Location
Location ANSS
The ANSS event ID is ak021gbh4rso and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak021gbh4rso/executive.
2021/12/21 22:42:14 60.124 -153.274 151.2 5.9 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2021/12/21 22:42:14:0 60.12 -153.27 151.2 5.9 Alaska
Stations used:
AK.CAST AK.FIRE AK.GHO AK.GLI AK.HIN AK.K20K AK.KNK AK.L18K
AK.L20K AK.M16K AK.N15K AK.N18K AK.N19K AK.O18K AK.O19K
AK.P16K AK.P17K AK.P23K AK.PWL AK.Q19K AK.RC01 AK.SAW
AK.SCM AK.SKN AK.SLK AK.SWD AT.PMR AV.ACH AV.ILS AV.RED
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 0.06 n 3
Best Fitting Double Couple
Mo = 7.76e+24 dyne-cm
Mw = 5.86
Z = 158 km
Plane Strike Dip Rake
NP1 75 75 30
NP2 336 61 163
Principal Axes:
Axis Value Plunge Azimuth
T 7.76e+24 32 299
N 0.00e+00 57 99
P -7.76e+24 9 203
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.06e+24
Mxy -5.14e+24
Mxz 2.80e+24
Myy 3.12e+24
Myz -2.55e+24
Mzz 1.94e+24
--------------
######----------------
###########-----------------
###############---------------
##################----------------
#####################---------------
##### ###############---------------
###### T ################---------------
###### #################-------------#
############################----------####
#############################-----########
#############################-############
#########################-----############
#################------------###########
-----------------------------###########
-----------------------------#########
----------------------------########
---------------------------#######
-------------------------#####
----- ---------------#####
-- P ---------------##
-------------
Global CMT Convention Moment Tensor:
R T P
1.94e+24 2.80e+24 2.55e+24
2.80e+24 -5.06e+24 5.14e+24
2.55e+24 5.14e+24 3.12e+24
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20211221224214/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 = 75
DIP = 75
RAKE = 30
MW = 5.86
HS = 158.0
The NDK file is 20211221224214.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 2021/12/21 22:42:14:0 60.12 -153.27 151.2 5.9 Alaska
Stations used:
AK.CAST AK.FIRE AK.GHO AK.GLI AK.HIN AK.K20K AK.KNK AK.L18K
AK.L20K AK.M16K AK.N15K AK.N18K AK.N19K AK.O18K AK.O19K
AK.P16K AK.P17K AK.P23K AK.PWL AK.Q19K AK.RC01 AK.SAW
AK.SCM AK.SKN AK.SLK AK.SWD AT.PMR AV.ACH AV.ILS AV.RED
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 0.06 n 3
Best Fitting Double Couple
Mo = 7.76e+24 dyne-cm
Mw = 5.86
Z = 158 km
Plane Strike Dip Rake
NP1 75 75 30
NP2 336 61 163
Principal Axes:
Axis Value Plunge Azimuth
T 7.76e+24 32 299
N 0.00e+00 57 99
P -7.76e+24 9 203
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.06e+24
Mxy -5.14e+24
Mxz 2.80e+24
Myy 3.12e+24
Myz -2.55e+24
Mzz 1.94e+24
--------------
######----------------
###########-----------------
###############---------------
##################----------------
#####################---------------
##### ###############---------------
###### T ################---------------
###### #################-------------#
############################----------####
#############################-----########
#############################-############
#########################-----############
#################------------###########
-----------------------------###########
-----------------------------#########
----------------------------########
---------------------------#######
-------------------------#####
----- ---------------#####
-- P ---------------##
-------------
Global CMT Convention Moment Tensor:
R T P
1.94e+24 2.80e+24 2.55e+24
2.80e+24 -5.06e+24 5.14e+24
2.55e+24 5.14e+24 3.12e+24
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20211221224214/index.html
|
W-phase Moment Tensor (Mww)
Moment 8.727e+17 N-m
Magnitude 5.89 Mww
Depth 150.5 km
Percent DC 97%
Half Duration 2.29 s
Catalog US
Data Source US 3
Contributor US 3
Nodal Planes
Plane Strike Dip Rake
NP1 336° 61° 160°
NP2 76° 73° 31°
Principal Axes
Axis Value Plunge Azimuth
T 8.664e+17 N-m 34° 299°
N 0.125e+17 N-m 55° 102°
P -8.789e+17 N-m 8° 204°
|
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 0.06 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 330 55 -35 4.89 0.1555
WVFGRD96 4.0 335 85 -15 4.91 0.1702
WVFGRD96 6.0 160 80 25 4.97 0.1821
WVFGRD96 8.0 160 80 30 5.04 0.1955
WVFGRD96 10.0 160 80 30 5.07 0.2039
WVFGRD96 12.0 160 80 25 5.09 0.2075
WVFGRD96 14.0 160 80 25 5.11 0.2092
WVFGRD96 16.0 160 85 25 5.13 0.2094
WVFGRD96 18.0 155 90 25 5.14 0.2083
WVFGRD96 20.0 155 90 25 5.16 0.2049
WVFGRD96 22.0 155 90 25 5.17 0.2007
WVFGRD96 24.0 335 90 -20 5.19 0.1948
WVFGRD96 26.0 335 90 -20 5.20 0.1878
WVFGRD96 28.0 335 90 -20 5.21 0.1795
WVFGRD96 30.0 150 55 -25 5.21 0.1701
WVFGRD96 32.0 150 55 -25 5.22 0.1633
WVFGRD96 34.0 150 55 -25 5.23 0.1546
WVFGRD96 36.0 150 55 -20 5.24 0.1446
WVFGRD96 38.0 85 80 25 5.27 0.1346
WVFGRD96 40.0 265 65 30 5.33 0.1321
WVFGRD96 42.0 260 65 20 5.35 0.1305
WVFGRD96 44.0 260 70 20 5.36 0.1296
WVFGRD96 46.0 255 75 15 5.38 0.1298
WVFGRD96 48.0 255 75 10 5.40 0.1311
WVFGRD96 50.0 255 80 -15 5.43 0.1332
WVFGRD96 52.0 255 80 -15 5.45 0.1374
WVFGRD96 54.0 255 80 -15 5.47 0.1422
WVFGRD96 56.0 255 80 -15 5.49 0.1473
WVFGRD96 58.0 255 80 -15 5.50 0.1537
WVFGRD96 60.0 255 80 -15 5.52 0.1605
WVFGRD96 62.0 255 80 -15 5.54 0.1682
WVFGRD96 64.0 255 80 -10 5.55 0.1784
WVFGRD96 66.0 255 80 -10 5.57 0.1895
WVFGRD96 68.0 255 80 -10 5.58 0.2014
WVFGRD96 70.0 255 80 -10 5.60 0.2143
WVFGRD96 72.0 255 80 -10 5.62 0.2293
WVFGRD96 74.0 255 85 -10 5.63 0.2465
WVFGRD96 76.0 255 85 -10 5.65 0.2650
WVFGRD96 78.0 250 90 -10 5.66 0.2843
WVFGRD96 80.0 250 90 -10 5.67 0.3042
WVFGRD96 82.0 250 90 -10 5.68 0.3226
WVFGRD96 84.0 70 85 10 5.69 0.3408
WVFGRD96 86.0 70 85 10 5.70 0.3564
WVFGRD96 88.0 70 80 15 5.71 0.3698
WVFGRD96 90.0 70 80 15 5.72 0.3829
WVFGRD96 92.0 70 80 15 5.73 0.4039
WVFGRD96 94.0 70 75 20 5.74 0.4278
WVFGRD96 96.0 70 75 20 5.75 0.4536
WVFGRD96 98.0 70 75 20 5.76 0.4799
WVFGRD96 100.0 70 75 20 5.77 0.5068
WVFGRD96 102.0 70 75 20 5.78 0.5320
WVFGRD96 104.0 70 75 25 5.79 0.5553
WVFGRD96 106.0 70 75 25 5.80 0.5738
WVFGRD96 108.0 70 75 25 5.80 0.5862
WVFGRD96 110.0 70 75 25 5.81 0.5943
WVFGRD96 112.0 70 75 25 5.81 0.6011
WVFGRD96 114.0 70 75 25 5.81 0.6071
WVFGRD96 116.0 70 75 25 5.82 0.6129
WVFGRD96 118.0 70 75 25 5.82 0.6182
WVFGRD96 120.0 70 75 25 5.83 0.6234
WVFGRD96 122.0 70 75 25 5.83 0.6281
WVFGRD96 124.0 75 70 30 5.83 0.6324
WVFGRD96 126.0 75 70 30 5.83 0.6365
WVFGRD96 128.0 75 70 30 5.83 0.6397
WVFGRD96 130.0 75 70 30 5.84 0.6436
WVFGRD96 132.0 75 70 30 5.84 0.6468
WVFGRD96 134.0 75 70 30 5.84 0.6494
WVFGRD96 136.0 75 70 30 5.84 0.6519
WVFGRD96 138.0 75 70 30 5.84 0.6538
WVFGRD96 140.0 75 75 30 5.85 0.6554
WVFGRD96 142.0 75 75 30 5.85 0.6571
WVFGRD96 144.0 75 75 30 5.85 0.6587
WVFGRD96 146.0 75 75 30 5.85 0.6602
WVFGRD96 148.0 75 75 30 5.85 0.6613
WVFGRD96 150.0 75 75 30 5.86 0.6621
WVFGRD96 152.0 75 75 30 5.86 0.6628
WVFGRD96 154.0 75 75 30 5.86 0.6635
WVFGRD96 156.0 75 75 30 5.86 0.6639
WVFGRD96 158.0 75 75 30 5.86 0.6640
WVFGRD96 160.0 75 75 30 5.87 0.6638
WVFGRD96 162.0 75 75 30 5.87 0.6636
WVFGRD96 164.0 75 75 30 5.87 0.6629
WVFGRD96 166.0 75 75 30 5.87 0.6619
WVFGRD96 168.0 75 75 30 5.87 0.6609
WVFGRD96 170.0 75 75 30 5.87 0.6601
The best solution is
WVFGRD96 158.0 75 75 30 5.86 0.6640
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 0.06 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 Thu Apr 25 03:31:04 AM CDT 2024
