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 ak022ctqflju and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak022ctqflju/executive.
2022/10/06 19:30:50 61.850 -147.578 28.7 4.1 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2022/10/06 19:30:50:0 61.85 -147.58 28.7 4.1 Alaska
Stations used:
AK.BMR AK.CCB AK.CUT AK.DHY AK.EYAK AK.FID AK.GHO AK.GLI
AK.HDA AK.HIN AK.K24K AK.KLU AK.KNK AK.L22K AK.MCAR AK.MCK
AK.POKR AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN AK.SLK AT.PMR
IM.IL31 IU.COLA
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 0.08 n 3
Best Fitting Double Couple
Mo = 1.66e+22 dyne-cm
Mw = 4.08
Z = 71 km
Plane Strike Dip Rake
NP1 360 85 175
NP2 90 85 5
Principal Axes:
Axis Value Plunge Azimuth
T 1.66e+22 7 315
N 0.00e+00 83 135
P -1.66e+22 0 45
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.51e+20
Mxy -1.65e+22
Mxz 1.42e+21
Myy -1.44e+15
Myz -1.44e+21
Mzz 2.51e+20
#######-------
###########-----------
#############----------- P
T #############-----------
## #############----------------
###################-----------------
####################------------------
#####################-------------------
#####################-------------------
######################--------------------
######################--------------------
---------#############-----------#########
----------------------####################
---------------------###################
---------------------###################
--------------------##################
-------------------#################
------------------################
----------------##############
---------------#############
------------##########
-------#######
Global CMT Convention Moment Tensor:
R T P
2.51e+20 1.42e+21 1.44e+21
1.42e+21 -2.51e+20 1.65e+22
1.44e+21 1.65e+22 -1.44e+15
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20221006193050/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 = 90
DIP = 85
RAKE = 5
MW = 4.08
HS = 71.0
The NDK file is 20221006193050.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 2022/10/06 19:30:50:0 61.85 -147.58 28.7 4.1 Alaska
Stations used:
AK.BMR AK.CCB AK.CUT AK.DHY AK.EYAK AK.FID AK.GHO AK.GLI
AK.HDA AK.HIN AK.K24K AK.KLU AK.KNK AK.L22K AK.MCAR AK.MCK
AK.POKR AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN AK.SLK AT.PMR
IM.IL31 IU.COLA
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 0.08 n 3
Best Fitting Double Couple
Mo = 1.66e+22 dyne-cm
Mw = 4.08
Z = 71 km
Plane Strike Dip Rake
NP1 360 85 175
NP2 90 85 5
Principal Axes:
Axis Value Plunge Azimuth
T 1.66e+22 7 315
N 0.00e+00 83 135
P -1.66e+22 0 45
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.51e+20
Mxy -1.65e+22
Mxz 1.42e+21
Myy -1.44e+15
Myz -1.44e+21
Mzz 2.51e+20
#######-------
###########-----------
#############----------- P
T #############-----------
## #############----------------
###################-----------------
####################------------------
#####################-------------------
#####################-------------------
######################--------------------
######################--------------------
---------#############-----------#########
----------------------####################
---------------------###################
---------------------###################
--------------------##################
-------------------#################
------------------################
----------------##############
---------------#############
------------##########
-------#######
Global CMT Convention Moment Tensor:
R T P
2.51e+20 1.42e+21 1.44e+21
1.42e+21 -2.51e+20 1.65e+22
1.44e+21 1.65e+22 -1.44e+15
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20221006193050/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.
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 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 -5 90 15 3.07 0.2284
WVFGRD96 2.0 170 70 -25 3.23 0.3181
WVFGRD96 3.0 175 80 -20 3.27 0.3544
WVFGRD96 4.0 175 85 -20 3.31 0.3795
WVFGRD96 5.0 175 80 -15 3.35 0.3979
WVFGRD96 6.0 -5 90 15 3.37 0.4118
WVFGRD96 7.0 -5 90 15 3.41 0.4241
WVFGRD96 8.0 175 90 -15 3.44 0.4339
WVFGRD96 9.0 175 90 -10 3.46 0.4365
WVFGRD96 10.0 275 70 15 3.51 0.4474
WVFGRD96 11.0 275 70 15 3.54 0.4635
WVFGRD96 12.0 270 75 5 3.54 0.4788
WVFGRD96 13.0 270 75 0 3.56 0.4934
WVFGRD96 14.0 270 75 -10 3.58 0.5080
WVFGRD96 15.0 270 75 -10 3.60 0.5227
WVFGRD96 16.0 270 75 -10 3.62 0.5373
WVFGRD96 17.0 270 75 -10 3.64 0.5521
WVFGRD96 18.0 270 75 -10 3.65 0.5662
WVFGRD96 19.0 270 75 -10 3.67 0.5807
WVFGRD96 20.0 270 75 -10 3.68 0.5955
WVFGRD96 21.0 270 75 -10 3.70 0.6095
WVFGRD96 22.0 270 75 -10 3.71 0.6219
WVFGRD96 23.0 270 75 -10 3.72 0.6348
WVFGRD96 24.0 270 75 -10 3.73 0.6459
WVFGRD96 25.0 270 75 -10 3.74 0.6554
WVFGRD96 26.0 270 75 -10 3.75 0.6634
WVFGRD96 27.0 270 75 -10 3.76 0.6695
WVFGRD96 28.0 270 75 -10 3.77 0.6764
WVFGRD96 29.0 270 75 -10 3.78 0.6808
WVFGRD96 30.0 270 75 -10 3.78 0.6843
WVFGRD96 31.0 270 75 -10 3.79 0.6853
WVFGRD96 32.0 270 75 -10 3.80 0.6850
WVFGRD96 33.0 270 75 -10 3.81 0.6842
WVFGRD96 34.0 270 75 -10 3.82 0.6821
WVFGRD96 35.0 270 75 -10 3.82 0.6798
WVFGRD96 36.0 270 80 -10 3.83 0.6775
WVFGRD96 37.0 270 80 -10 3.85 0.6764
WVFGRD96 38.0 270 75 -5 3.86 0.6768
WVFGRD96 39.0 270 80 -5 3.87 0.6789
WVFGRD96 40.0 270 75 -15 3.91 0.6868
WVFGRD96 41.0 270 75 -15 3.93 0.6885
WVFGRD96 42.0 270 75 -15 3.94 0.6891
WVFGRD96 43.0 270 75 -15 3.95 0.6900
WVFGRD96 44.0 270 75 -15 3.96 0.6901
WVFGRD96 45.0 270 75 -15 3.97 0.6900
WVFGRD96 46.0 270 75 -15 3.98 0.6892
WVFGRD96 47.0 270 75 -15 3.98 0.6877
WVFGRD96 48.0 270 75 -15 3.99 0.6880
WVFGRD96 49.0 270 75 -10 3.99 0.6880
WVFGRD96 50.0 270 75 -10 4.00 0.6887
WVFGRD96 51.0 270 75 -10 4.00 0.6891
WVFGRD96 52.0 270 75 -10 4.01 0.6910
WVFGRD96 53.0 270 75 -10 4.01 0.6912
WVFGRD96 54.0 270 75 -10 4.02 0.6914
WVFGRD96 55.0 270 75 -10 4.03 0.6924
WVFGRD96 56.0 270 75 -10 4.03 0.6917
WVFGRD96 57.0 270 80 -10 4.03 0.6925
WVFGRD96 58.0 270 80 -10 4.04 0.6928
WVFGRD96 59.0 270 80 -10 4.04 0.6925
WVFGRD96 60.0 270 80 -10 4.05 0.6931
WVFGRD96 61.0 270 80 -10 4.05 0.6915
WVFGRD96 62.0 270 80 -5 4.05 0.6920
WVFGRD96 63.0 270 80 -5 4.05 0.6933
WVFGRD96 64.0 270 90 -5 4.05 0.6917
WVFGRD96 65.0 90 85 5 4.06 0.6934
WVFGRD96 66.0 90 85 5 4.06 0.6936
WVFGRD96 67.0 90 85 5 4.06 0.6934
WVFGRD96 68.0 90 85 5 4.07 0.6933
WVFGRD96 69.0 90 85 5 4.07 0.6939
WVFGRD96 70.0 90 85 5 4.07 0.6940
WVFGRD96 71.0 90 85 5 4.08 0.6945
WVFGRD96 72.0 90 85 5 4.08 0.6932
WVFGRD96 73.0 90 85 5 4.08 0.6941
WVFGRD96 74.0 90 85 5 4.09 0.6934
WVFGRD96 75.0 90 85 5 4.09 0.6940
WVFGRD96 76.0 90 85 5 4.09 0.6929
WVFGRD96 77.0 90 85 5 4.09 0.6933
WVFGRD96 78.0 90 85 5 4.10 0.6931
WVFGRD96 79.0 90 85 5 4.10 0.6938
The best solution is
WVFGRD96 71.0 90 85 5 4.08 0.6945
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 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 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 Thu Apr 25 01:48:43 AM CDT 2024
