The ANSS event ID is ak012a40byc4 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak012a40byc4/executive.
2012/08/07 10:24:57 63.344 -145.184 5.4 4.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2012/08/07 10:24:57:0 63.34 -145.18 5.4 4.2 Alaska
Stations used:
AK.BMR AK.BPAW AK.BRLK AK.BWN AK.CAST AK.CCB AK.CNP AK.COLD
AK.DHY AK.DIV AK.DOT AK.EYAK AK.FID AK.FYU AK.GHO AK.GLI
AK.GLM AK.HDA AK.HIN AK.HOM AK.KAI AK.KLU AK.KNK AK.KTH
AK.MCK AK.MDM AK.MLY AK.NEA AK.PAX AK.PNL AK.PPD AK.PPLA
AK.RAG AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SGA
AK.SKN AK.SSN AK.TRF AK.WRH AT.MID AT.SVW2 AT.YKU2 CN.DAWY
IU.COLA US.EGAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 2.34e+22 dyne-cm
Mw = 4.18
Z = 12 km
Plane Strike Dip Rake
NP1 265 65 70
NP2 126 32 126
Principal Axes:
Axis Value Plunge Azimuth
T 2.34e+22 64 141
N 0.00e+00 18 274
P -2.34e+22 18 10
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.80e+22
Mxy -5.69e+21
Mxz -1.38e+22
Myy 1.13e+21
Myz 4.61e+21
Mzz 1.69e+22
-------- ---
------------ P -------
--------------- ----------
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#-----------------------------------
#-------------------------------------
##--------------------###---------------
##--------#########################-----
###----#################################--
###-######################################
#---######################################
-----################## ################
-----################# T ###############
------################ ###############
-------###############################
--------############################
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---------------#######------
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Global CMT Convention Moment Tensor:
R T P
1.69e+22 -1.38e+22 -4.61e+21
-1.38e+22 -1.80e+22 5.69e+21
-4.61e+21 5.69e+21 1.13e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120807102457/index.html
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STK = 265
DIP = 65
RAKE = 70
MW = 4.18
HS = 12.0
The NDK file is 20120807102457.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:
hp c 0.02 n 3 lp c 0.05 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 255 60 75 3.99 0.4841
WVFGRD96 1.0 255 60 75 4.02 0.4951
WVFGRD96 2.0 270 45 85 4.03 0.4933
WVFGRD96 3.0 250 80 65 4.15 0.5031
WVFGRD96 4.0 250 85 70 4.17 0.5316
WVFGRD96 5.0 255 80 70 4.15 0.5709
WVFGRD96 6.0 260 75 70 4.15 0.6077
WVFGRD96 7.0 265 70 75 4.16 0.6433
WVFGRD96 8.0 265 70 70 4.15 0.6726
WVFGRD96 9.0 270 65 75 4.16 0.6962
WVFGRD96 10.0 270 65 75 4.19 0.7111
WVFGRD96 11.0 265 65 70 4.19 0.7222
WVFGRD96 12.0 265 65 70 4.18 0.7246
WVFGRD96 13.0 265 65 65 4.18 0.7216
WVFGRD96 14.0 265 65 65 4.17 0.7146
WVFGRD96 15.0 265 65 65 4.17 0.7042
WVFGRD96 16.0 260 70 60 4.16 0.6929
WVFGRD96 17.0 260 70 60 4.16 0.6799
WVFGRD96 18.0 260 70 55 4.16 0.6654
WVFGRD96 19.0 260 70 55 4.16 0.6510
WVFGRD96 20.0 255 75 55 4.18 0.6396
WVFGRD96 21.0 255 75 55 4.18 0.6251
WVFGRD96 22.0 255 75 55 4.18 0.6100
WVFGRD96 23.0 255 75 55 4.18 0.5943
WVFGRD96 24.0 250 75 50 4.18 0.5789
WVFGRD96 25.0 250 75 50 4.19 0.5655
WVFGRD96 26.0 250 75 50 4.19 0.5521
WVFGRD96 27.0 50 65 -40 4.20 0.5384
WVFGRD96 28.0 50 65 -40 4.20 0.5276
WVFGRD96 29.0 50 65 -40 4.20 0.5167
The best solution is
WVFGRD96 12.0 265 65 70 4.18 0.7246
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
hp c 0.02 n 3 lp c 0.05 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