Location
Location ANSS
The ANSS event ID is ak02117d7jj0 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak02117d7jj0/executive.
2021/01/26 22:23:25 59.174 -151.663 44.6 4.3 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2021/01/26 22:23:25:0 59.17 -151.66 44.6 4.3 Alaska
Stations used:
AK.BRLK AK.CNP AK.DIV AK.GHO AK.GLI AK.KLU AK.KNK AK.M20K
AK.N19K AK.O18K AK.O19K AK.PWL AK.SAW AK.SLK AT.PMR AV.ILS
AV.RED II.KDAK
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 0.07 n 3
Best Fitting Double Couple
Mo = 3.80e+22 dyne-cm
Mw = 4.32
Z = 50 km
Plane Strike Dip Rake
NP1 200 75 -50
NP2 307 42 -157
Principal Axes:
Axis Value Plunge Azimuth
T 3.80e+22 20 261
N 0.00e+00 38 8
P -3.80e+22 45 150
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.35e+22
Mxy 1.34e+22
Mxz 1.46e+22
Myy 2.80e+22
Myz -2.15e+22
Mzz -1.46e+22
--------------
----------------######
------------------##########
--###########----#############
##################--##############
##################------############
##################----------##########
##################-------------#########
##################--------------########
##################----------------########
#################------------------#######
### ###########-------------------######
### T ##########---------------------#####
## ##########----------------------###
##############-----------------------###
#############---------- ----------##
###########----------- P ----------#
##########----------- ----------
########----------------------
#######---------------------
####------------------
--------------
Global CMT Convention Moment Tensor:
R T P
-1.46e+22 1.46e+22 2.15e+22
1.46e+22 -1.35e+22 -1.34e+22
2.15e+22 -1.34e+22 2.80e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210126222325/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 = 200
DIP = 75
RAKE = -50
MW = 4.32
HS = 50.0
The NDK file is 20210126222325.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 2021/01/26 22:23:25:0 59.17 -151.66 44.6 4.3 Alaska
Stations used:
AK.BRLK AK.CNP AK.DIV AK.GHO AK.GLI AK.KLU AK.KNK AK.M20K
AK.N19K AK.O18K AK.O19K AK.PWL AK.SAW AK.SLK AT.PMR AV.ILS
AV.RED II.KDAK
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 0.07 n 3
Best Fitting Double Couple
Mo = 3.80e+22 dyne-cm
Mw = 4.32
Z = 50 km
Plane Strike Dip Rake
NP1 200 75 -50
NP2 307 42 -157
Principal Axes:
Axis Value Plunge Azimuth
T 3.80e+22 20 261
N 0.00e+00 38 8
P -3.80e+22 45 150
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.35e+22
Mxy 1.34e+22
Mxz 1.46e+22
Myy 2.80e+22
Myz -2.15e+22
Mzz -1.46e+22
--------------
----------------######
------------------##########
--###########----#############
##################--##############
##################------############
##################----------##########
##################-------------#########
##################--------------########
##################----------------########
#################------------------#######
### ###########-------------------######
### T ##########---------------------#####
## ##########----------------------###
##############-----------------------###
#############---------- ----------##
###########----------- P ----------#
##########----------- ----------
########----------------------
#######---------------------
####------------------
--------------
Global CMT Convention Moment Tensor:
R T P
-1.46e+22 1.46e+22 2.15e+22
1.46e+22 -1.35e+22 -1.34e+22
2.15e+22 -1.34e+22 2.80e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210126222325/index.html
|
Regional Moment Tensor (Mwr)
Moment 4.818e+15 N-m
Magnitude 4.39 Mwr
Depth 50.0 km
Percent DC 67%
Half Duration -
Catalog US
Data Source US 3
Contributor US 3
Nodal Planes
Plane Strike Dip Rake
NP1 347 26 -116
NP2 195 66 -78
Principal Axes
Axis Value Plunge Azimuth
T 5.184e+15 N-m 20 276
N -0.843e+15 N-m 11 10
P -4.340e+15 N-m 66 127
|
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 0.07 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 355 45 95 3.72 0.3037
WVFGRD96 4.0 15 85 0 3.68 0.3539
WVFGRD96 6.0 15 75 -15 3.75 0.3874
WVFGRD96 8.0 200 65 -10 3.80 0.4233
WVFGRD96 10.0 200 65 -15 3.84 0.4537
WVFGRD96 12.0 200 65 -20 3.88 0.4794
WVFGRD96 14.0 200 65 -20 3.91 0.5026
WVFGRD96 16.0 205 70 -20 3.93 0.5251
WVFGRD96 18.0 205 70 -25 3.96 0.5485
WVFGRD96 20.0 205 70 -25 3.99 0.5718
WVFGRD96 22.0 205 70 -25 4.01 0.5962
WVFGRD96 24.0 205 75 -25 4.03 0.6180
WVFGRD96 26.0 205 75 -30 4.05 0.6460
WVFGRD96 28.0 205 75 -30 4.07 0.6727
WVFGRD96 30.0 205 75 -35 4.10 0.6955
WVFGRD96 32.0 200 75 -35 4.12 0.7156
WVFGRD96 34.0 200 75 -35 4.14 0.7348
WVFGRD96 36.0 200 75 -35 4.15 0.7496
WVFGRD96 38.0 200 75 -35 4.17 0.7612
WVFGRD96 40.0 200 75 -45 4.25 0.7698
WVFGRD96 42.0 200 75 -50 4.28 0.7786
WVFGRD96 44.0 200 75 -50 4.29 0.7836
WVFGRD96 46.0 200 75 -50 4.30 0.7891
WVFGRD96 48.0 200 75 -50 4.31 0.7930
WVFGRD96 50.0 200 75 -50 4.32 0.7936
WVFGRD96 52.0 200 75 -55 4.34 0.7928
WVFGRD96 54.0 200 75 -55 4.35 0.7905
WVFGRD96 56.0 200 75 -55 4.35 0.7872
WVFGRD96 58.0 200 75 -55 4.36 0.7826
WVFGRD96 60.0 200 75 -55 4.36 0.7798
WVFGRD96 62.0 200 75 -55 4.36 0.7755
WVFGRD96 64.0 200 75 -55 4.37 0.7693
WVFGRD96 66.0 200 75 -55 4.37 0.7616
WVFGRD96 68.0 200 75 -55 4.37 0.7566
WVFGRD96 70.0 200 75 -55 4.38 0.7522
WVFGRD96 72.0 200 75 -55 4.38 0.7463
WVFGRD96 74.0 200 75 -55 4.38 0.7415
WVFGRD96 76.0 200 75 -55 4.39 0.7377
WVFGRD96 78.0 200 75 -55 4.39 0.7320
WVFGRD96 80.0 200 75 -55 4.39 0.7289
WVFGRD96 82.0 200 75 -60 4.40 0.7247
WVFGRD96 84.0 200 75 -60 4.40 0.7198
WVFGRD96 86.0 200 75 -60 4.41 0.7172
WVFGRD96 88.0 200 75 -60 4.41 0.7113
WVFGRD96 90.0 205 75 -60 4.41 0.7087
WVFGRD96 92.0 205 75 -60 4.41 0.7026
WVFGRD96 94.0 230 90 -70 4.43 0.6982
WVFGRD96 96.0 55 85 70 4.44 0.6993
WVFGRD96 98.0 55 85 70 4.44 0.6992
The best solution is
WVFGRD96 50.0 200 75 -50 4.32 0.7936
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 0.07 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 09:01:52 PM CDT 2024
