Location
Location ANSS
The ANSS event ID is ak0217ti4bu9 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0217ti4bu9/executive.
2021/06/19 21:43:05 63.999 -149.780 12.7 4.4 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2021/06/19 21:43:05:0 64.00 -149.78 12.7 4.4 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.COLD AK.CUT AK.DHY AK.DIV AK.DOT
AK.EYAK AK.F21K AK.FID AK.G23K AK.G24K AK.GHO AK.GLB AK.GLI
AK.H21K AK.H22K AK.H23K AK.H24K AK.HARP AK.HDA AK.HIN
AK.I21K AK.I23K AK.I27K AK.J19K AK.J20K AK.J25K AK.K20K
AK.K24K AK.KLU AK.KNK AK.KTH AK.L19K AK.L20K AK.L26K
AK.M19K AK.M20K AK.M26K AK.M27K AK.MCAR AK.MCK AK.MLY
AK.NEA2 AK.PAX AK.POKR AK.PPD AK.PPLA AK.PWL AK.RC01
AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.SLK AK.SSN
AK.SWD AK.TRF AK.VRDI AK.WRH AT.MENT AT.PMR AV.RED AV.SPCP
AV.STLK 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 0.10 n 3
Best Fitting Double Couple
Mo = 3.20e+22 dyne-cm
Mw = 4.27
Z = 15 km
Plane Strike Dip Rake
NP1 40 55 65
NP2 259 42 121
Principal Axes:
Axis Value Plunge Azimuth
T 3.20e+22 69 255
N 0.00e+00 20 55
P -3.20e+22 7 148
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.22e+22
Mxy 1.53e+22
Mxz 4.34e+20
Myy -5.08e+21
Myz -1.26e+22
Mzz 2.72e+22
--------------
----------------------
---------------------------#
-----------------------------#
-----------------######--------###
-----------####################-####
--------#########################--###
------###########################-----##
----#############################-------
----#############################---------
---########### ###############----------
--############ T ##############-----------
-############# #############------------
############################------------
##########################--------------
#######################---------------
####################----------------
#################-----------------
###########-------------- --
#####------------------- P -
---------------------
--------------
Global CMT Convention Moment Tensor:
R T P
2.72e+22 4.34e+20 1.26e+22
4.34e+20 -2.22e+22 -1.53e+22
1.26e+22 -1.53e+22 -5.08e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210619214305/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 = 40
DIP = 55
RAKE = 65
MW = 4.27
HS = 15.0
The NDK file is 20210619214305.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 |
USGSMWR |
USGS/SLU Moment Tensor Solution
ENS 2021/06/19 21:43:05:0 64.00 -149.78 12.7 4.4 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.COLD AK.CUT AK.DHY AK.DIV AK.DOT
AK.EYAK AK.F21K AK.FID AK.G23K AK.G24K AK.GHO AK.GLB AK.GLI
AK.H21K AK.H22K AK.H23K AK.H24K AK.HARP AK.HDA AK.HIN
AK.I21K AK.I23K AK.I27K AK.J19K AK.J20K AK.J25K AK.K20K
AK.K24K AK.KLU AK.KNK AK.KTH AK.L19K AK.L20K AK.L26K
AK.M19K AK.M20K AK.M26K AK.M27K AK.MCAR AK.MCK AK.MLY
AK.NEA2 AK.PAX AK.POKR AK.PPD AK.PPLA AK.PWL AK.RC01
AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.SLK AK.SSN
AK.SWD AK.TRF AK.VRDI AK.WRH AT.MENT AT.PMR AV.RED AV.SPCP
AV.STLK 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 0.10 n 3
Best Fitting Double Couple
Mo = 3.20e+22 dyne-cm
Mw = 4.27
Z = 15 km
Plane Strike Dip Rake
NP1 40 55 65
NP2 259 42 121
Principal Axes:
Axis Value Plunge Azimuth
T 3.20e+22 69 255
N 0.00e+00 20 55
P -3.20e+22 7 148
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.22e+22
Mxy 1.53e+22
Mxz 4.34e+20
Myy -5.08e+21
Myz -1.26e+22
Mzz 2.72e+22
--------------
----------------------
---------------------------#
-----------------------------#
-----------------######--------###
-----------####################-####
--------#########################--###
------###########################-----##
----#############################-------
----#############################---------
---########### ###############----------
--############ T ##############-----------
-############# #############------------
############################------------
##########################--------------
#######################---------------
####################----------------
#################-----------------
###########-------------- --
#####------------------- P -
---------------------
--------------
Global CMT Convention Moment Tensor:
R T P
2.72e+22 4.34e+20 1.26e+22
4.34e+20 -2.22e+22 -1.53e+22
1.26e+22 -1.53e+22 -5.08e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210619214305/index.html
|
Regional Moment Tensor (Mwr)
Moment 3.430e+15 N-m
Magnitude 4.29 Mwr
Depth 15.0 km
Percent DC 91%
Half Duration -
Catalog US
Data Source US 2
Contributor US 2
Nodal Planes
Plane Strike Dip Rake
NP1 255° 35° 112°
NP2 48° 58° 75°
Principal Axes
Axis Value Plunge Azimuth
T 3.345e+15 N-m 73° 281°
N 0.165e+15 N-m 13° 56°
P -3.510e+15 N-m 12° 149°
|
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 0.10 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 230 45 -90 3.82 0.2522
WVFGRD96 2.0 50 45 -90 3.97 0.3382
WVFGRD96 3.0 75 70 -65 3.97 0.2522
WVFGRD96 4.0 75 75 -70 3.99 0.3060
WVFGRD96 5.0 75 80 -75 4.01 0.3684
WVFGRD96 6.0 75 75 -75 4.03 0.4210
WVFGRD96 7.0 75 75 -75 4.04 0.4613
WVFGRD96 8.0 75 75 -75 4.13 0.4923
WVFGRD96 9.0 35 30 55 4.15 0.5301
WVFGRD96 10.0 40 60 65 4.19 0.5817
WVFGRD96 11.0 40 55 65 4.21 0.6271
WVFGRD96 12.0 40 55 65 4.23 0.6616
WVFGRD96 13.0 40 55 65 4.25 0.6841
WVFGRD96 14.0 40 55 65 4.26 0.6962
WVFGRD96 15.0 40 55 65 4.27 0.6993
WVFGRD96 16.0 40 50 65 4.28 0.6964
WVFGRD96 17.0 40 50 65 4.29 0.6882
WVFGRD96 18.0 40 50 65 4.30 0.6751
WVFGRD96 19.0 40 45 60 4.31 0.6588
WVFGRD96 20.0 35 45 60 4.32 0.6407
WVFGRD96 21.0 30 45 55 4.33 0.6201
WVFGRD96 22.0 30 45 55 4.34 0.5989
WVFGRD96 23.0 30 45 55 4.34 0.5766
WVFGRD96 24.0 30 40 50 4.35 0.5530
WVFGRD96 25.0 30 40 50 4.35 0.5290
WVFGRD96 26.0 30 40 50 4.35 0.5045
WVFGRD96 27.0 230 60 80 4.36 0.4823
WVFGRD96 28.0 235 60 85 4.36 0.4619
WVFGRD96 29.0 60 30 90 4.36 0.4399
The best solution is
WVFGRD96 15.0 40 55 65 4.27 0.6993
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 0.10 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 Wed Apr 24 11:49:10 PM CDT 2024
