The ANSS event ID is us10008u1n and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us10008u1n/executive.
2017/05/21 19:33:38 59.768 -136.672 5.0 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/05/21 19:33:38:0 59.77 -136.67 5.0 3.9 Alaska
Stations used:
AK.BAGL AK.BAL AK.BARN AK.BCP AK.BESE AK.CTG AK.GLB AK.JIS
AK.KIAG AK.LOGN AK.MCAR AK.PAX AK.PIN AK.PNL AK.PTPK
AK.SAMH AK.TABL AK.VRDI AT.CRAG AT.SIT AT.SKAG CN.DAWY
CN.HYT CN.YUK2 CN.YUK3 CN.YUK4 CN.YUK5 CN.YUK6 CN.YUK7
CN.YUK8 NY.MAYO TA.K27K TA.K29M TA.L27K TA.L29M TA.M26K
TA.M27K TA.M29M TA.M30M TA.M31M TA.N30M TA.N31M TA.N32M
TA.O28M TA.O29M TA.O30N TA.P29M TA.P30M TA.P32M TA.P33M
TA.Q32M TA.R32K TA.R33M TA.S31K TA.S32K TA.S34M TA.T33K
TA.T35M TA.U33K TA.U35K TA.V35K US.WRAK
Filtering commands used:
cut o DIST/3.3 -20 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 9.23e+21 dyne-cm
Mw = 3.91
Z = 17 km
Plane Strike Dip Rake
NP1 26 78 144
NP2 125 55 15
Principal Axes:
Axis Value Plunge Azimuth
T 9.23e+21 34 340
N 0.00e+00 52 190
P -9.23e+21 15 80
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.35e+21
Mxy -3.55e+21
Mxz 3.60e+21
Myy -7.60e+21
Myz -3.72e+21
Mzz 2.24e+21
##############
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######### ###########-------
########### T ###########---------
-########### ##########-----------
--########################------------
----######################--------------
-----#####################---------- -
-------###################----------- P --
--------#################------------ --
---------###############------------------
-----------############-------------------
------------#########-------------------
--------------######--------------------
----------------##--------------------
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--------####################
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Global CMT Convention Moment Tensor:
R T P
2.24e+21 3.60e+21 3.72e+21
3.60e+21 5.35e+21 3.55e+21
3.72e+21 3.55e+21 -7.60e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170521193338/index.html
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STK = 125
DIP = 55
RAKE = 15
MW = 3.91
HS = 17.0
The NDK file is 20170521193338.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 -20 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 65 45 90 3.56 0.3336
WVFGRD96 2.0 70 45 95 3.66 0.3979
WVFGRD96 3.0 225 55 65 3.71 0.3667
WVFGRD96 4.0 210 85 55 3.72 0.3866
WVFGRD96 5.0 210 90 55 3.73 0.4279
WVFGRD96 6.0 30 90 -50 3.74 0.4613
WVFGRD96 7.0 25 85 -45 3.75 0.4849
WVFGRD96 8.0 30 90 -55 3.81 0.4990
WVFGRD96 9.0 25 85 -50 3.81 0.5158
WVFGRD96 10.0 125 45 15 3.83 0.5338
WVFGRD96 11.0 125 50 20 3.85 0.5564
WVFGRD96 12.0 125 50 20 3.86 0.5749
WVFGRD96 13.0 125 50 15 3.87 0.5884
WVFGRD96 14.0 125 50 15 3.88 0.5976
WVFGRD96 15.0 125 55 20 3.90 0.6033
WVFGRD96 16.0 125 55 20 3.91 0.6063
WVFGRD96 17.0 125 55 15 3.91 0.6068
WVFGRD96 18.0 125 55 15 3.92 0.6045
WVFGRD96 19.0 125 55 15 3.93 0.5999
WVFGRD96 20.0 125 55 15 3.94 0.5938
WVFGRD96 21.0 125 55 15 3.95 0.5850
WVFGRD96 22.0 125 55 15 3.96 0.5753
WVFGRD96 23.0 125 55 15 3.96 0.5648
WVFGRD96 24.0 125 50 10 3.96 0.5536
WVFGRD96 25.0 125 50 5 3.97 0.5431
WVFGRD96 26.0 120 50 -5 3.97 0.5322
WVFGRD96 27.0 120 50 -5 3.98 0.5219
WVFGRD96 28.0 115 50 -10 3.98 0.5121
WVFGRD96 29.0 115 50 -10 3.99 0.5027
The best solution is
WVFGRD96 17.0 125 55 15 3.91 0.6068
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 -20 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 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