Location
Location ANSS
The ANSS event ID is ak012awb7szn and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak012awb7szn/executive.
2012/08/24 21:58:09 63.920 -148.396 10.8 3.7 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2012/08/24 21:58:09:0 63.92 -148.40 10.8 3.7 Alaska
Stations used:
AK.BAL AK.BPAW AK.BRLK AK.BWN AK.CCB AK.COLD AK.CTG AK.DHY
AK.FID AK.FYU AK.GHO AK.GLI AK.GLM AK.HDA AK.HIN AK.HMT
AK.KLU AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA AK.PAX
AK.PPD AK.PPLA AK.RAG AK.RIDG AK.RND AK.SAW AK.SCM AK.SGA
AK.SKN AK.SSN AK.TGL AK.WAX AK.WRH AK.YAH AT.MID AT.SVW2
CN.DAWY IU.COLA US.EGAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 6.53e+21 dyne-cm
Mw = 3.81
Z = 17 km
Plane Strike Dip Rake
NP1 82 67 101
NP2 235 25 65
Principal Axes:
Axis Value Plunge Azimuth
T 6.53e+21 66 12
N 0.00e+00 10 258
P -6.53e+21 22 164
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.14e+21
Mxy 1.73e+21
Mxz 4.55e+21
Myy -3.96e+20
Myz -1.33e+20
Mzz 4.53e+21
--------------
----------------------
----------###############---
-------######################-
-------###########################
------##############################
-----############## ################
-----############### T #################
----################ #################
----######################################
---#####################################--
---##################################-----
---##############################---------
##-#######################--------------
##--------------------------------------
#-------------------------------------
------------------------------------
----------------------------------
------------------ ---------
----------------- P --------
-------------- -----
--------------
Global CMT Convention Moment Tensor:
R T P
4.53e+21 4.55e+21 1.33e+20
4.55e+21 -4.14e+21 -1.73e+21
1.33e+20 -1.73e+21 -3.96e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120824215809/index.html
|
Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is
STK = 235
DIP = 25
RAKE = 65
MW = 3.81
HS = 17.0
The NDK file is 20120824215809.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 2012/08/24 21:58:09:0 63.92 -148.40 10.8 3.7 Alaska
Stations used:
AK.BAL AK.BPAW AK.BRLK AK.BWN AK.CCB AK.COLD AK.CTG AK.DHY
AK.FID AK.FYU AK.GHO AK.GLI AK.GLM AK.HDA AK.HIN AK.HMT
AK.KLU AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA AK.PAX
AK.PPD AK.PPLA AK.RAG AK.RIDG AK.RND AK.SAW AK.SCM AK.SGA
AK.SKN AK.SSN AK.TGL AK.WAX AK.WRH AK.YAH AT.MID AT.SVW2
CN.DAWY IU.COLA US.EGAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 6.53e+21 dyne-cm
Mw = 3.81
Z = 17 km
Plane Strike Dip Rake
NP1 82 67 101
NP2 235 25 65
Principal Axes:
Axis Value Plunge Azimuth
T 6.53e+21 66 12
N 0.00e+00 10 258
P -6.53e+21 22 164
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.14e+21
Mxy 1.73e+21
Mxz 4.55e+21
Myy -3.96e+20
Myz -1.33e+20
Mzz 4.53e+21
--------------
----------------------
----------###############---
-------######################-
-------###########################
------##############################
-----############## ################
-----############### T #################
----################ #################
----######################################
---#####################################--
---##################################-----
---##############################---------
##-#######################--------------
##--------------------------------------
#-------------------------------------
------------------------------------
----------------------------------
------------------ ---------
----------------- P --------
-------------- -----
--------------
Global CMT Convention Moment Tensor:
R T P
4.53e+21 4.55e+21 1.33e+20
4.55e+21 -4.14e+21 -1.73e+21
1.33e+20 -1.73e+21 -3.96e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120824215809/index.html
|
NEIC Mwr
Moment
6.34e+14 N-m
Magnitude
3.8
Percent DC
89%
Depth
14.0 km
Principal Axes
Axis Value Plunge Azimuth
T 6.506 62 11
N -0.336 15 249
P -6.170 22 153
Nodal Planes
Plane Strike Dip Rake
NP1 225 34 61
NP2 78 64 118
|
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 0.5 275 40 -90 3.48 0.3569
WVFGRD96 1.0 275 40 -90 3.51 0.3355
WVFGRD96 2.0 115 50 -95 3.60 0.3930
WVFGRD96 3.0 330 55 -30 3.68 0.3515
WVFGRD96 4.0 335 60 -15 3.71 0.3299
WVFGRD96 5.0 175 20 -5 3.68 0.3727
WVFGRD96 6.0 180 20 0 3.68 0.4238
WVFGRD96 7.0 180 25 0 3.68 0.4597
WVFGRD96 8.0 185 20 5 3.75 0.4851
WVFGRD96 9.0 190 20 15 3.75 0.5145
WVFGRD96 10.0 200 20 25 3.75 0.5384
WVFGRD96 11.0 210 20 35 3.76 0.5582
WVFGRD96 12.0 210 25 40 3.77 0.5727
WVFGRD96 13.0 220 25 50 3.78 0.5850
WVFGRD96 14.0 220 25 50 3.79 0.5933
WVFGRD96 15.0 230 25 60 3.80 0.5977
WVFGRD96 16.0 230 25 60 3.80 0.5996
WVFGRD96 17.0 235 25 65 3.81 0.5996
WVFGRD96 18.0 230 25 60 3.81 0.5968
WVFGRD96 19.0 230 25 60 3.81 0.5920
WVFGRD96 20.0 230 25 60 3.81 0.5862
WVFGRD96 21.0 235 25 65 3.83 0.5801
WVFGRD96 22.0 235 25 65 3.83 0.5718
WVFGRD96 23.0 230 25 60 3.83 0.5626
WVFGRD96 24.0 230 25 60 3.84 0.5531
WVFGRD96 25.0 230 25 60 3.84 0.5429
WVFGRD96 26.0 230 25 60 3.84 0.5322
WVFGRD96 27.0 230 25 60 3.85 0.5213
WVFGRD96 28.0 225 25 55 3.85 0.5098
WVFGRD96 29.0 225 25 55 3.85 0.4977
The best solution is
WVFGRD96 17.0 235 25 65 3.81 0.5996
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 10:24:06 PM CDT 2024
