The ANSS event ID is ak0198eyu6cj and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0198eyu6cj/executive.
2019/07/02 19:30:01 59.226 -152.091 52.0 4.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/07/02 19:30:01:0 59.23 -152.09 52.0 4.6 Alaska
Stations used:
AK.BRLK AK.CNP AK.HOM AV.ILSW AV.RED AV.SPU TA.O18K TA.O19K
TA.P18K TA.P19K TA.Q19K TA.Q20K
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 = 1.07e+23 dyne-cm
Mw = 4.62
Z = 62 km
Plane Strike Dip Rake
NP1 230 75 -85
NP2 31 16 -108
Principal Axes:
Axis Value Plunge Azimuth
T 1.07e+23 30 316
N 0.00e+00 5 49
P -1.07e+23 60 147
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.24e+22
Mxy -2.78e+22
Mxz 7.24e+22
Myy 3.09e+22
Myz -5.76e+22
Mzz -5.34e+22
##############
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###########################-
#############################-
##### #####################----#
###### T #################---------#
####### ##############------------##
######################----------------##
####################------------------##
##################---------------------###
################-----------------------###
##############-------------------------###
############---------------------------###
##########------------- -----------###
########--------------- P -----------###
######---------------- ----------###
####-----------------------------###
##----------------------------####
--------------------------####
-----------------------#####
-----------------#####
##############
Global CMT Convention Moment Tensor:
R T P
-5.34e+22 7.24e+22 5.76e+22
7.24e+22 2.24e+22 2.78e+22
5.76e+22 2.78e+22 3.09e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190702193001/index.html
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STK = 230
DIP = 75
RAKE = -85
MW = 4.62
HS = 62.0
The NDK file is 20190702193001.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 2.0 220 50 90 3.97 0.2170
WVFGRD96 4.0 305 35 -15 3.97 0.2251
WVFGRD96 6.0 305 40 -15 4.00 0.2616
WVFGRD96 8.0 230 65 50 4.08 0.2823
WVFGRD96 10.0 230 65 45 4.12 0.3196
WVFGRD96 12.0 230 65 45 4.15 0.3393
WVFGRD96 14.0 230 65 40 4.17 0.3471
WVFGRD96 16.0 230 65 40 4.19 0.3502
WVFGRD96 18.0 220 70 -35 4.20 0.3475
WVFGRD96 20.0 225 75 -40 4.23 0.3492
WVFGRD96 22.0 225 85 -45 4.25 0.3509
WVFGRD96 24.0 50 85 45 4.27 0.3612
WVFGRD96 26.0 225 85 -45 4.29 0.3704
WVFGRD96 28.0 225 90 -50 4.30 0.3766
WVFGRD96 30.0 50 85 55 4.32 0.3855
WVFGRD96 32.0 50 85 60 4.34 0.3920
WVFGRD96 34.0 50 85 60 4.35 0.3972
WVFGRD96 36.0 230 90 -65 4.37 0.4034
WVFGRD96 38.0 55 90 65 4.37 0.4125
WVFGRD96 40.0 235 80 -75 4.52 0.4292
WVFGRD96 42.0 235 80 -75 4.54 0.4482
WVFGRD96 44.0 230 75 -75 4.55 0.4649
WVFGRD96 46.0 230 75 -80 4.57 0.4838
WVFGRD96 48.0 230 75 -80 4.58 0.4995
WVFGRD96 50.0 230 75 -80 4.59 0.5129
WVFGRD96 52.0 230 75 -80 4.59 0.5246
WVFGRD96 54.0 230 75 -80 4.60 0.5328
WVFGRD96 56.0 230 75 -80 4.60 0.5395
WVFGRD96 58.0 230 75 -85 4.62 0.5428
WVFGRD96 60.0 230 75 -85 4.62 0.5452
WVFGRD96 62.0 230 75 -85 4.62 0.5460
WVFGRD96 64.0 230 75 -85 4.63 0.5442
WVFGRD96 66.0 235 75 -85 4.63 0.5421
WVFGRD96 68.0 225 70 -85 4.63 0.5394
WVFGRD96 70.0 225 70 -85 4.63 0.5361
WVFGRD96 72.0 225 70 -85 4.63 0.5332
WVFGRD96 74.0 225 70 -85 4.63 0.5287
WVFGRD96 76.0 35 20 -100 4.64 0.5238
WVFGRD96 78.0 225 70 -85 4.64 0.5180
WVFGRD96 80.0 235 75 -95 4.67 0.5148
WVFGRD96 82.0 60 15 -80 4.66 0.5080
WVFGRD96 84.0 230 75 -95 4.66 0.5051
WVFGRD96 86.0 70 15 -75 4.67 0.4989
WVFGRD96 88.0 60 20 -75 4.66 0.4931
WVFGRD96 90.0 55 20 -75 4.65 0.4900
WVFGRD96 92.0 55 20 -75 4.66 0.4851
WVFGRD96 94.0 55 20 -75 4.66 0.4817
WVFGRD96 96.0 55 20 -75 4.66 0.4769
WVFGRD96 98.0 55 20 -75 4.66 0.4719
The best solution is
WVFGRD96 62.0 230 75 -85 4.62 0.5460
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