The ANSS event ID is ak011a43re2a and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak011a43re2a/executive.
2011/08/08 16:00:48 58.258 -151.469 46.1 4.3 Alaska
USGS/SLU Moment Tensor Solution
ENS 2011/08/08 16:00:48:0 58.26 -151.47 46.1 4.3 Alaska
Stations used:
AK.BMR AK.BRLK AK.CAST AK.CNP AK.DIV AK.EYAK AK.FID AK.GHO
AK.HOM AK.KNK AK.KTH AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM
AK.SSN AK.SWD AK.TRF AT.OHAK AT.PMR AT.SVW2 II.KDAK
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 5.01e+22 dyne-cm
Mw = 4.40
Z = 40 km
Plane Strike Dip Rake
NP1 15 57 -123
NP2 245 45 -50
Principal Axes:
Axis Value Plunge Azimuth
T 5.01e+22 7 128
N 0.00e+00 27 34
P -5.01e+22 62 231
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.41e+22
Mxy -2.93e+22
Mxz 9.63e+21
Myy 2.43e+22
Myz 2.06e+22
Mzz -3.84e+22
#############-
##################----
######################------
#######################-------
#################---------###-----
#############--------------########-
###########-----------------##########
#########--------------------###########
#######----------------------###########
#######-----------------------############
#####-------------------------############
####-------------------------#############
###----------- ------------#############
##----------- P -----------#############
#------------ -----------#############
-------------------------#############
-----------------------######### #
---------------------########## T
-----------------############
---------------#############
----------############
---###########
Global CMT Convention Moment Tensor:
R T P
-3.84e+22 9.63e+21 -2.06e+22
9.63e+21 1.41e+22 2.93e+22
-2.06e+22 2.93e+22 2.43e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110808160048/index.html
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STK = 245
DIP = 45
RAKE = -50
MW = 4.40
HS = 40.0
The NDK file is 20110808160048.ndk The waveform inversion is preferred.
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.
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.
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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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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 o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 10.0 75 80 -35 3.91 0.4244
WVFGRD96 12.0 245 60 -40 3.96 0.4656
WVFGRD96 14.0 245 55 -40 3.99 0.5145
WVFGRD96 16.0 245 55 -40 4.02 0.5594
WVFGRD96 18.0 245 55 -40 4.05 0.5978
WVFGRD96 20.0 245 55 -40 4.07 0.6300
WVFGRD96 22.0 245 55 -45 4.10 0.6578
WVFGRD96 24.0 245 55 -45 4.13 0.6833
WVFGRD96 26.0 245 55 -45 4.15 0.7048
WVFGRD96 28.0 245 50 -45 4.17 0.7226
WVFGRD96 30.0 245 50 -45 4.19 0.7372
WVFGRD96 32.0 245 50 -45 4.21 0.7468
WVFGRD96 34.0 250 50 -40 4.23 0.7517
WVFGRD96 36.0 250 50 -40 4.25 0.7511
WVFGRD96 38.0 250 50 -40 4.26 0.7432
WVFGRD96 40.0 245 45 -50 4.40 0.7591
WVFGRD96 42.0 245 40 -50 4.43 0.7584
WVFGRD96 44.0 245 40 -50 4.45 0.7510
WVFGRD96 46.0 245 40 -50 4.46 0.7382
WVFGRD96 48.0 245 35 -50 4.48 0.7223
WVFGRD96 50.0 250 35 -45 4.49 0.7045
WVFGRD96 52.0 250 35 -45 4.50 0.6838
WVFGRD96 54.0 250 35 -45 4.51 0.6594
WVFGRD96 56.0 255 35 -40 4.51 0.6361
WVFGRD96 58.0 255 35 -40 4.52 0.6110
WVFGRD96 60.0 255 30 -40 4.53 0.5863
WVFGRD96 62.0 260 30 -35 4.53 0.5634
WVFGRD96 64.0 260 30 -35 4.53 0.5400
WVFGRD96 66.0 265 30 -30 4.52 0.5173
WVFGRD96 68.0 265 30 -30 4.52 0.4954
WVFGRD96 70.0 250 55 -30 4.47 0.4843
WVFGRD96 72.0 255 55 -25 4.46 0.4740
WVFGRD96 74.0 255 60 -25 4.46 0.4635
WVFGRD96 76.0 255 60 -25 4.46 0.4531
WVFGRD96 78.0 255 65 -25 4.47 0.4432
WVFGRD96 80.0 255 70 -25 4.47 0.4339
WVFGRD96 82.0 255 85 -25 4.48 0.4286
WVFGRD96 84.0 255 85 -25 4.48 0.4245
WVFGRD96 86.0 265 45 20 4.38 0.4160
WVFGRD96 88.0 265 45 20 4.38 0.4138
WVFGRD96 90.0 265 45 25 4.37 0.4126
WVFGRD96 92.0 265 45 25 4.38 0.4116
WVFGRD96 94.0 265 45 25 4.38 0.4109
WVFGRD96 96.0 265 45 25 4.38 0.4097
WVFGRD96 98.0 270 45 30 4.38 0.4107
WVFGRD96 100.0 230 40 -60 4.48 0.4113
WVFGRD96 102.0 230 40 -60 4.48 0.4135
WVFGRD96 104.0 230 40 -60 4.48 0.4150
WVFGRD96 106.0 230 40 -60 4.48 0.4159
WVFGRD96 108.0 235 40 -55 4.49 0.4166
WVFGRD96 110.0 235 40 -55 4.49 0.4178
WVFGRD96 112.0 235 40 -55 4.49 0.4183
WVFGRD96 114.0 225 35 -75 4.47 0.4199
WVFGRD96 116.0 225 35 -75 4.48 0.4218
WVFGRD96 118.0 225 35 -75 4.48 0.4237
WVFGRD96 120.0 225 35 -75 4.48 0.4254
WVFGRD96 122.0 225 35 -75 4.48 0.4274
WVFGRD96 124.0 225 35 -80 4.48 0.4291
WVFGRD96 126.0 225 35 -80 4.48 0.4304
WVFGRD96 128.0 225 35 -80 4.48 0.4319
The best solution is
WVFGRD96 40.0 245 45 -50 4.40 0.7591
The mechanism corresponding to the best fit is
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The best fit as a function of depth is given in the following figure:
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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 o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 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. |
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:
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.
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