The ANSS event ID is ak017b7l1ix4 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak017b7l1ix4/executive.
2017/09/01 03:21:20 63.040 -150.969 132.0 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/09/01 03:21:20:0 63.04 -150.97 132.0 4.0 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.GHO AK.GLI AK.HDA
AK.KNK AK.KTH AK.MDM AK.MLY AK.NEA2 AK.PPD AK.PWL AK.RC01
AK.RND AK.SAW AK.SCM AK.SCRK AK.SSN AK.TRF AK.WRH AT.PMR
AT.TTA IM.IL31 IU.COLA TA.H21K TA.J20K TA.J25K TA.J26L
TA.K20K TA.L26K TA.M19K TA.M20K TA.M22K TA.N19K TA.N25K
TA.O22K TA.POKR TA.TCOL
Filtering commands used:
cut o DIST/3.3 -60 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 = 2.88e+22 dyne-cm
Mw = 4.24
Z = 136 km
Plane Strike Dip Rake
NP1 60 80 45
NP2 320 46 166
Principal Axes:
Axis Value Plunge Azimuth
T 2.88e+22 38 291
N 0.00e+00 44 70
P -2.88e+22 22 183
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.26e+22
Mxy -7.02e+21
Mxz 1.48e+22
Myy 1.56e+22
Myz -1.26e+22
Mzz 6.97e+21
--------------
----------------------
#########-------------------
###############---------------
####################--------------
#######################-------------
##########################---------###
###### ####################-----######
###### T #####################--########
####### ####################--##########
###########################------#########
########################----------########
#####################--------------#######
#################-----------------######
#############----------------------#####
########--------------------------####
#--------------------------------###
-------------------------------###
------------- -------------#
------------ P ------------#
--------- ----------
--------------
Global CMT Convention Moment Tensor:
R T P
6.97e+21 1.48e+22 1.26e+22
1.48e+22 -2.26e+22 7.02e+21
1.26e+22 7.02e+21 1.56e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170901032120/index.html
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STK = 60
DIP = 80
RAKE = 45
MW = 4.24
HS = 136.0
The NDK file is 20170901032120.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 -60 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 2.0 135 45 -45 3.29 0.1476
WVFGRD96 4.0 155 55 -10 3.32 0.1572
WVFGRD96 6.0 160 65 15 3.37 0.1709
WVFGRD96 8.0 160 65 15 3.45 0.1825
WVFGRD96 10.0 160 65 20 3.50 0.1899
WVFGRD96 12.0 160 65 25 3.54 0.1954
WVFGRD96 14.0 160 70 25 3.58 0.1980
WVFGRD96 16.0 160 65 25 3.60 0.1950
WVFGRD96 18.0 160 65 30 3.62 0.1884
WVFGRD96 20.0 160 65 35 3.64 0.1793
WVFGRD96 22.0 160 70 35 3.66 0.1692
WVFGRD96 24.0 250 70 30 3.66 0.1723
WVFGRD96 26.0 245 80 25 3.69 0.1762
WVFGRD96 28.0 245 80 25 3.70 0.1786
WVFGRD96 30.0 245 80 25 3.72 0.1802
WVFGRD96 32.0 245 80 25 3.74 0.1796
WVFGRD96 34.0 245 85 20 3.75 0.1776
WVFGRD96 36.0 70 80 15 3.78 0.1773
WVFGRD96 38.0 75 75 20 3.81 0.1754
WVFGRD96 40.0 250 90 -20 3.85 0.1741
WVFGRD96 42.0 75 75 20 3.89 0.1768
WVFGRD96 44.0 75 75 20 3.92 0.1780
WVFGRD96 46.0 75 75 20 3.94 0.1804
WVFGRD96 48.0 80 70 30 3.96 0.1844
WVFGRD96 50.0 80 70 30 3.98 0.1899
WVFGRD96 52.0 80 70 30 3.99 0.1960
WVFGRD96 54.0 80 70 30 4.00 0.2024
WVFGRD96 56.0 80 70 30 4.01 0.2087
WVFGRD96 58.0 60 75 -25 4.05 0.2171
WVFGRD96 60.0 60 75 -20 4.06 0.2298
WVFGRD96 62.0 60 75 -20 4.07 0.2432
WVFGRD96 64.0 60 75 -20 4.09 0.2560
WVFGRD96 66.0 60 75 -20 4.10 0.2679
WVFGRD96 68.0 55 75 -25 4.12 0.2847
WVFGRD96 70.0 55 75 -25 4.14 0.3013
WVFGRD96 72.0 55 75 -20 4.15 0.3242
WVFGRD96 74.0 55 75 -15 4.16 0.3627
WVFGRD96 76.0 60 65 20 4.13 0.4235
WVFGRD96 78.0 60 65 20 4.16 0.4934
WVFGRD96 80.0 60 70 25 4.17 0.5557
WVFGRD96 82.0 60 70 25 4.18 0.6036
WVFGRD96 84.0 60 70 30 4.18 0.6311
WVFGRD96 86.0 60 70 30 4.19 0.6390
WVFGRD96 88.0 60 75 30 4.19 0.6447
WVFGRD96 90.0 60 75 30 4.19 0.6484
WVFGRD96 92.0 60 75 35 4.19 0.6547
WVFGRD96 94.0 60 75 35 4.19 0.6593
WVFGRD96 96.0 60 75 35 4.20 0.6653
WVFGRD96 98.0 60 75 40 4.20 0.6699
WVFGRD96 100.0 60 75 40 4.20 0.6754
WVFGRD96 102.0 60 75 40 4.20 0.6809
WVFGRD96 104.0 60 75 40 4.20 0.6834
WVFGRD96 106.0 60 75 40 4.21 0.6889
WVFGRD96 108.0 60 75 40 4.21 0.6902
WVFGRD96 110.0 60 75 40 4.21 0.6950
WVFGRD96 112.0 60 75 40 4.21 0.6965
WVFGRD96 114.0 60 75 40 4.22 0.7010
WVFGRD96 116.0 60 75 40 4.22 0.7018
WVFGRD96 118.0 60 80 45 4.22 0.7060
WVFGRD96 120.0 60 80 45 4.23 0.7060
WVFGRD96 122.0 60 80 45 4.23 0.7098
WVFGRD96 124.0 60 80 45 4.23 0.7094
WVFGRD96 126.0 60 80 45 4.23 0.7135
WVFGRD96 128.0 60 80 45 4.23 0.7139
WVFGRD96 130.0 60 80 45 4.24 0.7152
WVFGRD96 132.0 60 80 45 4.24 0.7152
WVFGRD96 134.0 60 80 45 4.24 0.7156
WVFGRD96 136.0 60 80 45 4.24 0.7166
WVFGRD96 138.0 60 80 45 4.24 0.7148
WVFGRD96 140.0 60 80 45 4.24 0.7149
WVFGRD96 142.0 60 80 45 4.25 0.7137
WVFGRD96 144.0 60 80 45 4.25 0.7133
WVFGRD96 146.0 60 80 45 4.25 0.7115
WVFGRD96 148.0 60 80 45 4.25 0.7084
The best solution is
WVFGRD96 136.0 60 80 45 4.24 0.7166
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 -60 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