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 nm608397 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/nm608397/executive.
2011/02/18 08:13:35 35.271 -92.377 6.2 4.1 Arkansas
USGS/SLU Moment Tensor Solution
ENS 2011/02/18 08:13:35:0 35.27 -92.38 6.2 4.1 Arkansas
Stations used:
AG.CCAR AG.FCAR AG.HHAR AG.LCAR AG.WHAR AG.WLAR IU.WVT
NM.MGMO NM.MPH NM.OLIL NM.SIUC NM.SLM NM.UALR NM.USIN
NM.X102 NM.X201 TA.139A TA.140A TA.141A TA.236A TA.O34A
TA.O36A TA.O38A TA.O39A TA.O40A TA.P33A TA.P34A TA.P35A
TA.P36A TA.P38A TA.P39A TA.P40A TA.Q33A TA.Q34A TA.Q35A
TA.Q36A TA.Q37A TA.Q38A TA.Q39A TA.R32A TA.R33A TA.R34A
TA.R35A TA.R36A TA.R37A TA.R38A TA.R39A TA.R40A TA.S32A
TA.S33A TA.S34A TA.S35A TA.S36A TA.S37A TA.S38A TA.S39A
TA.S40A TA.T32A TA.T33A TA.T34A TA.T35A TA.T36A TA.T37A
TA.T38A TA.T39A TA.T40A TA.TUL1 TA.U31A TA.U32A TA.U33A
TA.U34A TA.U35A TA.U36A TA.U37A TA.U38A TA.U39A TA.U40A
TA.V32A TA.V33A TA.V34A TA.V35A TA.V36A TA.V37A TA.V38A
TA.V39A TA.W33A TA.W34A TA.W35A TA.W36A TA.W37B TA.W38A
TA.W39A TA.W40A TA.WHTX TA.X33A TA.X36A TA.X37A TA.X38A
TA.X40A TA.Y36A TA.Y37A TA.Y38A TA.Y39A TA.Y40A TA.Z37A
TA.Z38A TA.Z39A TA.Z40A US.KSU1 US.LRAL US.MIAR US.OXF
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 = 1.30e+22 dyne-cm
Mw = 4.01
Z = 3 km
Plane Strike Dip Rake
NP1 201 85 -170
NP2 110 80 -5
Principal Axes:
Axis Value Plunge Azimuth
T 1.30e+22 4 335
N 0.00e+00 79 227
P -1.30e+22 11 66
Moment Tensor: (dyne-cm)
Component Value
Mxx 8.56e+21
Mxy -9.67e+21
Mxz -2.32e+20
Myy -8.17e+21
Myz -2.48e+21
Mzz -3.88e+20
##############
# T #############-----
#### ############---------
###################-----------
#####################-------------
#####################------------
#####################------------- P -
-####################-------------- --
----################--------------------
--------#############---------------------
------------########----------------------
----------------###-----------------------
-------------------##---------------------
-----------------#########--------------
----------------##################------
---------------#######################
-------------#######################
------------######################
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----##################
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Global CMT Convention Moment Tensor:
R T P
-3.88e+20 -2.32e+20 2.48e+21
-2.32e+20 8.56e+21 9.67e+21
2.48e+21 9.67e+21 -8.17e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110218081335/index.html
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STK = 110
DIP = 80
RAKE = -5
MW = 4.01
HS = 3.0
The NDK file is 20110218081335.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 0.5 110 70 -20 3.85 0.4827
WVFGRD96 1.0 105 75 -20 3.88 0.5095
WVFGRD96 2.0 290 90 10 3.96 0.6268
WVFGRD96 3.0 110 80 -5 4.01 0.6566
WVFGRD96 4.0 110 75 -15 4.04 0.6489
WVFGRD96 5.0 110 80 -15 4.04 0.6368
WVFGRD96 6.0 110 85 -15 4.05 0.6259
WVFGRD96 7.0 290 90 15 4.06 0.6205
WVFGRD96 8.0 110 90 -15 4.07 0.6187
WVFGRD96 9.0 290 80 15 4.08 0.6198
WVFGRD96 10.0 290 75 15 4.09 0.6202
WVFGRD96 11.0 290 75 15 4.10 0.6188
WVFGRD96 12.0 290 75 15 4.11 0.6158
WVFGRD96 13.0 290 75 15 4.12 0.6118
WVFGRD96 14.0 290 75 10 4.12 0.6076
WVFGRD96 15.0 290 75 10 4.13 0.6028
WVFGRD96 16.0 290 75 10 4.14 0.5973
WVFGRD96 17.0 290 70 10 4.15 0.5915
WVFGRD96 18.0 290 70 10 4.16 0.5850
WVFGRD96 19.0 290 70 10 4.16 0.5780
WVFGRD96 20.0 290 70 10 4.18 0.5699
WVFGRD96 21.0 290 70 10 4.18 0.5617
WVFGRD96 22.0 290 70 10 4.19 0.5529
WVFGRD96 23.0 290 70 10 4.20 0.5440
WVFGRD96 24.0 290 65 10 4.20 0.5350
WVFGRD96 25.0 290 65 10 4.21 0.5265
WVFGRD96 26.0 290 65 10 4.22 0.5177
WVFGRD96 27.0 290 65 10 4.22 0.5088
WVFGRD96 28.0 290 65 10 4.23 0.4994
WVFGRD96 29.0 290 65 10 4.23 0.4902
The best solution is
WVFGRD96 3.0 110 80 -5 4.01 0.6566
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 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