The ANSS event ID is usp000fmp9 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000fmp9/executive.
2007/09/08 07:15:40 33.697 -108.811 5.0 3.6 New Mexico
USGS/SLU Moment Tensor Solution
ENS 2007/09/08 07:15:40:0 33.70 -108.81 5.0 3.6 New Mexico
Stations used:
TA.115A TA.116A TA.W13A TA.W14A TA.W15A TA.X13A TA.X14A
TA.X15A TA.Y13A TA.Y14A TA.Y22C TA.Z14A US.MVCO US.TPNV
US.WUAZ
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 2.95e+21 dyne-cm
Mw = 3.58
Z = 9 km
Plane Strike Dip Rake
NP1 250 80 65
NP2 140 27 157
Principal Axes:
Axis Value Plunge Azimuth
T 2.95e+21 49 133
N 0.00e+00 25 255
P -2.95e+21 31 0
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.60e+21
Mxy -6.47e+20
Mxz -2.29e+21
Myy 6.83e+20
Myz 1.06e+21
Mzz 9.15e+20
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------------- ------------
-------------- P -------------
#--------------- ---------------
##----------------------------------
##------------------------------------
###-------------------------------######
###-----------------------##############
####-----------------#####################
####------------##########################
#####-------##############################
#####---##################################
####-##################### ###########
#-----#################### T ###########
------################### ##########
------##############################
-------###########################
--------######################
----------##################
-------------######---
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Global CMT Convention Moment Tensor:
R T P
9.15e+20 -2.29e+21 -1.06e+21
-2.29e+21 -1.60e+21 6.47e+20
-1.06e+21 6.47e+20 6.83e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070908071540/index.html
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STK = 250
DIP = 80
RAKE = 65
MW = 3.58
HS = 9.0
The NDK file is 20070908071540.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:
cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 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 1.0 335 60 20 3.26 0.5030
WVFGRD96 2.0 335 50 15 3.37 0.5639
WVFGRD96 3.0 240 90 30 3.39 0.5732
WVFGRD96 4.0 325 20 0 3.56 0.6200
WVFGRD96 5.0 245 85 70 3.55 0.6447
WVFGRD96 6.0 250 80 65 3.53 0.6633
WVFGRD96 7.0 245 85 60 3.51 0.6713
WVFGRD96 8.0 250 80 65 3.58 0.6762
WVFGRD96 9.0 250 80 65 3.58 0.6786
WVFGRD96 10.0 250 80 60 3.58 0.6774
WVFGRD96 11.0 250 80 60 3.58 0.6745
WVFGRD96 12.0 60 90 55 3.56 0.6738
WVFGRD96 13.0 235 80 -50 3.57 0.6768
WVFGRD96 14.0 235 80 -50 3.58 0.6784
WVFGRD96 15.0 235 80 -50 3.59 0.6781
WVFGRD96 16.0 240 85 -50 3.60 0.6765
WVFGRD96 17.0 240 85 -50 3.61 0.6726
WVFGRD96 18.0 240 85 -50 3.62 0.6664
WVFGRD96 19.0 240 85 -50 3.63 0.6597
WVFGRD96 20.0 240 85 -50 3.64 0.6520
WVFGRD96 21.0 65 90 60 3.66 0.6410
WVFGRD96 22.0 65 90 60 3.67 0.6322
WVFGRD96 23.0 245 85 -60 3.68 0.6213
WVFGRD96 24.0 65 90 60 3.70 0.6097
WVFGRD96 25.0 65 90 60 3.71 0.5968
WVFGRD96 26.0 65 90 60 3.72 0.5814
WVFGRD96 27.0 65 90 60 3.73 0.5670
WVFGRD96 28.0 65 90 60 3.74 0.5501
WVFGRD96 29.0 245 90 -60 3.75 0.5328
The best solution is
WVFGRD96 9.0 250 80 65 3.58 0.6786
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
cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 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= 244.99
DIP= 75.00
RAKE= 60.00
OR
STK= 130.84
DIP= 33.23
RAKE= 151.81
DEPTH = 10.0 km
Mw = 3.67
Best Fit 0.9305 - 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. |
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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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