The ANSS event ID is ak018dtutpev and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018dtutpev/executive.
2018/10/28 03:46:39 59.621 -152.001 67.8 4.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/10/28 03:46:39:0 59.62 -152.00 67.8 4.2 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CNP AK.HOM AK.PWL AK.RC01 AV.ILSW
TA.N19K TA.O19K TA.O22K TA.P19K TA.Q19K TA.Q20K
Filtering commands used:
cut o DIST/3.5 -40 o DIST/3.5 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 5.01e+22 dyne-cm
Mw = 4.40
Z = 74 km
Plane Strike Dip Rake
NP1 40 85 40
NP2 306 50 173
Principal Axes:
Axis Value Plunge Azimuth
T 5.01e+22 31 271
N 0.00e+00 50 46
P -5.01e+22 23 166
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.00e+22
Mxy 9.40e+21
Mxz 1.78e+22
Myy 3.44e+22
Myz -2.65e+22
Mzz 5.59e+21
--------------
----------------------
---------------------------#
---------------------------###
###############-------------######
#####################-------########
#########################--###########
###########################-############
#########################-----##########
##### #################--------#########
##### T ###############-----------########
##### ##############-------------#######
####################----------------######
##################------------------####
################--------------------####
#############-----------------------##
###########------------------------#
########--------------------------
####------------- ----------
#--------------- P ---------
------------- ------
--------------
Global CMT Convention Moment Tensor:
R T P
5.59e+21 1.78e+22 2.65e+22
1.78e+22 -4.00e+22 -9.40e+21
2.65e+22 -9.40e+21 3.44e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181028034639/index.html
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STK = 40
DIP = 85
RAKE = 40
MW = 4.40
HS = 74.0
The NDK file is 20181028034639.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 +50 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 2.0 250 50 -80 3.59 0.2019
WVFGRD96 4.0 280 65 -35 3.59 0.2289
WVFGRD96 6.0 110 65 20 3.64 0.2607
WVFGRD96 8.0 115 60 25 3.73 0.2806
WVFGRD96 10.0 110 65 25 3.77 0.2866
WVFGRD96 12.0 55 80 -20 3.84 0.2914
WVFGRD96 14.0 55 85 -20 3.87 0.3004
WVFGRD96 16.0 55 85 -20 3.90 0.3045
WVFGRD96 18.0 235 85 15 3.93 0.3111
WVFGRD96 20.0 235 80 10 3.95 0.3194
WVFGRD96 22.0 235 80 10 3.98 0.3337
WVFGRD96 24.0 235 75 10 4.00 0.3455
WVFGRD96 26.0 225 70 5 4.02 0.3604
WVFGRD96 28.0 220 65 -5 4.03 0.3746
WVFGRD96 30.0 220 65 -5 4.05 0.3815
WVFGRD96 32.0 220 65 -5 4.06 0.3847
WVFGRD96 34.0 220 65 -5 4.08 0.3881
WVFGRD96 36.0 220 65 -5 4.09 0.3979
WVFGRD96 38.0 220 80 -15 4.11 0.4095
WVFGRD96 40.0 220 80 -25 4.17 0.4257
WVFGRD96 42.0 220 80 -25 4.20 0.4370
WVFGRD96 44.0 220 80 -25 4.22 0.4481
WVFGRD96 46.0 220 80 -30 4.24 0.4600
WVFGRD96 48.0 220 85 -35 4.27 0.4748
WVFGRD96 50.0 220 85 -35 4.28 0.4890
WVFGRD96 52.0 40 90 35 4.30 0.5039
WVFGRD96 54.0 220 90 -35 4.31 0.5175
WVFGRD96 56.0 40 90 35 4.32 0.5305
WVFGRD96 58.0 220 90 -35 4.33 0.5418
WVFGRD96 60.0 40 90 35 4.34 0.5516
WVFGRD96 62.0 40 90 35 4.35 0.5598
WVFGRD96 64.0 220 90 -35 4.36 0.5656
WVFGRD96 66.0 40 85 40 4.38 0.5757
WVFGRD96 68.0 220 90 -35 4.37 0.5734
WVFGRD96 70.0 40 85 40 4.39 0.5843
WVFGRD96 72.0 40 85 40 4.40 0.5859
WVFGRD96 74.0 40 85 40 4.40 0.5867
WVFGRD96 76.0 40 85 40 4.41 0.5861
WVFGRD96 78.0 40 85 40 4.41 0.5857
WVFGRD96 80.0 40 85 35 4.41 0.5826
WVFGRD96 82.0 40 85 35 4.41 0.5797
WVFGRD96 84.0 40 85 35 4.41 0.5774
WVFGRD96 86.0 40 85 35 4.42 0.5741
WVFGRD96 88.0 40 85 35 4.42 0.5691
WVFGRD96 90.0 40 85 35 4.42 0.5640
WVFGRD96 92.0 40 85 35 4.42 0.5602
WVFGRD96 94.0 40 85 35 4.42 0.5556
WVFGRD96 96.0 220 90 -30 4.41 0.5461
WVFGRD96 98.0 220 90 -30 4.41 0.5431
The best solution is
WVFGRD96 74.0 40 85 40 4.40 0.5867
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 +50 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