The ANSS event ID is ak016fec3aq6 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak016fec3aq6/executive.
2016/11/30 18:57:07 60.001 -152.673 98.0 4.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2016/11/30 18:57:07:0 60.00 -152.67 98.0 4.2 Alaska
Stations used:
AK.CNP AK.HOM AK.RC01 AK.SKN AK.SSN AT.SVW2 AV.ILSW TA.M19K
TA.M20K TA.M22K TA.N19K TA.O18K TA.O22K TA.P18K TA.P19K
Filtering commands used:
cut o DIST/3.7 -30 o DIST/3.7 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.06 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 2.88e+22 dyne-cm
Mw = 4.24
Z = 108 km
Plane Strike Dip Rake
NP1 310 70 142
NP2 55 55 25
Principal Axes:
Axis Value Plunge Azimuth
T 2.88e+22 41 267
N 0.00e+00 48 106
P -2.88e+22 9 5
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.78e+22
Mxy -1.94e+21
Mxz -5.18e+21
Myy 1.64e+22
Myz -1.47e+22
Mzz 1.15e+22
------- P ----
----------- --------
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#######---------------------------
############-----------------------#
################--------------------##
####################----------------####
#######################------------#####
##########################---------#######
####### ##################------########
####### T ###################----#########
####### ################################
############################---#########
##########################-------#######
######################-----------#####
##################---------------###
############---------------------#
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Global CMT Convention Moment Tensor:
R T P
1.15e+22 -5.18e+21 1.47e+22
-5.18e+21 -2.78e+22 1.94e+21
1.47e+22 1.94e+21 1.64e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20161130185707/index.html
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STK = 55
DIP = 55
RAKE = 25
MW = 4.24
HS = 108.0
The NDK file is 20161130185707.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/3.7 -30 o DIST/3.7 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 45 65 45 3.49 0.3097
WVFGRD96 4.0 50 55 50 3.58 0.3625
WVFGRD96 6.0 70 55 80 3.69 0.3994
WVFGRD96 8.0 65 55 75 3.73 0.4171
WVFGRD96 10.0 40 60 30 3.64 0.4168
WVFGRD96 12.0 40 60 25 3.65 0.4250
WVFGRD96 14.0 35 65 20 3.66 0.4351
WVFGRD96 16.0 35 65 15 3.68 0.4465
WVFGRD96 18.0 35 65 15 3.71 0.4571
WVFGRD96 20.0 35 65 15 3.73 0.4654
WVFGRD96 22.0 35 65 15 3.75 0.4727
WVFGRD96 24.0 35 65 15 3.78 0.4774
WVFGRD96 26.0 35 70 10 3.79 0.4817
WVFGRD96 28.0 35 70 10 3.81 0.4844
WVFGRD96 30.0 35 70 10 3.83 0.4862
WVFGRD96 32.0 35 70 10 3.85 0.4868
WVFGRD96 34.0 50 80 10 3.86 0.4888
WVFGRD96 36.0 35 75 -5 3.89 0.4933
WVFGRD96 38.0 35 75 -5 3.91 0.4996
WVFGRD96 40.0 35 70 -10 3.96 0.5062
WVFGRD96 42.0 50 85 0 3.97 0.5148
WVFGRD96 44.0 50 85 0 3.99 0.5240
WVFGRD96 46.0 230 90 0 4.01 0.5322
WVFGRD96 48.0 55 55 25 4.06 0.5465
WVFGRD96 50.0 55 55 25 4.07 0.5625
WVFGRD96 52.0 55 55 25 4.09 0.5787
WVFGRD96 54.0 55 55 25 4.10 0.5948
WVFGRD96 56.0 55 55 25 4.11 0.6101
WVFGRD96 58.0 55 55 25 4.12 0.6244
WVFGRD96 60.0 55 55 25 4.13 0.6390
WVFGRD96 62.0 55 55 25 4.14 0.6516
WVFGRD96 64.0 55 55 25 4.15 0.6637
WVFGRD96 66.0 55 55 25 4.16 0.6742
WVFGRD96 68.0 55 55 25 4.16 0.6844
WVFGRD96 70.0 55 55 25 4.17 0.6930
WVFGRD96 72.0 55 55 25 4.17 0.7003
WVFGRD96 74.0 55 55 25 4.18 0.7077
WVFGRD96 76.0 55 55 25 4.18 0.7135
WVFGRD96 78.0 55 55 25 4.19 0.7186
WVFGRD96 80.0 55 55 25 4.19 0.7225
WVFGRD96 82.0 55 55 25 4.20 0.7266
WVFGRD96 84.0 55 55 25 4.20 0.7300
WVFGRD96 86.0 55 55 25 4.21 0.7332
WVFGRD96 88.0 55 55 25 4.21 0.7356
WVFGRD96 90.0 55 55 25 4.21 0.7379
WVFGRD96 92.0 55 55 25 4.22 0.7396
WVFGRD96 94.0 55 55 25 4.22 0.7410
WVFGRD96 96.0 55 55 25 4.22 0.7424
WVFGRD96 98.0 55 55 25 4.23 0.7430
WVFGRD96 100.0 55 55 25 4.23 0.7437
WVFGRD96 102.0 55 55 25 4.23 0.7436
WVFGRD96 104.0 55 55 25 4.24 0.7440
WVFGRD96 106.0 55 55 25 4.24 0.7443
WVFGRD96 108.0 55 55 25 4.24 0.7444
WVFGRD96 110.0 55 55 25 4.25 0.7442
WVFGRD96 112.0 55 55 25 4.25 0.7436
WVFGRD96 114.0 55 55 25 4.25 0.7424
WVFGRD96 116.0 55 55 25 4.26 0.7409
WVFGRD96 118.0 55 55 25 4.26 0.7405
The best solution is
WVFGRD96 108.0 55 55 25 4.24 0.7444
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/3.7 -30 o DIST/3.7 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 n 3 br c 0.12 0.25 n 4 p 2
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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