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 usc000tbz7 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usc000tbz7/executive.
2015/01/05 22:06:44 36.261 -97.262 5.0 3.6 Oklahoma
USGS/SLU Moment Tensor Solution
ENS 2015/01/05 22:06:44:0 36.26 -97.26 5.0 3.6 Oklahoma
Stations used:
AG.WLAR GS.KAN08 GS.KAN10 GS.KAN12 GS.KAN14 GS.OK030
GS.OK031 N4.R32B N4.T35B N4.U38B N4.Z35B NM.UALR OK.CROK
OK.KAY1 OK.U32A OK.X37A TA.ABTX TA.TUL1 TA.W39A TA.WHTX
TA.X40A US.CBKS US.KSU1 US.WMOK
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 3.27e+21 dyne-cm
Mw = 3.61
Z = 4 km
Plane Strike Dip Rake
NP1 110 90 -35
NP2 200 55 -180
Principal Axes:
Axis Value Plunge Azimuth
T 3.27e+21 24 161
N 0.00e+00 55 290
P -3.27e+21 24 59
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.72e+21
Mxy -2.05e+21
Mxz -1.76e+21
Myy -1.72e+21
Myz -6.42e+20
Mzz 1.64e+14
##############
##############--------
##############--------------
#############-----------------
##############--------------------
#############----------------- ---
#############------------------ P ----
--###########------------------- -----
-------#####----------------------------
------------------------------------------
------------######------------------------
------------############------------------
-----------##################-------------
----------#######################-------
----------############################--
---------#############################
--------############################
-------###########################
-----############## ########
-----############# T #######
---############ ####
##############
Global CMT Convention Moment Tensor:
R T P
1.64e+14 -1.76e+21 6.42e+20
-1.76e+21 1.72e+21 2.05e+21
6.42e+20 2.05e+21 -1.72e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150105220644/index.html
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STK = 110
DIP = 90
RAKE = -35
MW = 3.61
HS = 4.0
The NDK file is 20150105220644.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 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 110 80 -25 3.41 0.3039
WVFGRD96 2.0 110 80 -35 3.54 0.3554
WVFGRD96 3.0 105 80 -40 3.59 0.3671
WVFGRD96 4.0 110 90 -35 3.61 0.3671
WVFGRD96 5.0 110 90 -35 3.63 0.3649
WVFGRD96 6.0 295 80 30 3.65 0.3642
WVFGRD96 7.0 295 80 25 3.66 0.3619
WVFGRD96 8.0 295 80 35 3.71 0.3575
WVFGRD96 9.0 295 75 30 3.71 0.3519
WVFGRD96 10.0 115 70 20 3.72 0.3464
WVFGRD96 11.0 115 70 15 3.72 0.3418
WVFGRD96 12.0 115 70 15 3.74 0.3368
WVFGRD96 13.0 115 75 20 3.75 0.3313
WVFGRD96 14.0 115 70 15 3.76 0.3257
WVFGRD96 15.0 115 70 15 3.76 0.3201
WVFGRD96 16.0 115 70 15 3.77 0.3145
WVFGRD96 17.0 115 70 15 3.78 0.3088
WVFGRD96 18.0 115 70 20 3.79 0.3030
WVFGRD96 19.0 115 70 20 3.80 0.2977
WVFGRD96 20.0 115 70 20 3.80 0.2924
WVFGRD96 21.0 120 70 25 3.82 0.2875
WVFGRD96 22.0 120 70 25 3.83 0.2828
WVFGRD96 23.0 120 65 25 3.83 0.2788
WVFGRD96 24.0 120 65 25 3.84 0.2749
WVFGRD96 25.0 120 65 25 3.85 0.2710
WVFGRD96 26.0 120 65 25 3.85 0.2675
WVFGRD96 27.0 120 65 25 3.86 0.2642
WVFGRD96 28.0 215 60 25 3.88 0.2613
WVFGRD96 29.0 210 60 20 3.88 0.2662
The best solution is
WVFGRD96 4.0 110 90 -35 3.61 0.3671
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 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 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 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