The ANSS event ID is ak018cv7zjua and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018cv7zjua/executive.
2018/10/07 09:25:54 61.317 -147.383 20.2 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/10/07 09:25:54:0 61.32 -147.38 20.2 4.0 Alaska
Stations used:
AK.BRLK AK.DHY AK.EYAK AK.FID AK.FIRE AK.GHO AK.GLB AK.GLI
AK.HIN AK.KLU AK.KNK AK.RAG AK.RC01 AK.SAW AK.SCM AK.SKN
AK.SSN AK.TGL AT.MID AT.PMR AV.SPU TA.M22K TA.N25K TA.O22K
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 1.45e+22 dyne-cm
Mw = 4.04
Z = 32 km
Plane Strike Dip Rake
NP1 231 71 -85
NP2 35 20 -105
Principal Axes:
Axis Value Plunge Azimuth
T 1.45e+22 26 317
N 0.00e+00 5 49
P -1.45e+22 64 150
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.15e+21
Mxy -4.65e+21
Mxz 9.01e+21
Myy 4.82e+21
Myz -6.74e+21
Mzz -8.97e+21
##############
######################
###########################-
### #######################-
##### T ####################-----#
###### ################---------##
######################--------------##
####################-----------------###
##################--------------------##
#################----------------------###
###############------------------------###
#############-------------------------####
###########---------------------------####
#########------------- -----------####
#######--------------- P -----------####
#####---------------- ----------####
###----------------------------#####
#----------------------------#####
-------------------------#####
---------------------#######
---------------#######
##############
Global CMT Convention Moment Tensor:
R T P
-8.97e+21 9.01e+21 6.74e+21
9.01e+21 4.15e+21 4.65e+21
6.74e+21 4.65e+21 4.82e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181007092554/index.html
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STK = 35
DIP = 20
RAKE = -105
MW = 4.04
HS = 32.0
The NDK file is 20181007092554.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 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 45 45 90 3.59 0.3212
WVFGRD96 4.0 230 40 -80 3.64 0.2385
WVFGRD96 6.0 60 90 -65 3.64 0.2781
WVFGRD96 8.0 60 90 -65 3.72 0.3168
WVFGRD96 10.0 250 80 60 3.76 0.3516
WVFGRD96 12.0 250 75 55 3.80 0.3766
WVFGRD96 14.0 230 85 -65 3.80 0.4170
WVFGRD96 16.0 230 80 -70 3.83 0.4589
WVFGRD96 18.0 65 25 -65 3.87 0.4990
WVFGRD96 20.0 230 75 -75 3.90 0.5377
WVFGRD96 22.0 230 75 -75 3.93 0.5740
WVFGRD96 24.0 230 75 -80 3.96 0.6046
WVFGRD96 26.0 230 70 -85 3.99 0.6326
WVFGRD96 28.0 230 70 -85 4.01 0.6521
WVFGRD96 30.0 35 20 -105 4.03 0.6640
WVFGRD96 32.0 35 20 -105 4.04 0.6640
WVFGRD96 34.0 40 20 -100 4.06 0.6551
WVFGRD96 36.0 50 20 -90 4.07 0.6391
WVFGRD96 38.0 230 70 -90 4.08 0.6201
WVFGRD96 40.0 55 15 -85 4.21 0.5995
WVFGRD96 42.0 55 20 -85 4.22 0.5769
WVFGRD96 44.0 55 20 -85 4.23 0.5575
WVFGRD96 46.0 50 20 -90 4.23 0.5348
WVFGRD96 48.0 40 25 -110 4.25 0.5109
WVFGRD96 50.0 240 65 -80 4.25 0.4916
WVFGRD96 52.0 240 65 -80 4.26 0.4716
WVFGRD96 54.0 240 65 -80 4.26 0.4509
WVFGRD96 56.0 240 65 -80 4.26 0.4294
WVFGRD96 58.0 240 60 -80 4.27 0.4141
WVFGRD96 60.0 240 60 -80 4.27 0.4025
WVFGRD96 62.0 240 60 -80 4.27 0.3907
WVFGRD96 64.0 235 60 -85 4.27 0.3779
WVFGRD96 66.0 40 55 -85 4.26 0.3783
WVFGRD96 68.0 45 55 -80 4.27 0.3819
WVFGRD96 70.0 45 55 -80 4.27 0.3845
WVFGRD96 72.0 45 55 -80 4.27 0.3856
WVFGRD96 74.0 45 55 -80 4.27 0.3866
WVFGRD96 76.0 45 55 -80 4.28 0.3864
WVFGRD96 78.0 45 55 -75 4.28 0.3861
WVFGRD96 80.0 45 55 -75 4.28 0.3875
WVFGRD96 82.0 45 55 -75 4.29 0.3882
WVFGRD96 84.0 45 55 -75 4.29 0.3886
WVFGRD96 86.0 45 55 -75 4.29 0.3879
WVFGRD96 88.0 45 55 -75 4.29 0.3858
WVFGRD96 90.0 45 55 -75 4.29 0.3832
WVFGRD96 92.0 45 55 -75 4.29 0.3804
WVFGRD96 94.0 45 55 -75 4.30 0.3813
WVFGRD96 96.0 45 55 -75 4.30 0.3817
WVFGRD96 98.0 45 55 -75 4.30 0.3818
The best solution is
WVFGRD96 32.0 35 20 -105 4.04 0.6640
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 -30 o DIST/3.3 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 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