1997/06/25 18:50:21 35.82N 129.19E 10 4.3 Korea
SLU Moment Tensor Solution
1997/06/25 18:50:21 35.82N 129.19E 10 4.3 Korea
Best Fitting Double Couple
Mo = 3.63e+22 dyne-cm
Mw = 4.34
Z = 11 km
Plane Strike Dip Rake
NP1 18 62 139
NP2 130 55 35
Principal Axes:
Axis Value Plunge Azimuth
T 3.63e+22 48 341
N 0.00e+00 42 169
P -3.63e+22 4 76
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.25e+22
Mxy -1.39e+22
Mxz 1.64e+22
Myy -3.21e+22
Myz -8.49e+21
Mzz 1.96e+22
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-########### ###########--------
--########### T ###########---------
---########### ###########----------
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------######################---------- P
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Harvard Convention
Moment Tensor:
R T F
1.96e+22 1.64e+22 8.49e+21
1.64e+22 1.25e+22 1.39e+22
8.49e+21 1.39e+22 -3.21e+22
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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 = 55
RAKE = 35
MW = 4.34
HS = 11
The waveform inversion is preferred. This solution agrees with the surface-wave solution. This solution is well determined. This earthquake was studied by Kim and Kraeva (1999) source parameter determination of local earthquakes in Korea using moment tensor inversion of single station data, Bull. Seism. Soc. Am. 89, 1077-1083. their solution was based on the inversion of the long period VH trace at INCN and gave Strike=135, Dip=52, Rake=22, Depth =7 and Mw=4.8. Our data set has little depth control, but the mechanism is similar to ours. However their moment magnitude is too large, because they used an incorrect instrument response.
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 0.01 3 lp c 0.025 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 305 55 5 4.22 0.3820
WVFGRD96 1.0 305 55 5 4.23 0.3884
WVFGRD96 2.0 305 45 5 4.29 0.4001
WVFGRD96 3.0 305 45 5 4.30 0.4091
WVFGRD96 4.0 305 50 10 4.29 0.4179
WVFGRD96 5.0 305 50 10 4.30 0.4268
WVFGRD96 6.0 310 50 25 4.31 0.4361
WVFGRD96 7.0 310 50 25 4.32 0.4453
WVFGRD96 8.0 130 55 35 4.33 0.4525
WVFGRD96 9.0 130 55 35 4.33 0.4591
WVFGRD96 10.0 130 55 35 4.34 0.4625
WVFGRD96 11.0 130 55 35 4.34 0.4636
WVFGRD96 12.0 130 55 35 4.34 0.4629
WVFGRD96 13.0 125 60 30 4.33 0.4613
WVFGRD96 14.0 125 60 25 4.33 0.4605
WVFGRD96 15.0 125 60 25 4.33 0.4599
WVFGRD96 16.0 125 60 25 4.34 0.4589
WVFGRD96 17.0 125 60 25 4.34 0.4569
WVFGRD96 18.0 125 60 25 4.35 0.4546
WVFGRD96 19.0 120 70 20 4.34 0.4536
The best solution is
WVFGRD96 11.0 130 55 35 4.34 0.4636
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 0.01 3 lp c 0.025 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= 21.67
DIP= 65.82
RAKE= 141.04
OR
STK= 129.99
DIP= 55.00
RAKE= 30.00
DEPTH = 17.0 km
Mw = 4.45
Best Fit 0.9374 - 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 SSE 237 909 eP_X MDJ 2 978 eP_X BJT 296 1235 eP_X
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) SSE 237 909 MDJ 2 978 BJT 296 1235
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 0.01 3 lp c 0.025 3
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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
