The ANSS event ID is ak0177jcjdxu and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0177jcjdxu/executive.
2017/06/13 07:39:36 63.868 -148.257 101.2 3.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/06/13 07:39:36:0 63.87 -148.26 101.2 3.8 Alaska
Stations used:
AK.BPAW AK.BWN AK.CAST AK.CCB AK.DHY AK.HDA AK.KNK AK.KTH
AK.MDM AK.MLY AK.NEA2 AK.PPD AK.PPLA AK.RC01 AK.RIDG AK.RND
AK.SAW AK.SCM AK.SCRK AK.TRF AT.PMR IM.IL31 IU.COLA TA.H23K
TA.H24K TA.I21K TA.I23K TA.J20K TA.J25K TA.J26L TA.K20K
TA.L26K TA.L27K TA.M26K
Filtering commands used:
cut o DIST/3.5 -40 o DIST/3.5 +40
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.10e+22 dyne-cm
Mw = 3.96
Z = 120 km
Plane Strike Dip Rake
NP1 352 86 150
NP2 85 60 5
Principal Axes:
Axis Value Plunge Azimuth
T 1.10e+22 24 305
N 0.00e+00 60 165
P -1.10e+22 17 43
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.46e+21
Mxy -9.24e+21
Mxz -3.27e+14
Myy 1.64e+21
Myz -5.48e+21
Mzz 8.28e+20
#####---------
#########-------------
############----------------
##############------------ -
################------------ P ---
### ############----------- ----
#### T ############-------------------
##### ############--------------------
####################--------------------
######################--------------------
######################-------------------#
######################-----------------###
-#####################---------------#####
---###################----------########
--------##############-----#############
---------------------#################
--------------------################
-------------------###############
------------------############
-----------------###########
--------------########
----------####
Global CMT Convention Moment Tensor:
R T P
8.28e+20 -3.27e+14 5.48e+21
-3.27e+14 -2.46e+21 9.24e+21
5.48e+21 9.24e+21 1.64e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170613073936/index.html
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STK = 85
DIP = 60
RAKE = 5
MW = 3.96
HS = 120.0
The NDK file is 20170613073936.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 +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 2.0 310 45 -65 3.02 0.2250
WVFGRD96 4.0 330 80 -35 3.04 0.2411
WVFGRD96 6.0 155 80 30 3.10 0.2715
WVFGRD96 8.0 155 80 30 3.18 0.2956
WVFGRD96 10.0 335 75 30 3.23 0.3080
WVFGRD96 12.0 335 75 30 3.27 0.3132
WVFGRD96 14.0 335 75 30 3.30 0.3108
WVFGRD96 16.0 310 85 25 3.34 0.3029
WVFGRD96 18.0 100 70 25 3.37 0.3046
WVFGRD96 20.0 100 70 25 3.40 0.3239
WVFGRD96 22.0 95 70 20 3.43 0.3405
WVFGRD96 24.0 95 70 20 3.45 0.3578
WVFGRD96 26.0 85 70 5 3.47 0.3753
WVFGRD96 28.0 85 70 5 3.48 0.3864
WVFGRD96 30.0 85 90 15 3.49 0.3934
WVFGRD96 32.0 90 75 20 3.51 0.4006
WVFGRD96 34.0 90 75 20 3.53 0.4038
WVFGRD96 36.0 90 70 20 3.55 0.4064
WVFGRD96 38.0 90 75 15 3.58 0.4103
WVFGRD96 40.0 85 50 0 3.67 0.4210
WVFGRD96 42.0 80 60 -15 3.68 0.4307
WVFGRD96 44.0 85 55 0 3.71 0.4410
WVFGRD96 46.0 85 55 0 3.73 0.4516
WVFGRD96 48.0 80 55 -10 3.75 0.4634
WVFGRD96 50.0 80 50 -10 3.77 0.4775
WVFGRD96 52.0 80 50 -10 3.79 0.4923
WVFGRD96 54.0 80 50 -10 3.80 0.5065
WVFGRD96 56.0 80 50 -10 3.81 0.5201
WVFGRD96 58.0 80 50 -5 3.81 0.5335
WVFGRD96 60.0 80 50 -5 3.82 0.5475
WVFGRD96 62.0 80 50 -5 3.83 0.5625
WVFGRD96 64.0 80 50 -5 3.84 0.5769
WVFGRD96 66.0 80 50 -5 3.84 0.5889
WVFGRD96 68.0 80 50 -5 3.85 0.6031
WVFGRD96 70.0 80 50 -5 3.86 0.6145
WVFGRD96 72.0 80 50 -5 3.86 0.6255
WVFGRD96 74.0 80 50 -5 3.87 0.6351
WVFGRD96 76.0 80 50 -5 3.87 0.6447
WVFGRD96 78.0 85 55 0 3.87 0.6519
WVFGRD96 80.0 85 55 0 3.88 0.6601
WVFGRD96 82.0 85 55 0 3.88 0.6682
WVFGRD96 84.0 85 55 0 3.89 0.6741
WVFGRD96 86.0 85 55 5 3.89 0.6801
WVFGRD96 88.0 85 55 5 3.89 0.6859
WVFGRD96 90.0 85 55 5 3.90 0.6918
WVFGRD96 92.0 85 60 5 3.90 0.6981
WVFGRD96 94.0 85 60 5 3.90 0.7038
WVFGRD96 96.0 85 60 5 3.91 0.7107
WVFGRD96 98.0 85 60 5 3.91 0.7166
WVFGRD96 100.0 85 60 5 3.92 0.7218
WVFGRD96 102.0 85 60 5 3.92 0.7259
WVFGRD96 104.0 85 60 5 3.92 0.7297
WVFGRD96 106.0 85 60 5 3.93 0.7351
WVFGRD96 108.0 85 60 5 3.93 0.7389
WVFGRD96 110.0 85 60 5 3.94 0.7418
WVFGRD96 112.0 85 60 5 3.94 0.7438
WVFGRD96 114.0 85 60 5 3.94 0.7462
WVFGRD96 116.0 85 60 5 3.95 0.7479
WVFGRD96 118.0 85 60 5 3.95 0.7499
WVFGRD96 120.0 85 60 5 3.96 0.7500
WVFGRD96 122.0 85 60 5 3.96 0.7490
WVFGRD96 124.0 85 60 5 3.96 0.7491
WVFGRD96 126.0 85 60 5 3.97 0.7494
WVFGRD96 128.0 85 65 5 3.97 0.7481
The best solution is
WVFGRD96 120.0 85 60 5 3.96 0.7500
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 +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