Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.
The ANSS event ID is ak0198gbkbu4 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0198gbkbu4/executive.
2019/07/03 01:13:20 65.818 -166.191 23.0 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/07/03 01:13:20:0 65.82 -166.19 23.0 4.1 Alaska
Stations used:
AK.ANM AK.GAMB AK.RDOG AK.TNA TA.C18K TA.E18K TA.E19K
TA.F15K TA.F17K TA.G16K TA.G18K TA.G19K TA.H17K TA.H18K
TA.I17K TA.J14K TA.J16K TA.J17K TA.K13K
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 = 9.23e+21 dyne-cm
Mw = 3.91
Z = 15 km
Plane Strike Dip Rake
NP1 100 65 -80
NP2 257 27 -110
Principal Axes:
Axis Value Plunge Azimuth
T 9.23e+21 19 183
N 0.00e+00 9 276
P -9.23e+21 68 30
Moment Tensor: (dyne-cm)
Component Value
Mxx 7.25e+21
Mxy -1.74e+20
Mxz -5.63e+21
Myy -2.87e+20
Myz -1.68e+21
Mzz -6.96e+21
##############
######################
############################
########-----------------#####
######-------------------------###
#####-----------------------------##
####---------------------------------#
####----------------- ---------------#
##------------------- P ----------------
-#-------------------- -----------------
--#---------------------------------------
-#####------------------------------------
-########---------------------------------
############---------------------------#
####################-------------#######
######################################
####################################
##################################
##############################
############ #############
######### T ##########
##### ######
Global CMT Convention Moment Tensor:
R T P
-6.96e+21 -5.63e+21 1.68e+21
-5.63e+21 7.25e+21 1.74e+20
1.68e+21 1.74e+20 -2.87e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190703011320/index.html
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STK = 100
DIP = 65
RAKE = -80
MW = 3.91
HS = 15.0
The NDK file is 20190703011320.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: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
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 1.0 120 90 0 3.46 0.3595
WVFGRD96 2.0 120 85 5 3.55 0.3934
WVFGRD96 3.0 305 90 -25 3.60 0.3729
WVFGRD96 4.0 300 80 -40 3.66 0.3780
WVFGRD96 5.0 120 75 -55 3.68 0.4243
WVFGRD96 6.0 120 80 -65 3.69 0.4744
WVFGRD96 7.0 115 75 -65 3.71 0.5230
WVFGRD96 8.0 115 75 -70 3.79 0.5636
WVFGRD96 9.0 110 70 -70 3.82 0.6130
WVFGRD96 10.0 105 65 -75 3.85 0.6536
WVFGRD96 11.0 105 65 -75 3.86 0.6858
WVFGRD96 12.0 100 65 -80 3.88 0.7085
WVFGRD96 13.0 100 65 -80 3.89 0.7221
WVFGRD96 14.0 100 65 -80 3.90 0.7284
WVFGRD96 15.0 100 65 -80 3.91 0.7284
WVFGRD96 16.0 100 65 -80 3.92 0.7231
WVFGRD96 17.0 105 60 -75 3.91 0.7137
WVFGRD96 18.0 105 65 -75 3.92 0.7013
WVFGRD96 19.0 105 65 -80 3.93 0.6864
WVFGRD96 20.0 105 65 -80 3.93 0.6694
WVFGRD96 21.0 105 65 -80 3.95 0.6535
WVFGRD96 22.0 105 65 -80 3.95 0.6334
WVFGRD96 23.0 105 65 -80 3.96 0.6116
WVFGRD96 24.0 105 65 -80 3.96 0.5891
WVFGRD96 25.0 285 20 -85 3.98 0.5663
WVFGRD96 26.0 285 20 -85 3.99 0.5456
WVFGRD96 27.0 290 20 -80 3.99 0.5240
WVFGRD96 28.0 290 20 -80 4.00 0.5018
WVFGRD96 29.0 290 20 -80 4.00 0.4790
WVFGRD96 30.0 300 75 -65 3.96 0.4627
WVFGRD96 31.0 295 75 -70 3.98 0.4504
WVFGRD96 32.0 295 65 -60 3.98 0.4411
WVFGRD96 33.0 295 65 -60 3.99 0.4362
WVFGRD96 34.0 295 65 -60 3.99 0.4305
WVFGRD96 35.0 295 65 -60 4.00 0.4241
WVFGRD96 36.0 290 65 -60 4.02 0.4179
WVFGRD96 37.0 290 65 -60 4.03 0.4115
WVFGRD96 38.0 290 65 -60 4.03 0.4056
WVFGRD96 39.0 290 65 -60 4.04 0.3993
WVFGRD96 40.0 285 65 -65 4.17 0.3988
WVFGRD96 41.0 285 65 -65 4.18 0.3898
WVFGRD96 42.0 285 65 -65 4.19 0.3810
WVFGRD96 43.0 205 40 30 4.18 0.3722
WVFGRD96 44.0 210 40 35 4.19 0.3672
WVFGRD96 45.0 230 45 40 4.18 0.3638
WVFGRD96 46.0 230 45 40 4.18 0.3611
WVFGRD96 47.0 230 45 35 4.17 0.3578
WVFGRD96 48.0 230 50 40 4.18 0.3548
WVFGRD96 49.0 230 50 40 4.19 0.3517
WVFGRD96 50.0 230 50 40 4.19 0.3478
WVFGRD96 51.0 230 50 40 4.19 0.3439
WVFGRD96 52.0 230 50 40 4.20 0.3396
WVFGRD96 53.0 230 50 40 4.20 0.3352
WVFGRD96 54.0 230 50 40 4.20 0.3301
WVFGRD96 55.0 230 50 40 4.21 0.3249
WVFGRD96 56.0 100 55 -85 4.23 0.3252
WVFGRD96 57.0 100 55 -85 4.23 0.3273
WVFGRD96 58.0 100 55 -85 4.23 0.3300
WVFGRD96 59.0 100 55 -85 4.23 0.3317
The best solution is
WVFGRD96 15.0 100 65 -80 3.91 0.7284
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