The ANSS event ID is ak0178d9n54j and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0178d9n54j/executive.
2017/07/01 16:51:51 62.225 -151.372 86.8 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/07/01 16:51:51:0 62.22 -151.37 86.8 4.0 Alaska
Stations used:
AK.BPAW AK.BWN AK.CAST AK.DHY AK.GHO AK.KNK AK.KTH AK.MLY
AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AK.TRF
AT.PMR AV.ILSW TA.J20K TA.K20K TA.L19K TA.M22K
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 = 1.06e+22 dyne-cm
Mw = 3.95
Z = 96 km
Plane Strike Dip Rake
NP1 255 60 -40
NP2 8 56 -143
Principal Axes:
Axis Value Plunge Azimuth
T 1.06e+22 2 312
N 0.00e+00 42 44
P -1.06e+22 48 220
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.99e+21
Mxy -7.56e+21
Mxz 4.34e+21
Myy 3.91e+21
Myz 3.04e+21
Mzz -5.90e+21
###########---
################------
###################--------
T ####################--------
# #####################---------
##########################----------
######################-----####-------
###############--------------#########--
###########------------------###########
#########---------------------############
######------------------------############
####-------------------------#############
###--------------------------#############
----------------------------############
------------ ------------#############
----------- P ------------############
---------- -----------############
----------------------############
-------------------###########
-----------------###########
------------##########
------########
Global CMT Convention Moment Tensor:
R T P
-5.90e+21 4.34e+21 -3.04e+21
4.34e+21 1.99e+21 7.56e+21
-3.04e+21 7.56e+21 3.91e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170701165151/index.html
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STK = 255
DIP = 60
RAKE = -40
MW = 3.95
HS = 96.0
The NDK file is 20170701165151.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 2.0 220 50 70 3.15 0.2681
WVFGRD96 4.0 185 75 -40 3.17 0.2351
WVFGRD96 6.0 -5 50 -30 3.24 0.2734
WVFGRD96 8.0 355 50 -30 3.33 0.3002
WVFGRD96 10.0 -5 55 -35 3.37 0.3160
WVFGRD96 12.0 0 60 -30 3.39 0.3172
WVFGRD96 14.0 0 60 -30 3.42 0.3079
WVFGRD96 16.0 0 60 -30 3.45 0.2909
WVFGRD96 18.0 315 50 -10 3.44 0.2792
WVFGRD96 20.0 110 55 25 3.50 0.2876
WVFGRD96 22.0 105 60 10 3.52 0.2946
WVFGRD96 24.0 105 65 15 3.55 0.3007
WVFGRD96 26.0 105 65 15 3.57 0.3049
WVFGRD96 28.0 105 65 20 3.60 0.3163
WVFGRD96 30.0 105 65 15 3.62 0.3322
WVFGRD96 32.0 105 60 10 3.63 0.3474
WVFGRD96 34.0 105 60 10 3.65 0.3586
WVFGRD96 36.0 105 60 5 3.67 0.3658
WVFGRD96 38.0 105 65 15 3.70 0.3704
WVFGRD96 40.0 100 55 -10 3.76 0.3816
WVFGRD96 42.0 100 50 -10 3.79 0.3824
WVFGRD96 44.0 100 55 -10 3.81 0.3785
WVFGRD96 46.0 280 60 0 3.84 0.3831
WVFGRD96 48.0 280 60 0 3.86 0.3948
WVFGRD96 50.0 275 55 -10 3.87 0.4080
WVFGRD96 52.0 270 55 -20 3.88 0.4274
WVFGRD96 54.0 270 55 -25 3.88 0.4488
WVFGRD96 56.0 270 55 -25 3.89 0.4695
WVFGRD96 58.0 270 55 -25 3.90 0.4886
WVFGRD96 60.0 265 55 -30 3.91 0.5058
WVFGRD96 62.0 265 55 -30 3.91 0.5220
WVFGRD96 64.0 265 55 -30 3.92 0.5362
WVFGRD96 66.0 265 55 -30 3.92 0.5489
WVFGRD96 68.0 265 55 -30 3.92 0.5585
WVFGRD96 70.0 260 55 -35 3.93 0.5663
WVFGRD96 72.0 260 55 -35 3.93 0.5741
WVFGRD96 74.0 260 55 -35 3.93 0.5791
WVFGRD96 76.0 260 55 -35 3.93 0.5836
WVFGRD96 78.0 260 55 -35 3.93 0.5865
WVFGRD96 80.0 260 55 -35 3.93 0.5889
WVFGRD96 82.0 260 60 -35 3.94 0.5904
WVFGRD96 84.0 260 60 -35 3.94 0.5929
WVFGRD96 86.0 260 60 -35 3.94 0.5943
WVFGRD96 88.0 260 60 -35 3.94 0.5955
WVFGRD96 90.0 260 60 -35 3.94 0.5962
WVFGRD96 92.0 255 60 -40 3.95 0.5960
WVFGRD96 94.0 255 60 -40 3.95 0.5969
WVFGRD96 96.0 255 60 -40 3.95 0.5975
WVFGRD96 98.0 255 60 -40 3.95 0.5968
WVFGRD96 100.0 255 60 -40 3.95 0.5959
WVFGRD96 102.0 255 60 -40 3.95 0.5950
WVFGRD96 104.0 255 60 -40 3.95 0.5946
WVFGRD96 106.0 255 65 -40 3.97 0.5932
WVFGRD96 108.0 255 65 -40 3.97 0.5906
WVFGRD96 110.0 255 65 -40 3.97 0.5896
WVFGRD96 112.0 255 65 -40 3.97 0.5889
WVFGRD96 114.0 255 65 -40 3.97 0.5862
WVFGRD96 116.0 255 65 -40 3.97 0.5840
WVFGRD96 118.0 255 65 -40 3.97 0.5824
The best solution is
WVFGRD96 96.0 255 60 -40 3.95 0.5975
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