The ANSS event ID is ak019d059k9u and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019d059k9u/executive.
2019/10/10 07:01:45 62.916 -143.681 7.3 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/10/10 07:01:45:0 62.92 -143.68 7.3 4.0 Alaska
Stations used:
AK.BCP AK.BMR AK.BPAW AK.BWN AK.CCB AK.CTG AK.DHY AK.DIV
AK.DOT AK.FID AK.GLB AK.GLI AK.GRNC AK.H24K AK.HDA AK.HIN
AK.I23K AK.I26K AK.J25K AK.J26L AK.KLU AK.KNK AK.KTH
AK.L26K AK.LOGN AK.M26K AK.M27K AK.MCAR AK.MCK AK.MESA
AK.NEA2 AK.PAX AK.PIN AK.PPD AK.PPLA AK.RIDG AK.RND AK.SAW
AK.SCM AK.SCRK AK.SKN AK.SSN AK.TABL AK.TGL AK.TRF AK.VRDI
AK.WAX AK.WRH AK.YAH AT.PMR IM.IL31 IU.COLA NY.MAYO TA.H27K
TA.I28M TA.I29M TA.I30M TA.J29N TA.J30M TA.K24K TA.K29M
TA.L29M TA.M22K TA.M29M TA.M30M TA.N25K TA.N30M TA.N31M
TA.O28M TA.O29M TA.POKR US.EGAK
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 = 6.31e+21 dyne-cm
Mw = 3.80
Z = 17 km
Plane Strike Dip Rake
NP1 164 85 165
NP2 255 75 5
Principal Axes:
Axis Value Plunge Azimuth
T 6.31e+21 14 118
N 0.00e+00 74 326
P -6.31e+21 7 210
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.29e+21
Mxy -5.19e+21
Mxz -3.90e+19
Myy 3.02e+21
Myz 1.69e+21
Mzz 2.75e+20
#-------------
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#################-----------------------
#################------#################
#################--#######################
############-------#######################
########------------######################
####-----------------#####################
#-------------------####################
---------------------############# ###
--------------------############# T ##
--------------------############ #
--------------------##############
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--- ------------##########
P -------------######
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Global CMT Convention Moment Tensor:
R T P
2.75e+20 -3.90e+19 -1.69e+21
-3.90e+19 -3.29e+21 5.19e+21
-1.69e+21 5.19e+21 3.02e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191010070145/index.html
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STK = 255
DIP = 75
RAKE = 5
MW = 3.80
HS = 17.0
The NDK file is 20191010070145.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 1.0 165 90 0 3.27 0.2817
WVFGRD96 2.0 70 85 -5 3.39 0.3784
WVFGRD96 3.0 70 85 5 3.45 0.4192
WVFGRD96 4.0 70 90 -20 3.50 0.4510
WVFGRD96 5.0 70 90 -20 3.54 0.4820
WVFGRD96 6.0 255 75 20 3.58 0.5165
WVFGRD96 7.0 255 70 15 3.61 0.5516
WVFGRD96 8.0 255 65 15 3.66 0.5890
WVFGRD96 9.0 255 65 15 3.68 0.6192
WVFGRD96 10.0 255 65 10 3.70 0.6447
WVFGRD96 11.0 255 65 10 3.72 0.6654
WVFGRD96 12.0 255 70 10 3.74 0.6808
WVFGRD96 13.0 255 70 10 3.75 0.6936
WVFGRD96 14.0 255 70 10 3.77 0.7024
WVFGRD96 15.0 255 70 5 3.78 0.7078
WVFGRD96 16.0 255 70 5 3.79 0.7106
WVFGRD96 17.0 255 75 5 3.80 0.7108
WVFGRD96 18.0 255 75 5 3.81 0.7101
WVFGRD96 19.0 255 75 5 3.83 0.7076
WVFGRD96 20.0 255 75 5 3.84 0.7031
WVFGRD96 21.0 255 75 5 3.85 0.6981
WVFGRD96 22.0 255 75 5 3.85 0.6917
WVFGRD96 23.0 255 75 5 3.86 0.6853
WVFGRD96 24.0 255 75 5 3.87 0.6780
WVFGRD96 25.0 255 75 5 3.88 0.6703
WVFGRD96 26.0 255 75 5 3.88 0.6623
WVFGRD96 27.0 250 75 0 3.89 0.6539
WVFGRD96 28.0 250 75 0 3.89 0.6455
WVFGRD96 29.0 250 75 -5 3.90 0.6375
The best solution is
WVFGRD96 17.0 255 75 5 3.80 0.7108
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