The ANSS event ID is usb000h79t and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usb000h79t/executive.
2013/05/28 04:36:00 56.198 -120.571 5.0 4.2 British Columbia
USGS/SLU Moment Tensor Solution
ENS 2013/05/28 04:36:00:0 56.20 -120.57 5.0 4.2 British Columbia
Stations used:
AK.JIS CN.BBB CN.BCBC CN.DLBC CN.EDZN CN.FNBB CN.LLLB
CN.MOBC CN.PGC CN.PHC CN.PNT CN.SHB CN.SLEB CN.UBRB CN.WHY
CN.WSLR CN.YKW3 US.WRAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.05 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 1.22e+22 dyne-cm
Mw = 3.99
Z = 2 km
Plane Strike Dip Rake
NP1 314 46 100
NP2 120 45 80
Principal Axes:
Axis Value Plunge Azimuth
T 1.22e+22 83 304
N 0.00e+00 7 127
P -1.22e+22 0 37
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.69e+21
Mxy -5.93e+21
Mxz 7.47e+20
Myy -4.29e+21
Myz -1.29e+21
Mzz 1.20e+22
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-------------------------- P
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-#########################------------
--###########################-----------
---############################---------
----############## ############---------
-----############# T #############--------
------############ ##############-------
-------#############################------
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Global CMT Convention Moment Tensor:
R T P
1.20e+22 7.47e+20 1.29e+21
7.47e+20 -7.69e+21 5.93e+21
1.29e+21 5.93e+21 -4.29e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130528043600/index.html
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STK = 120
DIP = 45
RAKE = 80
MW = 3.99
HS = 2.0
The NDK file is 20130528043600.ndk The waveform inversion is preferred.
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The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) 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's 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 are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 115 45 70 3.92 0.6238
WVFGRD96 1.0 315 45 100 3.95 0.6252
WVFGRD96 2.0 120 45 80 3.99 0.6337
WVFGRD96 3.0 305 35 90 4.07 0.6191
WVFGRD96 4.0 120 70 85 4.18 0.5913
WVFGRD96 5.0 120 70 85 4.18 0.5874
WVFGRD96 6.0 120 70 85 4.16 0.5794
WVFGRD96 7.0 120 65 85 4.16 0.5678
WVFGRD96 8.0 120 70 85 4.19 0.5748
WVFGRD96 9.0 310 20 95 4.18 0.5649
WVFGRD96 10.0 105 15 60 4.15 0.5553
WVFGRD96 11.0 95 15 50 4.13 0.5511
WVFGRD96 12.0 55 20 5 4.12 0.5500
WVFGRD96 13.0 40 20 -10 4.11 0.5522
WVFGRD96 14.0 30 20 -20 4.11 0.5551
WVFGRD96 15.0 15 20 -40 4.12 0.5593
WVFGRD96 16.0 140 65 -85 4.14 0.5659
WVFGRD96 17.0 140 60 -80 4.16 0.5770
WVFGRD96 18.0 140 60 -80 4.15 0.5863
WVFGRD96 19.0 140 60 -80 4.15 0.5933
WVFGRD96 20.0 140 60 -75 4.16 0.5969
WVFGRD96 21.0 140 60 -75 4.16 0.5979
WVFGRD96 22.0 140 60 -75 4.16 0.5996
WVFGRD96 23.0 140 55 -75 4.17 0.5997
WVFGRD96 24.0 140 55 -75 4.17 0.5986
WVFGRD96 25.0 140 55 -75 4.17 0.5961
WVFGRD96 26.0 140 55 -75 4.17 0.5931
WVFGRD96 27.0 145 55 -70 4.17 0.5891
WVFGRD96 28.0 145 55 -65 4.18 0.5855
WVFGRD96 29.0 145 55 -65 4.18 0.5814
The best solution is
WVFGRD96 2.0 120 45 80 3.99 0.6337
The mechanism corresponding 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 component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number 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, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. 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 lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).
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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| Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample. |
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| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. 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. |
A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:
Time_shift = A + B cos Azimuth + C Sin Azimuth
The time shifts for this inversion lead to the next figure:
The derived shift in origin time and epicentral coordinates are given at the bottom of 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.
NODAL PLANES
STK= 119.99
DIP= 50.00
RAKE= 80.00
OR
STK= 315.33
DIP= 41.03
RAKE= 101.70
DEPTH = 2.0 km
Mw = 4.09
Best Fit 0.8844 - P-T axis plot gives solutions with FIT greater than FIT90
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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 next set of figures compares the observed and predicted surface wave radiation patterns as a function of period. The observed spectral amplitude have been corrected for anleastic attenuation by multiplication by a factor of eγr and corrected for surface wave geometrical spreading to a reference distance of 1000km. If the theoretical γ values are not perfect, then observations at large distance will plot as outliers.
Given the mechanisms and depth, the observed spectral amplitudes can be compared to the theoretical predictions of a model with infinite Q. The difference can be used to estimated the anelastic attenuation coefficent at each period according to the model
Obs = e-γ r PreThe γ values can be use to invert for the Q structure. The next figure compares the network average group velocities to those of the model and as well as the γ derived from this data set and that of the model.
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The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).
MODEL.01
Model after 8 iterations
ISOTROPIC
KGS
FLAT EARTH
1-D
CONSTANT VELOCITY
LINE08
LINE09
LINE10
LINE11
H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC) QP QS ETAP ETAS FREFP FREFS
1.9000 3.4065 2.0089 2.2150 0.302E-02 0.679E-02 0.00 0.00 1.00 1.00
6.1000 5.5445 3.2953 2.6089 0.349E-02 0.784E-02 0.00 0.00 1.00 1.00
13.0000 6.2708 3.7396 2.7812 0.212E-02 0.476E-02 0.00 0.00 1.00 1.00
19.0000 6.4075 3.7680 2.8223 0.111E-02 0.249E-02 0.00 0.00 1.00 1.00
0.0000 7.9000 4.6200 3.2760 0.164E-10 0.370E-10 0.00 0.00 1.00 1.00