.0 120 75 40 3.21 0.4196
WVFGRD96 6.0 125 75 40 3.23 0.4493
WVFGRD96 7.0 125 75 40 3.26 0.4704
WVFGRD96 8.0 125 75 45 3.28 0.4843
WVFGRD96 9.0 130 65 50 3.31 0.4935
WVFGRD96 10.0 130 65 50 3.33 0.4975
WVFGRD96 11.0 130 65 50 3.35 0.4959
WVFGRD96 12.0 130 70 55 3.36 0.4904
WVFGRD96 13.0 130 65 55 3.38 0.4824
WVFGRD96 14.0 135 65 60 3.40 0.4705
WVFGRD96 15.0 135 60 60 3.41 0.4545
WVFGRD96 16.0 130 65 60 3.42 0.4351
WVFGRD96 17.0 140 65 70 3.45 0.4139
WVFGRD96 18.0 140 65 70 3.46 0.3934
WVFGRD96 19.0 140 65 70 3.46 0.3714
WVFGRD96 20.0 140 65 70 3.47 0.3492
WVFGRD96 21.0 140 65 70 3.47 0.3262
WVFGRD96 22.0 135 65 70 3.48 0.3064
WVFGRD96 23.0 145 65 75 3.49 0.2894
WVFGRD96 24.0 145 65 75 3.49 0.2757
WVFGRD96 25.0 325 55 65 3.47 0.2830
The best solution is
WVFGRD96 10.0 130 65 50 3.33 0.4975
The mechanism correspond to the best fit is
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The best fit as a function of depth is given in the following figure:
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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 0.05 3 lp c 0.20 3
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| 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. |
NODAL PLANES
STK= 21.66
DIP= 60.50
RAKE= 132.39
OR
STK= 139.99
DIP= 50.00
RAKE= 40.00
DEPTH = 6.0 km
Mw = 3.30
Best Fit 0.8747 - P-T axis plot gives solutions with FIT greater than FIT90
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The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion BRD 207 91 iP_- SEO 130 209 eP_X SES 150 243 eP_+ CHC 112 259 eP_X DGY 109 333 iP_+
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.
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.
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| 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. |
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| 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. |
The distribution of broadband stations with azimuth and distance is
Sta Az(deg) Dist(km) BRD 207 91 SEO 130 209 SES 150 243 CHC 112 259 DGY 109 333 KWJ 156 427 ULJ 119 440 DAG 133 468
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:
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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 0.02 3 lp c 0.10 3 br c 0.13 0.2 n 4 p 2
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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.
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Here we tabulate the reasons for not using certain digital data sets
