Location
Location ANSS
The ANSS event ID is ak023qqz70l and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak023qqz70l/executive.
2023/01/16 17:43:27 61.720 -149.552 34.8 4.5 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2023/01/16 17:43:27:0 61.72 -149.55 34.8 4.5 Alaska
Stations used:
AK.CAST AK.DHY AK.EYAK AK.FIRE AK.GHO AK.GLI AK.KLU AK.KNK
AK.L22K AK.M19K AK.MCK AK.PAX AK.PWL AK.RC01 AK.RND AK.SAW
AK.SCM AK.SKN AK.SLK AT.PMR AV.RED AV.STLK
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 0.07 n 3
Best Fitting Double Couple
Mo = 6.84e+22 dyne-cm
Mw = 4.49
Z = 46 km
Plane Strike Dip Rake
NP1 165 55 -85
NP2 336 35 -97
Principal Axes:
Axis Value Plunge Azimuth
T 6.84e+22 10 251
N 0.00e+00 4 342
P -6.84e+22 79 94
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.73e+21
Mxy 2.02e+22
Mxz -2.73e+21
Myy 5.73e+22
Myz -2.34e+22
Mzz -6.40e+22
-#############
####-------###########
######------------##########
######---------------#########
########-----------------#########
#########-------------------########
#########---------------------########
##########----------------------########
##########-----------------------#######
############----------------------########
############----------- ---------#######
############----------- P ---------#######
#############---------- ---------#######
# ########----------------------######
# T #########---------------------######
##########--------------------#####
#############-------------------####
#############-----------------####
############---------------###
#############------------###
############---------#
###########---
Global CMT Convention Moment Tensor:
R T P
-6.40e+22 -2.73e+21 2.34e+22
-2.73e+21 6.73e+21 -2.02e+22
2.34e+22 -2.02e+22 5.73e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230116174327/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 = 165
DIP = 55
RAKE = -85
MW = 4.49
HS = 46.0
The NDK file is 20230116174327.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 2023/01/16 17:43:27:0 61.72 -149.55 34.8 4.5 Alaska
Stations used:
AK.CAST AK.DHY AK.EYAK AK.FIRE AK.GHO AK.GLI AK.KLU AK.KNK
AK.L22K AK.M19K AK.MCK AK.PAX AK.PWL AK.RC01 AK.RND AK.SAW
AK.SCM AK.SKN AK.SLK AT.PMR AV.RED AV.STLK
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 0.07 n 3
Best Fitting Double Couple
Mo = 6.84e+22 dyne-cm
Mw = 4.49
Z = 46 km
Plane Strike Dip Rake
NP1 165 55 -85
NP2 336 35 -97
Principal Axes:
Axis Value Plunge Azimuth
T 6.84e+22 10 251
N 0.00e+00 4 342
P -6.84e+22 79 94
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.73e+21
Mxy 2.02e+22
Mxz -2.73e+21
Myy 5.73e+22
Myz -2.34e+22
Mzz -6.40e+22
-#############
####-------###########
######------------##########
######---------------#########
########-----------------#########
#########-------------------########
#########---------------------########
##########----------------------########
##########-----------------------#######
############----------------------########
############----------- ---------#######
############----------- P ---------#######
#############---------- ---------#######
# ########----------------------######
# T #########---------------------######
##########--------------------#####
#############-------------------####
#############-----------------####
############---------------###
#############------------###
############---------#
###########---
Global CMT Convention Moment Tensor:
R T P
-6.40e+22 -2.73e+21 2.34e+22
-2.73e+21 6.73e+21 -2.02e+22
2.34e+22 -2.02e+22 5.73e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230116174327/index.html
|
Regional Moment Tensor (Mwr)
Moment 7.637e+15 N-m
Magnitude 4.52 Mwr
Depth 45.0 km
Percent DC 95%
Half Duration -
Catalog US
Data Source US 3
Contributor US 3
Nodal Planes
Plane Strike Dip Rake
NP1 337 32 -102
NP2 171 58 -82
Principal Axes
Axis Value Plunge Azimuth
T 7.538e+15 N-m 13 255
N 0.195e+15 N-m 6 347
P -7.732e+15 N-m 75 103
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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 0.07 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 340 45 85 3.72 0.2125
WVFGRD96 2.0 340 45 90 3.84 0.2682
WVFGRD96 3.0 170 40 -80 3.89 0.2443
WVFGRD96 4.0 180 40 -70 3.91 0.2457
WVFGRD96 5.0 35 55 -30 3.88 0.2492
WVFGRD96 6.0 320 75 70 3.94 0.2600
WVFGRD96 7.0 325 70 75 3.95 0.2774
WVFGRD96 8.0 60 35 30 4.00 0.2953
WVFGRD96 9.0 60 30 30 4.02 0.3141
WVFGRD96 10.0 60 35 30 4.03 0.3294
WVFGRD96 11.0 55 40 30 4.03 0.3409
WVFGRD96 12.0 55 40 30 4.04 0.3499
WVFGRD96 13.0 55 40 25 4.04 0.3561
WVFGRD96 14.0 55 40 25 4.05 0.3604
WVFGRD96 15.0 15 55 -45 4.07 0.3708
WVFGRD96 16.0 15 55 -45 4.08 0.3830
WVFGRD96 17.0 15 55 -45 4.09 0.3933
WVFGRD96 18.0 15 55 -45 4.10 0.4022
WVFGRD96 19.0 15 55 -45 4.11 0.4096
WVFGRD96 20.0 20 55 -40 4.12 0.4165
WVFGRD96 21.0 20 55 -40 4.13 0.4215
WVFGRD96 22.0 20 55 -40 4.14 0.4276
WVFGRD96 23.0 15 50 -45 4.15 0.4328
WVFGRD96 24.0 15 50 -45 4.16 0.4376
WVFGRD96 25.0 15 50 -45 4.17 0.4413
WVFGRD96 26.0 15 50 -45 4.18 0.4443
WVFGRD96 27.0 15 50 -45 4.19 0.4483
WVFGRD96 28.0 10 45 -50 4.20 0.4524
WVFGRD96 29.0 10 45 -50 4.21 0.4573
WVFGRD96 30.0 10 45 -50 4.22 0.4633
WVFGRD96 31.0 195 50 -45 4.26 0.4731
WVFGRD96 32.0 190 50 -50 4.27 0.4841
WVFGRD96 33.0 0 40 -65 4.25 0.4961
WVFGRD96 34.0 0 40 -65 4.26 0.5085
WVFGRD96 35.0 0 40 -65 4.27 0.5187
WVFGRD96 36.0 -15 35 -85 4.29 0.5286
WVFGRD96 37.0 -15 35 -85 4.31 0.5388
WVFGRD96 38.0 -15 35 -85 4.32 0.5475
WVFGRD96 39.0 -20 35 -90 4.34 0.5560
WVFGRD96 40.0 345 35 -85 4.43 0.5735
WVFGRD96 41.0 165 55 -85 4.45 0.5799
WVFGRD96 42.0 165 55 -85 4.46 0.5851
WVFGRD96 43.0 165 55 -85 4.47 0.5897
WVFGRD96 44.0 165 55 -85 4.48 0.5928
WVFGRD96 45.0 165 55 -85 4.48 0.5937
WVFGRD96 46.0 165 55 -85 4.49 0.5948
WVFGRD96 47.0 165 55 -85 4.50 0.5934
WVFGRD96 48.0 165 55 -85 4.50 0.5915
WVFGRD96 49.0 165 55 -85 4.51 0.5886
WVFGRD96 50.0 165 55 -85 4.51 0.5831
WVFGRD96 51.0 165 55 -85 4.51 0.5778
WVFGRD96 52.0 165 55 -85 4.51 0.5713
WVFGRD96 53.0 170 60 -80 4.52 0.5638
WVFGRD96 54.0 170 60 -80 4.53 0.5563
WVFGRD96 55.0 170 60 -80 4.53 0.5474
WVFGRD96 56.0 170 60 -80 4.53 0.5382
WVFGRD96 57.0 170 60 -80 4.53 0.5290
WVFGRD96 58.0 170 60 -80 4.53 0.5178
WVFGRD96 59.0 170 60 -80 4.53 0.5076
The best solution is
WVFGRD96 46.0 165 55 -85 4.49 0.5948
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 0.07 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 Mon Apr 22 09:12:59 PM CDT 2024
