Location
Location ANSS
The ANSS event ID is ak022ytdd55 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak022ytdd55/executive.
2022/01/21 05:18:26 60.316 -152.360 115.3 5.1 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2022/01/21 05:18:26:0 60.32 -152.36 115.3 5.1 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CNP AK.CUT AK.DHY AK.FIRE AK.GHO AK.GLI
AK.HIN AK.HOM AK.K20K AK.KNK AK.L18K AK.L20K AK.L22K
AK.M20K AK.MCK AK.N18K AK.N19K AK.O18K AK.O19K AK.P17K
AK.P23K AK.R18K AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SLK
AK.SWD AT.PMR AV.ILS AV.STLK 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 0.10 n 3
Best Fitting Double Couple
Mo = 4.27e+23 dyne-cm
Mw = 5.02
Z = 96 km
Plane Strike Dip Rake
NP1 55 85 25
NP2 323 65 174
Principal Axes:
Axis Value Plunge Azimuth
T 4.27e+23 21 282
N 0.00e+00 65 66
P -4.27e+23 14 186
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.83e+23
Mxy -1.17e+23
Mxz 1.26e+23
Myy 3.52e+23
Myz -1.29e+23
Mzz 3.13e+22
--------------
----------------------
####------------------------
#########---------------------
##############--------------------
#################----------------###
####################------------######
#######################--------#########
## ###################----############
### T ####################################
### ###################---##############
######################-------#############
###################-----------############
###############---------------##########
#############------------------#########
#########----------------------#######
####--------------------------######
-----------------------------#####
----------------------------##
---------- --------------#
------- P ------------
--- --------
Global CMT Convention Moment Tensor:
R T P
3.13e+22 1.26e+23 1.29e+23
1.26e+23 -3.83e+23 1.17e+23
1.29e+23 1.17e+23 3.52e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20220121051826/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 = 55
DIP = 85
RAKE = 25
MW = 5.02
HS = 96.0
The NDK file is 20220121051826.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 2022/01/21 05:18:26:0 60.32 -152.36 115.3 5.1 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CNP AK.CUT AK.DHY AK.FIRE AK.GHO AK.GLI
AK.HIN AK.HOM AK.K20K AK.KNK AK.L18K AK.L20K AK.L22K
AK.M20K AK.MCK AK.N18K AK.N19K AK.O18K AK.O19K AK.P17K
AK.P23K AK.R18K AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SLK
AK.SWD AT.PMR AV.ILS AV.STLK 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 0.10 n 3
Best Fitting Double Couple
Mo = 4.27e+23 dyne-cm
Mw = 5.02
Z = 96 km
Plane Strike Dip Rake
NP1 55 85 25
NP2 323 65 174
Principal Axes:
Axis Value Plunge Azimuth
T 4.27e+23 21 282
N 0.00e+00 65 66
P -4.27e+23 14 186
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.83e+23
Mxy -1.17e+23
Mxz 1.26e+23
Myy 3.52e+23
Myz -1.29e+23
Mzz 3.13e+22
--------------
----------------------
####------------------------
#########---------------------
##############--------------------
#################----------------###
####################------------######
#######################--------#########
## ###################----############
### T ####################################
### ###################---##############
######################-------#############
###################-----------############
###############---------------##########
#############------------------#########
#########----------------------#######
####--------------------------######
-----------------------------#####
----------------------------##
---------- --------------#
------- P ------------
--- --------
Global CMT Convention Moment Tensor:
R T P
3.13e+22 1.26e+23 1.29e+23
1.26e+23 -3.83e+23 1.17e+23
1.29e+23 1.17e+23 3.52e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20220121051826/index.html
|
W-phase Moment Tensor (Mww)
Moment 5.174e+16 N-m
Magnitude 5.08 Mww
Depth 100.5 km
Percent DC 97%
Half Duration 0.89 s
Catalog US
Data Source US 3
Contributor US 3
Nodal Planes
Plane Strike Dip Rake
NP1 325° 86° 177°
NP2 55° 87° 4°
Principal Axes
Axis Value Plunge Azimuth
T 5.208e+16 N-m 5° 280°
N -0.070e+16 N-m 85° 93°
P -5.138e+16 N-m 1° 190°
|
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 2.0 325 85 0 4.07 0.2649
WVFGRD96 4.0 325 80 5 4.19 0.3204
WVFGRD96 6.0 325 80 -10 4.26 0.3459
WVFGRD96 8.0 325 80 -10 4.34 0.3643
WVFGRD96 10.0 325 80 -10 4.39 0.3609
WVFGRD96 12.0 325 85 -5 4.43 0.3505
WVFGRD96 14.0 145 85 5 4.45 0.3327
WVFGRD96 16.0 325 85 0 4.47 0.3093
WVFGRD96 18.0 325 85 5 4.48 0.2810
WVFGRD96 20.0 145 80 -5 4.49 0.2511
WVFGRD96 22.0 235 75 10 4.51 0.2492
WVFGRD96 24.0 235 80 5 4.52 0.2515
WVFGRD96 26.0 240 80 5 4.54 0.2521
WVFGRD96 28.0 240 85 -10 4.55 0.2508
WVFGRD96 30.0 240 90 -10 4.56 0.2485
WVFGRD96 32.0 60 90 15 4.57 0.2470
WVFGRD96 34.0 240 90 -15 4.58 0.2440
WVFGRD96 36.0 240 90 -20 4.61 0.2429
WVFGRD96 38.0 240 90 -20 4.63 0.2470
WVFGRD96 40.0 60 90 30 4.70 0.2592
WVFGRD96 42.0 60 85 20 4.72 0.2659
WVFGRD96 44.0 60 85 20 4.74 0.2750
WVFGRD96 46.0 60 80 20 4.76 0.2857
WVFGRD96 48.0 60 80 15 4.78 0.2989
WVFGRD96 50.0 60 80 15 4.80 0.3145
WVFGRD96 52.0 60 80 20 4.82 0.3357
WVFGRD96 54.0 60 80 15 4.83 0.3598
WVFGRD96 56.0 60 80 20 4.86 0.3892
WVFGRD96 58.0 55 85 20 4.88 0.4221
WVFGRD96 60.0 55 85 20 4.89 0.4586
WVFGRD96 62.0 55 85 20 4.91 0.4957
WVFGRD96 64.0 55 85 25 4.93 0.5343
WVFGRD96 66.0 55 90 25 4.95 0.5712
WVFGRD96 68.0 55 90 25 4.96 0.6056
WVFGRD96 70.0 235 90 -25 4.97 0.6300
WVFGRD96 72.0 55 90 25 4.98 0.6413
WVFGRD96 74.0 235 90 -25 4.98 0.6464
WVFGRD96 76.0 55 90 25 4.99 0.6514
WVFGRD96 78.0 55 85 25 4.99 0.6547
WVFGRD96 80.0 55 85 25 4.99 0.6572
WVFGRD96 82.0 55 85 25 5.00 0.6590
WVFGRD96 84.0 55 85 25 5.00 0.6613
WVFGRD96 86.0 55 85 25 5.01 0.6619
WVFGRD96 88.0 55 85 25 5.01 0.6622
WVFGRD96 90.0 55 85 25 5.01 0.6621
WVFGRD96 92.0 55 85 25 5.02 0.6634
WVFGRD96 94.0 55 85 25 5.02 0.6642
WVFGRD96 96.0 55 85 25 5.02 0.6642
WVFGRD96 98.0 55 85 25 5.03 0.6625
WVFGRD96 100.0 55 85 25 5.03 0.6594
WVFGRD96 102.0 55 85 25 5.03 0.6591
WVFGRD96 104.0 55 85 25 5.03 0.6584
WVFGRD96 106.0 55 85 25 5.04 0.6560
WVFGRD96 108.0 55 85 25 5.04 0.6523
WVFGRD96 110.0 55 85 25 5.04 0.6515
WVFGRD96 112.0 55 85 25 5.05 0.6486
WVFGRD96 114.0 55 85 25 5.05 0.6434
WVFGRD96 116.0 55 85 25 5.05 0.6422
WVFGRD96 118.0 55 80 20 5.05 0.6399
WVFGRD96 120.0 55 80 20 5.05 0.6353
WVFGRD96 122.0 55 80 20 5.05 0.6343
WVFGRD96 124.0 55 80 20 5.06 0.6312
WVFGRD96 126.0 55 80 20 5.06 0.6269
WVFGRD96 128.0 55 80 20 5.06 0.6260
WVFGRD96 130.0 55 80 20 5.06 0.6216
WVFGRD96 132.0 55 80 20 5.06 0.6187
WVFGRD96 134.0 55 80 20 5.07 0.6161
WVFGRD96 136.0 55 80 20 5.07 0.6114
WVFGRD96 138.0 55 80 20 5.07 0.6091
WVFGRD96 140.0 55 80 25 5.07 0.6033
WVFGRD96 142.0 55 80 25 5.08 0.6022
WVFGRD96 144.0 55 80 25 5.08 0.5968
WVFGRD96 146.0 55 80 25 5.08 0.5932
WVFGRD96 148.0 55 80 25 5.08 0.5893
The best solution is
WVFGRD96 96.0 55 85 25 5.02 0.6642
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 09:14:39 PM CDT 2024
