The ANSS event ID is ak018da4zt40 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018da4zt40/executive.
2018/10/16 11:22:12 61.536 -146.398 30.1 4.3 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/10/16 11:22:12:0 61.54 -146.40 30.1 4.3 Alaska
Stations used:
AK.BMR AK.DHY AK.DIV AK.EYAK AK.FID AK.GHO AK.GLI AK.KLU
AK.KNK AK.MCAR AK.RC01 AK.SAW AK.SCM AK.VRDI AT.PMR TA.M22K
TA.M24K TA.N25K
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 3.43e+22 dyne-cm
Mw = 4.29
Z = 42 km
Plane Strike Dip Rake
NP1 240 60 -75
NP2 32 33 -114
Principal Axes:
Axis Value Plunge Azimuth
T 3.43e+22 14 319
N 0.00e+00 13 52
P -3.43e+22 71 184
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.49e+22
Mxy -1.63e+22
Mxz 1.66e+22
Myy 1.38e+22
Myz -4.44e+21
Mzz -2.87e+22
##############
######################
# ######################--
## T #######################--
#### ########################---
#######################----------###
###################---------------####
################-------------------#####
#############----------------------#####
############------------------------######
##########--------------------------######
########----------------------------######
######------------- -------------#######
####-------------- P ------------#######
###--------------- -----------########
##----------------------------########
----------------------------########
-------------------------#########
---------------------#########
-----------------###########
---------#############
##############
Global CMT Convention Moment Tensor:
R T P
-2.87e+22 1.66e+22 4.44e+21
1.66e+22 1.49e+22 1.63e+22
4.44e+21 1.63e+22 1.38e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181016112212/index.html
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STK = 240
DIP = 60
RAKE = -75
MW = 4.29
HS = 42.0
The NDK file is 20181016112212.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 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 115 45 90 3.74 0.2934
WVFGRD96 4.0 60 65 -35 3.73 0.3074
WVFGRD96 6.0 300 20 65 3.81 0.3511
WVFGRD96 8.0 295 20 60 3.90 0.3944
WVFGRD96 10.0 260 40 -50 3.87 0.4261
WVFGRD96 12.0 265 50 -50 3.90 0.4504
WVFGRD96 14.0 265 55 -50 3.92 0.4724
WVFGRD96 16.0 265 55 -50 3.94 0.4851
WVFGRD96 18.0 270 60 -50 3.96 0.4890
WVFGRD96 20.0 105 60 50 3.98 0.4971
WVFGRD96 22.0 110 60 55 4.01 0.5044
WVFGRD96 24.0 110 60 55 4.03 0.5095
WVFGRD96 26.0 105 65 50 4.03 0.5086
WVFGRD96 28.0 250 60 -60 4.06 0.5165
WVFGRD96 30.0 245 60 -60 4.09 0.5314
WVFGRD96 32.0 245 60 -65 4.11 0.5436
WVFGRD96 34.0 240 60 -70 4.14 0.5726
WVFGRD96 36.0 240 60 -70 4.15 0.5909
WVFGRD96 38.0 235 55 -75 4.17 0.5962
WVFGRD96 40.0 235 60 -80 4.28 0.6148
WVFGRD96 42.0 240 60 -75 4.29 0.6162
WVFGRD96 44.0 240 60 -75 4.30 0.6124
WVFGRD96 46.0 240 60 -75 4.31 0.6056
WVFGRD96 48.0 240 55 -75 4.32 0.5974
WVFGRD96 50.0 240 55 -75 4.32 0.5868
WVFGRD96 52.0 240 55 -70 4.33 0.5741
WVFGRD96 54.0 245 55 -70 4.33 0.5622
WVFGRD96 56.0 245 55 -70 4.33 0.5498
WVFGRD96 58.0 245 55 -70 4.33 0.5381
WVFGRD96 60.0 245 55 -70 4.34 0.5253
WVFGRD96 62.0 250 55 -65 4.34 0.5147
WVFGRD96 64.0 250 50 -65 4.34 0.5057
WVFGRD96 66.0 250 50 -65 4.35 0.5003
WVFGRD96 68.0 250 50 -65 4.35 0.4951
WVFGRD96 70.0 250 50 -65 4.35 0.4900
WVFGRD96 72.0 250 50 -65 4.36 0.4844
WVFGRD96 74.0 250 50 -65 4.36 0.4790
WVFGRD96 76.0 255 50 -60 4.36 0.4728
WVFGRD96 78.0 255 50 -60 4.37 0.4675
WVFGRD96 80.0 255 50 -60 4.37 0.4613
WVFGRD96 82.0 255 50 -60 4.37 0.4551
WVFGRD96 84.0 255 50 -60 4.38 0.4492
WVFGRD96 86.0 250 50 -65 4.38 0.4437
WVFGRD96 88.0 250 50 -65 4.38 0.4378
WVFGRD96 90.0 250 50 -65 4.38 0.4308
WVFGRD96 92.0 250 50 -65 4.38 0.4206
WVFGRD96 94.0 250 50 -65 4.38 0.4071
WVFGRD96 96.0 250 50 -65 4.38 0.3930
WVFGRD96 98.0 240 45 -80 4.37 0.3801
The best solution is
WVFGRD96 42.0 240 60 -75 4.29 0.6162
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 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 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