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 ak022ctr3qn3 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak022ctr3qn3/executive.
2022/10/06 20:43:13 61.820 -147.573 34.1 4.8 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2022/10/06 20:43:13:0 61.82 -147.57 34.1 4.8 Alaska
Stations used:
AK.BMR AK.CAST AK.CUT AK.DHY AK.DIV AK.EYAK AK.FID AK.FIRE
AK.GHO AK.GLB AK.GLI AK.GRNC AK.HARP AK.HDA AK.HIN AK.I21K
AK.I23K AK.J20K AK.J25K AK.J26L AK.K20K AK.K24K AK.KLU
AK.KNK AK.L20K AK.L22K AK.L26K AK.LOGN AK.MCAR AK.MCK
AK.MLY AK.NEA2 AK.P23K AK.PAX AK.PIN AK.POKR AK.PPLA AK.PWL
AK.RC01 AK.RIDG AK.SAW AK.SCM AK.SKN AK.SLK AK.SSN AK.SWD
AK.TABL AK.VRDI AK.WAX AK.WRH 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 = 1.57e+23 dyne-cm
Mw = 4.73
Z = 45 km
Plane Strike Dip Rake
NP1 11 58 -138
NP2 255 55 -40
Principal Axes:
Axis Value Plunge Azimuth
T 1.57e+23 2 132
N 0.00e+00 39 41
P -1.57e+23 51 225
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.91e+22
Mxy -1.09e+23
Mxz 5.11e+22
Myy 5.55e+22
Myz 5.76e+22
Mzz -9.46e+22
############--
#################-----
#####################-------
######################--------
#########################---------
##########################----------
################-----------#######----
############-----------------##########-
#########--------------------###########
#######----------------------#############
#####------------------------#############
###--------------------------#############
##---------------------------#############
------------ ------------#############
------------ P ------------#############
----------- -----------#############
-----------------------#############
---------------------######### #
------------------########## T
----------------###########
-----------###########
-----#########
Global CMT Convention Moment Tensor:
R T P
-9.46e+22 5.11e+22 -5.76e+22
5.11e+22 3.91e+22 1.09e+23
-5.76e+22 1.09e+23 5.55e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20221006204313/index.html
|
Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is
STK = 255
DIP = 55
RAKE = -40
MW = 4.73
HS = 45.0
The NDK file is 20221006204313.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 |
USGSW |
SLUFM |
USGS/SLU Moment Tensor Solution
ENS 2022/10/06 20:43:13:0 61.82 -147.57 34.1 4.8 Alaska
Stations used:
AK.BMR AK.CAST AK.CUT AK.DHY AK.DIV AK.EYAK AK.FID AK.FIRE
AK.GHO AK.GLB AK.GLI AK.GRNC AK.HARP AK.HDA AK.HIN AK.I21K
AK.I23K AK.J20K AK.J25K AK.J26L AK.K20K AK.K24K AK.KLU
AK.KNK AK.L20K AK.L22K AK.L26K AK.LOGN AK.MCAR AK.MCK
AK.MLY AK.NEA2 AK.P23K AK.PAX AK.PIN AK.POKR AK.PPLA AK.PWL
AK.RC01 AK.RIDG AK.SAW AK.SCM AK.SKN AK.SLK AK.SSN AK.SWD
AK.TABL AK.VRDI AK.WAX AK.WRH 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 = 1.57e+23 dyne-cm
Mw = 4.73
Z = 45 km
Plane Strike Dip Rake
NP1 11 58 -138
NP2 255 55 -40
Principal Axes:
Axis Value Plunge Azimuth
T 1.57e+23 2 132
N 0.00e+00 39 41
P -1.57e+23 51 225
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.91e+22
Mxy -1.09e+23
Mxz 5.11e+22
Myy 5.55e+22
Myz 5.76e+22
Mzz -9.46e+22
############--
#################-----
#####################-------
######################--------
#########################---------
##########################----------
################-----------#######----
############-----------------##########-
#########--------------------###########
#######----------------------#############
#####------------------------#############
###--------------------------#############
##---------------------------#############
------------ ------------#############
------------ P ------------#############
----------- -----------#############
-----------------------#############
---------------------######### #
------------------########## T
----------------###########
-----------###########
-----#########
Global CMT Convention Moment Tensor:
R T P
-9.46e+22 5.11e+22 -5.76e+22
5.11e+22 3.91e+22 1.09e+23
-5.76e+22 1.09e+23 5.55e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20221006204313/index.html
|
W-phase Moment Tensor (Mww)
Moment 2.063e+16 N-m
Magnitude 4.81 Mww
Depth 40.5 km
Percent DC 75%
Half Duration 0.65 s
Catalog US
Data Source US 2
Contributor US 2
Nodal Planes
Plane Strike Dip Rake
NP1 243 42 -52
NP2 16 58 -119
Principal Axes
Axis Value Plunge Azimuth
T 2.187e+16 N-m 9 126
N -0.277e+16 N-m 25 32
P -1.910e+16 N-m 64 234
|
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 20 45 60 3.90 0.2017
WVFGRD96 2.0 20 45 60 4.04 0.2768
WVFGRD96 3.0 30 40 75 4.13 0.3023
WVFGRD96 4.0 5 55 30 4.08 0.2982
WVFGRD96 5.0 355 85 -25 4.07 0.3082
WVFGRD96 6.0 355 85 -25 4.10 0.3214
WVFGRD96 7.0 350 75 -20 4.13 0.3338
WVFGRD96 8.0 350 75 -25 4.17 0.3404
WVFGRD96 9.0 95 65 25 4.20 0.3504
WVFGRD96 10.0 95 65 25 4.21 0.3640
WVFGRD96 11.0 90 70 20 4.23 0.3760
WVFGRD96 12.0 90 70 20 4.25 0.3879
WVFGRD96 13.0 90 70 20 4.26 0.3984
WVFGRD96 14.0 90 75 20 4.28 0.4084
WVFGRD96 15.0 265 70 -25 4.30 0.4197
WVFGRD96 16.0 265 70 -25 4.32 0.4329
WVFGRD96 17.0 265 70 -25 4.34 0.4456
WVFGRD96 18.0 265 70 -25 4.35 0.4579
WVFGRD96 19.0 265 70 -20 4.36 0.4707
WVFGRD96 20.0 265 70 -20 4.38 0.4841
WVFGRD96 21.0 265 70 -25 4.40 0.4969
WVFGRD96 22.0 265 70 -25 4.41 0.5089
WVFGRD96 23.0 265 70 -25 4.42 0.5204
WVFGRD96 24.0 265 70 -25 4.44 0.5316
WVFGRD96 25.0 265 70 -25 4.45 0.5425
WVFGRD96 26.0 265 70 -25 4.46 0.5523
WVFGRD96 27.0 265 70 -25 4.47 0.5616
WVFGRD96 28.0 265 70 -25 4.48 0.5692
WVFGRD96 29.0 260 65 -30 4.49 0.5761
WVFGRD96 30.0 265 65 -25 4.50 0.5863
WVFGRD96 31.0 265 65 -25 4.51 0.5964
WVFGRD96 32.0 265 65 -25 4.52 0.6055
WVFGRD96 33.0 265 65 -25 4.53 0.6116
WVFGRD96 34.0 260 60 -30 4.54 0.6157
WVFGRD96 35.0 260 60 -30 4.55 0.6179
WVFGRD96 36.0 260 60 -30 4.56 0.6171
WVFGRD96 37.0 260 60 -30 4.57 0.6146
WVFGRD96 38.0 260 60 -30 4.59 0.6102
WVFGRD96 39.0 265 65 -25 4.60 0.6053
WVFGRD96 40.0 255 55 -40 4.68 0.6420
WVFGRD96 41.0 255 55 -40 4.69 0.6491
WVFGRD96 42.0 255 55 -40 4.70 0.6536
WVFGRD96 43.0 255 55 -40 4.71 0.6564
WVFGRD96 44.0 255 55 -45 4.73 0.6577
WVFGRD96 45.0 255 55 -40 4.73 0.6582
WVFGRD96 46.0 255 55 -40 4.74 0.6574
WVFGRD96 47.0 255 55 -40 4.75 0.6552
WVFGRD96 48.0 255 55 -40 4.76 0.6524
WVFGRD96 49.0 255 55 -40 4.76 0.6486
WVFGRD96 50.0 255 55 -40 4.77 0.6445
WVFGRD96 51.0 255 55 -40 4.78 0.6394
WVFGRD96 52.0 255 55 -40 4.78 0.6338
WVFGRD96 53.0 255 55 -40 4.79 0.6280
WVFGRD96 54.0 255 55 -40 4.79 0.6214
WVFGRD96 55.0 255 55 -40 4.79 0.6140
WVFGRD96 56.0 255 55 -40 4.80 0.6069
WVFGRD96 57.0 260 60 -30 4.79 0.6003
WVFGRD96 58.0 260 60 -30 4.79 0.5950
WVFGRD96 59.0 260 60 -30 4.80 0.5890
WVFGRD96 60.0 260 60 -30 4.80 0.5835
WVFGRD96 61.0 260 60 -30 4.80 0.5780
WVFGRD96 62.0 260 60 -30 4.80 0.5714
WVFGRD96 63.0 260 60 -30 4.81 0.5656
WVFGRD96 64.0 260 60 -30 4.81 0.5603
WVFGRD96 65.0 260 65 -25 4.81 0.5550
WVFGRD96 66.0 260 65 -25 4.81 0.5507
WVFGRD96 67.0 260 65 -25 4.81 0.5463
WVFGRD96 68.0 260 65 -25 4.81 0.5424
WVFGRD96 69.0 260 65 -25 4.81 0.5373
WVFGRD96 70.0 260 65 -25 4.81 0.5335
WVFGRD96 71.0 260 65 -25 4.82 0.5297
WVFGRD96 72.0 265 70 -15 4.80 0.5266
WVFGRD96 73.0 265 70 -15 4.80 0.5245
WVFGRD96 74.0 265 70 -15 4.80 0.5230
WVFGRD96 75.0 265 70 -15 4.81 0.5206
WVFGRD96 76.0 265 70 -15 4.81 0.5189
WVFGRD96 77.0 265 70 -15 4.81 0.5172
WVFGRD96 78.0 265 70 -15 4.81 0.5154
WVFGRD96 79.0 90 70 -20 4.78 0.5095
The best solution is
WVFGRD96 45.0 255 55 -40 4.73 0.6582
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 02:00:28 AM CDT 2024
