The ANSS event ID is ak015e8x3c2v and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak015e8x3c2v/executive.
2015/11/06 14:26:49 61.996 -149.876 46.1 4.4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2015/11/06 14:26:49:0 62.00 -149.88 46.1 4.4 Alaska
Stations used:
AK.CUT AK.GHO AK.KLU AK.KNK AK.KTH AK.PWL AK.RC01 AK.RND
AK.SAW AK.SCM AK.SKN AK.TRF AT.PMR TA.M24K TA.O22K
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 5.19e+22 dyne-cm
Mw = 4.41
Z = 56 km
Plane Strike Dip Rake
NP1 230 50 -60
NP2 8 48 -121
Principal Axes:
Axis Value Plunge Azimuth
T 5.19e+22 1 299
N 0.00e+00 23 30
P -5.19e+22 67 207
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.40e+21
Mxy -2.52e+22
Mxz 1.67e+22
Myy 3.79e+22
Myz 7.76e+21
Mzz -4.42e+22
###########---
#################-----
#####################-------
#####################--######-
################---------########
T #############-------------########
##########----------------#########
############------------------##########
##########---------------------#########
##########----------------------##########
#########-----------------------##########
#######------------------------###########
#######---------- -----------###########
#####----------- P -----------##########
####------------ ----------###########
###------------------------###########
##------------------------##########
#-----------------------##########
--------------------##########
------------------##########
-------------#########
------########
Global CMT Convention Moment Tensor:
R T P
-4.42e+22 1.67e+22 -7.76e+21
1.67e+22 6.40e+21 2.52e+22
-7.76e+21 2.52e+22 3.79e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20151106142649/index.html
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STK = 230
DIP = 50
RAKE = -60
MW = 4.41
HS = 56.0
The NDK file is 20151106142649.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 +70 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.06 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 210 45 85 3.78 0.4173
WVFGRD96 4.0 45 50 -80 3.84 0.3431
WVFGRD96 6.0 205 70 75 3.85 0.3383
WVFGRD96 8.0 180 20 30 3.95 0.3884
WVFGRD96 10.0 175 25 25 3.97 0.4308
WVFGRD96 12.0 180 25 30 3.98 0.4561
WVFGRD96 14.0 120 35 40 3.97 0.4705
WVFGRD96 16.0 180 25 30 4.01 0.4800
WVFGRD96 18.0 180 25 25 4.03 0.4822
WVFGRD96 20.0 175 30 20 4.04 0.4804
WVFGRD96 22.0 130 35 80 4.10 0.4782
WVFGRD96 24.0 260 40 -30 4.04 0.4779
WVFGRD96 26.0 70 70 -50 4.08 0.4940
WVFGRD96 28.0 255 45 -30 4.08 0.5087
WVFGRD96 30.0 250 45 -35 4.10 0.5292
WVFGRD96 32.0 250 50 -35 4.12 0.5515
WVFGRD96 34.0 245 50 -40 4.15 0.5732
WVFGRD96 36.0 245 50 -40 4.17 0.5908
WVFGRD96 38.0 240 50 -45 4.21 0.6080
WVFGRD96 40.0 235 45 -55 4.30 0.6589
WVFGRD96 42.0 235 45 -55 4.32 0.6733
WVFGRD96 44.0 230 45 -60 4.34 0.6868
WVFGRD96 46.0 230 45 -60 4.36 0.7006
WVFGRD96 48.0 230 45 -60 4.37 0.7117
WVFGRD96 50.0 225 45 -70 4.38 0.7183
WVFGRD96 52.0 235 50 -55 4.39 0.7243
WVFGRD96 54.0 230 50 -60 4.41 0.7298
WVFGRD96 56.0 230 50 -60 4.41 0.7313
WVFGRD96 58.0 225 50 -70 4.42 0.7300
WVFGRD96 60.0 220 50 -80 4.42 0.7261
WVFGRD96 62.0 220 50 -80 4.43 0.7198
WVFGRD96 64.0 215 50 -90 4.43 0.7112
WVFGRD96 66.0 35 35 -90 4.44 0.7046
WVFGRD96 68.0 40 35 -85 4.44 0.6996
WVFGRD96 70.0 40 35 -85 4.44 0.6921
WVFGRD96 72.0 45 35 -80 4.44 0.6832
WVFGRD96 74.0 50 35 -75 4.45 0.6740
WVFGRD96 76.0 55 35 -70 4.45 0.6642
WVFGRD96 78.0 55 35 -70 4.45 0.6540
WVFGRD96 80.0 60 35 -65 4.45 0.6438
WVFGRD96 82.0 65 35 -60 4.45 0.6331
WVFGRD96 84.0 65 35 -60 4.45 0.6228
WVFGRD96 86.0 70 35 -55 4.46 0.6121
WVFGRD96 88.0 70 35 -55 4.45 0.6021
WVFGRD96 90.0 70 35 -50 4.45 0.5918
WVFGRD96 92.0 70 35 -50 4.45 0.5819
WVFGRD96 94.0 75 35 -45 4.46 0.5718
WVFGRD96 96.0 75 35 -45 4.46 0.5632
WVFGRD96 98.0 75 35 -45 4.45 0.5540
The best solution is
WVFGRD96 56.0 230 50 -60 4.41 0.7313
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 +70 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.06 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