Location
Location ANSS
The ANSS event ID is ak0139iodhbh and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0139iodhbh/executive.
2013/07/26 20:37:19 58.016 -151.750 40.3 4.7 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2013/07/26 20:37:19:0 58.02 -151.75 40.3 4.7 Alaska
Stations used:
AK.BRLK AK.CHI AK.CNP AK.FID AK.GHO AK.GLI AK.HIN AK.HOM
AK.KNK AK.RC01 AK.SII AK.SKN AK.SSN AK.SWD AT.CHGN AT.OHAK
AT.PMR AT.SVW2 II.KDAK
Filtering commands used:
cut a -30 a 180
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 7.59e+22 dyne-cm
Mw = 4.52
Z = 40 km
Plane Strike Dip Rake
NP1 220 50 -90
NP2 40 40 -90
Principal Axes:
Axis Value Plunge Azimuth
T 7.59e+22 5 310
N 0.00e+00 -0 220
P -7.59e+22 85 130
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.09e+22
Mxy -3.68e+22
Mxz 8.47e+21
Myy 4.38e+22
Myz -1.01e+22
Mzz -7.47e+22
##############
######################
#####################------#
################------------#
T ############-----------------##
# ##########-------------------###
#############---------------------####
############-----------------------#####
###########------------------------#####
###########-------------------------######
##########----------- -----------#######
#########------------ P -----------#######
########------------- ----------########
######--------------------------########
######-------------------------#########
#####-----------------------##########
####---------------------###########
###-------------------############
#----------------#############
#-----------################
######################
##############
Global CMT Convention Moment Tensor:
R T P
-7.47e+22 8.47e+21 1.01e+22
8.47e+21 3.09e+22 3.68e+22
1.01e+22 3.68e+22 4.38e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130726203719/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 = 40
DIP = 40
RAKE = -90
MW = 4.52
HS = 40.0
The NDK file is 20130726203719.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 2013/07/26 20:37:19:0 58.02 -151.75 40.3 4.7 Alaska
Stations used:
AK.BRLK AK.CHI AK.CNP AK.FID AK.GHO AK.GLI AK.HIN AK.HOM
AK.KNK AK.RC01 AK.SII AK.SKN AK.SSN AK.SWD AT.CHGN AT.OHAK
AT.PMR AT.SVW2 II.KDAK
Filtering commands used:
cut a -30 a 180
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 7.59e+22 dyne-cm
Mw = 4.52
Z = 40 km
Plane Strike Dip Rake
NP1 220 50 -90
NP2 40 40 -90
Principal Axes:
Axis Value Plunge Azimuth
T 7.59e+22 5 310
N 0.00e+00 -0 220
P -7.59e+22 85 130
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.09e+22
Mxy -3.68e+22
Mxz 8.47e+21
Myy 4.38e+22
Myz -1.01e+22
Mzz -7.47e+22
##############
######################
#####################------#
################------------#
T ############-----------------##
# ##########-------------------###
#############---------------------####
############-----------------------#####
###########------------------------#####
###########-------------------------######
##########----------- -----------#######
#########------------ P -----------#######
########------------- ----------########
######--------------------------########
######-------------------------#########
#####-----------------------##########
####---------------------###########
###-------------------############
#----------------#############
#-----------################
######################
##############
Global CMT Convention Moment Tensor:
R T P
-7.47e+22 8.47e+21 1.01e+22
8.47e+21 3.09e+22 3.68e+22
1.01e+22 3.68e+22 4.38e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130726203719/index.html
|
USGS/SLU Regional Moment Solution
13/07/26 20:37:19.00
Epicenter: 58.024 -151.745
MW 4.6
USGS/SLU REGIONAL MOMENT TENSOR
Depth 40 No. of sta: 30
Moment Tensor; Scale 10**15 Nm
Mrr=-7.74 Mtt= 2.11
Mpp= 5.63 Mrt=-1.46
Mrp=-0.62 Mtp= 4.58
Principal axes:
T Val= 8.89 Plg= 5 Azm=125
N -0.94 7 215
P -7.96 82 1
Best Double Couple:Mo=8.5*10**15
NP1:Strike= 41 Dip=50 Slip= -81
NP2: 207 41 -101
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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 -30 a 180
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 1.0 115 45 85 3.75 0.2859
WVFGRD96 2.0 75 45 85 3.95 0.3658
WVFGRD96 3.0 255 45 85 4.03 0.3897
WVFGRD96 4.0 215 85 5 3.96 0.3739
WVFGRD96 5.0 215 85 0 3.99 0.3818
WVFGRD96 6.0 35 90 0 4.01 0.3819
WVFGRD96 7.0 30 75 -20 4.04 0.3907
WVFGRD96 8.0 30 70 -25 4.08 0.4042
WVFGRD96 9.0 30 70 -25 4.09 0.4078
WVFGRD96 10.0 30 70 -20 4.09 0.4083
WVFGRD96 11.0 30 65 -20 4.10 0.4092
WVFGRD96 12.0 75 75 -40 4.06 0.4212
WVFGRD96 13.0 70 70 -40 4.08 0.4392
WVFGRD96 14.0 70 65 -40 4.09 0.4557
WVFGRD96 15.0 70 65 -40 4.10 0.4716
WVFGRD96 16.0 70 65 -40 4.11 0.4867
WVFGRD96 17.0 70 65 -40 4.12 0.5006
WVFGRD96 18.0 70 65 -40 4.13 0.5164
WVFGRD96 19.0 70 65 -40 4.14 0.5320
WVFGRD96 20.0 35 30 -90 4.22 0.5534
WVFGRD96 21.0 35 30 -90 4.24 0.5732
WVFGRD96 22.0 215 60 -90 4.24 0.5921
WVFGRD96 23.0 40 35 -85 4.25 0.6106
WVFGRD96 24.0 40 35 -85 4.25 0.6275
WVFGRD96 25.0 40 35 -85 4.26 0.6430
WVFGRD96 26.0 40 35 -85 4.27 0.6570
WVFGRD96 27.0 40 35 -85 4.28 0.6693
WVFGRD96 28.0 40 35 -85 4.29 0.6802
WVFGRD96 29.0 45 40 -80 4.29 0.6905
WVFGRD96 30.0 50 40 -75 4.29 0.7000
WVFGRD96 31.0 50 40 -75 4.30 0.7090
WVFGRD96 32.0 50 40 -75 4.31 0.7159
WVFGRD96 33.0 45 40 -80 4.32 0.7199
WVFGRD96 34.0 45 40 -80 4.33 0.7224
WVFGRD96 35.0 45 40 -80 4.34 0.7228
WVFGRD96 36.0 45 40 -80 4.35 0.7205
WVFGRD96 37.0 45 40 -80 4.36 0.7159
WVFGRD96 38.0 50 45 -80 4.37 0.7113
WVFGRD96 39.0 45 45 -80 4.38 0.7064
WVFGRD96 40.0 40 40 -90 4.52 0.7422
WVFGRD96 41.0 40 40 -90 4.52 0.7395
WVFGRD96 42.0 220 45 -95 4.52 0.7339
WVFGRD96 43.0 220 45 -95 4.53 0.7272
WVFGRD96 44.0 225 45 -90 4.54 0.7186
WVFGRD96 45.0 45 45 -90 4.54 0.7087
WVFGRD96 46.0 220 45 -90 4.55 0.6979
WVFGRD96 47.0 40 45 -90 4.55 0.6860
WVFGRD96 48.0 45 50 -90 4.55 0.6733
WVFGRD96 49.0 45 50 -90 4.56 0.6608
WVFGRD96 50.0 45 50 -90 4.56 0.6484
WVFGRD96 51.0 45 50 -90 4.56 0.6353
WVFGRD96 52.0 225 40 -90 4.56 0.6212
WVFGRD96 53.0 225 40 -90 4.56 0.6075
WVFGRD96 54.0 45 50 -90 4.56 0.5929
WVFGRD96 55.0 45 50 -90 4.56 0.5792
WVFGRD96 56.0 225 40 -90 4.56 0.5650
WVFGRD96 57.0 45 50 -90 4.56 0.5517
WVFGRD96 58.0 45 50 -90 4.55 0.5381
WVFGRD96 59.0 45 50 -90 4.55 0.5254
The best solution is
WVFGRD96 40.0 40 40 -90 4.52 0.7422
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 -30 a 180
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 07:11:44 PM CDT 2024
