Location
Location ANSS
The ANSS event ID is ak01234mu8s7 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak01234mu8s7/executive.
2012/03/08 10:57:42 61.007 -150.914 10.0 4.0 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2012/03/08 10:57:42:0 61.01 -150.91 10.0 4.0 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.DHY AK.DIV AK.EYAK AK.GLI AK.KLU
AK.KTH AK.MCK AK.MLY AK.PPLA AK.RIDG AK.RND AK.SAW AK.SCM
AK.SKN AK.SSN AK.SWD AK.TRF IU.COLA
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 1.55e+22 dyne-cm
Mw = 4.06
Z = 30 km
Plane Strike Dip Rake
NP1 43 78 118
NP2 155 30 25
Principal Axes:
Axis Value Plunge Azimuth
T 1.55e+22 50 343
N 0.00e+00 27 217
P -1.55e+22 28 111
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.36e+21
Mxy 2.34e+21
Mxz 9.63e+21
Myy -1.00e+22
Myz -8.10e+21
Mzz 5.67e+21
##############
-#####################
--##########################
--###########################-
---########## ##############----
----########## T #############------
----########### ############--------
-----########################-----------
-----#######################------------
------######################--------------
------####################----------------
------##################------------------
------#################----------- -----
------##############------------- P ----
-------###########--------------- ----
-------########-----------------------
-------#####------------------------
----------------------------------
---####-----------------------
#########-------------------
#########-------------
##############
Global CMT Convention Moment Tensor:
R T P
5.67e+21 9.63e+21 8.10e+21
9.63e+21 4.36e+21 -2.34e+21
8.10e+21 -2.34e+21 -1.00e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120308105742/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 = 155
DIP = 30
RAKE = 25
MW = 4.06
HS = 30.0
The NDK file is 20120308105742.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 2012/03/08 10:57:42:0 61.01 -150.91 10.0 4.0 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.DHY AK.DIV AK.EYAK AK.GLI AK.KLU
AK.KTH AK.MCK AK.MLY AK.PPLA AK.RIDG AK.RND AK.SAW AK.SCM
AK.SKN AK.SSN AK.SWD AK.TRF IU.COLA
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 1.55e+22 dyne-cm
Mw = 4.06
Z = 30 km
Plane Strike Dip Rake
NP1 43 78 118
NP2 155 30 25
Principal Axes:
Axis Value Plunge Azimuth
T 1.55e+22 50 343
N 0.00e+00 27 217
P -1.55e+22 28 111
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.36e+21
Mxy 2.34e+21
Mxz 9.63e+21
Myy -1.00e+22
Myz -8.10e+21
Mzz 5.67e+21
##############
-#####################
--##########################
--###########################-
---########## ##############----
----########## T #############------
----########### ############--------
-----########################-----------
-----#######################------------
------######################--------------
------####################----------------
------##################------------------
------#################----------- -----
------##############------------- P ----
-------###########--------------- ----
-------########-----------------------
-------#####------------------------
----------------------------------
---####-----------------------
#########-------------------
#########-------------
##############
Global CMT Convention Moment Tensor:
R T P
5.67e+21 9.63e+21 8.10e+21
9.63e+21 4.36e+21 -2.34e+21
8.10e+21 -2.34e+21 -1.00e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120308105742/index.html
|
USGS/SLU Regional Moment Solution
12/03/08 10:57:44.07
Epicenter: 60.996 -150.950
MW 4.1
USGS/SLU REGIONAL MOMENT TENSOR
Depth 26 No. of sta: 67
Moment Tensor; Scale 10**15 Nm
Mrr= 0.91 Mtt= 0.56
Mpp=-1.46 Mrt= 0.90
Mrp= 0.45 Mtp=-0.04
Principal axes:
T Val= 1.68 Plg=51 Azm=350
N -0.11 36 196
P -1.57 13 96
Best Double Couple:Mo=1.6*10**15
NP1:Strike=149 Dip=45 Slip= 34
NP2: 34 67 130
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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 0.5 265 45 -90 3.51 0.2878
WVFGRD96 1.0 85 45 -90 3.57 0.3107
WVFGRD96 2.0 265 45 -90 3.67 0.3879
WVFGRD96 3.0 265 45 -85 3.76 0.4134
WVFGRD96 4.0 115 60 -35 3.78 0.3851
WVFGRD96 5.0 120 75 -20 3.80 0.3547
WVFGRD96 6.0 125 90 -10 3.80 0.3297
WVFGRD96 7.0 130 50 0 3.78 0.3289
WVFGRD96 8.0 135 40 5 3.83 0.3428
WVFGRD96 9.0 140 35 15 3.84 0.3723
WVFGRD96 10.0 140 30 10 3.85 0.4033
WVFGRD96 11.0 145 30 15 3.85 0.4335
WVFGRD96 12.0 145 30 20 3.87 0.4615
WVFGRD96 13.0 150 30 25 3.88 0.4883
WVFGRD96 14.0 150 30 25 3.90 0.5133
WVFGRD96 15.0 150 30 25 3.91 0.5366
WVFGRD96 16.0 155 30 30 3.92 0.5582
WVFGRD96 17.0 155 30 30 3.93 0.5782
WVFGRD96 18.0 155 30 30 3.94 0.5964
WVFGRD96 19.0 155 30 30 3.95 0.6130
WVFGRD96 20.0 155 30 30 3.96 0.6283
WVFGRD96 21.0 160 30 30 3.97 0.6410
WVFGRD96 22.0 160 30 30 3.98 0.6540
WVFGRD96 23.0 160 30 30 3.99 0.6654
WVFGRD96 24.0 160 30 30 4.00 0.6753
WVFGRD96 25.0 160 30 30 4.01 0.6839
WVFGRD96 26.0 160 30 30 4.02 0.6908
WVFGRD96 27.0 160 30 30 4.03 0.6959
WVFGRD96 28.0 160 30 30 4.04 0.6996
WVFGRD96 29.0 155 30 25 4.05 0.7017
WVFGRD96 30.0 155 30 25 4.06 0.7021
WVFGRD96 31.0 155 30 25 4.07 0.7009
WVFGRD96 32.0 155 30 25 4.08 0.6979
WVFGRD96 33.0 155 30 25 4.08 0.6937
WVFGRD96 34.0 155 30 25 4.09 0.6882
WVFGRD96 35.0 155 30 25 4.09 0.6815
WVFGRD96 36.0 150 35 25 4.11 0.6750
WVFGRD96 37.0 150 35 25 4.12 0.6683
WVFGRD96 38.0 150 40 25 4.13 0.6619
WVFGRD96 39.0 150 40 25 4.13 0.6550
WVFGRD96 40.0 155 25 25 4.24 0.6418
WVFGRD96 41.0 155 25 25 4.24 0.6291
WVFGRD96 42.0 150 30 20 4.25 0.6156
WVFGRD96 43.0 150 30 20 4.25 0.6029
WVFGRD96 44.0 150 30 20 4.26 0.5896
WVFGRD96 45.0 150 30 20 4.26 0.5760
WVFGRD96 46.0 150 30 20 4.26 0.5622
WVFGRD96 47.0 150 30 20 4.26 0.5480
WVFGRD96 48.0 145 35 15 4.27 0.5348
WVFGRD96 49.0 145 35 15 4.27 0.5214
The best solution is
WVFGRD96 30.0 155 30 25 4.06 0.7021
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 08:29:02 PM CDT 2024
