The ANSS event ID is ak0178n1jqxy and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0178n1jqxy/executive.
2017/07/07 07:58:17 60.475 -151.759 71.3 4.3 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/07/07 07:58:17:0 60.47 -151.76 71.3 4.3 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CNP AK.CUT AK.FIRE AK.GHO AK.HOM AK.KNK
AK.RC01 AK.SSN AK.SWD AT.PMR AT.SVW2 AV.ILSW TA.M19K
TA.M20K TA.M22K TA.N18K TA.N19K TA.O18K TA.O19K TA.O22K
TA.P19K TA.Q19K
Filtering commands used:
cut o DIST/3.5 -40 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 = 3.94e+22 dyne-cm
Mw = 4.33
Z = 72 km
Plane Strike Dip Rake
NP1 205 80 -30
NP2 301 61 -168
Principal Axes:
Axis Value Plunge Azimuth
T 3.94e+22 13 256
N 0.00e+00 59 8
P -3.94e+22 28 159
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.45e+22
Mxy 1.90e+22
Mxz 1.32e+22
Myy 3.12e+22
Myz -1.43e+22
Mzz -6.73e+21
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--------------------########
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#############-------################
###################-##################
####################----################
###################--------#############
###################-----------############
###################-------------##########
##################----------------########
## ############------------------#######
# T ###########--------------------#####
# ##########----------------------####
#############-----------------------##
###########-------------------------
##########----------- ----------
#######------------ P --------
######------------ -------
##--------------------
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Global CMT Convention Moment Tensor:
R T P
-6.73e+21 1.32e+22 1.43e+22
1.32e+22 -2.45e+22 -1.90e+22
1.43e+22 -1.90e+22 3.12e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170707075817/index.html
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STK = 205
DIP = 80
RAKE = -30
MW = 4.33
HS = 72.0
The NDK file is 20170707075817.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 -40 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 2.0 120 80 -15 3.43 0.2150
WVFGRD96 4.0 305 90 -20 3.53 0.2473
WVFGRD96 6.0 125 85 20 3.60 0.2639
WVFGRD96 8.0 215 70 5 3.68 0.2871
WVFGRD96 10.0 215 70 5 3.73 0.2990
WVFGRD96 12.0 35 75 -5 3.77 0.3085
WVFGRD96 14.0 35 75 -5 3.81 0.3125
WVFGRD96 16.0 35 75 -10 3.84 0.3145
WVFGRD96 18.0 35 75 -10 3.87 0.3169
WVFGRD96 20.0 35 80 -10 3.90 0.3224
WVFGRD96 22.0 35 75 -5 3.92 0.3322
WVFGRD96 24.0 35 75 -5 3.94 0.3471
WVFGRD96 26.0 35 80 0 3.96 0.3649
WVFGRD96 28.0 35 80 0 3.98 0.3856
WVFGRD96 30.0 35 80 0 4.00 0.4017
WVFGRD96 32.0 35 80 0 4.02 0.4138
WVFGRD96 34.0 35 80 0 4.03 0.4189
WVFGRD96 36.0 215 85 -10 4.05 0.4208
WVFGRD96 38.0 215 85 -10 4.08 0.4286
WVFGRD96 40.0 210 80 -20 4.14 0.4431
WVFGRD96 42.0 210 80 -20 4.17 0.4495
WVFGRD96 44.0 210 80 -20 4.19 0.4532
WVFGRD96 46.0 210 80 -20 4.21 0.4579
WVFGRD96 48.0 210 85 -20 4.22 0.4663
WVFGRD96 50.0 210 85 -25 4.24 0.4763
WVFGRD96 52.0 210 85 -25 4.25 0.4860
WVFGRD96 54.0 210 85 -25 4.26 0.4955
WVFGRD96 56.0 210 85 -25 4.27 0.5044
WVFGRD96 58.0 210 85 -25 4.28 0.5112
WVFGRD96 60.0 210 85 -25 4.29 0.5177
WVFGRD96 62.0 205 80 -30 4.30 0.5241
WVFGRD96 64.0 205 80 -30 4.31 0.5312
WVFGRD96 66.0 205 80 -30 4.31 0.5344
WVFGRD96 68.0 205 80 -30 4.32 0.5367
WVFGRD96 70.0 205 80 -30 4.32 0.5380
WVFGRD96 72.0 205 80 -30 4.33 0.5402
WVFGRD96 74.0 205 80 -30 4.33 0.5388
WVFGRD96 76.0 30 90 25 4.32 0.5347
WVFGRD96 78.0 205 80 -30 4.33 0.5365
WVFGRD96 80.0 30 90 25 4.33 0.5311
WVFGRD96 82.0 205 85 -30 4.34 0.5315
WVFGRD96 84.0 205 85 -30 4.34 0.5280
WVFGRD96 86.0 30 85 25 4.34 0.5245
WVFGRD96 88.0 30 85 25 4.34 0.5211
WVFGRD96 90.0 205 85 -30 4.35 0.5161
WVFGRD96 92.0 30 85 25 4.34 0.5135
WVFGRD96 94.0 210 90 -25 4.34 0.5078
WVFGRD96 96.0 210 90 -25 4.35 0.5044
WVFGRD96 98.0 210 90 -30 4.35 0.4994
The best solution is
WVFGRD96 72.0 205 80 -30 4.33 0.5402
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 -40 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 CUS.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 CUS Model with Q from simple gamma values 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 1.0000 5.0000 2.8900 2.5000 0.172E-02 0.387E-02 0.00 0.00 1.00 1.00 9.0000 6.1000 3.5200 2.7300 0.160E-02 0.363E-02 0.00 0.00 1.00 1.00 10.0000 6.4000 3.7000 2.8200 0.149E-02 0.336E-02 0.00 0.00 1.00 1.00 20.0000 6.7000 3.8700 2.9020 0.000E-04 0.000E-04 0.00 0.00 1.00 1.00 0.0000 8.1500 4.7000 3.3640 0.194E-02 0.431E-02 0.00 0.00 1.00 1.00