Location
Location ANSS
The ANSS event ID is ak017a8wiw06 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak017a8wiw06/executive.
2017/08/11 06:18:17 60.068 -152.470 95.7 4.9 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2017/08/11 06:18:17:0 60.07 -152.47 95.7 4.9 Alaska
Stations used:
AK.BRLK AK.CNP AK.CUT AK.HOM AK.RC01 AK.SSN AT.PMR AT.SVW2
AT.TTA TA.L19K TA.M19K TA.M20K TA.M22K TA.N18K TA.N19K
TA.O18K TA.O19K TA.P18K TA.P19K TA.Q19K
Filtering commands used:
cut o DIST/3.4 -30 o DIST/3.4 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 3.02e+23 dyne-cm
Mw = 4.92
Z = 98 km
Plane Strike Dip Rake
NP1 255 65 65
NP2 123 35 132
Principal Axes:
Axis Value Plunge Azimuth
T 3.02e+23 62 126
N 0.00e+00 23 266
P -3.02e+23 16 3
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.53e+23
Mxy -4.78e+22
Mxz -1.56e+23
Myy 4.38e+22
Myz 9.76e+22
Mzz 2.10e+23
------ -----
---------- P ---------
------------- ------------
------------------------------
----------------------------------
------------------------------------
#-------------------------------------
##------------------#################---
##------------##########################
###--------###############################
####---###################################
##########################################
###---#################### #############
------################### T ############
-------################## ############
--------##############################
---------###########################
-----------######################-
-------------###############--
----------------------------
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
2.10e+23 -1.56e+23 -9.76e+22
-1.56e+23 -2.53e+23 4.78e+22
-9.76e+22 4.78e+22 4.38e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170811061817/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 = 255
DIP = 65
RAKE = 65
MW = 4.92
HS = 98.0
The NDK file is 20170811061817.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 2017/08/11 06:18:17:0 60.07 -152.47 95.7 4.9 Alaska
Stations used:
AK.BRLK AK.CNP AK.CUT AK.HOM AK.RC01 AK.SSN AT.PMR AT.SVW2
AT.TTA TA.L19K TA.M19K TA.M20K TA.M22K TA.N18K TA.N19K
TA.O18K TA.O19K TA.P18K TA.P19K TA.Q19K
Filtering commands used:
cut o DIST/3.4 -30 o DIST/3.4 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 3.02e+23 dyne-cm
Mw = 4.92
Z = 98 km
Plane Strike Dip Rake
NP1 255 65 65
NP2 123 35 132
Principal Axes:
Axis Value Plunge Azimuth
T 3.02e+23 62 126
N 0.00e+00 23 266
P -3.02e+23 16 3
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.53e+23
Mxy -4.78e+22
Mxz -1.56e+23
Myy 4.38e+22
Myz 9.76e+22
Mzz 2.10e+23
------ -----
---------- P ---------
------------- ------------
------------------------------
----------------------------------
------------------------------------
#-------------------------------------
##------------------#################---
##------------##########################
###--------###############################
####---###################################
##########################################
###---#################### #############
------################### T ############
-------################## ############
--------##############################
---------###########################
-----------######################-
-------------###############--
----------------------------
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
2.10e+23 -1.56e+23 -9.76e+22
-1.56e+23 -2.53e+23 4.78e+22
-9.76e+22 4.78e+22 4.38e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170811061817/index.html
|
Regional Moment Tensor (Mwr)
Moment 3.483e+16 N-m
Magnitude 5.0 Mwr
Depth 102.0 km
Percent DC 91 %
Half Duration –
Catalog US
Data Source US3
Contributor US3
Nodal Planes
Plane Strike Dip Rake
NP1 255 62 72
NP2 110 33 120
Principal Axes
Axis Value Plunge Azimuth
T 3.560e+16 N-m 68 128
N -0.158e+16 N-m 16 264
P -3.401e+16 N-m 15 358
|
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/3.4 -30 o DIST/3.4 +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 2.0 90 45 -90 4.09 0.2494
WVFGRD96 4.0 305 55 -35 4.09 0.2544
WVFGRD96 6.0 145 60 30 4.15 0.2992
WVFGRD96 8.0 235 40 45 4.27 0.3301
WVFGRD96 10.0 230 40 40 4.31 0.3639
WVFGRD96 12.0 225 40 35 4.35 0.3789
WVFGRD96 14.0 220 45 25 4.37 0.3839
WVFGRD96 16.0 220 45 25 4.40 0.3834
WVFGRD96 18.0 220 50 20 4.42 0.3812
WVFGRD96 20.0 220 55 20 4.44 0.3827
WVFGRD96 22.0 225 55 20 4.47 0.3871
WVFGRD96 24.0 225 60 20 4.48 0.3976
WVFGRD96 26.0 225 60 20 4.51 0.4114
WVFGRD96 28.0 225 65 25 4.52 0.4272
WVFGRD96 30.0 230 60 30 4.55 0.4425
WVFGRD96 32.0 230 60 30 4.56 0.4533
WVFGRD96 34.0 235 60 35 4.58 0.4588
WVFGRD96 36.0 220 55 -25 4.61 0.4642
WVFGRD96 38.0 220 55 -25 4.64 0.4680
WVFGRD96 40.0 240 60 45 4.70 0.4882
WVFGRD96 42.0 240 55 40 4.73 0.4955
WVFGRD96 44.0 240 55 40 4.75 0.4990
WVFGRD96 46.0 240 55 40 4.77 0.5053
WVFGRD96 48.0 240 60 40 4.77 0.5103
WVFGRD96 50.0 265 45 80 4.84 0.5245
WVFGRD96 52.0 95 45 95 4.85 0.5384
WVFGRD96 54.0 265 50 80 4.85 0.5485
WVFGRD96 56.0 265 50 80 4.86 0.5639
WVFGRD96 58.0 260 50 75 4.86 0.5768
WVFGRD96 60.0 260 50 75 4.87 0.5886
WVFGRD96 62.0 270 55 80 4.88 0.6030
WVFGRD96 64.0 265 55 75 4.88 0.6167
WVFGRD96 66.0 265 55 75 4.89 0.6285
WVFGRD96 68.0 265 55 75 4.89 0.6395
WVFGRD96 70.0 260 60 70 4.89 0.6496
WVFGRD96 72.0 260 60 70 4.89 0.6590
WVFGRD96 74.0 260 60 70 4.89 0.6691
WVFGRD96 76.0 260 60 70 4.90 0.6792
WVFGRD96 78.0 260 60 70 4.90 0.6884
WVFGRD96 80.0 260 60 70 4.90 0.6955
WVFGRD96 82.0 260 60 70 4.90 0.7017
WVFGRD96 84.0 260 60 70 4.90 0.7071
WVFGRD96 86.0 260 60 70 4.91 0.7121
WVFGRD96 88.0 255 65 70 4.91 0.7166
WVFGRD96 90.0 255 65 70 4.91 0.7205
WVFGRD96 92.0 255 65 70 4.91 0.7244
WVFGRD96 94.0 255 65 65 4.92 0.7269
WVFGRD96 96.0 255 65 65 4.92 0.7277
WVFGRD96 98.0 255 65 65 4.92 0.7292
WVFGRD96 100.0 255 65 65 4.92 0.7291
WVFGRD96 102.0 255 65 65 4.93 0.7288
WVFGRD96 104.0 255 65 65 4.93 0.7276
WVFGRD96 106.0 255 65 65 4.93 0.7247
WVFGRD96 108.0 250 70 65 4.93 0.7243
WVFGRD96 110.0 250 70 65 4.93 0.7215
WVFGRD96 112.0 250 70 65 4.93 0.7203
WVFGRD96 114.0 250 70 65 4.93 0.7171
WVFGRD96 116.0 250 70 65 4.93 0.7152
WVFGRD96 118.0 250 70 65 4.94 0.7116
The best solution is
WVFGRD96 98.0 255 65 65 4.92 0.7292
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/3.4 -30 o DIST/3.4 +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 Sat Apr 27 03:59:45 PM CDT 2024
