The ANSS event ID is ak0177oakw2n and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0177oakw2n/executive.
2017/06/16 06:43:53 61.385 -146.609 32.5 3.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/06/16 06:43:53:0 61.38 -146.61 32.5 3.6 Alaska
Stations used:
AK.DHY AK.EYAK AK.GHO AK.GLB AK.GLI AK.KLU AK.KNK AK.MCAR
AK.PWL AK.SAW AK.SCM AK.VRDI AT.PMR TA.N25K
Filtering commands used:
cut o DIST/3.5 -30 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 = 2.95e+21 dyne-cm
Mw = 3.58
Z = 49 km
Plane Strike Dip Rake
NP1 181 76 -122
NP2 70 35 -25
Principal Axes:
Axis Value Plunge Azimuth
T 2.95e+21 24 295
N 0.00e+00 31 190
P -2.95e+21 49 56
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.88e+19
Mxy -1.55e+21
Mxz -3.49e+20
Myy 1.12e+21
Myz -2.20e+21
Mzz -1.17e+21
#######-------
##########------------
############----------------
#############-----------------
###############-------------------
###############---------------------
### ##########---------- ---------
#### T ##########---------- P ----------
#### ##########---------- ---------#
##################----------------------##
#################-----------------------##
#################----------------------###
#################---------------------####
################--------------------####
#################-----------------######
-###############---------------#######
--#############-------------########
----##########---------###########
-------######--###############
------------################
---------#############
------########
Global CMT Convention Moment Tensor:
R T P
-1.17e+21 -3.49e+20 2.20e+21
-3.49e+20 4.88e+19 1.55e+21
2.20e+21 1.55e+21 1.12e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170616064353/index.html
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STK = 70
DIP = 35
RAKE = -25
MW = 3.58
HS = 49.0
The NDK file is 20170616064353.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 -30 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 1.0 40 50 90 2.70 0.1690
WVFGRD96 2.0 40 50 85 2.86 0.2282
WVFGRD96 3.0 0 70 -50 2.86 0.2137
WVFGRD96 4.0 0 70 -50 2.89 0.2428
WVFGRD96 5.0 235 75 45 2.97 0.2715
WVFGRD96 6.0 235 75 40 3.00 0.2935
WVFGRD96 7.0 235 75 40 3.02 0.3084
WVFGRD96 8.0 240 70 40 3.09 0.3172
WVFGRD96 9.0 240 75 45 3.10 0.3241
WVFGRD96 10.0 240 75 40 3.12 0.3299
WVFGRD96 11.0 240 75 40 3.13 0.3317
WVFGRD96 12.0 245 70 40 3.15 0.3324
WVFGRD96 13.0 280 55 25 3.11 0.3325
WVFGRD96 14.0 280 55 25 3.13 0.3408
WVFGRD96 15.0 280 55 25 3.15 0.3483
WVFGRD96 16.0 280 55 25 3.16 0.3561
WVFGRD96 17.0 280 55 25 3.18 0.3638
WVFGRD96 18.0 280 55 25 3.20 0.3701
WVFGRD96 19.0 275 55 20 3.21 0.3757
WVFGRD96 20.0 280 55 25 3.23 0.3824
WVFGRD96 21.0 275 55 20 3.24 0.3887
WVFGRD96 22.0 275 55 20 3.25 0.3935
WVFGRD96 23.0 280 55 25 3.27 0.3978
WVFGRD96 24.0 280 55 25 3.28 0.4015
WVFGRD96 25.0 280 55 25 3.29 0.4039
WVFGRD96 26.0 95 50 15 3.29 0.4144
WVFGRD96 27.0 90 50 0 3.30 0.4284
WVFGRD96 28.0 90 45 0 3.32 0.4447
WVFGRD96 29.0 90 45 0 3.33 0.4602
WVFGRD96 30.0 85 45 -5 3.35 0.4729
WVFGRD96 31.0 85 40 -5 3.36 0.4829
WVFGRD96 32.0 85 40 -5 3.37 0.4907
WVFGRD96 33.0 85 40 -5 3.37 0.4978
WVFGRD96 34.0 85 40 -5 3.38 0.5058
WVFGRD96 35.0 85 40 -10 3.39 0.5124
WVFGRD96 36.0 75 35 -25 3.40 0.5232
WVFGRD96 37.0 75 35 -25 3.41 0.5340
WVFGRD96 38.0 70 35 -30 3.42 0.5442
WVFGRD96 39.0 70 40 -30 3.44 0.5541
WVFGRD96 40.0 65 30 -30 3.54 0.5619
WVFGRD96 41.0 65 30 -30 3.55 0.5693
WVFGRD96 42.0 65 30 -35 3.55 0.5767
WVFGRD96 43.0 65 30 -35 3.55 0.5834
WVFGRD96 44.0 65 30 -35 3.56 0.5878
WVFGRD96 45.0 65 35 -35 3.57 0.5919
WVFGRD96 46.0 65 35 -35 3.57 0.5959
WVFGRD96 47.0 65 35 -35 3.58 0.5962
WVFGRD96 48.0 70 35 -25 3.58 0.5996
WVFGRD96 49.0 70 35 -25 3.58 0.6008
WVFGRD96 50.0 70 35 -25 3.58 0.5995
WVFGRD96 51.0 70 35 -25 3.59 0.6006
WVFGRD96 52.0 70 40 -25 3.59 0.5990
WVFGRD96 53.0 70 40 -25 3.60 0.5987
WVFGRD96 54.0 70 40 -25 3.60 0.5973
WVFGRD96 55.0 70 40 -25 3.60 0.5937
WVFGRD96 56.0 70 40 -25 3.60 0.5920
WVFGRD96 57.0 70 40 -20 3.61 0.5886
WVFGRD96 58.0 70 40 -20 3.61 0.5879
WVFGRD96 59.0 70 35 -20 3.61 0.5860
The best solution is
WVFGRD96 49.0 70 35 -25 3.58 0.6008
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 -30 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