Location
Location ANSS
The ANSS event ID is ak012b4fi0t8 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak012b4fi0t8/executive.
2012/08/29 12:50:55 60.334 -150.735 65.8 4.4 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2012/08/29 12:50:55:0 60.33 -150.74 65.8 4.4 Alaska
Stations used:
AK.BMR AK.BPAW AK.BRLK AK.CNP AK.DOT AK.FIB AK.FID AK.GHO
AK.GLI AK.HIN AK.HOM AK.KNK AK.KTH AK.RC01 AK.RIDG AK.SAW
AK.SCM AK.SKN AK.SSN AT.SVW2 II.KDAK IU.COLA
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 7.85e+22 dyne-cm
Mw = 4.53
Z = 72 km
Plane Strike Dip Rake
NP1 281 72 154
NP2 20 65 20
Principal Axes:
Axis Value Plunge Azimuth
T 7.85e+22 31 239
N 0.00e+00 58 69
P -7.85e+22 5 332
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.54e+22
Mxy 5.78e+22
Mxz -2.34e+22
Myy 2.48e+22
Myz -2.69e+22
Mzz 2.06e+22
--------------
P ----------------###
--- ----------------######
-----------------------#######
--------------------------########
---------------------------#########
---------------------------###########
----------------------------############
-###################--------############
############################-#############
############################------########
###########################----------#####
###########################-------------##
###### ################---------------
###### T ###############----------------
##### ##############----------------
####################----------------
##################----------------
###############---------------
############----------------
#######---------------
--------------
Global CMT Convention Moment Tensor:
R T P
2.06e+22 -2.34e+22 2.69e+22
-2.34e+22 -4.54e+22 -5.78e+22
2.69e+22 -5.78e+22 2.48e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120829125055/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 = 20
DIP = 65
RAKE = 20
MW = 4.53
HS = 72.0
The NDK file is 20120829125055.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 2012/08/29 12:50:55:0 60.33 -150.74 65.8 4.4 Alaska
Stations used:
AK.BMR AK.BPAW AK.BRLK AK.CNP AK.DOT AK.FIB AK.FID AK.GHO
AK.GLI AK.HIN AK.HOM AK.KNK AK.KTH AK.RC01 AK.RIDG AK.SAW
AK.SCM AK.SKN AK.SSN AT.SVW2 II.KDAK IU.COLA
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 7.85e+22 dyne-cm
Mw = 4.53
Z = 72 km
Plane Strike Dip Rake
NP1 281 72 154
NP2 20 65 20
Principal Axes:
Axis Value Plunge Azimuth
T 7.85e+22 31 239
N 0.00e+00 58 69
P -7.85e+22 5 332
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.54e+22
Mxy 5.78e+22
Mxz -2.34e+22
Myy 2.48e+22
Myz -2.69e+22
Mzz 2.06e+22
--------------
P ----------------###
--- ----------------######
-----------------------#######
--------------------------########
---------------------------#########
---------------------------###########
----------------------------############
-###################--------############
############################-#############
############################------########
###########################----------#####
###########################-------------##
###### ################---------------
###### T ###############----------------
##### ##############----------------
####################----------------
##################----------------
###############---------------
############----------------
#######---------------
--------------
Global CMT Convention Moment Tensor:
R T P
2.06e+22 -2.34e+22 2.69e+22
-2.34e+22 -4.54e+22 -5.78e+22
2.69e+22 -5.78e+22 2.48e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120829125055/index.html
|
USGS/SLU Regional Moment Solution
12/08/29 12:50:51.00
Epicenter: 60.269 -150.709
MW 4.5
USGS/SLU REGIONAL MOMENT TENSOR
Depth 67 No. of sta: 53
Moment Tensor; Scale 10**15 Nm
Mrr= 1.50 Mtt=-3.34
Mpp= 1.84 Mrt=-2.69
Mrp= 3.18 Mtp=-4.53
Principal axes:
T Val= 7.36 Plg=35 Azm=238
N -1.30 54 72
P -6.06 7 333
Best Double Couple:Mo=6.8*10**15
NP1:Strike= 21 Dip=60 Slip= 22
NP2: 280 71 149
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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:
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 10 65 -25 3.66 0.1594
WVFGRD96 1.0 25 85 5 3.66 0.1721
WVFGRD96 2.0 25 75 15 3.79 0.2304
WVFGRD96 3.0 25 65 10 3.85 0.2540
WVFGRD96 4.0 20 65 -5 3.87 0.2769
WVFGRD96 5.0 20 65 -5 3.90 0.2979
WVFGRD96 6.0 20 65 -5 3.93 0.3152
WVFGRD96 7.0 20 70 -5 3.95 0.3303
WVFGRD96 8.0 20 65 -10 3.99 0.3433
WVFGRD96 9.0 20 65 -10 4.00 0.3531
WVFGRD96 10.0 20 65 -10 4.01 0.3605
WVFGRD96 11.0 20 70 -15 4.03 0.3671
WVFGRD96 12.0 20 70 -15 4.05 0.3726
WVFGRD96 13.0 20 70 -15 4.06 0.3768
WVFGRD96 14.0 20 70 -15 4.07 0.3800
WVFGRD96 15.0 20 70 -15 4.08 0.3832
WVFGRD96 16.0 15 70 -20 4.09 0.3869
WVFGRD96 17.0 15 70 -20 4.10 0.3901
WVFGRD96 18.0 15 70 -20 4.11 0.3933
WVFGRD96 19.0 15 70 -20 4.12 0.3967
WVFGRD96 20.0 15 70 -20 4.13 0.3999
WVFGRD96 21.0 15 70 -20 4.14 0.4025
WVFGRD96 22.0 15 70 -15 4.14 0.4049
WVFGRD96 23.0 15 70 -20 4.15 0.4072
WVFGRD96 24.0 15 70 -15 4.16 0.4099
WVFGRD96 25.0 15 70 -15 4.17 0.4122
WVFGRD96 26.0 15 70 -15 4.17 0.4142
WVFGRD96 27.0 15 70 -10 4.18 0.4159
WVFGRD96 28.0 15 70 -10 4.19 0.4174
WVFGRD96 29.0 15 70 -10 4.20 0.4191
WVFGRD96 30.0 15 70 -10 4.20 0.4205
WVFGRD96 31.0 15 70 -10 4.21 0.4215
WVFGRD96 32.0 15 70 -5 4.22 0.4220
WVFGRD96 33.0 15 70 -5 4.23 0.4222
WVFGRD96 34.0 15 70 -5 4.24 0.4221
WVFGRD96 35.0 15 70 0 4.25 0.4233
WVFGRD96 36.0 15 70 0 4.26 0.4248
WVFGRD96 37.0 15 70 0 4.27 0.4264
WVFGRD96 38.0 15 70 0 4.28 0.4276
WVFGRD96 39.0 15 70 0 4.29 0.4286
WVFGRD96 40.0 15 60 5 4.34 0.4301
WVFGRD96 41.0 15 60 5 4.35 0.4306
WVFGRD96 42.0 15 60 5 4.35 0.4310
WVFGRD96 43.0 15 65 5 4.36 0.4315
WVFGRD96 44.0 15 65 5 4.37 0.4321
WVFGRD96 45.0 15 65 10 4.38 0.4328
WVFGRD96 46.0 15 65 10 4.39 0.4335
WVFGRD96 47.0 20 65 15 4.40 0.4346
WVFGRD96 48.0 20 65 15 4.40 0.4363
WVFGRD96 49.0 20 65 20 4.42 0.4389
WVFGRD96 50.0 20 65 20 4.42 0.4423
WVFGRD96 51.0 20 65 20 4.43 0.4455
WVFGRD96 52.0 20 65 20 4.44 0.4484
WVFGRD96 53.0 20 65 20 4.44 0.4514
WVFGRD96 54.0 20 65 20 4.45 0.4546
WVFGRD96 55.0 20 65 20 4.46 0.4574
WVFGRD96 56.0 20 65 20 4.46 0.4599
WVFGRD96 57.0 20 65 20 4.47 0.4627
WVFGRD96 58.0 20 65 20 4.47 0.4656
WVFGRD96 59.0 20 65 20 4.48 0.4680
WVFGRD96 60.0 20 65 20 4.48 0.4695
WVFGRD96 61.0 20 65 20 4.49 0.4713
WVFGRD96 62.0 20 65 20 4.49 0.4737
WVFGRD96 63.0 20 65 20 4.50 0.4749
WVFGRD96 64.0 20 65 20 4.50 0.4761
WVFGRD96 65.0 20 65 20 4.50 0.4775
WVFGRD96 66.0 20 65 20 4.51 0.4784
WVFGRD96 67.0 20 65 20 4.51 0.4794
WVFGRD96 68.0 20 65 20 4.51 0.4795
WVFGRD96 69.0 20 65 20 4.52 0.4804
WVFGRD96 70.0 20 65 20 4.52 0.4809
WVFGRD96 71.0 20 65 20 4.52 0.4803
WVFGRD96 72.0 20 65 20 4.53 0.4811
WVFGRD96 73.0 20 65 20 4.53 0.4805
WVFGRD96 74.0 20 65 20 4.53 0.4802
WVFGRD96 75.0 20 65 20 4.54 0.4802
WVFGRD96 76.0 20 65 20 4.54 0.4786
WVFGRD96 77.0 20 65 20 4.54 0.4789
WVFGRD96 78.0 20 65 20 4.54 0.4777
WVFGRD96 79.0 20 65 20 4.55 0.4768
WVFGRD96 80.0 20 65 20 4.55 0.4758
WVFGRD96 81.0 20 65 20 4.55 0.4743
WVFGRD96 82.0 20 65 20 4.55 0.4735
WVFGRD96 83.0 20 65 20 4.56 0.4716
WVFGRD96 84.0 20 65 20 4.56 0.4708
WVFGRD96 85.0 20 65 20 4.56 0.4689
WVFGRD96 86.0 20 65 20 4.56 0.4676
WVFGRD96 87.0 20 65 20 4.56 0.4661
WVFGRD96 88.0 25 65 20 4.56 0.4643
WVFGRD96 89.0 25 65 20 4.57 0.4632
WVFGRD96 90.0 25 65 20 4.57 0.4616
WVFGRD96 91.0 25 65 20 4.57 0.4603
WVFGRD96 92.0 25 65 20 4.57 0.4587
WVFGRD96 93.0 25 65 20 4.57 0.4571
WVFGRD96 94.0 25 70 20 4.58 0.4556
WVFGRD96 95.0 25 70 20 4.58 0.4543
WVFGRD96 96.0 25 70 20 4.58 0.4531
WVFGRD96 97.0 25 70 20 4.58 0.4516
WVFGRD96 98.0 25 70 20 4.58 0.4505
WVFGRD96 99.0 25 70 20 4.59 0.4490
The best solution is
WVFGRD96 72.0 20 65 20 4.53 0.4811
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
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 10:40:06 PM CDT 2024
