Location
SLU Location
Because the moment tensor depth was 25 km, first arrivals were read and the event was located using elocate with the WUS model used for the moment tensor solution. The depth is 31 km with the WUS model and 25 with the CUS model. Thus the moment tensor depth is accepted. The first motions were poor, but the few good readings support the moment tensor solution. The relocation results are
in the file elocate.txt.
Location ANSS
The ANSS event ID is us1000heyv and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/us1000heyv/executive.
2018/10/20 13:04:30 35.309 -101.761 5.0 3.7 Texas
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2018/10/20 13:04:30:0 35.31 -101.76 5.0 3.7 Texas
Stations used:
GS.KAN01 GS.KAN14 GS.OK038 N4.ABTX N4.KSCO N4.MSTX N4.R32B
O2.DOVR O2.PERK TA.T25A TX.DKNS TX.PB06 TX.PLPT TX.POST
TX.RTBA TX.SMWD TX.SN05 TX.SN07 US.AMTX US.CBKS US.SDCO
US.WMOK YX.UNM2 YX.UNM5
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.12 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 4.32e+21 dyne-cm
Mw = 3.69
Z = 25 km
Plane Strike Dip Rake
NP1 285 70 -35
NP2 28 57 -156
Principal Axes:
Axis Value Plunge Azimuth
T 4.32e+21 8 339
N 0.00e+00 50 79
P -4.32e+21 39 243
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.15e+21
Mxy -2.48e+21
Mxz 1.52e+21
Myy -1.55e+21
Myz 1.66e+21
Mzz -1.59e+21
#############
## T ################-
##### #################---
##########################----
############################------
#############################-------
##############################--------
####---------##################---------
----------------------#########---------
-----------------------------##-----------
------------------------------###---------
------------------------------######------
-----------------------------#########----
------- -----------------############-
------- P ----------------##############
------ --------------###############
---------------------###############
------------------################
--------------################
-----------#################
----##################
##############
Global CMT Convention Moment Tensor:
R T P
-1.59e+21 1.52e+21 -1.66e+21
1.52e+21 3.15e+21 2.48e+21
-1.66e+21 2.48e+21 -1.55e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181020130430/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 = 285
DIP = 70
RAKE = -35
MW = 3.69
HS = 25.0
The NDK file is 20181020130430.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 2018/10/20 13:04:30:0 35.31 -101.76 5.0 3.7 Texas
Stations used:
GS.KAN01 GS.KAN14 GS.OK038 N4.ABTX N4.KSCO N4.MSTX N4.R32B
O2.DOVR O2.PERK TA.T25A TX.DKNS TX.PB06 TX.PLPT TX.POST
TX.RTBA TX.SMWD TX.SN05 TX.SN07 US.AMTX US.CBKS US.SDCO
US.WMOK YX.UNM2 YX.UNM5
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.12 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 4.32e+21 dyne-cm
Mw = 3.69
Z = 25 km
Plane Strike Dip Rake
NP1 285 70 -35
NP2 28 57 -156
Principal Axes:
Axis Value Plunge Azimuth
T 4.32e+21 8 339
N 0.00e+00 50 79
P -4.32e+21 39 243
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.15e+21
Mxy -2.48e+21
Mxz 1.52e+21
Myy -1.55e+21
Myz 1.66e+21
Mzz -1.59e+21
#############
## T ################-
##### #################---
##########################----
############################------
#############################-------
##############################--------
####---------##################---------
----------------------#########---------
-----------------------------##-----------
------------------------------###---------
------------------------------######------
-----------------------------#########----
------- -----------------############-
------- P ----------------##############
------ --------------###############
---------------------###############
------------------################
--------------################
-----------#################
----##################
##############
Global CMT Convention Moment Tensor:
R T P
-1.59e+21 1.52e+21 -1.66e+21
1.52e+21 3.15e+21 2.48e+21
-1.66e+21 2.48e+21 -1.55e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181020130430/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/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.12 n 3
br c 0.12 0.25 n 4 p 2
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 305 75 -20 3.05 0.1688
WVFGRD96 2.0 280 50 -70 3.24 0.2595
WVFGRD96 3.0 290 80 -15 3.22 0.2435
WVFGRD96 4.0 110 85 -35 3.27 0.2455
WVFGRD96 5.0 125 70 55 3.37 0.2901
WVFGRD96 6.0 125 70 60 3.40 0.3428
WVFGRD96 7.0 125 70 60 3.42 0.3812
WVFGRD96 8.0 135 65 75 3.53 0.4089
WVFGRD96 9.0 135 60 75 3.54 0.4326
WVFGRD96 10.0 135 60 75 3.55 0.4540
WVFGRD96 11.0 270 45 -55 3.54 0.4732
WVFGRD96 12.0 270 45 -55 3.54 0.4939
WVFGRD96 13.0 275 50 -50 3.55 0.5150
WVFGRD96 14.0 275 50 -45 3.56 0.5335
WVFGRD96 15.0 280 60 -40 3.56 0.5533
WVFGRD96 16.0 280 60 -40 3.57 0.5754
WVFGRD96 17.0 280 60 -35 3.59 0.5957
WVFGRD96 18.0 285 65 -35 3.60 0.6143
WVFGRD96 19.0 285 65 -35 3.62 0.6324
WVFGRD96 20.0 285 65 -35 3.63 0.6484
WVFGRD96 21.0 285 65 -35 3.65 0.6606
WVFGRD96 22.0 285 65 -35 3.66 0.6695
WVFGRD96 23.0 280 65 -40 3.66 0.6774
WVFGRD96 24.0 285 70 -35 3.68 0.6820
WVFGRD96 25.0 285 70 -35 3.69 0.6852
WVFGRD96 26.0 285 70 -40 3.70 0.6840
WVFGRD96 27.0 285 70 -40 3.71 0.6831
WVFGRD96 28.0 285 75 -40 3.71 0.6811
WVFGRD96 29.0 285 75 -40 3.72 0.6786
WVFGRD96 30.0 285 75 -40 3.73 0.6711
WVFGRD96 31.0 285 75 -40 3.74 0.6649
WVFGRD96 32.0 285 75 -40 3.75 0.6550
WVFGRD96 33.0 280 75 -45 3.75 0.6435
WVFGRD96 34.0 280 75 -45 3.77 0.6322
WVFGRD96 35.0 280 75 -45 3.78 0.6165
WVFGRD96 36.0 280 75 -45 3.78 0.6004
WVFGRD96 37.0 285 80 -40 3.80 0.5839
WVFGRD96 38.0 280 75 -40 3.81 0.5705
WVFGRD96 39.0 280 75 -40 3.82 0.5589
The best solution is
WVFGRD96 25.0 285 70 -35 3.69 0.6852
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/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.12 n 3
br c 0.12 0.25 n 4 p 2
|
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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 Fri Apr 26 03:17:55 AM CDT 2024
