Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.
The ANSS event ID is usp000j7yp and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000j7yp/executive.
2011/09/12 14:18:34 32.822 -100.871 7.9 4.30 Texas
USGS/SLU Moment Tensor Solution
ENS 2011/09/12 14:18:34:6 32.82 -100.87 7.9 4.3 Texas
Stations used:
TA.233A TA.234A TA.236A TA.333A TA.334A TA.335A TA.435B
TA.436A TA.534A TA.633A TA.634A TA.ABTX TA.MSTX TA.W35A
TA.WHTX TA.X35A TA.Y36A TA.Z36A US.MNTX US.WMOK
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 = 2.85e+21 dyne-cm
Mw = 3.57
Z = 10 km
Plane Strike Dip Rake
NP1 300 70 30
NP2 199 62 157
Principal Axes:
Axis Value Plunge Azimuth
T 2.85e+21 35 162
N 0.00e+00 54 331
P -2.85e+21 5 68
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.32e+21
Mxy -1.56e+21
Mxz -1.37e+21
Myy -2.24e+21
Myz 1.85e+20
Mzz 9.16e+20
#############-
##############--------
###############-------------
###############---------------
###############-------------------
-----##########---------------------
-------------##---------------------
---------------####------------------ P
--------------########---------------
--------------############----------------
--------------###############-------------
-------------##################-----------
-------------####################---------
------------######################------
-----------#########################----
----------##########################--
---------###########################
--------############ ###########
------############ T #########
------########### ########
---###################
##############
Global CMT Convention Moment Tensor:
R T P
9.16e+20 -1.37e+21 -1.85e+20
-1.37e+21 1.32e+21 1.56e+21
-1.85e+20 1.56e+21 -2.24e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110912141834/index.html
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STK = 300
DIP = 70
RAKE = 30
MW = 3.57
HS = 10.0
The NDK file is 20110912141834.ndk The waveform inversion is preferred.
Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.
Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Horizontal components. Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Vertical components (research). Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
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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 0.5 115 80 -15 3.42 0.5932
WVFGRD96 1.0 115 80 -10 3.43 0.6071
WVFGRD96 2.0 295 90 15 3.45 0.6227
WVFGRD96 3.0 295 70 10 3.47 0.6331
WVFGRD96 4.0 115 70 0 3.48 0.6440
WVFGRD96 5.0 110 70 -15 3.49 0.6514
WVFGRD96 6.0 110 70 -10 3.50 0.6583
WVFGRD96 7.0 110 70 -10 3.51 0.6634
WVFGRD96 8.0 110 75 -25 3.53 0.6677
WVFGRD96 9.0 300 70 30 3.55 0.6696
WVFGRD96 10.0 300 70 30 3.57 0.6714
WVFGRD96 11.0 300 70 30 3.57 0.6706
WVFGRD96 12.0 300 75 25 3.57 0.6688
WVFGRD96 13.0 300 75 25 3.58 0.6659
WVFGRD96 14.0 300 75 25 3.59 0.6622
WVFGRD96 15.0 300 75 25 3.60 0.6572
WVFGRD96 16.0 300 75 25 3.60 0.6509
WVFGRD96 17.0 300 80 25 3.61 0.6434
WVFGRD96 18.0 300 80 25 3.62 0.6357
WVFGRD96 19.0 300 80 25 3.63 0.6265
WVFGRD96 20.0 305 75 25 3.65 0.6154
WVFGRD96 21.0 305 80 25 3.66 0.6045
WVFGRD96 22.0 305 80 30 3.68 0.5934
WVFGRD96 23.0 120 85 -40 3.71 0.5815
WVFGRD96 24.0 120 85 -40 3.71 0.5702
WVFGRD96 25.0 120 85 -45 3.73 0.5583
WVFGRD96 26.0 300 90 45 3.73 0.5447
WVFGRD96 27.0 300 90 50 3.75 0.5334
WVFGRD96 28.0 120 85 -50 3.76 0.5227
WVFGRD96 29.0 120 85 -50 3.77 0.5095
The best solution is
WVFGRD96 10.0 300 70 30 3.57 0.6714
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. |
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