The ANSS event ID is ak017ekiw14d and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak017ekiw14d/executive.
2017/11/13 16:00:37 60.561 -142.656 19.1 4.3 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/11/13 16:00:37:0 60.56 -142.66 19.1 4.3 Alaska
Stations used:
AK.BARN AK.BERG AK.BMR AK.BPAW AK.BWN AK.CAST AK.CCB AK.CRQ
AK.CTG AK.CUT AK.DHY AK.DIV AK.GHO AK.GLB AK.GLI AK.GRNC
AK.HDA AK.HMT AK.ISLE AK.KNK AK.KTH AK.LOGN AK.MCAR AK.MCK
AK.MDM AK.MESA AK.NEA2 AK.PAX AK.PPD AK.PPLA AK.RND AK.SAW
AK.SCM AK.SCRK AK.SKN AK.SSN AK.SSP AK.TABL AK.TGL AK.VRDI
AK.WRH AK.YAH AT.MENT AT.PMR CN.DAWY CN.HYT CN.WHY IM.IL31
IU.COLA NY.FARO NY.MAYO TA.I28M TA.I29M TA.J25K TA.J26L
TA.J30M TA.K27K TA.K29M TA.L26K TA.L27K TA.L29M TA.M20K
TA.M22K TA.M24K TA.M26K TA.M27K TA.M29M TA.M30M TA.M31M
TA.N25K TA.N30M TA.N31M TA.O29M TA.O30N TA.P33M TA.POKR
US.EGAK
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 2.99e+22 dyne-cm
Mw = 4.25
Z = 24 km
Plane Strike Dip Rake
NP1 275 85 -91
NP2 110 5 -75
Principal Axes:
Axis Value Plunge Azimuth
T 2.99e+22 40 6
N 0.00e+00 1 275
P -2.99e+22 50 184
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.85e+21
Mxy 1.09e+21
Mxz 2.93e+22
Myy 1.53e+20
Myz 2.48e+21
Mzz -5.01e+21
##############
######################
############################
############### ############
################# T ##############
################## ###############
######################################
########################################
########################################
-#########################################
#-----------------------##################
#-------------------------------------####
#-----------------------------------------
----------------------------------------
#---------------------------------------
#---------------- ------------------
#--------------- P -----------------
#-------------- ----------------
#-----------------------------
##-------------------------#
##-------------------#
###---------##
Global CMT Convention Moment Tensor:
R T P
-5.01e+21 2.93e+22 -2.48e+21
2.93e+22 4.85e+21 -1.09e+21
-2.48e+21 -1.09e+21 1.53e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171113160037/index.html
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STK = 110
DIP = 5
RAKE = -75
MW = 4.25
HS = 24.0
The NDK file is 20171113160037.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 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 245 40 85 3.66 0.1910
WVFGRD96 2.0 70 50 90 3.84 0.2779
WVFGRD96 3.0 240 40 80 3.88 0.2429
WVFGRD96 4.0 90 70 -50 3.83 0.2090
WVFGRD96 5.0 195 20 15 3.84 0.2368
WVFGRD96 6.0 190 20 10 3.85 0.2743
WVFGRD96 7.0 185 15 0 3.86 0.3113
WVFGRD96 8.0 185 10 0 3.96 0.3395
WVFGRD96 9.0 180 10 -5 3.98 0.3778
WVFGRD96 10.0 170 5 -15 4.00 0.4137
WVFGRD96 11.0 95 90 90 4.02 0.4504
WVFGRD96 12.0 175 0 -10 4.04 0.4853
WVFGRD96 13.0 105 0 -80 4.06 0.5177
WVFGRD96 14.0 150 0 -35 4.08 0.5470
WVFGRD96 15.0 115 0 -70 4.10 0.5741
WVFGRD96 16.0 150 5 -35 4.12 0.5977
WVFGRD96 17.0 110 5 -75 4.14 0.6197
WVFGRD96 18.0 245 -5 60 4.16 0.6375
WVFGRD96 19.0 110 5 -75 4.17 0.6525
WVFGRD96 20.0 95 90 90 4.19 0.6636
WVFGRD96 21.0 110 5 -75 4.21 0.6737
WVFGRD96 22.0 120 5 -65 4.23 0.6804
WVFGRD96 23.0 245 -5 60 4.24 0.6850
WVFGRD96 24.0 110 5 -75 4.25 0.6867
WVFGRD96 25.0 95 90 90 4.27 0.6812
WVFGRD96 26.0 110 5 -75 4.28 0.6820
WVFGRD96 27.0 110 5 -75 4.29 0.6751
WVFGRD96 28.0 220 -5 35 4.30 0.6639
WVFGRD96 29.0 175 5 -10 4.31 0.6533
The best solution is
WVFGRD96 24.0 110 5 -75 4.25 0.6867
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 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 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