Location
Location SLU
This was a small event and thus the waveforms were not the best.
An initial solution using the WUS velocity model yielded a solution with E-W tension axes rather than the expected E-W pressure axes. First times and P-wave polarities were picked as a test of the mechanism. The first motions gave exactly the opposite solution as the waveform velocity model. A discussion of the
model selection and other information is given at Model Selection.
The result of using the CRUST 1.0 model for 35N 98W is good agreement between P-wave first motions and the waveform inversion solution and also good agreement in the depths. This emphasizes the need to have good local velocity models, especially when processing small events.
Location ANSS
2020/03/06 01:42:14 34.972 -97.712 1.0 3.6 Oklahoma
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2020/03/06 01:42:14:0 34.97 -97.71 1.0 3.6 Oklahoma
Stations used:
GM.IWM01 GS.OK029 GS.OK038 GS.OK052 N4.TUL3 N4.Z35B O2.ARC2
O2.CALT O2.CHAN O2.CRES O2.DRUM O2.DUST O2.ERNS O2.PERK
O2.PERY O2.PW05 O2.PW09 O2.SC04 O2.SC07 O2.SC11 O2.SC13
O2.SC14 O2.SC16 O2.SC17 O2.SC20 O2.SMNL OK.W35A TX.WTFS
US.WMOK
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.04 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 1.88e+21 dyne-cm
Mw = 3.45
Z = 10 km
Plane Strike Dip Rake
NP1 234 72 -154
NP2 135 65 -20
Principal Axes:
Axis Value Plunge Azimuth
T 1.88e+21 5 3
N 0.00e+00 58 266
P -1.88e+21 31 96
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.85e+21
Mxy 2.47e+20
Mxz 2.36e+20
Myy -1.36e+21
Myz -8.22e+20
Mzz -4.94e+20
####### T ####
########### ########
############################
##############################
---#############################--
----#######################---------
------##################--------------
--------#############-------------------
--------###########---------------------
-----------######-------------------------
------------###------------------- -----
---------------------------------- P -----
-----------####------------------- -----
--------########------------------------
-------###########----------------------
----###############-------------------
--##################----------------
#######################-----------
#########################-----
############################
######################
##############
Global CMT Convention Moment Tensor:
R T P
-4.94e+20 2.36e+20 8.22e+20
2.36e+20 1.85e+21 -2.47e+20
8.22e+20 -2.47e+20 -1.36e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200306014214/index.html
|
Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is
STK = 135
DIP = 65
RAKE = -20
MW = 3.45
HS = 10.0
The NDK file is 20200306014214.ndk
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
SLUFM |
USGS/SLU Moment Tensor Solution
ENS 2020/03/06 01:42:14:0 34.97 -97.71 1.0 3.6 Oklahoma
Stations used:
GM.IWM01 GS.OK029 GS.OK038 GS.OK052 N4.TUL3 N4.Z35B O2.ARC2
O2.CALT O2.CHAN O2.CRES O2.DRUM O2.DUST O2.ERNS O2.PERK
O2.PERY O2.PW05 O2.PW09 O2.SC04 O2.SC07 O2.SC11 O2.SC13
O2.SC14 O2.SC16 O2.SC17 O2.SC20 O2.SMNL OK.W35A TX.WTFS
US.WMOK
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.04 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 1.88e+21 dyne-cm
Mw = 3.45
Z = 10 km
Plane Strike Dip Rake
NP1 234 72 -154
NP2 135 65 -20
Principal Axes:
Axis Value Plunge Azimuth
T 1.88e+21 5 3
N 0.00e+00 58 266
P -1.88e+21 31 96
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.85e+21
Mxy 2.47e+20
Mxz 2.36e+20
Myy -1.36e+21
Myz -8.22e+20
Mzz -4.94e+20
####### T ####
########### ########
############################
##############################
---#############################--
----#######################---------
------##################--------------
--------#############-------------------
--------###########---------------------
-----------######-------------------------
------------###------------------- -----
---------------------------------- P -----
-----------####------------------- -----
--------########------------------------
-------###########----------------------
----###############-------------------
--##################----------------
#######################-----------
#########################-----
############################
######################
##############
Global CMT Convention Moment Tensor:
R T P
-4.94e+20 2.36e+20 8.22e+20
2.36e+20 1.85e+21 -2.47e+20
8.22e+20 -2.47e+20 -1.36e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200306014214/index.html
|
First motions and takeoff angles from an elocate run.
|
Magnitudes
mLg Magnitude
(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.
ML Magnitude
(a) ML computed using the IASPEI formula for Horizontal components; (b) 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.
(a) ML computed using the IASPEI formula for Vertical components (research); (b) 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.
Context
The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. 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).
Waveform Inversion using wvfgrd96
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the
and stations used for 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 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 +40
rtr
taper w 0.1
hp c 0.04 n 3
lp c 0.08 n 3
The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 3.0 315 70 -15 3.17 0.3545
WVFGRD96 4.0 315 70 -15 3.21 0.3684
WVFGRD96 5.0 315 75 -20 3.25 0.3789
WVFGRD96 6.0 315 75 -20 3.29 0.3852
WVFGRD96 7.0 140 90 -25 3.34 0.3906
WVFGRD96 8.0 135 60 -20 3.41 0.3993
WVFGRD96 9.0 135 65 -25 3.44 0.4029
WVFGRD96 10.0 135 65 -20 3.45 0.4030
WVFGRD96 11.0 135 65 -20 3.48 0.3998
WVFGRD96 12.0 135 65 -20 3.50 0.3936
WVFGRD96 13.0 135 65 -20 3.51 0.3851
WVFGRD96 14.0 135 65 -20 3.53 0.3746
WVFGRD96 15.0 135 65 -20 3.55 0.3629
WVFGRD96 16.0 140 90 15 3.54 0.3470
WVFGRD96 17.0 140 90 15 3.55 0.3353
WVFGRD96 18.0 320 60 5 3.59 0.3231
WVFGRD96 19.0 320 55 10 3.61 0.3121
The best solution is
WVFGRD96 10.0 135 65 -20 3.45 0.4030
The mechanism correspond 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 and because the velocity model used in the predictions may not be perfect.
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 +40
rtr
taper w 0.1
hp c 0.04 n 3
lp c 0.08 n 3
|
|
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.
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Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms.
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.
Discussion
Acknowledgements
Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Iris stations and the Transportable Array of EarthScope.
Velocity Model
The 35.98 model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:
MODEL.01
CRUST1.0 at (35.01, -98.01)
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
0.500 2.500 1.070 2.110 0.0 0.0 0.0 0.0 1.0 1.0
4.000 4.600 2.590 2.460 0.0 0.0 0.0 0.0 1.0 1.0
3.000 5.000 2.880 2.540 0.0 0.0 0.0 0.0 1.0 1.0
11.270 6.100 3.530 2.740 0.0 0.0 0.0 0.0 1.0 1.0
12.670 6.500 3.710 2.830 0.0 0.0 0.0 0.0 1.0 1.0
11.260 6.900 3.930 2.920 0.0 0.0 0.0 0.0 1.0 1.0
0.000 8.160 4.530 3.360 0.0 0.0 0.0 0.0 1.0 1.0
Quality Control
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
Last Changed Fri Mar 6 07:52:52 CST 2020
