The ANSS event ID is ak0181749ev4 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0181749ev4/executive.
2018/01/26 07:20:31 62.159 -147.818 38.5 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/01/26 07:20:31:0 62.16 -147.82 38.5 4.1 Alaska
Stations used:
AK.BMR AK.CUT AK.DHY AK.GHO AK.KLU AK.KNK AK.KTH AK.MCK
AK.RND AK.SAW AK.SCM AK.TRF AT.PMR TA.L26K TA.M24K TA.N25K
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 1.97e+22 dyne-cm
Mw = 4.13
Z = 54 km
Plane Strike Dip Rake
NP1 202 70 -105
NP2 60 25 -55
Principal Axes:
Axis Value Plunge Azimuth
T 1.97e+22 23 304
N 0.00e+00 14 208
P -1.97e+22 62 89
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.14e+21
Mxy -7.75e+21
Mxz 3.87e+21
Myy 7.23e+21
Myz -1.41e+22
Mzz -1.24e+22
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#### T ##########--------------------#
##### #########----------------------#
################-----------------------#
#################---------- ----------##
################----------- P ---------###
###############------------ ---------###
###############-----------------------####
#############------------------------###
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##########--------------------######
-########------------------#######
-######----------------#######
-----#-----------###########
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Global CMT Convention Moment Tensor:
R T P
-1.24e+22 3.87e+21 1.41e+22
3.87e+21 5.14e+21 7.75e+21
1.41e+22 7.75e+21 7.23e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180126072031/index.html
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STK = 60
DIP = 25
RAKE = -55
MW = 4.13
HS = 54.0
The NDK file is 20180126072031.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 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 175 45 90 3.47 0.3507
WVFGRD96 4.0 315 40 35 3.54 0.3224
WVFGRD96 6.0 90 35 -5 3.58 0.3950
WVFGRD96 8.0 95 35 5 3.64 0.4200
WVFGRD96 10.0 135 30 45 3.64 0.4265
WVFGRD96 12.0 195 65 -60 3.66 0.4533
WVFGRD96 14.0 195 65 -60 3.68 0.4685
WVFGRD96 16.0 200 65 -55 3.69 0.4794
WVFGRD96 18.0 200 70 -50 3.72 0.4885
WVFGRD96 20.0 200 70 -50 3.74 0.4958
WVFGRD96 22.0 200 65 -50 3.77 0.5003
WVFGRD96 24.0 200 60 -50 3.79 0.5056
WVFGRD96 26.0 200 60 -50 3.81 0.5136
WVFGRD96 28.0 200 60 -50 3.83 0.5171
WVFGRD96 30.0 205 65 -45 3.86 0.5141
WVFGRD96 32.0 80 50 -35 3.85 0.5244
WVFGRD96 34.0 75 45 -40 3.87 0.5440
WVFGRD96 36.0 75 45 -40 3.89 0.5616
WVFGRD96 38.0 70 40 -45 3.91 0.5754
WVFGRD96 40.0 65 35 -50 4.03 0.5882
WVFGRD96 42.0 70 35 -45 4.05 0.5958
WVFGRD96 44.0 65 30 -50 4.07 0.6122
WVFGRD96 46.0 65 30 -50 4.08 0.6260
WVFGRD96 48.0 65 30 -50 4.10 0.6376
WVFGRD96 50.0 60 25 -55 4.11 0.6479
WVFGRD96 52.0 60 25 -55 4.12 0.6536
WVFGRD96 54.0 60 25 -55 4.13 0.6545
WVFGRD96 56.0 65 25 -45 4.13 0.6524
WVFGRD96 58.0 65 25 -45 4.13 0.6485
WVFGRD96 60.0 65 25 -45 4.14 0.6434
WVFGRD96 62.0 35 15 -75 4.16 0.6386
WVFGRD96 64.0 35 15 -75 4.16 0.6341
WVFGRD96 66.0 40 15 -65 4.16 0.6273
WVFGRD96 68.0 40 15 -65 4.16 0.6220
WVFGRD96 70.0 35 15 -70 4.16 0.6131
WVFGRD96 72.0 45 15 -60 4.16 0.6005
WVFGRD96 74.0 35 15 -70 4.17 0.5936
WVFGRD96 76.0 35 15 -70 4.17 0.5835
WVFGRD96 78.0 35 15 -65 4.17 0.5732
WVFGRD96 80.0 30 15 -70 4.17 0.5652
WVFGRD96 82.0 30 15 -70 4.17 0.5565
WVFGRD96 84.0 30 15 -70 4.17 0.5483
WVFGRD96 86.0 30 15 -70 4.17 0.5399
WVFGRD96 88.0 25 15 -75 4.18 0.5309
WVFGRD96 90.0 25 15 -75 4.18 0.5218
WVFGRD96 92.0 25 15 -75 4.17 0.5115
WVFGRD96 94.0 25 15 -75 4.17 0.5023
WVFGRD96 96.0 25 15 -75 4.17 0.4929
WVFGRD96 98.0 55 20 -40 4.18 0.4841
The best solution is
WVFGRD96 54.0 60 25 -55 4.13 0.6545
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 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2
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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