The ANSS event ID is usp000ghd9 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000ghd9/executive.
2008/09/20 05:16:11 63.588 -129.029 10.0 5.2 NWT, Canada
USGS/SLU Moment Tensor Solution
ENS 2008/09/20 05:16:11:0 63.59 -129.03 10.0 5.2 NWT, Canada
Stations used:
AK.PNL AT.SKAG CN.CTLN CN.DAWY CN.DLBC CN.FNBB CN.GALN
CN.ILKN CN.INK US.EGAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 6.68e+23 dyne-cm
Mw = 5.15
Z = 12 km
Plane Strike Dip Rake
NP1 250 75 20
NP2 155 71 164
Principal Axes:
Axis Value Plunge Azimuth
T 6.68e+23 25 113
N 0.00e+00 65 285
P -6.68e+23 3 22
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.91e+23
Mxy -4.28e+23
Mxz -1.30e+23
Myy 3.77e+23
Myz 2.20e+23
Mzz 1.14e+23
------------ P
##-------------- ---
#####-----------------------
######------------------------
########--------------------------
#########---------------------------
###########---------------------------
############-----------------###########
#############---------##################
###############---########################
#############--###########################
##########------##########################
#######----------#########################
###--------------################ ####
#-----------------############### T ####
------------------############## ###
------------------##################
-------------------###############
------------------############
-------------------#########
------------------####
--------------
Global CMT Convention Moment Tensor:
R T P
1.14e+23 -1.30e+23 -2.20e+23
-1.30e+23 -4.91e+23 4.28e+23
-2.20e+23 4.28e+23 3.77e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080920051611/index.html
|
STK = 250
DIP = 75
RAKE = 20
MW = 5.15
HS = 12.0
The NDK file is 20080920051611.ndk The waveform inversion is preferred.
|
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.
|
|
|
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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 65 80 -25 4.95 0.4615
WVFGRD96 1.0 65 80 -25 4.98 0.4757
WVFGRD96 2.0 60 70 -10 5.03 0.4859
WVFGRD96 3.0 60 80 20 5.07 0.4905
WVFGRD96 4.0 245 75 30 5.08 0.5024
WVFGRD96 5.0 245 75 25 5.09 0.5180
WVFGRD96 6.0 245 75 25 5.10 0.5303
WVFGRD96 7.0 250 70 25 5.09 0.5358
WVFGRD96 8.0 245 75 20 5.12 0.5514
WVFGRD96 9.0 250 75 20 5.11 0.5570
WVFGRD96 10.0 245 75 20 5.15 0.5709
WVFGRD96 11.0 250 75 20 5.14 0.5706
WVFGRD96 12.0 250 75 20 5.15 0.5724
WVFGRD96 13.0 250 75 15 5.16 0.5714
WVFGRD96 14.0 250 75 15 5.17 0.5656
WVFGRD96 15.0 250 75 15 5.18 0.5631
WVFGRD96 16.0 250 75 15 5.19 0.5538
WVFGRD96 17.0 250 75 15 5.20 0.5429
WVFGRD96 18.0 245 80 15 5.23 0.5349
WVFGRD96 19.0 245 80 15 5.24 0.5200
WVFGRD96 20.0 240 85 15 5.28 0.5053
WVFGRD96 21.0 245 80 15 5.26 0.4905
WVFGRD96 22.0 65 75 0 5.25 0.4760
WVFGRD96 23.0 60 75 -5 5.28 0.4612
WVFGRD96 24.0 60 75 -10 5.28 0.4468
WVFGRD96 25.0 60 75 -10 5.28 0.4320
WVFGRD96 26.0 60 75 -10 5.28 0.4208
WVFGRD96 27.0 60 75 -10 5.29 0.4096
WVFGRD96 28.0 60 75 -10 5.29 0.3988
WVFGRD96 29.0 60 75 -10 5.30 0.3886
WVFGRD96 30.0 60 75 -10 5.30 0.3787
WVFGRD96 31.0 330 80 10 5.32 0.3680
WVFGRD96 32.0 330 80 10 5.33 0.3614
WVFGRD96 33.0 330 80 10 5.34 0.3540
WVFGRD96 34.0 330 80 10 5.35 0.3462
WVFGRD96 35.0 330 80 10 5.36 0.3378
WVFGRD96 36.0 330 80 10 5.37 0.3295
WVFGRD96 37.0 60 75 -10 5.37 0.3229
WVFGRD96 38.0 60 80 -10 5.39 0.3168
WVFGRD96 39.0 60 80 -5 5.42 0.3107
WVFGRD96 40.0 60 75 -10 5.45 0.3041
WVFGRD96 41.0 60 75 -5 5.47 0.2998
WVFGRD96 42.0 60 80 -10 5.48 0.2948
WVFGRD96 43.0 60 80 5 5.50 0.2890
WVFGRD96 44.0 60 80 5 5.51 0.2828
WVFGRD96 45.0 60 80 5 5.52 0.2761
WVFGRD96 46.0 65 80 10 5.50 0.2690
WVFGRD96 47.0 65 80 10 5.51 0.2634
WVFGRD96 48.0 60 85 5 5.55 0.2576
WVFGRD96 49.0 240 90 -10 5.55 0.2504
WVFGRD96 50.0 60 85 5 5.56 0.2454
The best solution is
WVFGRD96 12.0 250 75 20 5.15 0.5724
The mechanism corresponding to the best fit is
|
|
|
The best fit as a function of depth is given in the following figure:
|
|
|
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 0.10 n 3
|
| 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. |
|
| 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.
|
|
|
The surface-wave determined focal mechanism is shown here.
NODAL PLANES
STK= 249.99
DIP= 75.00
RAKE= 19.99
OR
STK= 154.61
DIP= 70.72
RAKE= 164.08
DEPTH = 11.0 km
Mw = 5.21
Best Fit 0.8420 - P-T axis plot gives solutions with FIT greater than FIT90
 |
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.
|
|
|
|
| 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. |
|
|
The CUS.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 CUS Model with Q from simple gamma values 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 1.0000 5.0000 2.8900 2.5000 0.172E-02 0.387E-02 0.00 0.00 1.00 1.00 9.0000 6.1000 3.5200 2.7300 0.160E-02 0.363E-02 0.00 0.00 1.00 1.00 10.0000 6.4000 3.7000 2.8200 0.149E-02 0.336E-02 0.00 0.00 1.00 1.00 20.0000 6.7000 3.8700 2.9020 0.000E-04 0.000E-04 0.00 0.00 1.00 1.00 0.0000 8.1500 4.7000 3.3640 0.194E-02 0.431E-02 0.00 0.00 1.00 1.00
