The ANSS event ID is ak0178yhumx4 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0178yhumx4/executive.
2017/07/14 00:09:15 63.078 -150.663 118.2 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/07/14 00:09:15:0 63.08 -150.66 118.2 4.0 Alaska
Stations used:
AK.BPAW AK.BWN AK.CAST AK.DHY AK.GHO AK.GLI AK.KLU AK.KNK
AK.KTH AK.MDM AK.MLY AK.NEA2 AK.PAX AK.RC01 AK.SAW AK.SCM
AK.TRF AT.PMR IU.COLA TA.H21K TA.H23K TA.I21K TA.I23K
TA.J20K TA.J25K TA.K20K TA.L19K TA.M20K TA.M22K TA.POKR
TA.TCOL
Filtering commands used:
cut o DIST/3.4 -50 o DIST/3.4 +60
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.30e+22 dyne-cm
Mw = 4.01
Z = 122 km
Plane Strike Dip Rake
NP1 234 87 -125
NP2 140 35 -5
Principal Axes:
Axis Value Plunge Azimuth
T 1.30e+22 33 353
N 0.00e+00 35 236
P -1.30e+22 38 113
Moment Tensor: (dyne-cm)
Component Value
Mxx 7.77e+21
Mxy 1.82e+21
Mxz 8.40e+21
Myy -6.71e+21
Myz -6.54e+21
Mzz -1.07e+21
##############
######################
-########## ##############
-########### T ###############
--############ #################
--##############################----
---###########################--------
----########################------------
----#####################---------------
-----###################------------------
------###############---------------------
------############------------------------
-------#########--------------- --------
-------######----------------- P -------
--------##-------------------- -------
-------#------------------------------
----#####---------------------------
##########------------------------
###########-------------------
#############---------------
######################
##############
Global CMT Convention Moment Tensor:
R T P
-1.07e+21 8.40e+21 6.54e+21
8.40e+21 7.77e+21 -1.82e+21
6.54e+21 -1.82e+21 -6.71e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170714000915/index.html
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STK = 140
DIP = 35
RAKE = -5
MW = 4.01
HS = 122.0
The NDK file is 20170714000915.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/3.4 -50 o DIST/3.4 +60 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 2.0 235 70 -30 2.94 0.1222
WVFGRD96 4.0 250 65 30 3.03 0.1419
WVFGRD96 6.0 250 70 30 3.08 0.1554
WVFGRD96 8.0 255 65 30 3.17 0.1685
WVFGRD96 10.0 250 75 30 3.20 0.1723
WVFGRD96 12.0 60 90 -30 3.23 0.1739
WVFGRD96 14.0 60 90 -30 3.26 0.1737
WVFGRD96 16.0 60 90 -30 3.28 0.1702
WVFGRD96 18.0 60 90 -30 3.31 0.1646
WVFGRD96 20.0 60 90 -30 3.32 0.1566
WVFGRD96 22.0 55 65 -20 3.36 0.1476
WVFGRD96 24.0 95 45 -5 3.43 0.1434
WVFGRD96 26.0 95 45 -5 3.45 0.1444
WVFGRD96 28.0 95 45 -10 3.48 0.1453
WVFGRD96 30.0 95 45 -10 3.50 0.1472
WVFGRD96 32.0 95 50 -15 3.52 0.1520
WVFGRD96 34.0 100 60 -25 3.51 0.1584
WVFGRD96 36.0 100 60 -25 3.54 0.1666
WVFGRD96 38.0 105 60 -25 3.57 0.1749
WVFGRD96 40.0 105 60 -35 3.63 0.1740
WVFGRD96 42.0 135 45 35 3.71 0.1729
WVFGRD96 44.0 135 50 30 3.73 0.1844
WVFGRD96 46.0 135 50 30 3.76 0.2004
WVFGRD96 48.0 135 50 30 3.79 0.2197
WVFGRD96 50.0 135 50 25 3.81 0.2430
WVFGRD96 52.0 135 50 25 3.84 0.2695
WVFGRD96 54.0 140 55 25 3.85 0.2980
WVFGRD96 56.0 145 55 25 3.86 0.3266
WVFGRD96 58.0 145 60 25 3.88 0.3564
WVFGRD96 60.0 145 60 20 3.89 0.3809
WVFGRD96 62.0 145 60 20 3.90 0.3966
WVFGRD96 64.0 150 40 10 3.87 0.4218
WVFGRD96 66.0 150 40 10 3.88 0.4501
WVFGRD96 68.0 150 35 5 3.88 0.4756
WVFGRD96 70.0 150 35 5 3.89 0.4928
WVFGRD96 72.0 150 35 5 3.90 0.5016
WVFGRD96 74.0 150 40 5 3.91 0.5107
WVFGRD96 76.0 150 40 5 3.92 0.5204
WVFGRD96 78.0 140 35 0 3.93 0.5303
WVFGRD96 80.0 140 35 0 3.94 0.5396
WVFGRD96 82.0 140 35 0 3.94 0.5474
WVFGRD96 84.0 140 35 0 3.95 0.5564
WVFGRD96 86.0 135 35 -5 3.95 0.5641
WVFGRD96 88.0 135 35 -5 3.96 0.5732
WVFGRD96 90.0 140 40 0 3.96 0.5813
WVFGRD96 92.0 140 40 0 3.97 0.5889
WVFGRD96 94.0 140 40 0 3.97 0.5975
WVFGRD96 96.0 140 40 0 3.98 0.6025
WVFGRD96 98.0 140 40 0 3.98 0.6109
WVFGRD96 100.0 140 35 -5 3.98 0.6156
WVFGRD96 102.0 140 35 -5 3.98 0.6217
WVFGRD96 104.0 140 35 -5 3.98 0.6268
WVFGRD96 106.0 140 35 -5 3.99 0.6317
WVFGRD96 108.0 140 35 -5 3.99 0.6352
WVFGRD96 110.0 140 35 -5 3.99 0.6381
WVFGRD96 112.0 140 35 -5 4.00 0.6415
WVFGRD96 114.0 140 35 -5 4.00 0.6429
WVFGRD96 116.0 140 35 -5 4.00 0.6438
WVFGRD96 118.0 140 35 -5 4.00 0.6456
WVFGRD96 120.0 140 35 -5 4.01 0.6448
WVFGRD96 122.0 140 35 -5 4.01 0.6461
WVFGRD96 124.0 135 30 -10 4.02 0.6435
WVFGRD96 126.0 135 30 -10 4.02 0.6438
WVFGRD96 128.0 135 30 -10 4.02 0.6421
The best solution is
WVFGRD96 122.0 140 35 -5 4.01 0.6461
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/3.4 -50 o DIST/3.4 +60 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