The ANSS event ID is ak016bfwm167 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak016bfwm167/executive.
2016/09/05 06:37:39 63.587 -150.756 11.2 3.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2016/09/05 06:37:39:0 63.59 -150.76 11.2 3.6 Alaska
Stations used:
AK.BWN AK.CAST AK.CCB AK.CHUM AK.COLD AK.CUT AK.DHY AK.DOT
AK.FID AK.GCSA AK.GHO AK.HDA AK.KLU AK.KNK AK.MCK AK.MDM
AK.MLY AK.NEA2 AK.PAX AK.PPD AK.PPLA AK.PWL AK.RIDG AK.RND
AK.SAW AK.SCM AK.SKN AK.SSN AK.TRF AK.WAT1 AK.WAT6 AK.WAT7
AK.WRH AT.PMR IU.COLA TA.F21K TA.G21K TA.G23K TA.H21K
TA.H22K TA.H23K TA.H24K TA.HARP TA.I21K TA.I26K TA.J20K
TA.J25K TA.J26L TA.K20K TA.K24K TA.L19K TA.L20K TA.L26K
TA.M19K TA.M20K TA.M22K TA.M23K TA.M26K TA.N20K TA.N25K
TA.POKR XV.F1TN XV.F6TP XV.F7TV XV.F8KN XV.FAPT XV.FPAP
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 2.75e+21 dyne-cm
Mw = 3.56
Z = 14 km
Plane Strike Dip Rake
NP1 77 56 113
NP2 220 40 60
Principal Axes:
Axis Value Plunge Azimuth
T 2.75e+21 69 38
N 0.00e+00 19 244
P -2.75e+21 9 151
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.84e+21
Mxy 1.31e+21
Mxz 1.07e+21
Myy -5.07e+20
Myz 3.61e+20
Mzz 2.35e+21
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-----------#########################
----------############################
---------############# ###############
--------############## T ###############
--------############### ##############--
-------################################---
-------##############################-----
------#############################-------
-----##########################---------
##---#######################------------
####--###############-----------------
###---------------------------------
##--------------------------------
#-----------------------------
#--------------------- ---
------------------- P
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Global CMT Convention Moment Tensor:
R T P
2.35e+21 1.07e+21 -3.61e+20
1.07e+21 -1.84e+21 -1.31e+21
-3.61e+20 -1.31e+21 -5.07e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160905063739/index.html
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STK = 220
DIP = 40
RAKE = 60
MW = 3.56
HS = 14.0
The NDK file is 20160905063739.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 +70 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 90 45 -90 3.13 0.2401
WVFGRD96 2.0 90 45 -95 3.29 0.3188
WVFGRD96 3.0 355 45 -20 3.24 0.2772
WVFGRD96 4.0 195 25 20 3.32 0.3272
WVFGRD96 5.0 200 25 30 3.34 0.3837
WVFGRD96 6.0 200 30 30 3.35 0.4334
WVFGRD96 7.0 205 30 40 3.38 0.4747
WVFGRD96 8.0 210 30 45 3.46 0.5035
WVFGRD96 9.0 215 30 55 3.48 0.5398
WVFGRD96 10.0 215 35 55 3.50 0.5691
WVFGRD96 11.0 220 35 60 3.52 0.5918
WVFGRD96 12.0 220 35 60 3.53 0.6042
WVFGRD96 13.0 220 40 60 3.55 0.6126
WVFGRD96 14.0 220 40 60 3.56 0.6149
WVFGRD96 15.0 220 40 60 3.57 0.6100
WVFGRD96 16.0 220 40 60 3.58 0.5992
WVFGRD96 17.0 220 40 60 3.59 0.5836
WVFGRD96 18.0 215 45 55 3.60 0.5648
WVFGRD96 19.0 220 35 55 3.61 0.5459
WVFGRD96 20.0 220 35 55 3.61 0.5292
WVFGRD96 21.0 220 35 55 3.63 0.5113
WVFGRD96 22.0 215 35 50 3.63 0.4919
WVFGRD96 23.0 215 35 50 3.64 0.4716
WVFGRD96 24.0 215 35 50 3.64 0.4505
WVFGRD96 25.0 215 35 50 3.64 0.4296
WVFGRD96 26.0 215 35 50 3.65 0.4091
WVFGRD96 27.0 210 35 40 3.65 0.3907
WVFGRD96 28.0 215 35 45 3.65 0.3726
WVFGRD96 29.0 215 35 45 3.66 0.3544
The best solution is
WVFGRD96 14.0 220 40 60 3.56 0.6149
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 +70 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