Location
Location ANSS
The ANSS event ID is ak018exsi6u4 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak018exsi6u4/executive.
2018/11/21 18:21:43 59.955 -153.266 143.3 5.6 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2018/11/21 18:21:43:0 59.96 -153.27 143.3 5.6 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CNP AK.GHO AK.HOM AK.KNK AK.PPLA AK.PWL
AK.RC01 AK.SAW AK.SKN AK.SSN AT.OHAK AT.PMR AT.SVW2 AV.ACH
II.KDAK TA.L16K TA.L18K TA.L19K TA.M16K TA.M17K TA.M19K
TA.M22K TA.N17K TA.N18K TA.N19K TA.O16K TA.O18K TA.O19K
TA.O22K TA.P18K TA.P19K TA.Q19K TA.Q20K TA.R17L
Filtering commands used:
cut o DIST/3.5 -50 o DIST/3.5 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 2.24e+24 dyne-cm
Mw = 5.50
Z = 144 km
Plane Strike Dip Rake
NP1 60 65 30
NP2 316 63 152
Principal Axes:
Axis Value Plunge Azimuth
T 2.24e+24 38 279
N 0.00e+00 52 96
P -2.24e+24 1 188
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.16e+24
Mxy -5.07e+23
Mxz 2.13e+23
Myy 1.31e+24
Myz -1.07e+24
Mzz 8.57e+23
--------------
----------------------
----------------------------
#######-----------------------
#############---------------------
#################-------------------
####################----------------##
#######################-------------####
###### ################----------#####
####### T ##################------########
####### ###################----#########
##############################-###########
############################----##########
#########################-------########
#####################------------#######
################----------------######
#########-----------------------####
-------------------------------###
-----------------------------#
----------------------------
------- ------------
--- P --------
Global CMT Convention Moment Tensor:
R T P
8.57e+23 2.13e+23 1.07e+24
2.13e+23 -2.16e+24 5.07e+23
1.07e+24 5.07e+23 1.31e+24
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181121182143/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 = 60
DIP = 65
RAKE = 30
MW = 5.50
HS = 144.0
The NDK file is 20181121182143.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 2018/11/21 18:21:43:0 59.96 -153.27 143.3 5.6 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CNP AK.GHO AK.HOM AK.KNK AK.PPLA AK.PWL
AK.RC01 AK.SAW AK.SKN AK.SSN AT.OHAK AT.PMR AT.SVW2 AV.ACH
II.KDAK TA.L16K TA.L18K TA.L19K TA.M16K TA.M17K TA.M19K
TA.M22K TA.N17K TA.N18K TA.N19K TA.O16K TA.O18K TA.O19K
TA.O22K TA.P18K TA.P19K TA.Q19K TA.Q20K TA.R17L
Filtering commands used:
cut o DIST/3.5 -50 o DIST/3.5 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 2.24e+24 dyne-cm
Mw = 5.50
Z = 144 km
Plane Strike Dip Rake
NP1 60 65 30
NP2 316 63 152
Principal Axes:
Axis Value Plunge Azimuth
T 2.24e+24 38 279
N 0.00e+00 52 96
P -2.24e+24 1 188
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.16e+24
Mxy -5.07e+23
Mxz 2.13e+23
Myy 1.31e+24
Myz -1.07e+24
Mzz 8.57e+23
--------------
----------------------
----------------------------
#######-----------------------
#############---------------------
#################-------------------
####################----------------##
#######################-------------####
###### ################----------#####
####### T ##################------########
####### ###################----#########
##############################-###########
############################----##########
#########################-------########
#####################------------#######
################----------------######
#########-----------------------####
-------------------------------###
-----------------------------#
----------------------------
------- ------------
--- P --------
Global CMT Convention Moment Tensor:
R T P
8.57e+23 2.13e+23 1.07e+24
2.13e+23 -2.16e+24 5.07e+23
1.07e+24 5.07e+23 1.31e+24
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181121182143/index.html
|
W-phase Moment Tensor (Mww)
Moment 2.388e+17 N-m
Magnitude 5.52 Mww
Depth 140.5 km
Percent DC 98%
Half Duration 1.48 s
Catalog US
Data Source US 3
Contributor US 3
Nodal Planes
Plane Strike Dip Rake
NP1 319 63 153
NP2 62 66 30
Principal Axes
Axis Value Plunge Azimuth
T 2.376e+17 N-m 37 282
N 0.025e+17 N-m 53 98
P -2.400e+17 N-m 2 190
|
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/3.5 -50 o DIST/3.5 +70
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 2.0 135 65 -35 4.55 0.1778
WVFGRD96 4.0 320 75 10 4.57 0.1982
WVFGRD96 6.0 320 75 15 4.63 0.2124
WVFGRD96 8.0 325 65 20 4.69 0.2210
WVFGRD96 10.0 325 65 20 4.72 0.2210
WVFGRD96 12.0 325 65 25 4.74 0.2216
WVFGRD96 14.0 320 70 20 4.76 0.2222
WVFGRD96 16.0 225 70 -20 4.77 0.2235
WVFGRD96 18.0 225 70 -15 4.79 0.2252
WVFGRD96 20.0 225 70 -15 4.81 0.2249
WVFGRD96 22.0 55 70 25 4.82 0.2239
WVFGRD96 24.0 55 70 25 4.84 0.2228
WVFGRD96 26.0 55 70 20 4.85 0.2206
WVFGRD96 28.0 55 70 20 4.87 0.2170
WVFGRD96 30.0 55 70 20 4.88 0.2122
WVFGRD96 32.0 50 75 15 4.90 0.2069
WVFGRD96 34.0 50 75 10 4.92 0.2014
WVFGRD96 36.0 50 75 10 4.94 0.1960
WVFGRD96 38.0 50 75 5 4.96 0.1898
WVFGRD96 40.0 250 65 40 5.02 0.1911
WVFGRD96 42.0 50 70 5 5.03 0.1867
WVFGRD96 44.0 50 75 -10 5.04 0.1860
WVFGRD96 46.0 50 75 -5 5.06 0.1861
WVFGRD96 48.0 50 75 -10 5.07 0.1865
WVFGRD96 50.0 50 75 -10 5.08 0.1877
WVFGRD96 52.0 50 75 -5 5.10 0.1898
WVFGRD96 54.0 50 75 -5 5.11 0.1924
WVFGRD96 56.0 50 75 -5 5.12 0.1955
WVFGRD96 58.0 50 75 -5 5.14 0.1995
WVFGRD96 60.0 50 75 -5 5.15 0.2039
WVFGRD96 62.0 60 80 15 5.17 0.2094
WVFGRD96 64.0 60 80 15 5.18 0.2184
WVFGRD96 66.0 60 80 20 5.20 0.2309
WVFGRD96 68.0 60 80 20 5.21 0.2462
WVFGRD96 70.0 60 75 20 5.23 0.2627
WVFGRD96 72.0 60 75 20 5.25 0.2797
WVFGRD96 74.0 60 75 20 5.26 0.2967
WVFGRD96 76.0 60 75 25 5.28 0.3144
WVFGRD96 78.0 60 75 25 5.29 0.3324
WVFGRD96 80.0 60 75 25 5.31 0.3495
WVFGRD96 82.0 60 75 25 5.32 0.3687
WVFGRD96 84.0 60 75 30 5.34 0.3925
WVFGRD96 86.0 60 70 30 5.35 0.4170
WVFGRD96 88.0 60 70 30 5.36 0.4435
WVFGRD96 90.0 60 70 30 5.38 0.4697
WVFGRD96 92.0 60 70 30 5.39 0.4948
WVFGRD96 94.0 65 65 35 5.40 0.5219
WVFGRD96 96.0 65 65 35 5.41 0.5460
WVFGRD96 98.0 65 65 35 5.42 0.5654
WVFGRD96 100.0 65 65 35 5.43 0.5802
WVFGRD96 102.0 65 65 35 5.43 0.5912
WVFGRD96 104.0 65 65 35 5.44 0.6015
WVFGRD96 106.0 65 65 35 5.44 0.6105
WVFGRD96 108.0 65 65 35 5.45 0.6193
WVFGRD96 110.0 65 65 35 5.45 0.6266
WVFGRD96 112.0 65 65 35 5.46 0.6339
WVFGRD96 114.0 65 65 35 5.46 0.6402
WVFGRD96 116.0 65 65 35 5.47 0.6456
WVFGRD96 118.0 65 65 35 5.47 0.6506
WVFGRD96 120.0 65 65 35 5.47 0.6546
WVFGRD96 122.0 65 65 35 5.48 0.6595
WVFGRD96 124.0 65 65 35 5.48 0.6634
WVFGRD96 126.0 65 65 35 5.48 0.6667
WVFGRD96 128.0 65 65 35 5.48 0.6697
WVFGRD96 130.0 65 65 35 5.49 0.6721
WVFGRD96 132.0 65 65 35 5.49 0.6742
WVFGRD96 134.0 65 65 35 5.49 0.6756
WVFGRD96 136.0 60 65 30 5.49 0.6765
WVFGRD96 138.0 60 65 30 5.50 0.6776
WVFGRD96 140.0 60 65 30 5.50 0.6782
WVFGRD96 142.0 60 65 30 5.50 0.6785
WVFGRD96 144.0 60 65 30 5.50 0.6792
WVFGRD96 146.0 60 65 30 5.50 0.6789
WVFGRD96 148.0 60 65 30 5.51 0.6784
WVFGRD96 150.0 60 65 30 5.51 0.6777
WVFGRD96 152.0 60 65 30 5.51 0.6765
WVFGRD96 154.0 60 65 30 5.51 0.6748
WVFGRD96 156.0 60 65 30 5.51 0.6733
WVFGRD96 158.0 60 65 30 5.51 0.6717
WVFGRD96 160.0 60 65 30 5.51 0.6698
WVFGRD96 162.0 60 65 30 5.52 0.6676
WVFGRD96 164.0 60 65 30 5.52 0.6649
WVFGRD96 166.0 60 65 30 5.52 0.6625
WVFGRD96 168.0 60 65 30 5.52 0.6604
The best solution is
WVFGRD96 144.0 60 65 30 5.50 0.6792
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/3.5 -50 o DIST/3.5 +70
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 Fri Apr 26 04:11:41 AM CDT 2024
