The ANSS event ID is ak0171ff8vv1 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0171ff8vv1/executive.
2017/01/31 09:38:37 63.082 -150.943 132.9 5.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/01/31 09:38:37:0 63.08 -150.94 132.9 5.2 Alaska
Stations used:
AK.BPAW AK.BRLK AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV
AK.DOT AK.FIRE AK.GHO AK.GLB AK.HDA AK.HOM AK.KLU AK.KNK
AK.KTH AK.MDM AK.MLY AK.NEA2 AK.PAX AK.PPD AK.PWL AK.RC01
AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.SSN AK.SWD
AK.TRF AK.WRH AT.MENT AT.PMR AT.TTA IM.IL31 IU.COLA TA.H21K
TA.H24K TA.I21K TA.I23K TA.K20K TA.L19K TA.L26K TA.M20K
TA.M26K TA.N25K TA.POKR TA.TCOL
Filtering commands used:
cut o DIST/4.0 -60 o DIST/4.0 +40
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 6.68e+23 dyne-cm
Mw = 5.15
Z = 130 km
Plane Strike Dip Rake
NP1 45 80 70
NP2 289 22 153
Principal Axes:
Axis Value Plunge Azimuth
T 6.68e+23 51 292
N 0.00e+00 20 49
P -6.68e+23 32 152
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.33e+23
Mxy 1.07e+23
Mxz 3.89e+23
Myy 1.18e+23
Myz -4.45e+23
Mzz 2.15e+23
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------#############---------
---#####################-----#
--###########################-####
-#############################--####
##############################----####
#############################--------###
######### ################----------##
########## T ##############------------###
########## ############---------------##
########################----------------##
######################-------------------#
###################---------------------
#################-----------------------
##############------------------------
##########------------- ----------
#######--------------- P ---------
##------------------ -------
----------------------------
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Global CMT Convention Moment Tensor:
R T P
2.15e+23 3.89e+23 4.45e+23
3.89e+23 -3.33e+23 -1.07e+23
4.45e+23 -1.07e+23 1.18e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170131093837/index.html
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STK = 45
DIP = 80
RAKE = 70
MW = 5.15
HS = 130.0
The NDK file is 20170131093837.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/4.0 -60 o DIST/4.0 +40 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 70.0 45 65 -45 5.01 0.2420
WVFGRD96 72.0 50 75 -40 5.02 0.2466
WVFGRD96 74.0 45 55 45 4.94 0.2669
WVFGRD96 76.0 45 60 50 4.96 0.3227
WVFGRD96 78.0 40 65 45 5.00 0.3882
WVFGRD96 80.0 40 70 50 5.02 0.4550
WVFGRD96 82.0 40 70 50 5.04 0.5153
WVFGRD96 84.0 45 70 50 5.05 0.5641
WVFGRD96 86.0 45 70 55 5.07 0.5970
WVFGRD96 88.0 45 70 55 5.07 0.6102
WVFGRD96 90.0 45 70 55 5.07 0.6194
WVFGRD96 92.0 45 70 55 5.08 0.6249
WVFGRD96 94.0 45 75 60 5.09 0.6328
WVFGRD96 96.0 50 75 65 5.09 0.6385
WVFGRD96 98.0 50 75 65 5.10 0.6464
WVFGRD96 100.0 50 75 65 5.10 0.6528
WVFGRD96 102.0 50 75 65 5.10 0.6584
WVFGRD96 104.0 50 75 65 5.11 0.6642
WVFGRD96 106.0 50 75 65 5.11 0.6690
WVFGRD96 108.0 50 75 65 5.11 0.6737
WVFGRD96 110.0 50 75 65 5.11 0.6771
WVFGRD96 112.0 45 80 70 5.13 0.6804
WVFGRD96 114.0 45 80 70 5.13 0.6841
WVFGRD96 116.0 45 80 70 5.13 0.6861
WVFGRD96 118.0 45 80 70 5.14 0.6896
WVFGRD96 120.0 45 80 70 5.14 0.6906
WVFGRD96 122.0 45 80 70 5.14 0.6933
WVFGRD96 124.0 45 80 70 5.14 0.6930
WVFGRD96 126.0 45 80 70 5.14 0.6959
WVFGRD96 128.0 45 80 70 5.14 0.6951
WVFGRD96 130.0 45 80 70 5.15 0.6963
WVFGRD96 132.0 45 80 70 5.15 0.6954
WVFGRD96 134.0 45 80 70 5.15 0.6942
WVFGRD96 136.0 50 80 70 5.15 0.6950
WVFGRD96 138.0 45 80 70 5.15 0.6922
WVFGRD96 140.0 50 80 70 5.15 0.6922
WVFGRD96 142.0 45 80 70 5.15 0.6901
WVFGRD96 144.0 45 80 70 5.16 0.6894
WVFGRD96 146.0 45 80 70 5.16 0.6877
WVFGRD96 148.0 45 80 70 5.16 0.6843
WVFGRD96 150.0 45 80 70 5.16 0.6834
WVFGRD96 152.0 45 80 70 5.16 0.6793
WVFGRD96 154.0 45 80 70 5.16 0.6774
WVFGRD96 156.0 50 75 70 5.15 0.6759
WVFGRD96 158.0 50 75 70 5.16 0.6719
The best solution is
WVFGRD96 130.0 45 80 70 5.15 0.6963
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/4.0 -60 o DIST/4.0 +40 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