Location
Location ANSS
The ANSS event ID is ak0215dsbmgn and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0215dsbmgn/executive.
2021/04/27 17:54:36 61.342 -149.980 43.8 4.8 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2021/04/27 17:54:36:0 61.34 -149.98 43.8 4.8 Alaska
Stations used:
AK.BRLK AK.CNP AK.CUT AK.DHY AK.EYAK AK.FIRE AK.GHO AK.GLI
AK.HOM AK.MCK AK.PPLA AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN
AK.SLK AK.SSN AT.PMR AV.ILS AV.RED AV.SPCP TA.M22K
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 = 1.32e+23 dyne-cm
Mw = 4.68
Z = 49 km
Plane Strike Dip Rake
NP1 175 50 -70
NP2 325 44 -112
Principal Axes:
Axis Value Plunge Azimuth
T 1.32e+23 3 251
N 0.00e+00 15 342
P -1.32e+23 74 150
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.92e+21
Mxy 4.46e+22
Mxz 2.70e+22
Myy 1.15e+23
Myz -2.40e+22
Mzz -1.22e+23
----##########
------################
########----################
########---------#############
#########------------#############
#########---------------############
##########-----------------###########
##########-------------------###########
##########---------------------#########
###########----------------------#########
###########----------------------#########
###########---------- ----------########
###########---------- P -----------#######
#########--------- -----------######
T #########-----------------------######
##########----------------------#####
##########----------------------####
##########---------------------###
#########-------------------##
##########----------------##
########--------------
#######-------
Global CMT Convention Moment Tensor:
R T P
-1.22e+23 2.70e+22 2.40e+22
2.70e+22 6.92e+21 -4.46e+22
2.40e+22 -4.46e+22 1.15e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210427175436/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 = 175
DIP = 50
RAKE = -70
MW = 4.68
HS = 49.0
The NDK file is 20210427175436.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 2021/04/27 17:54:36:0 61.34 -149.98 43.8 4.8 Alaska
Stations used:
AK.BRLK AK.CNP AK.CUT AK.DHY AK.EYAK AK.FIRE AK.GHO AK.GLI
AK.HOM AK.MCK AK.PPLA AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN
AK.SLK AK.SSN AT.PMR AV.ILS AV.RED AV.SPCP TA.M22K
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 = 1.32e+23 dyne-cm
Mw = 4.68
Z = 49 km
Plane Strike Dip Rake
NP1 175 50 -70
NP2 325 44 -112
Principal Axes:
Axis Value Plunge Azimuth
T 1.32e+23 3 251
N 0.00e+00 15 342
P -1.32e+23 74 150
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.92e+21
Mxy 4.46e+22
Mxz 2.70e+22
Myy 1.15e+23
Myz -2.40e+22
Mzz -1.22e+23
----##########
------################
########----################
########---------#############
#########------------#############
#########---------------############
##########-----------------###########
##########-------------------###########
##########---------------------#########
###########----------------------#########
###########----------------------#########
###########---------- ----------########
###########---------- P -----------#######
#########--------- -----------######
T #########-----------------------######
##########----------------------#####
##########----------------------####
##########---------------------###
#########-------------------##
##########----------------##
########--------------
#######-------
Global CMT Convention Moment Tensor:
R T P
-1.22e+23 2.70e+22 2.40e+22
2.70e+22 6.92e+21 -4.46e+22
2.40e+22 -4.46e+22 1.15e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210427175436/index.html
|
W-phase Moment Tensor (Mww)
Moment 1.515e+16 N-m
Magnitude 4.72 Mww
Depth 45.5 km
Percent DC 70%
Half Duration 0.59 s
Catalog US
Data Source US 3
Contributor US 3
Nodal Planes
Plane Strike Dip Rake
NP1 337 41 -107
NP2 179 51 -76
Principal Axes
Axis Value Plunge Azimuth
T 1.379e+16 N-m 5 259
N 0.241e+16 N-m 11 350
P -1.621e+16 N-m 78 143
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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 0.10 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 175 35 110 3.78 0.1485
WVFGRD96 2.0 280 55 -85 3.96 0.2156
WVFGRD96 3.0 285 55 -80 4.02 0.2440
WVFGRD96 4.0 315 75 -50 3.99 0.2592
WVFGRD96 5.0 315 80 65 4.04 0.2924
WVFGRD96 6.0 275 90 60 4.06 0.3213
WVFGRD96 7.0 275 90 60 4.07 0.3427
WVFGRD96 8.0 270 90 60 4.15 0.3574
WVFGRD96 9.0 90 85 -60 4.17 0.3718
WVFGRD96 10.0 315 80 60 4.18 0.3819
WVFGRD96 11.0 315 80 60 4.20 0.3898
WVFGRD96 12.0 320 75 60 4.22 0.3958
WVFGRD96 13.0 320 75 60 4.23 0.3990
WVFGRD96 14.0 320 75 60 4.25 0.4000
WVFGRD96 15.0 320 80 55 4.26 0.3998
WVFGRD96 16.0 320 80 55 4.27 0.4005
WVFGRD96 17.0 185 55 35 4.28 0.4010
WVFGRD96 18.0 185 50 30 4.29 0.4018
WVFGRD96 19.0 185 50 30 4.30 0.4015
WVFGRD96 20.0 185 50 30 4.31 0.4004
WVFGRD96 21.0 190 45 35 4.33 0.3989
WVFGRD96 22.0 190 45 35 4.34 0.3955
WVFGRD96 23.0 190 45 35 4.35 0.3928
WVFGRD96 24.0 230 40 30 4.36 0.3907
WVFGRD96 25.0 230 35 30 4.37 0.3903
WVFGRD96 26.0 230 35 30 4.38 0.3909
WVFGRD96 27.0 230 35 30 4.39 0.3899
WVFGRD96 28.0 230 35 30 4.39 0.3882
WVFGRD96 29.0 230 35 30 4.40 0.3853
WVFGRD96 30.0 225 40 30 4.41 0.3836
WVFGRD96 31.0 185 55 -65 4.43 0.4087
WVFGRD96 32.0 185 55 -65 4.45 0.4436
WVFGRD96 33.0 185 55 -65 4.46 0.4744
WVFGRD96 34.0 185 55 -65 4.47 0.5047
WVFGRD96 35.0 185 55 -65 4.48 0.5284
WVFGRD96 36.0 180 50 -65 4.49 0.5478
WVFGRD96 37.0 185 55 -65 4.50 0.5639
WVFGRD96 38.0 185 55 -65 4.51 0.5778
WVFGRD96 39.0 185 55 -60 4.52 0.5928
WVFGRD96 40.0 180 50 -70 4.60 0.5845
WVFGRD96 41.0 180 50 -70 4.62 0.5954
WVFGRD96 42.0 180 50 -70 4.63 0.6069
WVFGRD96 43.0 180 50 -70 4.64 0.6152
WVFGRD96 44.0 180 50 -70 4.65 0.6210
WVFGRD96 45.0 175 50 -70 4.66 0.6267
WVFGRD96 46.0 175 50 -70 4.67 0.6296
WVFGRD96 47.0 175 50 -75 4.67 0.6307
WVFGRD96 48.0 175 50 -70 4.68 0.6307
WVFGRD96 49.0 175 50 -70 4.68 0.6318
WVFGRD96 50.0 175 50 -75 4.69 0.6293
WVFGRD96 51.0 175 50 -70 4.69 0.6272
WVFGRD96 52.0 175 50 -75 4.69 0.6244
WVFGRD96 53.0 180 55 -65 4.70 0.6217
WVFGRD96 54.0 180 55 -65 4.70 0.6175
WVFGRD96 55.0 180 55 -65 4.70 0.6136
WVFGRD96 56.0 180 55 -65 4.70 0.6101
WVFGRD96 57.0 180 55 -65 4.71 0.6043
WVFGRD96 58.0 180 55 -65 4.71 0.5986
WVFGRD96 59.0 180 55 -65 4.71 0.5940
The best solution is
WVFGRD96 49.0 175 50 -70 4.68 0.6318
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 10:22:42 PM CDT 2024
