Location
Location ANSS
The ANSS event ID is ak0212o88mqd and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0212o88mqd/executive.
2021/02/27 18:59:25 61.319 -149.933 46.0 5.3 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2021/02/27 18:59:25:0 61.32 -149.93 46.0 5.3 Alaska
Stations used:
AK.BMR AK.BPAW AK.BRLK AK.CAST AK.CNP AK.CUT AK.DHY AK.GHO
AK.GLI AK.HOM AK.KNK AK.L20K AK.L22K AK.M20K AK.P23K AK.PAX
AK.PPLA AK.RAG AK.SAW AK.SCM AK.SKN AK.SLK AT.PMR AV.ILS
AV.RED AV.SPCP TA.O22K
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 = 9.77e+23 dyne-cm
Mw = 5.26
Z = 48 km
Plane Strike Dip Rake
NP1 190 60 -75
NP2 342 33 -114
Principal Axes:
Axis Value Plunge Azimuth
T 9.77e+23 14 269
N 0.00e+00 13 2
P -9.77e+23 71 134
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.03e+22
Mxy 6.60e+22
Mxz 2.06e+23
Myy 8.68e+23
Myz -4.43e+23
Mzz -8.17e+23
####-----#####
###########--#########
############-------#########
############----------########
#############-------------########
#############---------------########
#############------------------#######
##############------------------########
#############--------------------#######
##############---------------------#######
# #########----------------------#######
# T #########---------- ---------#######
# #########---------- P ---------#######
############---------- ---------######
############----------------------######
###########----------------------#####
##########---------------------#####
##########--------------------####
########-------------------###
########----------------####
######--------------##
###-----------
Global CMT Convention Moment Tensor:
R T P
-8.17e+23 2.06e+23 4.43e+23
2.06e+23 -5.03e+22 -6.60e+22
4.43e+23 -6.60e+22 8.68e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210227185925/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 = 190
DIP = 60
RAKE = -75
MW = 5.26
HS = 48.0
The NDK file is 20210227185925.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 2021/02/27 18:59:25:0 61.32 -149.93 46.0 5.3 Alaska
Stations used:
AK.BMR AK.BPAW AK.BRLK AK.CAST AK.CNP AK.CUT AK.DHY AK.GHO
AK.GLI AK.HOM AK.KNK AK.L20K AK.L22K AK.M20K AK.P23K AK.PAX
AK.PPLA AK.RAG AK.SAW AK.SCM AK.SKN AK.SLK AT.PMR AV.ILS
AV.RED AV.SPCP TA.O22K
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 = 9.77e+23 dyne-cm
Mw = 5.26
Z = 48 km
Plane Strike Dip Rake
NP1 190 60 -75
NP2 342 33 -114
Principal Axes:
Axis Value Plunge Azimuth
T 9.77e+23 14 269
N 0.00e+00 13 2
P -9.77e+23 71 134
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.03e+22
Mxy 6.60e+22
Mxz 2.06e+23
Myy 8.68e+23
Myz -4.43e+23
Mzz -8.17e+23
####-----#####
###########--#########
############-------#########
############----------########
#############-------------########
#############---------------########
#############------------------#######
##############------------------########
#############--------------------#######
##############---------------------#######
# #########----------------------#######
# T #########---------- ---------#######
# #########---------- P ---------#######
############---------- ---------######
############----------------------######
###########----------------------#####
##########---------------------#####
##########--------------------####
########-------------------###
########----------------####
######--------------##
###-----------
Global CMT Convention Moment Tensor:
R T P
-8.17e+23 2.06e+23 4.43e+23
2.06e+23 -5.03e+22 -6.60e+22
4.43e+23 -6.60e+22 8.68e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210227185925/index.html
|
W-phase Moment Tensor (Mww)
Moment 1.228e+17 N-m
Magnitude 5.33 Mww
Depth 45.5 km
Percent DC 99%
Half Duration 1.11 s
Catalog US
Data Source US 3
Contributor US 3
Nodal Planes
Plane Strike Dip Rake
NP1 181 56 -91
NP2 4 34 -88
Principal Axes
Axis Value Plunge Azimuth
T 1.226e+17 N-m 11 272
N 0.006e+17 N-m 1 182
P -1.231e+17 N-m 79 86
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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 2.0 0 40 90 4.54 0.2732
WVFGRD96 4.0 155 50 60 4.59 0.2199
WVFGRD96 6.0 160 85 55 4.57 0.2319
WVFGRD96 8.0 165 80 65 4.65 0.2640
WVFGRD96 10.0 260 40 15 4.68 0.2891
WVFGRD96 12.0 260 45 20 4.72 0.3063
WVFGRD96 14.0 260 45 20 4.74 0.3183
WVFGRD96 16.0 250 45 25 4.76 0.3248
WVFGRD96 18.0 235 40 -25 4.78 0.3324
WVFGRD96 20.0 230 40 -30 4.80 0.3431
WVFGRD96 22.0 35 55 -50 4.84 0.3549
WVFGRD96 24.0 35 55 -50 4.87 0.3734
WVFGRD96 26.0 35 55 -45 4.89 0.3914
WVFGRD96 28.0 210 50 -50 4.92 0.4168
WVFGRD96 30.0 205 50 -55 4.95 0.4549
WVFGRD96 32.0 200 50 -60 4.98 0.4944
WVFGRD96 34.0 200 55 -55 5.01 0.5270
WVFGRD96 36.0 195 55 -60 5.03 0.5509
WVFGRD96 38.0 200 60 -55 5.06 0.5711
WVFGRD96 40.0 195 60 -65 5.17 0.5931
WVFGRD96 42.0 195 60 -65 5.20 0.6279
WVFGRD96 44.0 195 60 -70 5.23 0.6514
WVFGRD96 46.0 195 60 -70 5.25 0.6662
WVFGRD96 48.0 190 60 -75 5.26 0.6715
WVFGRD96 50.0 190 60 -80 5.28 0.6704
WVFGRD96 52.0 185 60 -80 5.28 0.6631
WVFGRD96 54.0 -5 30 -95 5.29 0.6486
WVFGRD96 56.0 180 60 -90 5.29 0.6312
WVFGRD96 58.0 0 30 -90 5.30 0.6113
WVFGRD96 60.0 0 30 -90 5.29 0.5893
WVFGRD96 62.0 0 30 -85 5.29 0.5673
WVFGRD96 64.0 5 30 -80 5.29 0.5453
WVFGRD96 66.0 5 30 -80 5.29 0.5245
WVFGRD96 68.0 20 35 -60 5.29 0.5043
WVFGRD96 70.0 30 40 -45 5.30 0.4905
WVFGRD96 72.0 200 75 -50 5.30 0.4845
WVFGRD96 74.0 200 75 -50 5.30 0.4756
WVFGRD96 76.0 200 75 -50 5.30 0.4663
WVFGRD96 78.0 205 80 -45 5.30 0.4583
WVFGRD96 80.0 200 75 -50 5.30 0.4509
WVFGRD96 82.0 205 80 -45 5.31 0.4443
WVFGRD96 84.0 205 80 -50 5.31 0.4383
WVFGRD96 86.0 205 80 -50 5.31 0.4325
WVFGRD96 88.0 205 75 -45 5.29 0.4269
WVFGRD96 90.0 205 75 -45 5.29 0.4222
WVFGRD96 92.0 205 75 -45 5.29 0.4169
WVFGRD96 94.0 205 75 -45 5.29 0.4118
WVFGRD96 96.0 205 75 -45 5.29 0.4074
WVFGRD96 98.0 205 75 -45 5.29 0.4023
The best solution is
WVFGRD96 48.0 190 60 -75 5.26 0.6715
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 Wed Apr 24 09:31:48 PM CDT 2024
