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 ok2020gpqn and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ok2020gpqn/executive.
2020/04/03 22:35:30 36.374 -97.721 5.0 3.65 Oklahoma
USGS/SLU Moment Tensor Solution
ENS 2020/04/03 22:35:30:0 36.37 -97.72 5.0 3.7 Oklahoma
Stations used:
GS.OK029 GS.OK038 GS.OK048 GS.OK052 O2.CALT O2.CHAN O2.CRES
O2.DOVR O2.FW03 O2.FW06 O2.MRSH O2.PERY O2.PW05 O2.PW09
O2.PW18 O2.SC19 O2.SMNL OK.AMES OK.CROK
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
Best Fitting Double Couple
Mo = 2.24e+21 dyne-cm
Mw = 3.50
Z = 3 km
Plane Strike Dip Rake
NP1 25 90 -175
NP2 295 85 0
Principal Axes:
Axis Value Plunge Azimuth
T 2.24e+21 4 160
N 0.00e+00 85 25
P -2.24e+21 4 250
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.71e+21
Mxy -1.43e+21
Mxz -8.25e+19
Myy -1.71e+21
Myz 1.77e+20
Mzz 0.00e+00
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---------------------#--------------------
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--------------------##########------------
---------------##############--------
P --------------##################-----
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############ T
Global CMT Convention Moment Tensor:
R T P
0.00e+00 -8.25e+19 -1.77e+20
-8.25e+19 1.71e+21 1.43e+21
-1.77e+20 1.43e+21 -1.71e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200403223530/index.html
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STK = 295
DIP = 85
RAKE = 0
MW = 3.50
HS = 3.0
The NDK file is 20200403223530.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 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 295 90 -25 3.33 0.4558
WVFGRD96 2.0 115 90 -10 3.45 0.5465
WVFGRD96 3.0 295 85 0 3.50 0.5659
WVFGRD96 4.0 295 85 0 3.54 0.5595
WVFGRD96 5.0 295 80 0 3.58 0.5412
WVFGRD96 6.0 290 80 -20 3.61 0.5197
WVFGRD96 7.0 295 90 -25 3.65 0.4961
WVFGRD96 8.0 295 90 35 3.70 0.4713
WVFGRD96 9.0 115 90 -30 3.71 0.4481
WVFGRD96 10.0 110 85 -30 3.72 0.4254
WVFGRD96 11.0 110 85 -30 3.73 0.4037
WVFGRD96 12.0 110 85 -30 3.74 0.3829
WVFGRD96 13.0 110 85 -35 3.76 0.3636
WVFGRD96 14.0 110 90 -35 3.77 0.3463
WVFGRD96 15.0 110 90 -35 3.78 0.3305
WVFGRD96 16.0 110 90 -35 3.79 0.3170
WVFGRD96 17.0 115 90 -35 3.80 0.3047
WVFGRD96 18.0 115 90 -35 3.80 0.2943
WVFGRD96 19.0 115 90 -35 3.81 0.2848
WVFGRD96 20.0 115 90 -35 3.81 0.2776
WVFGRD96 21.0 115 90 -35 3.82 0.2701
WVFGRD96 22.0 115 90 -35 3.82 0.2653
WVFGRD96 23.0 115 90 -35 3.83 0.2617
WVFGRD96 24.0 115 90 -35 3.83 0.2595
WVFGRD96 25.0 205 65 20 3.82 0.2639
WVFGRD96 26.0 205 75 -10 3.83 0.2715
WVFGRD96 27.0 205 75 -10 3.84 0.2792
WVFGRD96 28.0 205 75 -10 3.85 0.2874
WVFGRD96 29.0 205 75 -10 3.86 0.2952
The best solution is
WVFGRD96 3.0 295 85 0 3.50 0.5659
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
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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