Location
Location ANSS
The ANSS event ID is ak023dtuk2yv and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak023dtuk2yv/executive.
2023/10/28 03:01:49 59.018 -136.460 7.2 5.1 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2023/10/28 03:01:49:0 59.02 -136.46 7.2 5.1 Alaska
Stations used:
AK.BAGL AK.BAL AK.BARK AK.BARN AK.BERG AK.BESE AK.BMR
AK.CRQ AK.DOT AK.GRNC AK.HARP AK.ISLE AK.JIS AK.KIAG AK.KLU
AK.LOGN AK.M27K AK.MCAR AK.MESA AK.PAX AK.PIN AK.PNL
AK.PS11 AK.PS12 AK.PTPK AK.R32K AK.S31K AK.S32K AK.SAMH
AK.TGL AK.U33K AK.V35K AK.VRDI AK.WAX AT.CRAG AT.MENT
AT.SIT AT.SKAG AV.EDCR AV.EDES AV.EDSO AV.N25K CN.ATLI
CN.BVCY CN.DAWY CN.HYT CN.PLBC CN.RUBB CN.WHY CN.YUK3
CN.YUK4 CN.YUK5 CN.YUK6 CN.YUK7 CN.YUK8 NY.MAYO NY.MMPY
NY.WTLY US.WRAK
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 0.06 n 3
Best Fitting Double Couple
Mo = 3.59e+23 dyne-cm
Mw = 4.97
Z = 3 km
Plane Strike Dip Rake
NP1 347 48 109
NP2 140 45 70
Principal Axes:
Axis Value Plunge Azimuth
T 3.59e+23 76 327
N 0.00e+00 14 154
P -3.59e+23 2 64
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.39e+22
Mxy -1.51e+23
Mxz 6.65e+22
Myy -2.83e+23
Myz -5.58e+22
Mzz 3.37e+23
#####---------
############----------
-################-----------
-###################----------
--#####################-----------
---######################----------
----#######################--------- P
-----########################--------
------########### ##########----------
-------########### T ##########-----------
--------########## ##########-----------
--------########################----------
---------#######################----------
---------######################---------
-----------####################---------
-----------###################--------
------------################--------
--------------#############-------
---------------#########------
------------------####------
------------------####
------------##
Global CMT Convention Moment Tensor:
R T P
3.37e+23 6.65e+22 5.58e+22
6.65e+22 -5.39e+22 1.51e+23
5.58e+22 1.51e+23 -2.83e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20231028030149/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 = 140
DIP = 45
RAKE = 70
MW = 4.97
HS = 3.0
The NDK file is 20231028030149.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 |
USGS/SLU Moment Tensor Solution
ENS 2023/10/28 03:01:49:0 59.02 -136.46 7.2 5.1 Alaska
Stations used:
AK.BAGL AK.BAL AK.BARK AK.BARN AK.BERG AK.BESE AK.BMR
AK.CRQ AK.DOT AK.GRNC AK.HARP AK.ISLE AK.JIS AK.KIAG AK.KLU
AK.LOGN AK.M27K AK.MCAR AK.MESA AK.PAX AK.PIN AK.PNL
AK.PS11 AK.PS12 AK.PTPK AK.R32K AK.S31K AK.S32K AK.SAMH
AK.TGL AK.U33K AK.V35K AK.VRDI AK.WAX AT.CRAG AT.MENT
AT.SIT AT.SKAG AV.EDCR AV.EDES AV.EDSO AV.N25K CN.ATLI
CN.BVCY CN.DAWY CN.HYT CN.PLBC CN.RUBB CN.WHY CN.YUK3
CN.YUK4 CN.YUK5 CN.YUK6 CN.YUK7 CN.YUK8 NY.MAYO NY.MMPY
NY.WTLY US.WRAK
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 0.06 n 3
Best Fitting Double Couple
Mo = 3.59e+23 dyne-cm
Mw = 4.97
Z = 3 km
Plane Strike Dip Rake
NP1 347 48 109
NP2 140 45 70
Principal Axes:
Axis Value Plunge Azimuth
T 3.59e+23 76 327
N 0.00e+00 14 154
P -3.59e+23 2 64
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.39e+22
Mxy -1.51e+23
Mxz 6.65e+22
Myy -2.83e+23
Myz -5.58e+22
Mzz 3.37e+23
#####---------
############----------
-################-----------
-###################----------
--#####################-----------
---######################----------
----#######################--------- P
-----########################--------
------########### ##########----------
-------########### T ##########-----------
--------########## ##########-----------
--------########################----------
---------#######################----------
---------######################---------
-----------####################---------
-----------###################--------
------------################--------
--------------#############-------
---------------#########------
------------------####------
------------------####
------------##
Global CMT Convention Moment Tensor:
R T P
3.37e+23 6.65e+22 5.58e+22
6.65e+22 -5.39e+22 1.51e+23
5.58e+22 1.51e+23 -2.83e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20231028030149/index.html
|
W-phase Moment Tensor (Mww)
Moment 4.917e+16 N-m
Magnitude 5.06 Mww
Depth 11.5 km
Percent DC 85%
Half Duration 0.86 s
Catalog US
Data Source US 3
Contributor US 3
Nodal Planes
Plane Strike Dip Rake
NP1 319 55 70
NP2 171 39 116
Principal Axes
Axis Value Plunge Azimuth
T 4.716e+16 N-m 72 179
N 0.379e+16 N-m 16 330
P -5.095e+16 N-m 8 62
|
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 0.06 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 125 45 40 4.90 0.5791
WVFGRD96 2.0 135 50 60 4.93 0.6252
WVFGRD96 3.0 140 45 70 4.97 0.6330
WVFGRD96 4.0 145 45 75 4.98 0.5996
WVFGRD96 5.0 140 45 70 4.96 0.5361
WVFGRD96 6.0 295 70 30 4.85 0.5164
WVFGRD96 7.0 285 65 -20 4.87 0.5261
WVFGRD96 8.0 285 65 -20 4.88 0.5373
WVFGRD96 9.0 285 65 -20 4.88 0.5480
WVFGRD96 10.0 285 65 -20 4.90 0.5512
WVFGRD96 11.0 285 65 -20 4.90 0.5585
WVFGRD96 12.0 285 65 -20 4.91 0.5623
WVFGRD96 13.0 285 65 -20 4.91 0.5634
WVFGRD96 14.0 285 65 -20 4.92 0.5628
WVFGRD96 15.0 285 65 -20 4.92 0.5614
WVFGRD96 16.0 285 65 -20 4.93 0.5589
WVFGRD96 17.0 285 65 -20 4.93 0.5554
WVFGRD96 18.0 285 65 -20 4.94 0.5515
WVFGRD96 19.0 285 65 -20 4.94 0.5475
WVFGRD96 20.0 285 65 -20 4.96 0.5400
WVFGRD96 21.0 285 65 -20 4.96 0.5331
WVFGRD96 22.0 285 65 -20 4.96 0.5262
WVFGRD96 23.0 285 65 -20 4.97 0.5188
WVFGRD96 24.0 285 65 -20 4.97 0.5112
WVFGRD96 25.0 285 65 -20 4.98 0.5040
WVFGRD96 26.0 285 65 -15 4.98 0.4962
WVFGRD96 27.0 285 65 -15 4.99 0.4885
WVFGRD96 28.0 285 65 -15 4.99 0.4814
WVFGRD96 29.0 285 65 -15 5.00 0.4739
The best solution is
WVFGRD96 3.0 140 45 70 4.97 0.6330
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 0.06 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 CUS.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
CUS Model with Q from simple gamma values
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
1.0000 5.0000 2.8900 2.5000 0.172E-02 0.387E-02 0.00 0.00 1.00 1.00
9.0000 6.1000 3.5200 2.7300 0.160E-02 0.363E-02 0.00 0.00 1.00 1.00
10.0000 6.4000 3.7000 2.8200 0.149E-02 0.336E-02 0.00 0.00 1.00 1.00
20.0000 6.7000 3.8700 2.9020 0.000E-04 0.000E-04 0.00 0.00 1.00 1.00
0.0000 8.1500 4.7000 3.3640 0.194E-02 0.431E-02 0.00 0.00 1.00 1.00
Last Changed Tue Apr 23 04:43:08 AM CDT 2024
