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 nm609922 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/nm609922/executive.
2013/05/24 05:44:24 35.298 -92.715 5.6 3.4 Arkansas
USGS/SLU Moment Tensor Solution
ENS 2013/05/24 05:44:24:0 35.30 -92.71 5.6 3.4 Arkansas
Stations used:
AG.CCAR AG.FCAR AG.HHAR AG.WLAR IU.CCM NM.CLTN NM.HALT
NM.MGMO NM.PLAL NM.UALR TA.TUL1 TA.U40A TA.V48A TA.W39A
TA.W41B TA.X40A TA.X48A TA.Z41A US.LRAL US.MIAR
Filtering commands used:
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 1.76e+21 dyne-cm
Mw = 3.43
Z = 2 km
Plane Strike Dip Rake
NP1 187 71 -159
NP2 90 70 -20
Principal Axes:
Axis Value Plunge Azimuth
T 1.76e+21 1 318
N 0.00e+00 62 227
P -1.76e+21 28 49
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.86e+20
Mxy -1.55e+21
Mxz -4.61e+20
Myy -1.36e+14
Myz -5.65e+20
Mzz -3.86e+20
#########-----
###########-----------
T ###########---------------
###########----------------
###############------------ ----
###############------------- P -----
################------------- ------
################------------------------
################------------------------
#################-------------------------
################--------------------------
--##############------------------------##
------##########--------------------######
-------------##------------#############
---------------#########################
--------------########################
-------------#######################
------------######################
----------####################
----------##################
-------###############
----##########
Global CMT Convention Moment Tensor:
R T P
-3.86e+20 -4.61e+20 5.65e+20
-4.61e+20 3.86e+20 1.55e+21
5.65e+20 1.55e+21 -1.36e+14
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130524054424/index.html
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STK = 90
DIP = 70
RAKE = -20
MW = 3.43
HS = 2.0
The NDK file is 20130524054424.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 0.03 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 90 65 -10 3.37 0.4182
WVFGRD96 1.0 90 70 -15 3.39 0.4355
WVFGRD96 2.0 90 70 -20 3.43 0.4528
WVFGRD96 3.0 90 70 -20 3.45 0.4518
WVFGRD96 4.0 90 70 -15 3.45 0.4432
WVFGRD96 5.0 95 90 -25 3.47 0.4331
WVFGRD96 6.0 275 85 25 3.47 0.4319
WVFGRD96 7.0 90 90 -30 3.49 0.4311
WVFGRD96 8.0 90 90 -30 3.50 0.4336
WVFGRD96 9.0 90 90 -30 3.51 0.4368
WVFGRD96 10.0 275 85 30 3.52 0.4425
WVFGRD96 11.0 275 85 30 3.53 0.4431
WVFGRD96 12.0 275 80 30 3.54 0.4448
WVFGRD96 13.0 275 85 30 3.55 0.4446
WVFGRD96 14.0 275 80 30 3.56 0.4440
WVFGRD96 15.0 275 80 30 3.56 0.4418
WVFGRD96 16.0 275 80 30 3.57 0.4397
WVFGRD96 17.0 275 80 30 3.58 0.4369
WVFGRD96 18.0 275 80 30 3.59 0.4334
WVFGRD96 19.0 275 80 30 3.59 0.4290
WVFGRD96 20.0 275 80 30 3.61 0.4249
WVFGRD96 21.0 275 80 30 3.61 0.4191
WVFGRD96 22.0 275 80 30 3.62 0.4133
WVFGRD96 23.0 275 80 30 3.62 0.4075
WVFGRD96 24.0 275 60 10 3.62 0.4039
WVFGRD96 25.0 275 60 10 3.63 0.4026
WVFGRD96 26.0 275 60 10 3.63 0.4018
WVFGRD96 27.0 275 60 5 3.64 0.4001
WVFGRD96 28.0 275 60 5 3.64 0.3988
WVFGRD96 29.0 275 60 5 3.65 0.3959
The best solution is
WVFGRD96 2.0 90 70 -20 3.43 0.4528
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 0.03 n 3 lp c 0.08 n 3
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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