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 usp000hwz2 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000hwz2/executive.
2011/03/13 20:16:21 32.995 -100.767 5.0 3.8 Texas
USGS/SLU Moment Tensor Solution
ENS 2011/03/13 20:16:21:0 32.99 -100.77 5.0 3.8 Texas
Stations used:
IM.TX31 IU.ANMO TA.133A TA.134A TA.135A TA.137A TA.233A
TA.234A TA.237A TA.333A TA.334A TA.335A TA.336A TA.433A
TA.434A TA.435B TA.533A TA.534A TA.535A TA.633A TA.634A
TA.636A TA.832A TA.833A TA.ABTX TA.MSTX TA.S30A TA.TUL1
TA.V32A TA.V36A TA.WHTX TA.X34A TA.X35A TA.Y33A TA.Y34A
TA.Y35A TA.Y36A TA.Y37A TA.Z33A TA.Z34A TA.Z35A TA.Z36A
TA.Z37A US.AMTX US.JCT US.MNTX X4.GC09 X4.GC14 X4.GC15
X4.GC18 XR.SC04 XR.SC50
Filtering commands used:
hp c 0.02 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 6.76e+21 dyne-cm
Mw = 3.82
Z = 2 km
Plane Strike Dip Rake
NP1 70 60 -50
NP2 191 48 -138
Principal Axes:
Axis Value Plunge Azimuth
T 6.76e+21 7 133
N 0.00e+00 34 227
P -6.76e+21 55 33
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.54e+21
Mxy -4.32e+21
Mxz -3.18e+21
Myy 2.94e+21
Myz -1.16e+21
Mzz -4.49e+21
#########-----
##########------------
###########-----------------
##########--------------------
###########-----------------------
###########-------------------------
###########----------- -------------
###########------------ P -------------#
##########------------- ------------##
###########---------------------------####
###########-------------------------######
##########-------------------------#######
##########----------------------##########
#########-------------------############
#########---------------################
--#######---------####################
--------############################
--------###################### #
------###################### T
------#####################
----##################
--############
Global CMT Convention Moment Tensor:
R T P
-4.49e+21 -3.18e+21 1.16e+21
-3.18e+21 1.54e+21 4.32e+21
1.16e+21 4.32e+21 2.94e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110313201621/index.html
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STK = 70
DIP = 60
RAKE = -50
MW = 3.82
HS = 2.0
The NDK file is 20110313201621.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:
hp c 0.02 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 80 70 -35 3.72 0.3825
WVFGRD96 1.0 75 65 -40 3.75 0.3983
WVFGRD96 2.0 70 60 -50 3.82 0.4136
WVFGRD96 3.0 70 65 -55 3.86 0.3980
WVFGRD96 4.0 70 70 -55 3.88 0.3788
WVFGRD96 5.0 80 85 -45 3.85 0.3710
WVFGRD96 6.0 270 70 30 3.83 0.3734
WVFGRD96 7.0 270 70 30 3.83 0.3792
WVFGRD96 8.0 270 70 30 3.84 0.3833
WVFGRD96 9.0 270 70 30 3.84 0.3858
WVFGRD96 10.0 270 70 35 3.86 0.3838
WVFGRD96 11.0 95 65 40 3.87 0.3859
WVFGRD96 12.0 95 65 40 3.88 0.3870
WVFGRD96 13.0 95 65 40 3.88 0.3870
WVFGRD96 14.0 95 65 40 3.89 0.3861
WVFGRD96 15.0 95 65 40 3.90 0.3845
WVFGRD96 16.0 95 65 40 3.91 0.3824
WVFGRD96 17.0 95 65 40 3.91 0.3799
WVFGRD96 18.0 95 65 40 3.92 0.3771
WVFGRD96 19.0 270 70 40 3.93 0.3722
WVFGRD96 20.0 95 65 45 3.96 0.3693
WVFGRD96 21.0 95 65 45 3.97 0.3651
WVFGRD96 22.0 95 65 45 3.98 0.3603
WVFGRD96 23.0 275 65 50 3.99 0.3581
WVFGRD96 24.0 275 65 50 4.00 0.3540
WVFGRD96 25.0 275 65 50 4.01 0.3495
WVFGRD96 26.0 275 65 50 4.02 0.3443
WVFGRD96 27.0 275 65 50 4.03 0.3390
WVFGRD96 28.0 275 65 55 4.04 0.3333
WVFGRD96 29.0 10 60 40 4.00 0.3246
The best solution is
WVFGRD96 2.0 70 60 -50 3.82 0.4136
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 0.10 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 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