Location
SLU Location
To check the ANSS location or to compare the observed P-wave first motions to the moment tensor solution, P- and S-wave first arrival times were manually read together with the P-wave first motions. The subsequent output of the program elocate is given in the file elocate.txt. The first motion plot is shown below.
Location ANSS
The ANSS event ID is ak0169aax3e0 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0169aax3e0/executive.
2016/07/20 14:04:28 61.725 -148.552 29.3 3.4 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2016/07/20 14:04:28:0 61.72 -148.55 29.3 3.4 Alaska
Stations used:
AK.GHO AK.KNK AK.RC01 AK.SAW AK.SCM AT.PMR TA.M22K TA.O22K
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.43e+21 dyne-cm
Mw = 3.37
Z = 37 km
Plane Strike Dip Rake
NP1 316 52 -102
NP2 155 40 -75
Principal Axes:
Axis Value Plunge Azimuth
T 1.43e+21 6 54
N 0.00e+00 10 323
P -1.43e+21 79 176
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.25e+20
Mxy 6.73e+20
Mxz 3.58e+20
Myy 9.34e+20
Myz 9.75e+19
Mzz -1.36e+21
##############
-#####################
---#########################
###--------##################
####-------------############## T
#####----------------########### #
######------------------##############
######---------------------#############
######-----------------------###########
#######------------------------###########
########------------------------##########
########------------ ----------#########
#########----------- P -----------########
########----------- ------------######
#########-------------------------######
#########-------------------------####
##########-----------------------###
##########----------------------##
##########--------------------
############----------------
############----------
##############
Global CMT Convention Moment Tensor:
R T P
-1.36e+21 3.58e+20 -9.75e+19
3.58e+20 4.25e+20 -6.73e+20
-9.75e+19 -6.73e+20 9.34e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160720140428/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 = 155
DIP = 40
RAKE = -75
MW = 3.37
HS = 37.0
The NDK file is 20160720140428.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 |
SLUFM |
USGS/SLU Moment Tensor Solution
ENS 2016/07/20 14:04:28:0 61.72 -148.55 29.3 3.4 Alaska
Stations used:
AK.GHO AK.KNK AK.RC01 AK.SAW AK.SCM AT.PMR TA.M22K TA.O22K
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.43e+21 dyne-cm
Mw = 3.37
Z = 37 km
Plane Strike Dip Rake
NP1 316 52 -102
NP2 155 40 -75
Principal Axes:
Axis Value Plunge Azimuth
T 1.43e+21 6 54
N 0.00e+00 10 323
P -1.43e+21 79 176
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.25e+20
Mxy 6.73e+20
Mxz 3.58e+20
Myy 9.34e+20
Myz 9.75e+19
Mzz -1.36e+21
##############
-#####################
---#########################
###--------##################
####-------------############## T
#####----------------########### #
######------------------##############
######---------------------#############
######-----------------------###########
#######------------------------###########
########------------------------##########
########------------ ----------#########
#########----------- P -----------########
########----------- ------------######
#########-------------------------######
#########-------------------------####
##########-----------------------###
##########----------------------##
##########--------------------
############----------------
############----------
##############
Global CMT Convention Moment Tensor:
R T P
-1.36e+21 3.58e+20 -9.75e+19
3.58e+20 4.25e+20 -6.73e+20
-9.75e+19 -6.73e+20 9.34e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160720140428/index.html
|
First motions and takeoff angles from an elocate run.
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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 -30 o DIST/3.3 +70
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 1.0 150 50 -90 2.68 0.2181
WVFGRD96 2.0 330 45 -90 2.84 0.2771
WVFGRD96 3.0 355 50 -60 2.87 0.2636
WVFGRD96 4.0 305 85 -70 2.92 0.2899
WVFGRD96 5.0 125 90 75 2.91 0.3140
WVFGRD96 6.0 265 30 -25 2.93 0.3302
WVFGRD96 7.0 260 30 -35 2.95 0.3414
WVFGRD96 8.0 250 25 35 3.03 0.3475
WVFGRD96 9.0 300 40 50 3.03 0.3554
WVFGRD96 10.0 135 60 70 3.05 0.3619
WVFGRD96 11.0 130 60 65 3.07 0.3649
WVFGRD96 12.0 130 60 65 3.09 0.3621
WVFGRD96 13.0 290 45 30 3.08 0.3555
WVFGRD96 14.0 285 35 5 3.08 0.3504
WVFGRD96 15.0 15 85 50 3.09 0.3486
WVFGRD96 16.0 15 85 45 3.11 0.3473
WVFGRD96 17.0 275 35 -15 3.12 0.3461
WVFGRD96 18.0 275 35 -20 3.14 0.3444
WVFGRD96 19.0 165 45 -60 3.17 0.3451
WVFGRD96 20.0 165 45 -60 3.19 0.3653
WVFGRD96 21.0 165 45 -60 3.21 0.3818
WVFGRD96 22.0 165 45 -60 3.23 0.3991
WVFGRD96 23.0 165 45 -60 3.24 0.4120
WVFGRD96 24.0 170 45 -60 3.25 0.4216
WVFGRD96 25.0 150 40 -80 3.26 0.4301
WVFGRD96 26.0 150 40 -80 3.27 0.4344
WVFGRD96 27.0 145 45 -90 3.28 0.4452
WVFGRD96 28.0 145 45 -90 3.29 0.4597
WVFGRD96 29.0 330 50 -85 3.30 0.4786
WVFGRD96 30.0 325 50 -90 3.31 0.5012
WVFGRD96 31.0 325 50 -90 3.32 0.5207
WVFGRD96 32.0 150 40 -85 3.33 0.5384
WVFGRD96 33.0 325 50 -95 3.34 0.5554
WVFGRD96 34.0 155 40 -80 3.35 0.5685
WVFGRD96 35.0 155 40 -80 3.36 0.5793
WVFGRD96 36.0 155 40 -80 3.36 0.5838
WVFGRD96 37.0 155 40 -75 3.37 0.5848
WVFGRD96 38.0 155 40 -75 3.38 0.5810
WVFGRD96 39.0 155 40 -75 3.39 0.5731
WVFGRD96 40.0 160 40 -75 3.47 0.5614
WVFGRD96 41.0 160 40 -75 3.48 0.5601
WVFGRD96 42.0 160 40 -75 3.49 0.5564
WVFGRD96 43.0 160 40 -75 3.50 0.5517
WVFGRD96 44.0 160 40 -75 3.51 0.5451
WVFGRD96 45.0 165 45 -65 3.51 0.5406
WVFGRD96 46.0 165 45 -65 3.52 0.5329
WVFGRD96 47.0 165 45 -65 3.53 0.5273
WVFGRD96 48.0 165 45 -65 3.53 0.5198
WVFGRD96 49.0 165 45 -65 3.53 0.5154
The best solution is
WVFGRD96 37.0 155 40 -75 3.37 0.5848
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 -30 o DIST/3.3 +70
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 Fri Apr 26 06:48:48 PM CDT 2024
