The ANSS event ID is ak01777o1bzn and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak01777o1bzn/executive.
2017/06/06 01:11:25 61.257 -151.979 103.6 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/06/06 01:11:25:0 61.26 -151.98 103.6 3.9 Alaska
Stations used:
AK.CAST AK.CUT AK.GLI AK.PWL AK.RC01 AK.SCM TA.K20K TA.M19K
TA.M20K TA.M22K TA.O22K
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 9.55e+21 dyne-cm
Mw = 3.92
Z = 104 km
Plane Strike Dip Rake
NP1 45 65 55
NP2 284 42 141
Principal Axes:
Axis Value Plunge Azimuth
T 9.55e+21 55 269
N 0.00e+00 31 61
P -9.55e+21 13 160
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.96e+21
Mxy 3.00e+21
Mxz 1.92e+21
Myy 1.97e+21
Myz -5.19e+21
Mzz 5.99e+21
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------------------------------
---------#########-------------###
----#####################-------####
--############################--######
-###############################-#######
################################---#####
###############################-------####
########### ################---------###
########### T ###############-----------##
########### #############--------------#
########################----------------
######################------------------
###################-------------------
################--------------------
###########-----------------------
#####-------------------------
------------------- ------
---------------- P ---
------------
Global CMT Convention Moment Tensor:
R T P
5.99e+21 1.92e+21 5.19e+21
1.92e+21 -7.96e+21 -3.00e+21
5.19e+21 -3.00e+21 1.97e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170606011125/index.html
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STK = 45
DIP = 65
RAKE = 55
MW = 3.92
HS = 104.0
The NDK file is 20170606011125.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 -30 o DIST/3.3 +40 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 85 40 -85 3.10 0.2057
WVFGRD96 4.0 115 45 -35 3.11 0.2319
WVFGRD96 6.0 310 50 -5 3.13 0.2662
WVFGRD96 8.0 310 50 0 3.22 0.2961
WVFGRD96 10.0 310 50 0 3.27 0.3222
WVFGRD96 12.0 315 55 10 3.32 0.3403
WVFGRD96 14.0 315 55 10 3.36 0.3479
WVFGRD96 16.0 320 55 20 3.40 0.3431
WVFGRD96 18.0 225 70 40 3.42 0.3536
WVFGRD96 20.0 220 75 35 3.46 0.3681
WVFGRD96 22.0 220 75 35 3.50 0.3834
WVFGRD96 24.0 220 75 30 3.53 0.4021
WVFGRD96 26.0 220 75 25 3.56 0.4196
WVFGRD96 28.0 225 70 25 3.58 0.4293
WVFGRD96 30.0 225 70 25 3.59 0.4304
WVFGRD96 32.0 225 60 20 3.62 0.4348
WVFGRD96 34.0 225 60 20 3.63 0.4307
WVFGRD96 36.0 215 85 -5 3.64 0.4375
WVFGRD96 38.0 215 85 -5 3.68 0.4431
WVFGRD96 40.0 215 75 -10 3.73 0.4386
WVFGRD96 42.0 35 80 -20 3.75 0.4286
WVFGRD96 44.0 35 80 -20 3.77 0.4207
WVFGRD96 46.0 35 80 -20 3.78 0.4166
WVFGRD96 48.0 220 90 15 3.79 0.4085
WVFGRD96 50.0 220 80 15 3.80 0.4153
WVFGRD96 52.0 35 80 -20 3.81 0.4249
WVFGRD96 54.0 40 90 -20 3.82 0.4333
WVFGRD96 56.0 220 85 20 3.83 0.4423
WVFGRD96 58.0 40 90 -20 3.83 0.4503
WVFGRD96 60.0 40 90 -20 3.84 0.4576
WVFGRD96 62.0 220 90 20 3.85 0.4637
WVFGRD96 64.0 45 55 30 3.87 0.4716
WVFGRD96 66.0 50 55 35 3.88 0.4866
WVFGRD96 68.0 50 55 40 3.88 0.4990
WVFGRD96 70.0 50 55 40 3.89 0.5130
WVFGRD96 72.0 50 55 40 3.89 0.5238
WVFGRD96 74.0 50 55 40 3.89 0.5316
WVFGRD96 76.0 55 55 45 3.90 0.5411
WVFGRD96 78.0 55 55 45 3.90 0.5496
WVFGRD96 80.0 55 55 45 3.91 0.5559
WVFGRD96 82.0 50 60 45 3.90 0.5607
WVFGRD96 84.0 50 60 45 3.90 0.5649
WVFGRD96 86.0 50 60 45 3.91 0.5718
WVFGRD96 88.0 50 60 50 3.91 0.5772
WVFGRD96 90.0 50 60 50 3.91 0.5811
WVFGRD96 92.0 50 60 50 3.91 0.5866
WVFGRD96 94.0 50 60 50 3.91 0.5893
WVFGRD96 96.0 50 60 50 3.91 0.5899
WVFGRD96 98.0 45 65 50 3.91 0.5923
WVFGRD96 100.0 45 65 50 3.92 0.5947
WVFGRD96 102.0 45 65 50 3.92 0.5962
WVFGRD96 104.0 45 65 55 3.92 0.5970
WVFGRD96 106.0 45 65 55 3.92 0.5967
WVFGRD96 108.0 45 65 55 3.92 0.5965
WVFGRD96 110.0 40 70 55 3.93 0.5962
WVFGRD96 112.0 40 70 55 3.93 0.5964
WVFGRD96 114.0 40 70 55 3.93 0.5961
WVFGRD96 116.0 40 70 55 3.93 0.5941
WVFGRD96 118.0 45 65 50 3.92 0.5919
The best solution is
WVFGRD96 104.0 45 65 55 3.92 0.5970
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 -30 o DIST/3.3 +40 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