The ANSS event ID is ak017646eggp and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak017646eggp/executive.
2017/05/13 13:31:08 62.172 -147.822 37.8 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/05/13 13:31:08:0 62.17 -147.82 37.8 4.1 Alaska
Stations used:
AK.BPAW AK.CAST AK.CUT AK.DIV AK.EYAK AK.FIRE AK.GHO AK.HDA
AK.KLU AK.KNK AK.MCK AK.PAX AK.RC01 AK.RIDG AK.SAW AK.SCM
AK.SSN AK.SWD AK.TRF AK.WAT1 AK.WAT7 AT.PMR TA.L26K TA.M22K
TA.M23K TA.N20K TA.N25K TA.P23K
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.91e+22 dyne-cm
Mw = 4.12
Z = 52 km
Plane Strike Dip Rake
NP1 193 67 -99
NP2 35 25 -70
Principal Axes:
Axis Value Plunge Azimuth
T 1.91e+22 21 290
N 0.00e+00 8 197
P -1.91e+22 67 86
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.92e+21
Mxy -5.50e+21
Mxz 1.76e+21
Myy 1.18e+22
Myz -1.28e+22
Mzz -1.37e+22
###########---
#############---------
##############-------------#
##############---------------#
###############-----------------##
###############-------------------##
## ##########--------------------###
### T ##########---------------------###
### #########----------------------###
################--------- ----------####
###############---------- P ----------####
###############---------- ---------#####
##############-----------------------#####
#############----------------------#####
#############---------------------######
############--------------------######
###########-------------------######
##########-----------------#######
########---------------#######
########------------########
-####-------##########
--############
Global CMT Convention Moment Tensor:
R T P
-1.37e+22 1.76e+21 1.28e+22
1.76e+21 1.92e+21 5.50e+21
1.28e+22 5.50e+21 1.18e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170513133108/index.html
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STK = 35
DIP = 25
RAKE = -70
MW = 4.12
HS = 52.0
The NDK file is 20170513133108.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 -40 o DIST/3.3 +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 30 45 90 3.41 0.2586
WVFGRD96 4.0 180 45 35 3.42 0.2573
WVFGRD96 6.0 350 45 25 3.45 0.2907
WVFGRD96 8.0 260 80 50 3.53 0.3163
WVFGRD96 10.0 255 85 45 3.56 0.3331
WVFGRD96 12.0 75 90 -40 3.60 0.3440
WVFGRD96 14.0 70 80 -40 3.64 0.3540
WVFGRD96 16.0 75 90 -35 3.67 0.3633
WVFGRD96 18.0 80 85 -30 3.71 0.3724
WVFGRD96 20.0 80 85 -30 3.74 0.3834
WVFGRD96 22.0 75 80 -30 3.76 0.3951
WVFGRD96 24.0 80 80 -30 3.78 0.4075
WVFGRD96 26.0 75 70 -30 3.80 0.4189
WVFGRD96 28.0 75 65 -30 3.82 0.4361
WVFGRD96 30.0 75 60 -30 3.84 0.4575
WVFGRD96 32.0 70 55 -35 3.86 0.4778
WVFGRD96 34.0 70 55 -35 3.87 0.4968
WVFGRD96 36.0 70 55 -35 3.89 0.5106
WVFGRD96 38.0 60 35 -45 3.91 0.5328
WVFGRD96 40.0 55 30 -50 4.03 0.5595
WVFGRD96 42.0 50 30 -55 4.05 0.5757
WVFGRD96 44.0 50 30 -55 4.07 0.5862
WVFGRD96 46.0 50 30 -55 4.08 0.5959
WVFGRD96 48.0 45 30 -65 4.10 0.6029
WVFGRD96 50.0 50 30 -55 4.10 0.6068
WVFGRD96 52.0 35 25 -70 4.12 0.6074
WVFGRD96 54.0 45 30 -60 4.11 0.6071
WVFGRD96 56.0 45 30 -60 4.12 0.6063
WVFGRD96 58.0 45 30 -60 4.12 0.6050
WVFGRD96 60.0 45 30 -60 4.13 0.6019
WVFGRD96 62.0 45 30 -55 4.13 0.5972
WVFGRD96 64.0 50 35 -50 4.13 0.5936
WVFGRD96 66.0 50 35 -50 4.13 0.5908
WVFGRD96 68.0 50 35 -50 4.14 0.5846
WVFGRD96 70.0 50 35 -50 4.14 0.5796
WVFGRD96 72.0 55 40 -45 4.15 0.5740
WVFGRD96 74.0 55 40 -45 4.15 0.5683
WVFGRD96 76.0 55 40 -45 4.15 0.5621
WVFGRD96 78.0 55 40 -45 4.16 0.5554
WVFGRD96 80.0 55 40 -45 4.16 0.5496
WVFGRD96 82.0 55 40 -45 4.16 0.5420
WVFGRD96 84.0 55 40 -45 4.17 0.5353
WVFGRD96 86.0 60 45 -40 4.17 0.5278
WVFGRD96 88.0 60 45 -40 4.18 0.5225
WVFGRD96 90.0 60 45 -40 4.18 0.5157
WVFGRD96 92.0 65 50 -30 4.19 0.5120
WVFGRD96 94.0 65 50 -30 4.19 0.5068
WVFGRD96 96.0 65 50 -30 4.19 0.5033
WVFGRD96 98.0 65 50 -30 4.20 0.4975
The best solution is
WVFGRD96 52.0 35 25 -70 4.12 0.6074
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 -40 o DIST/3.3 +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