The ANSS event ID is ak018732l063 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018732l063/executive.
2018/06/03 23:21:41 62.842 -148.364 64.5 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/06/03 23:21:41:0 62.84 -148.36 64.5 3.9 Alaska
Stations used:
AK.BPAW AK.BWN AK.CAST AK.CUT AK.DHY AK.DIV AK.FID AK.GHO
AK.GLI AK.HDA AK.KLU AK.KTH AK.MCK AK.NEA2 AK.PAX AK.RC01
AK.RND AK.SAW AK.SCM AK.SKN AK.TRF AT.PMR TA.J25K TA.M22K
TA.M24K TA.N25K
Filtering commands used:
cut o DIST/3.5 -40 o DIST/3.5 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.06e+22 dyne-cm
Mw = 3.95
Z = 74 km
Plane Strike Dip Rake
NP1 232 74 -143
NP2 130 55 -20
Principal Axes:
Axis Value Plunge Azimuth
T 1.06e+22 12 357
N 0.00e+00 50 252
P -1.06e+22 37 96
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.00e+22
Mxy 2.60e+20
Mxz 2.72e+21
Myy -6.62e+21
Myz -5.17e+21
Mzz -3.40e+21
##### ######
######### T ##########
############ #############
##############################
-#############################----
--########################----------
---#####################--------------
-----#################------------------
------#############---------------------
--------##########------------------------
---------#######----------------- ------
----------####------------------- P ------
-----------#--------------------- ------
---------###----------------------------
--------######--------------------------
-----##########-----------------------
---##############-------------------
-###################--------------
##############################
############################
######################
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Global CMT Convention Moment Tensor:
R T P
-3.40e+21 2.72e+21 5.17e+21
2.72e+21 1.00e+22 -2.60e+20
5.17e+21 -2.60e+20 -6.62e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180603232141/index.html
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STK = 130
DIP = 55
RAKE = -20
MW = 3.95
HS = 74.0
The NDK file is 20180603232141.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/3.5 -40 o DIST/3.5 +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 2.0 55 50 45 3.06 0.1826
WVFGRD96 4.0 45 70 20 3.10 0.2046
WVFGRD96 6.0 40 75 -30 3.18 0.2314
WVFGRD96 8.0 40 75 -35 3.26 0.2486
WVFGRD96 10.0 225 85 35 3.31 0.2551
WVFGRD96 12.0 225 85 35 3.35 0.2559
WVFGRD96 14.0 315 65 -20 3.37 0.2527
WVFGRD96 16.0 315 65 -15 3.41 0.2695
WVFGRD96 18.0 310 65 -20 3.45 0.2860
WVFGRD96 20.0 310 65 -15 3.48 0.3024
WVFGRD96 22.0 310 65 -15 3.51 0.3183
WVFGRD96 24.0 310 70 -10 3.54 0.3313
WVFGRD96 26.0 130 80 -20 3.56 0.3435
WVFGRD96 28.0 130 80 -20 3.58 0.3505
WVFGRD96 30.0 130 70 -10 3.60 0.3538
WVFGRD96 32.0 135 60 5 3.63 0.3656
WVFGRD96 34.0 125 70 -15 3.65 0.3835
WVFGRD96 36.0 125 70 -15 3.67 0.3997
WVFGRD96 38.0 125 65 -10 3.71 0.4182
WVFGRD96 40.0 125 60 -10 3.78 0.4328
WVFGRD96 42.0 125 55 -10 3.81 0.4378
WVFGRD96 44.0 125 60 -10 3.83 0.4430
WVFGRD96 46.0 120 60 -20 3.85 0.4567
WVFGRD96 48.0 120 60 -20 3.87 0.4729
WVFGRD96 50.0 120 60 -25 3.87 0.4896
WVFGRD96 52.0 120 55 -25 3.89 0.5039
WVFGRD96 54.0 125 60 -20 3.89 0.5169
WVFGRD96 56.0 125 60 -20 3.90 0.5277
WVFGRD96 58.0 125 60 -20 3.91 0.5360
WVFGRD96 60.0 125 55 -20 3.92 0.5436
WVFGRD96 62.0 125 55 -20 3.92 0.5486
WVFGRD96 64.0 125 55 -20 3.93 0.5527
WVFGRD96 66.0 125 55 -20 3.93 0.5555
WVFGRD96 68.0 125 55 -20 3.93 0.5566
WVFGRD96 70.0 130 55 -20 3.94 0.5583
WVFGRD96 72.0 130 55 -20 3.94 0.5590
WVFGRD96 74.0 130 55 -20 3.95 0.5590
WVFGRD96 76.0 130 55 -20 3.95 0.5579
WVFGRD96 78.0 130 55 -20 3.95 0.5568
WVFGRD96 80.0 130 55 -20 3.95 0.5556
WVFGRD96 82.0 130 55 -20 3.96 0.5553
WVFGRD96 84.0 130 55 -20 3.96 0.5545
WVFGRD96 86.0 130 55 -20 3.96 0.5511
WVFGRD96 88.0 130 55 -20 3.96 0.5487
WVFGRD96 90.0 130 50 -20 3.98 0.5475
WVFGRD96 92.0 130 50 -20 3.98 0.5448
WVFGRD96 94.0 130 50 -20 3.98 0.5418
WVFGRD96 96.0 130 50 -20 3.98 0.5403
WVFGRD96 98.0 130 50 -20 3.98 0.5373
The best solution is
WVFGRD96 74.0 130 55 -20 3.95 0.5590
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/3.5 -40 o DIST/3.5 +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