Location
SLU Location
To check the ANSS location or to compare the observed P-wave first motions to the moment tensor solution, P- and S-wave first arrival times were manually read together with the P-wave first motions. The subsequent output of the program elocate is given in the file elocate.txt. The first motion plot is shown below.
Location ANSS
The ANSS event ID is ok2021gurr and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ok2021gurr/executive.
2021/04/07 16:51:49 35.059 -96.317 28.6 3.61 Oklahoma
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2021/04/07 16:51:49:0 35.06 -96.32 28.6 3.6 Oklahoma
Stations used:
GS.OK048 N4.TUL3 O2.ARC2 O2.CHAN O2.CRES O2.DRIP O2.DRUM
O2.DUST O2.ERNS O2.MRSH O2.PERK O2.PERY O2.PW05 O2.PW09
O2.PW18 O2.SC13 O2.SMNL O2.STIG OK.CHOK OK.DEOK OK.QUOK
Filtering commands used:
cut o DIST/3.5 -10 o DIST/3.5 +10
rtr
taper w 0.1
hp c 0.5 n 3
lp c 0.50 n 3
Best Fitting Double Couple
Mo = 8.81e+20 dyne-cm
Mw = 3.23
Z = 29 km
Plane Strike Dip Rake
NP1 237 86 -150
NP2 145 60 -5
Principal Axes:
Axis Value Plunge Azimuth
T 8.81e+20 17 7
N 0.00e+00 60 245
P -8.81e+20 24 105
Moment Tensor: (dyne-cm)
Component Value
Mxx 7.36e+20
Mxy 2.91e+20
Mxz 3.37e+20
Myy -6.70e+20
Myz -2.83e+20
Mzz -6.65e+19
######## ###
############ T #######
--############# ##########
---###########################
-----#############################
------###########################---
-------########################-------
---------####################-----------
---------################---------------
-----------############-------------------
------------########----------------------
-------------#####------------------------
--------------#-------------------- ----
-----------###-------------------- P ---
--------#######------------------- ---
-----###########----------------------
--###############-------------------
##################----------------
##################------------
#####################-------
######################
##############
Global CMT Convention Moment Tensor:
R T P
-6.65e+19 3.37e+20 2.83e+20
3.37e+20 7.36e+20 -2.91e+20
2.83e+20 -2.91e+20 -6.70e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210407165149/index.html
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Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is
STK = 145
DIP = 60
RAKE = -5
MW = 3.23
HS = 29.0
The NDK file is 20210407165149.ndk
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
| SLU |
SLUFM |
USGS/SLU Moment Tensor Solution
ENS 2021/04/07 16:51:49:0 35.06 -96.32 28.6 3.6 Oklahoma
Stations used:
GS.OK048 N4.TUL3 O2.ARC2 O2.CHAN O2.CRES O2.DRIP O2.DRUM
O2.DUST O2.ERNS O2.MRSH O2.PERK O2.PERY O2.PW05 O2.PW09
O2.PW18 O2.SC13 O2.SMNL O2.STIG OK.CHOK OK.DEOK OK.QUOK
Filtering commands used:
cut o DIST/3.5 -10 o DIST/3.5 +10
rtr
taper w 0.1
hp c 0.5 n 3
lp c 0.50 n 3
Best Fitting Double Couple
Mo = 8.81e+20 dyne-cm
Mw = 3.23
Z = 29 km
Plane Strike Dip Rake
NP1 237 86 -150
NP2 145 60 -5
Principal Axes:
Axis Value Plunge Azimuth
T 8.81e+20 17 7
N 0.00e+00 60 245
P -8.81e+20 24 105
Moment Tensor: (dyne-cm)
Component Value
Mxx 7.36e+20
Mxy 2.91e+20
Mxz 3.37e+20
Myy -6.70e+20
Myz -2.83e+20
Mzz -6.65e+19
######## ###
############ T #######
--############# ##########
---###########################
-----#############################
------###########################---
-------########################-------
---------####################-----------
---------################---------------
-----------############-------------------
------------########----------------------
-------------#####------------------------
--------------#-------------------- ----
-----------###-------------------- P ---
--------#######------------------- ---
-----###########----------------------
--###############-------------------
##################----------------
##################------------
#####################-------
######################
##############
Global CMT Convention Moment Tensor:
R T P
-6.65e+19 3.37e+20 2.83e+20
3.37e+20 7.36e+20 -2.91e+20
2.83e+20 -2.91e+20 -6.70e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210407165149/index.html
|
First motions and takeoff angles from an elocate run.
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Magnitudes
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.
mLg Magnitude
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.
ML Magnitude
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.
Context
The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.
Waveform Inversion using wvfgrd96
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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Location of broadband stations used for waveform inversion
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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/3.5 -10 o DIST/3.5 +10
rtr
taper w 0.1
hp c 0.5 n 3
lp c 0.50 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 280 0 -105 2.65 0.1415
WVFGRD96 2.0 55 55 -25 2.83 0.1297
WVFGRD96 3.0 170 60 55 2.94 0.1884
WVFGRD96 4.0 175 55 65 2.97 0.2374
WVFGRD96 5.0 175 50 60 2.98 0.2607
WVFGRD96 6.0 170 55 50 3.00 0.2716
WVFGRD96 7.0 165 50 35 2.99 0.2731
WVFGRD96 8.0 170 45 40 3.07 0.2830
WVFGRD96 9.0 165 55 40 3.12 0.3142
WVFGRD96 10.0 165 55 40 3.15 0.3259
WVFGRD96 11.0 160 55 35 3.18 0.3433
WVFGRD96 12.0 160 55 30 3.20 0.3553
WVFGRD96 13.0 155 55 25 3.21 0.3618
WVFGRD96 14.0 155 55 20 3.22 0.3815
WVFGRD96 15.0 155 55 25 3.23 0.3777
WVFGRD96 16.0 155 55 20 3.24 0.4026
WVFGRD96 17.0 155 55 20 3.24 0.4015
WVFGRD96 18.0 150 55 15 3.24 0.3989
WVFGRD96 19.0 150 55 15 3.24 0.4109
WVFGRD96 20.0 150 55 15 3.23 0.3949
WVFGRD96 21.0 150 50 15 3.23 0.3878
WVFGRD96 22.0 150 55 10 3.21 0.3659
WVFGRD96 23.0 150 55 10 3.20 0.3502
WVFGRD96 24.0 145 60 -5 3.19 0.3263
WVFGRD96 25.0 140 65 -20 3.21 0.3564
WVFGRD96 26.0 140 60 -15 3.21 0.3623
WVFGRD96 27.0 140 60 -10 3.22 0.3933
WVFGRD96 28.0 140 60 -10 3.23 0.4092
WVFGRD96 29.0 145 60 -5 3.23 0.4159
WVFGRD96 30.0 145 60 -5 3.23 0.4008
WVFGRD96 31.0 145 60 -5 3.22 0.3955
WVFGRD96 32.0 140 65 -20 3.20 0.3752
WVFGRD96 33.0 140 65 -20 3.20 0.3957
WVFGRD96 34.0 140 65 -20 3.19 0.3929
WVFGRD96 35.0 140 65 -25 3.19 0.3958
WVFGRD96 36.0 140 60 -20 3.18 0.4043
WVFGRD96 37.0 140 60 -20 3.18 0.3999
WVFGRD96 38.0 140 60 -20 3.18 0.3982
WVFGRD96 39.0 135 60 -20 3.21 0.3821
WVFGRD96 40.0 330 60 -5 3.26 0.3389
WVFGRD96 41.0 140 60 -25 3.31 0.2990
WVFGRD96 42.0 150 70 5 3.32 0.2795
WVFGRD96 43.0 150 70 10 3.34 0.2573
WVFGRD96 44.0 330 70 -5 3.35 0.2726
WVFGRD96 45.0 330 70 -5 3.36 0.2671
WVFGRD96 46.0 335 70 5 3.37 0.2670
WVFGRD96 47.0 150 65 10 3.37 0.2338
WVFGRD96 48.0 155 55 30 3.39 0.2143
WVFGRD96 49.0 155 55 30 3.40 0.2214
The best solution is
WVFGRD96 29.0 145 60 -5 3.23 0.4159
The mechanism corresponding to the best fit is
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Figure 1. Waveform inversion focal mechanism
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The best fit as a function of depth is given in the following figure:
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Figure 2. Depth sensitivity for waveform mechanism
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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/3.5 -10 o DIST/3.5 +10
rtr
taper w 0.1
hp c 0.5 n 3
lp c 0.50 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.
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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:
- The origin time and epicentral distance are incorrect
- The velocity model used for the inversion is incorrect
- The velocity model used to define the P-arrival time is not the
same as the velocity model used for the waveform inversion
(assuming that the initial trace alignment is based on the
P arrival time)
Assuming only a mislocation, the time shifts are fit to a functional form:
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.
Velocity Model
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
Last Changed Wed Apr 24 10:07:01 PM CDT 2024
