The ANSS event ID is ak01776c1re1 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak01776c1re1/executive.
2017/06/05 20:51:09 63.042 -149.673 81.9 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/06/05 20:51:09:0 63.04 -149.67 81.9 3.7 Alaska
Stations used:
AK.BPAW AK.CUT AK.KNK AK.MCK AK.RND AK.SAW AK.SCM AT.PMR
TA.J25K TA.M20K
Filtering commands used:
cut o DIST/3.3 -40 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 = 5.13e+21 dyne-cm
Mw = 3.74
Z = 104 km
Plane Strike Dip Rake
NP1 314 79 134
NP2 55 45 15
Principal Axes:
Axis Value Plunge Azimuth
T 5.13e+21 39 263
N 0.00e+00 43 124
P -5.13e+21 22 12
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.18e+21
Mxy -5.74e+20
Mxz -2.01e+21
Myy 2.85e+21
Myz -2.87e+21
Mzz 1.33e+21
--------------
------------- ------
---------------- P ---------
#---------------- ----------
#######---------------------------
##########-------------------------#
##############----------------------##
#################--------------------###
###################------------------###
######################---------------#####
####### ##############------------######
####### T ################---------#######
####### #################-------########
############################----########
########################################
###########################---########
#######################-------######
##################-------------###
----######--------------------
----------------------------
----------------------
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Global CMT Convention Moment Tensor:
R T P
1.33e+21 -2.01e+21 2.87e+21
-2.01e+21 -4.18e+21 5.74e+20
2.87e+21 5.74e+20 2.85e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170605205109/index.html
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STK = 55
DIP = 45
RAKE = 15
MW = 3.74
HS = 104.0
The NDK file is 20170605205109.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 +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 2.0 340 85 0 2.77 0.2649
WVFGRD96 4.0 345 55 25 2.91 0.2966
WVFGRD96 6.0 340 60 20 2.96 0.3302
WVFGRD96 8.0 340 60 15 3.04 0.3547
WVFGRD96 10.0 335 55 0 3.09 0.3639
WVFGRD96 12.0 335 60 -5 3.12 0.3617
WVFGRD96 14.0 335 60 -5 3.15 0.3461
WVFGRD96 16.0 335 65 -5 3.17 0.3222
WVFGRD96 18.0 220 25 -45 3.28 0.3069
WVFGRD96 20.0 220 25 -45 3.32 0.3170
WVFGRD96 22.0 225 30 -40 3.34 0.3297
WVFGRD96 24.0 230 35 -30 3.34 0.3420
WVFGRD96 26.0 235 40 -25 3.36 0.3578
WVFGRD96 28.0 240 45 -15 3.36 0.3761
WVFGRD96 30.0 245 50 -10 3.39 0.3984
WVFGRD96 32.0 245 55 -10 3.40 0.4197
WVFGRD96 34.0 250 60 -5 3.44 0.4397
WVFGRD96 36.0 250 60 -5 3.46 0.4533
WVFGRD96 38.0 250 60 0 3.49 0.4633
WVFGRD96 40.0 250 50 -5 3.59 0.4813
WVFGRD96 42.0 250 55 -5 3.60 0.4829
WVFGRD96 44.0 250 55 -5 3.62 0.4838
WVFGRD96 46.0 250 55 5 3.62 0.4862
WVFGRD96 48.0 250 55 5 3.64 0.4874
WVFGRD96 50.0 250 60 5 3.65 0.4881
WVFGRD96 52.0 250 65 15 3.66 0.4916
WVFGRD96 54.0 250 65 15 3.68 0.4994
WVFGRD96 56.0 250 65 15 3.69 0.5059
WVFGRD96 58.0 250 65 10 3.70 0.5119
WVFGRD96 60.0 250 70 10 3.70 0.5166
WVFGRD96 62.0 245 75 20 3.70 0.5242
WVFGRD96 64.0 245 80 15 3.69 0.5342
WVFGRD96 66.0 245 80 15 3.70 0.5446
WVFGRD96 68.0 245 80 15 3.71 0.5536
WVFGRD96 70.0 240 85 20 3.72 0.5599
WVFGRD96 72.0 240 85 15 3.71 0.5694
WVFGRD96 74.0 240 85 15 3.71 0.5767
WVFGRD96 76.0 240 90 15 3.71 0.5851
WVFGRD96 78.0 60 90 -15 3.72 0.5913
WVFGRD96 80.0 60 90 -15 3.72 0.5966
WVFGRD96 82.0 235 90 15 3.73 0.5996
WVFGRD96 84.0 55 85 -15 3.73 0.6075
WVFGRD96 86.0 235 90 15 3.74 0.6071
WVFGRD96 88.0 55 85 -15 3.74 0.6129
WVFGRD96 90.0 55 80 -10 3.72 0.6152
WVFGRD96 92.0 55 80 -10 3.72 0.6159
WVFGRD96 94.0 55 80 -10 3.73 0.6197
WVFGRD96 96.0 55 40 10 3.74 0.6217
WVFGRD96 98.0 60 40 15 3.73 0.6236
WVFGRD96 100.0 60 40 15 3.73 0.6234
WVFGRD96 102.0 60 40 15 3.74 0.6254
WVFGRD96 104.0 55 45 15 3.74 0.6273
WVFGRD96 106.0 55 45 15 3.74 0.6248
WVFGRD96 108.0 60 45 20 3.73 0.6246
WVFGRD96 110.0 60 45 20 3.73 0.6226
WVFGRD96 112.0 60 45 20 3.73 0.6227
WVFGRD96 114.0 60 50 20 3.72 0.6207
WVFGRD96 116.0 60 50 20 3.72 0.6209
WVFGRD96 118.0 60 50 20 3.72 0.6194
The best solution is
WVFGRD96 104.0 55 45 15 3.74 0.6273
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 +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