2008/07/08 16:08:34 39.4850 73.1350 40.0 5.20
USGS Felt map for this earthquake
USGS/SLU Moment Tensor Solution
2008/07/08 16:08:34 39.4850 73.1350 40.0 5.20
Best Fitting Double Couple
Mo = 8.61e+22 dyne-cm
Mw = 4.59
Z = 12 km
Plane Strike Dip Rake
NP1 190 65 90
NP2 10 25 90
Principal Axes:
Axis Value Plunge Azimuth
T 8.61e+22 70 100
N 0.00e+00 -0 190
P -8.61e+22 20 280
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.99e+21
Mxy 1.13e+22
Mxz -9.61e+21
Myy -6.40e+22
Myz 5.45e+22
Mzz 6.60e+22
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-- ----------#####################----
--- P ---------######################-----
--- ---------########## #########-----
--------------########### T #########-----
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Harvard Convention
Moment Tensor:
R T F
6.60e+22 -9.61e+21 -5.45e+22
-9.61e+21 -1.99e+21 -1.13e+22
-5.45e+22 -1.13e+22 -6.40e+22
Details of the solution is found at
http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20080708160834/index.html
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STK = 190
DIP = 65
RAKE = 90
MW = 4.59
HS = 12
The surface-wave is preferred. Rake is least well determined.
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.
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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 0.02 n 3 lp c 0.05 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
WVFGRD96 0.0 0 0 0 0.00-2.0000
The best solution is
WVFGRD96 0.0 0 0 0 0.00-2.0000
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 0.02 n 3 lp c 0.05 n 3 br c 0.12 0.25 n 4 p 2
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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. |
The following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
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The surface-wave determined focal mechanism is shown here.
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The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion KZA 31 339 -12345 AAK 18 368 -12345 KBK 23 384 -12345 USP 15 435 -12345 KKAR 332 458 -12345
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)
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:
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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.
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
The VELOCITY_MODEL used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
DATE=Fri Jul 11 09:16:44 MDT 2008