Location
Location ANSS
The ANSS event ID is ak017f7s3c06 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak017f7s3c06/executive.
2017/11/27 22:18:30 60.555 -147.430 16.6 5.3 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2017/11/27 22:18:30:0 60.56 -147.43 16.6 5.3 Alaska
Stations used:
AK.BARN AK.BRLK AK.BWN AK.CAST AK.CNP AK.CUT AK.DIV AK.EYAK
AK.GHO AK.GLB AK.GRNC AK.HDA AK.HIN AK.KLU AK.KNK AK.KTH
AK.MCAR AK.MCK AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM AK.SWD
AK.TGL AK.TRF AK.VRDI AK.WAX AK.WRH AT.MENT AT.PMR TA.L26K
TA.M22K TA.M24K TA.M27K TA.N25K TA.O22K TA.Q20K
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 0.10 n 3
Best Fitting Double Couple
Mo = 5.07e+23 dyne-cm
Mw = 5.07
Z = 29 km
Plane Strike Dip Rake
NP1 210 85 -70
NP2 313 21 -166
Principal Axes:
Axis Value Plunge Azimuth
T 5.07e+23 37 282
N 0.00e+00 20 28
P -5.07e+23 46 141
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.29e+23
Mxy 5.05e+22
Mxz 2.48e+23
Myy 2.12e+23
Myz -3.99e+23
Mzz -8.27e+22
--------------
---#########---------#
-###################---#####
######################--######
#######################------#####
#######################--------#####
#######################-----------####
######################--------------####
###### ############----------------###
####### T ###########-----------------####
####### ##########-------------------###
###################--------------------###
##################---------------------###
################----------------------##
###############---------- ----------##
#############----------- P ----------#
###########------------ ---------#
#########------------------------#
######------------------------
#####-----------------------
#---------------------
--------------
Global CMT Convention Moment Tensor:
R T P
-8.27e+22 2.48e+23 3.99e+23
2.48e+23 -1.29e+23 -5.05e+22
3.99e+23 -5.05e+22 2.12e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171127221830/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 = 210
DIP = 85
RAKE = -70
MW = 5.07
HS = 29.0
The NDK file is 20171127221830.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 2017/11/27 22:18:30:0 60.56 -147.43 16.6 5.3 Alaska
Stations used:
AK.BARN AK.BRLK AK.BWN AK.CAST AK.CNP AK.CUT AK.DIV AK.EYAK
AK.GHO AK.GLB AK.GRNC AK.HDA AK.HIN AK.KLU AK.KNK AK.KTH
AK.MCAR AK.MCK AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM AK.SWD
AK.TGL AK.TRF AK.VRDI AK.WAX AK.WRH AT.MENT AT.PMR TA.L26K
TA.M22K TA.M24K TA.M27K TA.N25K TA.O22K TA.Q20K
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 0.10 n 3
Best Fitting Double Couple
Mo = 5.07e+23 dyne-cm
Mw = 5.07
Z = 29 km
Plane Strike Dip Rake
NP1 210 85 -70
NP2 313 21 -166
Principal Axes:
Axis Value Plunge Azimuth
T 5.07e+23 37 282
N 0.00e+00 20 28
P -5.07e+23 46 141
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.29e+23
Mxy 5.05e+22
Mxz 2.48e+23
Myy 2.12e+23
Myz -3.99e+23
Mzz -8.27e+22
--------------
---#########---------#
-###################---#####
######################--######
#######################------#####
#######################--------#####
#######################-----------####
######################--------------####
###### ############----------------###
####### T ###########-----------------####
####### ##########-------------------###
###################--------------------###
##################---------------------###
################----------------------##
###############---------- ----------##
#############----------- P ----------#
###########------------ ---------#
#########------------------------#
######------------------------
#####-----------------------
#---------------------
--------------
Global CMT Convention Moment Tensor:
R T P
-8.27e+22 2.48e+23 3.99e+23
2.48e+23 -1.29e+23 -5.05e+22
3.99e+23 -5.05e+22 2.12e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171127221830/index.html
|
W-phase Moment Tensor (Mww)
Moment 1.059e+17 N-m
Magnitude 5.3 Mww
Depth 23.5 km
Percent DC 87 %
Half Duration 1.38 s
Catalog US
Data Source US3
Contributor US3
Nodal Planes
Plane Strike Dip Rake
NP1 334 21 -151
NP2 216 80 -71
Principal Axes
Axis Value Plunge Azimuth
T 1.023e+17 N-m 33 291
N 0.069e+17 N-m 18 33
P -1.092e+17 N-m 51 148
|
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 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 30 45 85 4.40 0.1888
WVFGRD96 2.0 35 40 90 4.55 0.2543
WVFGRD96 3.0 35 40 90 4.59 0.2105
WVFGRD96 4.0 300 35 -5 4.55 0.2061
WVFGRD96 5.0 315 25 20 4.58 0.2408
WVFGRD96 6.0 310 30 15 4.59 0.2697
WVFGRD96 7.0 315 30 20 4.61 0.2922
WVFGRD96 8.0 40 85 60 4.69 0.3148
WVFGRD96 9.0 40 85 60 4.71 0.3465
WVFGRD96 10.0 40 85 55 4.74 0.3766
WVFGRD96 11.0 215 90 -55 4.76 0.4040
WVFGRD96 12.0 35 90 55 4.78 0.4310
WVFGRD96 13.0 215 90 -55 4.80 0.4560
WVFGRD96 14.0 215 85 -55 4.82 0.4804
WVFGRD96 15.0 215 85 -55 4.85 0.5050
WVFGRD96 16.0 215 85 -55 4.87 0.5278
WVFGRD96 17.0 215 85 -60 4.88 0.5497
WVFGRD96 18.0 210 80 -60 4.90 0.5705
WVFGRD96 19.0 210 80 -65 4.91 0.5897
WVFGRD96 20.0 210 80 -65 4.93 0.6081
WVFGRD96 21.0 210 80 -65 4.96 0.6244
WVFGRD96 22.0 210 80 -65 4.97 0.6397
WVFGRD96 23.0 210 80 -65 4.99 0.6540
WVFGRD96 24.0 210 80 -65 5.01 0.6667
WVFGRD96 25.0 210 80 -65 5.02 0.6782
WVFGRD96 26.0 215 85 -65 5.04 0.6880
WVFGRD96 27.0 215 85 -65 5.05 0.6955
WVFGRD96 28.0 210 85 -70 5.06 0.7009
WVFGRD96 29.0 210 85 -70 5.07 0.7032
WVFGRD96 30.0 210 85 -70 5.09 0.7030
WVFGRD96 31.0 210 85 -65 5.10 0.6998
WVFGRD96 32.0 210 85 -65 5.10 0.6946
WVFGRD96 33.0 210 85 -65 5.11 0.6871
WVFGRD96 34.0 35 90 65 5.12 0.6712
WVFGRD96 35.0 35 90 65 5.12 0.6637
WVFGRD96 36.0 35 90 65 5.13 0.6548
WVFGRD96 37.0 210 85 -65 5.13 0.6454
WVFGRD96 38.0 35 90 60 5.14 0.6333
WVFGRD96 39.0 215 90 -60 5.14 0.6198
WVFGRD96 40.0 35 90 70 5.27 0.6037
WVFGRD96 41.0 35 90 65 5.26 0.5879
WVFGRD96 42.0 35 90 65 5.26 0.5736
WVFGRD96 43.0 215 90 -65 5.26 0.5576
WVFGRD96 44.0 35 90 60 5.26 0.5433
WVFGRD96 45.0 40 85 60 5.27 0.5304
WVFGRD96 46.0 40 85 60 5.27 0.5180
WVFGRD96 47.0 45 80 60 5.27 0.5077
WVFGRD96 48.0 45 80 55 5.27 0.4986
WVFGRD96 49.0 45 80 55 5.28 0.4887
The best solution is
WVFGRD96 29.0 210 85 -70 5.07 0.7032
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 0.10 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 Sat Apr 27 08:55:57 PM CDT 2024
