Location
Location ANSS
The ANSS event ID is ak01386kg7tb and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak01386kg7tb/executive.
2013/06/27 11:40:47 61.328 -150.019 55.3 4.2 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2013/06/27 11:40:47:0 61.33 -150.02 55.3 4.2 Alaska
Stations used:
AK.BPAW AK.BWN AK.GHO AK.GLI AK.HDA AK.HIN AK.KNK AK.KTH
AK.MCK AK.RC01 AK.SAW AK.SCM AK.SKN AK.WRH II.KDAK IU.COLA
Filtering commands used:
cut a -10 a 150
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 3.67e+22 dyne-cm
Mw = 4.31
Z = 58 km
Plane Strike Dip Rake
NP1 200 65 -35
NP2 306 59 -150
Principal Axes:
Axis Value Plunge Azimuth
T 3.67e+22 4 254
N 0.00e+00 48 349
P -3.67e+22 42 161
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.56e+22
Mxy 1.57e+22
Mxz 1.66e+22
Myy 3.18e+22
Myz -8.38e+21
Mzz -1.61e+22
--------------
---------------#######
----------------############
---------------###############
##########------##################
####################################
################-----#################
################---------###############
###############------------#############
###############---------------############
###############-----------------##########
##############-------------------#########
###########--------------------########
T ##########----------------------######
##########-----------------------#####
###########------------------------###
#########----------- -----------##
########----------- P ------------
#######---------- ----------
######----------------------
###-------------------
--------------
Global CMT Convention Moment Tensor:
R T P
-1.61e+22 1.66e+22 8.38e+21
1.66e+22 -1.56e+22 -1.57e+22
8.38e+21 -1.57e+22 3.18e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130627114047/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 = 200
DIP = 65
RAKE = -35
MW = 4.31
HS = 58.0
The NDK file is 20130627114047.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 |
USGSMT |
USGS/SLU Moment Tensor Solution
ENS 2013/06/27 11:40:47:0 61.33 -150.02 55.3 4.2 Alaska
Stations used:
AK.BPAW AK.BWN AK.GHO AK.GLI AK.HDA AK.HIN AK.KNK AK.KTH
AK.MCK AK.RC01 AK.SAW AK.SCM AK.SKN AK.WRH II.KDAK IU.COLA
Filtering commands used:
cut a -10 a 150
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 3.67e+22 dyne-cm
Mw = 4.31
Z = 58 km
Plane Strike Dip Rake
NP1 200 65 -35
NP2 306 59 -150
Principal Axes:
Axis Value Plunge Azimuth
T 3.67e+22 4 254
N 0.00e+00 48 349
P -3.67e+22 42 161
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.56e+22
Mxy 1.57e+22
Mxz 1.66e+22
Myy 3.18e+22
Myz -8.38e+21
Mzz -1.61e+22
--------------
---------------#######
----------------############
---------------###############
##########------##################
####################################
################-----#################
################---------###############
###############------------#############
###############---------------############
###############-----------------##########
##############-------------------#########
###########--------------------########
T ##########----------------------######
##########-----------------------#####
###########------------------------###
#########----------- -----------##
########----------- P ------------
#######---------- ----------
######----------------------
###-------------------
--------------
Global CMT Convention Moment Tensor:
R T P
-1.61e+22 1.66e+22 8.38e+21
1.66e+22 -1.56e+22 -1.57e+22
8.38e+21 -1.57e+22 3.18e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130627114047/index.html
|
us ak10746531-neic-mwr
Type
Mwr
Moment
3.62e+15 N-m
Magnitude
4.3
Percent DC
77%
Depth
56.0 km
Author
neic
Updated
2013-06-27 16:56:42 UTC
Principal Axes
Axis Value Plunge Azimuth
T 3.413 8 266
N 0.388 40 3
P -3.801 49 166
Nodal Planes
Plane Strike Dip Rake
NP1 206 64 -45
NP2 320 50 -146
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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 a -10 a 150
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 160 45 60 3.48 0.1538
WVFGRD96 1.0 160 50 65 3.53 0.1632
WVFGRD96 2.0 165 45 65 3.64 0.2076
WVFGRD96 3.0 160 50 60 3.71 0.2215
WVFGRD96 4.0 160 50 55 3.74 0.2160
WVFGRD96 5.0 150 65 40 3.73 0.2042
WVFGRD96 6.0 305 70 -25 3.72 0.2057
WVFGRD96 7.0 325 85 30 3.74 0.2123
WVFGRD96 8.0 330 80 40 3.79 0.2195
WVFGRD96 9.0 325 85 35 3.80 0.2250
WVFGRD96 10.0 140 90 -35 3.80 0.2289
WVFGRD96 11.0 140 90 -35 3.81 0.2326
WVFGRD96 12.0 320 90 35 3.82 0.2362
WVFGRD96 13.0 305 90 35 3.84 0.2390
WVFGRD96 14.0 305 90 35 3.85 0.2417
WVFGRD96 15.0 305 90 35 3.86 0.2449
WVFGRD96 16.0 305 85 35 3.87 0.2463
WVFGRD96 17.0 305 85 35 3.88 0.2478
WVFGRD96 18.0 305 85 35 3.89 0.2493
WVFGRD96 19.0 305 85 35 3.90 0.2508
WVFGRD96 20.0 40 70 -30 3.90 0.2515
WVFGRD96 21.0 40 70 -30 3.91 0.2579
WVFGRD96 22.0 40 70 -30 3.92 0.2647
WVFGRD96 23.0 40 70 -30 3.93 0.2710
WVFGRD96 24.0 40 70 -30 3.94 0.2768
WVFGRD96 25.0 40 70 -30 3.95 0.2826
WVFGRD96 26.0 40 70 -30 3.96 0.2874
WVFGRD96 27.0 40 70 -30 3.96 0.2923
WVFGRD96 28.0 40 70 -30 3.97 0.2967
WVFGRD96 29.0 40 70 -30 3.98 0.3007
WVFGRD96 30.0 40 70 -25 3.99 0.3048
WVFGRD96 31.0 215 65 -15 4.03 0.3157
WVFGRD96 32.0 215 65 -15 4.04 0.3238
WVFGRD96 33.0 215 65 -20 4.05 0.3317
WVFGRD96 34.0 215 65 -20 4.06 0.3389
WVFGRD96 35.0 215 65 -20 4.07 0.3459
WVFGRD96 36.0 215 65 -20 4.09 0.3523
WVFGRD96 37.0 215 65 -20 4.10 0.3583
WVFGRD96 38.0 215 65 -20 4.11 0.3640
WVFGRD96 39.0 210 65 -20 4.13 0.3704
WVFGRD96 40.0 210 60 -30 4.18 0.3661
WVFGRD96 41.0 210 60 -30 4.19 0.3727
WVFGRD96 42.0 210 60 -30 4.20 0.3783
WVFGRD96 43.0 210 60 -30 4.21 0.3827
WVFGRD96 44.0 210 60 -30 4.22 0.3860
WVFGRD96 45.0 210 60 -30 4.23 0.3891
WVFGRD96 46.0 210 60 -35 4.24 0.3914
WVFGRD96 47.0 210 60 -35 4.24 0.3932
WVFGRD96 48.0 210 60 -35 4.25 0.3943
WVFGRD96 49.0 205 65 -30 4.26 0.3955
WVFGRD96 50.0 205 65 -30 4.27 0.3976
WVFGRD96 51.0 205 65 -30 4.27 0.3991
WVFGRD96 52.0 205 65 -30 4.28 0.4005
WVFGRD96 53.0 205 65 -35 4.28 0.4016
WVFGRD96 54.0 200 65 -35 4.30 0.4024
WVFGRD96 55.0 200 65 -35 4.30 0.4038
WVFGRD96 56.0 200 65 -35 4.30 0.4047
WVFGRD96 57.0 200 65 -35 4.31 0.4054
WVFGRD96 58.0 200 65 -35 4.31 0.4054
WVFGRD96 59.0 200 65 -35 4.32 0.4047
WVFGRD96 60.0 200 65 -35 4.32 0.4037
WVFGRD96 61.0 200 65 -35 4.32 0.4026
WVFGRD96 62.0 200 65 -35 4.32 0.4013
WVFGRD96 63.0 200 65 -35 4.33 0.3992
WVFGRD96 64.0 200 65 -35 4.33 0.3968
WVFGRD96 65.0 200 70 -35 4.34 0.3959
WVFGRD96 66.0 200 70 -35 4.34 0.3953
WVFGRD96 67.0 200 70 -35 4.34 0.3935
WVFGRD96 68.0 200 70 -35 4.35 0.3914
WVFGRD96 69.0 200 70 -35 4.35 0.3899
WVFGRD96 70.0 200 70 -35 4.35 0.3876
WVFGRD96 71.0 200 70 -35 4.35 0.3857
WVFGRD96 72.0 200 70 -35 4.35 0.3831
WVFGRD96 73.0 200 70 -35 4.35 0.3808
WVFGRD96 74.0 200 70 -35 4.35 0.3780
WVFGRD96 75.0 200 70 -35 4.35 0.3754
WVFGRD96 76.0 200 70 -35 4.35 0.3732
WVFGRD96 77.0 200 75 -35 4.37 0.3701
WVFGRD96 78.0 200 75 -35 4.37 0.3684
WVFGRD96 79.0 200 75 -35 4.37 0.3661
The best solution is
WVFGRD96 58.0 200 65 -35 4.31 0.4054
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 a -10 a 150
rtr
taper w 0.1
hp c 0.02 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 06:19:52 PM CDT 2024
