Location
Location ANSS
The ANSS event ID is ak022fwcrh7q and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak022fwcrh7q/executive.
2022/12/12 08:09:01 59.278 -150.643 36.7 4.7 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2022/12/12 08:09:01:0 59.28 -150.64 36.7 4.7 Alaska
Stations used:
AK.BRLK AK.CNP AK.DIV AK.FID AK.GHO AK.GLI AK.HOM AK.KLU
AK.KNK AK.N19K AK.O18K AK.O19K AK.Q19K AK.SAW AK.SLK AK.SWD
AV.ACH AV.ILS AV.RED
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 0.08 n 3
Best Fitting Double Couple
Mo = 1.07e+23 dyne-cm
Mw = 4.62
Z = 44 km
Plane Strike Dip Rake
NP1 225 60 -45
NP2 342 52 -141
Principal Axes:
Axis Value Plunge Azimuth
T 1.07e+23 5 285
N 0.00e+00 38 18
P -1.07e+23 52 189
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.28e+22
Mxy -3.28e+22
Mxz 5.36e+22
Myy 9.84e+22
Myz -7.98e+15
Mzz -6.56e+22
#-------------
#########-------------
###############-------------
##################-----#######
##################################
##################------############
#################---------############
##############------------############
T ############--------------############
##########-----------------############
############-------------------###########
###########--------------------###########
#########----------------------###########
#######------------------------#########
#######------------------------#########
#####----------- -----------########
####----------- P ----------########
##------------ ----------#######
-------------------------#####
-----------------------#####
-------------------###
--------------
Global CMT Convention Moment Tensor:
R T P
-6.56e+22 5.36e+22 7.98e+15
5.36e+22 -3.28e+22 3.28e+22
7.98e+15 3.28e+22 9.84e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20221212080901/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 = 225
DIP = 60
RAKE = -45
MW = 4.62
HS = 44.0
The NDK file is 20221212080901.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 2022/12/12 08:09:01:0 59.28 -150.64 36.7 4.7 Alaska
Stations used:
AK.BRLK AK.CNP AK.DIV AK.FID AK.GHO AK.GLI AK.HOM AK.KLU
AK.KNK AK.N19K AK.O18K AK.O19K AK.Q19K AK.SAW AK.SLK AK.SWD
AV.ACH AV.ILS AV.RED
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 0.08 n 3
Best Fitting Double Couple
Mo = 1.07e+23 dyne-cm
Mw = 4.62
Z = 44 km
Plane Strike Dip Rake
NP1 225 60 -45
NP2 342 52 -141
Principal Axes:
Axis Value Plunge Azimuth
T 1.07e+23 5 285
N 0.00e+00 38 18
P -1.07e+23 52 189
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.28e+22
Mxy -3.28e+22
Mxz 5.36e+22
Myy 9.84e+22
Myz -7.98e+15
Mzz -6.56e+22
#-------------
#########-------------
###############-------------
##################-----#######
##################################
##################------############
#################---------############
##############------------############
T ############--------------############
##########-----------------############
############-------------------###########
###########--------------------###########
#########----------------------###########
#######------------------------#########
#######------------------------#########
#####----------- -----------########
####----------- P ----------########
##------------ ----------#######
-------------------------#####
-----------------------#####
-------------------###
--------------
Global CMT Convention Moment Tensor:
R T P
-6.56e+22 5.36e+22 7.98e+15
5.36e+22 -3.28e+22 3.28e+22
7.98e+15 3.28e+22 9.84e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20221212080901/index.html
|
Regional Moment Tensor (Mwr)
Moment 1.179e+16 N-m
Magnitude 4.65 Mwr
Depth 47.0 km
Percent DC 96%
Half Duration -
Catalog US
Data Source US 3
Contributor US 3
Nodal Planes
Plane Strike Dip Rake
NP1 347 50 -128
NP2 218 53 -53
Principal Axes
Axis Value Plunge Azimuth
T 1.168e+16 N-m 1 283
N 0.022e+16 N-m 28 13
P -1.189e+16 N-m 62 191
|
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 0.08 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 200 60 35 3.86 0.2251
WVFGRD96 2.0 45 45 70 4.02 0.2892
WVFGRD96 3.0 10 45 -90 4.08 0.2849
WVFGRD96 4.0 185 70 -35 4.07 0.2938
WVFGRD96 5.0 190 75 -35 4.08 0.3209
WVFGRD96 6.0 50 90 45 4.07 0.3424
WVFGRD96 7.0 50 90 40 4.09 0.3658
WVFGRD96 8.0 55 85 45 4.16 0.3847
WVFGRD96 9.0 230 90 -45 4.17 0.4006
WVFGRD96 10.0 230 90 -40 4.18 0.4142
WVFGRD96 11.0 55 80 40 4.20 0.4258
WVFGRD96 12.0 230 90 -40 4.21 0.4338
WVFGRD96 13.0 230 90 -40 4.23 0.4409
WVFGRD96 14.0 55 85 35 4.25 0.4478
WVFGRD96 15.0 230 90 -35 4.26 0.4539
WVFGRD96 16.0 55 85 35 4.27 0.4595
WVFGRD96 17.0 50 90 35 4.28 0.4645
WVFGRD96 18.0 230 85 -35 4.30 0.4723
WVFGRD96 19.0 230 85 -35 4.31 0.4795
WVFGRD96 20.0 230 85 -35 4.32 0.4866
WVFGRD96 21.0 230 85 -35 4.33 0.4924
WVFGRD96 22.0 230 80 -35 4.35 0.4976
WVFGRD96 23.0 230 80 -35 4.35 0.5034
WVFGRD96 24.0 230 80 -35 4.36 0.5084
WVFGRD96 25.0 230 80 -35 4.37 0.5118
WVFGRD96 26.0 225 70 -35 4.38 0.5163
WVFGRD96 27.0 230 70 -35 4.40 0.5205
WVFGRD96 28.0 225 65 -35 4.40 0.5246
WVFGRD96 29.0 225 65 -35 4.41 0.5287
WVFGRD96 30.0 230 75 -35 4.41 0.5364
WVFGRD96 31.0 230 75 -35 4.42 0.5483
WVFGRD96 32.0 230 70 -35 4.44 0.5614
WVFGRD96 33.0 225 65 -40 4.44 0.5760
WVFGRD96 34.0 225 65 -40 4.45 0.5899
WVFGRD96 35.0 225 65 -40 4.46 0.6017
WVFGRD96 36.0 230 65 -40 4.48 0.6129
WVFGRD96 37.0 230 65 -40 4.49 0.6226
WVFGRD96 38.0 225 60 -40 4.49 0.6333
WVFGRD96 39.0 225 60 -40 4.50 0.6390
WVFGRD96 40.0 225 60 -45 4.59 0.6510
WVFGRD96 41.0 225 60 -45 4.60 0.6557
WVFGRD96 42.0 225 60 -45 4.61 0.6598
WVFGRD96 43.0 225 60 -45 4.62 0.6611
WVFGRD96 44.0 225 60 -45 4.62 0.6632
WVFGRD96 45.0 220 55 -50 4.63 0.6627
WVFGRD96 46.0 220 55 -50 4.64 0.6615
WVFGRD96 47.0 220 55 -50 4.64 0.6601
WVFGRD96 48.0 220 55 -50 4.65 0.6583
WVFGRD96 49.0 225 55 -45 4.65 0.6562
WVFGRD96 50.0 225 55 -45 4.65 0.6539
WVFGRD96 51.0 225 55 -45 4.66 0.6512
WVFGRD96 52.0 225 55 -45 4.66 0.6483
WVFGRD96 53.0 225 55 -45 4.66 0.6444
WVFGRD96 54.0 225 55 -45 4.66 0.6406
WVFGRD96 55.0 225 55 -45 4.67 0.6366
WVFGRD96 56.0 225 55 -45 4.67 0.6323
WVFGRD96 57.0 220 50 -45 4.67 0.6278
WVFGRD96 58.0 220 50 -45 4.67 0.6237
WVFGRD96 59.0 225 50 -45 4.68 0.6181
The best solution is
WVFGRD96 44.0 225 60 -45 4.62 0.6632
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 0.08 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 03:38:16 AM CDT 2024
