The ANSS event ID is ak017cvc1n30 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak017cvc1n30/executive.
2017/10/07 16:07:15 63.108 -151.492 9.3 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/10/07 16:07:15:0 63.11 -151.49 9.3 4.0 Alaska
Stations used:
AK.BMR AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.FID
AK.GHO AK.GLI AK.HDA AK.HIN AK.KLU AK.KNK AK.KTH AK.MCK
AK.MDM AK.MLY AK.NEA2 AK.PAX AK.PPLA AK.PWL AK.RC01 AK.RND
AK.SAW AK.SCM AK.SKN AK.SSN AK.SWD AK.TRF AK.WRH AT.PMR
AT.SVW2 AT.TTA AV.ILSW IM.IL31 IU.COLA TA.H21K TA.H23K
TA.H24K TA.I23K TA.J20K TA.J25K TA.J26L TA.L19K TA.L26K
TA.M19K TA.M22K TA.N18K TA.N19K TA.N25K TA.O22K TA.P19K
TA.POKR
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 = 1.17e+22 dyne-cm
Mw = 3.98
Z = 14 km
Plane Strike Dip Rake
NP1 47 57 130
NP2 170 50 45
Principal Axes:
Axis Value Plunge Azimuth
T 1.17e+22 57 14
N 0.00e+00 33 203
P -1.17e+22 4 110
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.93e+21
Mxy 4.58e+21
Mxz 5.51e+21
Myy -1.01e+22
Myz 4.93e+20
Mzz 8.18e+21
--############
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--------########################--
--------########## ############---
---------########## T ############----
----------########## ###########------
----------#######################-------
-----------#######################--------
-----------#####################----------
-----------####################-----------
-----------###################------------
-----------################----------
-----------##############------------ P
-----------###########--------------
-----------#######------------------
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Global CMT Convention Moment Tensor:
R T P
8.18e+21 5.51e+21 -4.93e+20
5.51e+21 1.93e+21 -4.58e+21
-4.93e+20 -4.58e+21 -1.01e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171007160715/index.html
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STK = 170
DIP = 50
RAKE = 45
MW = 3.98
HS = 14.0
The NDK file is 20171007160715.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 1.0 105 45 -90 3.52 0.2790
WVFGRD96 2.0 105 45 -90 3.67 0.3576
WVFGRD96 3.0 325 30 -35 3.71 0.2940
WVFGRD96 4.0 170 35 10 3.72 0.3435
WVFGRD96 5.0 155 20 -25 3.74 0.3999
WVFGRD96 6.0 145 20 -35 3.75 0.4491
WVFGRD96 7.0 150 25 -30 3.76 0.4854
WVFGRD96 8.0 140 20 -40 3.85 0.5135
WVFGRD96 9.0 175 40 45 3.89 0.5504
WVFGRD96 10.0 175 45 50 3.92 0.5885
WVFGRD96 11.0 175 45 50 3.94 0.6168
WVFGRD96 12.0 175 50 50 3.95 0.6349
WVFGRD96 13.0 175 50 50 3.97 0.6457
WVFGRD96 14.0 170 50 45 3.98 0.6501
WVFGRD96 15.0 170 50 45 3.99 0.6489
WVFGRD96 16.0 170 50 45 4.00 0.6428
WVFGRD96 17.0 170 50 45 4.01 0.6332
WVFGRD96 18.0 170 50 45 4.02 0.6211
WVFGRD96 19.0 170 50 45 4.03 0.6076
WVFGRD96 20.0 170 45 50 4.04 0.5924
WVFGRD96 21.0 170 40 50 4.05 0.5765
WVFGRD96 22.0 170 40 50 4.06 0.5623
WVFGRD96 23.0 165 40 45 4.07 0.5469
WVFGRD96 24.0 165 40 45 4.08 0.5302
WVFGRD96 25.0 165 40 45 4.08 0.5128
WVFGRD96 26.0 165 40 45 4.09 0.4947
WVFGRD96 27.0 165 40 45 4.09 0.4760
WVFGRD96 28.0 175 50 55 4.10 0.4597
WVFGRD96 29.0 175 50 55 4.10 0.4424
The best solution is
WVFGRD96 14.0 170 50 45 3.98 0.6501
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