The ANSS event ID is ak019g4iwye2 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019g4iwye2/executive.
2019/12/17 02:15:27 60.604 -152.294 104.0 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/12/17 02:15:27:0 60.60 -152.29 104.0 4.1 Alaska
Stations used:
AK.BRLK AK.CNP AK.CUT AK.HOM AK.KNK AK.L19K AK.O18K AK.O19K
AK.PPLA AK.Q19K AK.RC01 AK.SAW AK.SKN AK.SSN AV.ILSW
AV.STLK TA.M22K TA.P19K
Filtering commands used:
cut o DIST/3.6 -20 o DIST/3.6 +40
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 3.31e+22 dyne-cm
Mw = 4.28
Z = 130 km
Plane Strike Dip Rake
NP1 332 69 148
NP2 75 60 25
Principal Axes:
Axis Value Plunge Azimuth
T 3.31e+22 38 291
N 0.00e+00 52 122
P -3.31e+22 5 25
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.43e+22
Mxy -1.95e+22
Mxz 2.88e+21
Myy 1.22e+22
Myz -1.63e+22
Mzz 1.21e+22
--------------
###--------------- P -
########------------- ----
############------------------
###############-------------------
##################------------------
####################------------------
###### ##############-----------------
###### T ###############----------------
####### ################-------------###
###########################-----------####
############################--------######
############################-----#########
############################-###########
--#######################----###########
-------#########------------##########
----------------------------########
---------------------------#######
-------------------------#####
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---------------------#
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Global CMT Convention Moment Tensor:
R T P
1.21e+22 2.88e+21 1.63e+22
2.88e+21 -2.43e+22 1.95e+22
1.63e+22 1.95e+22 1.22e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191217021527/index.html
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STK = 75
DIP = 60
RAKE = 25
MW = 4.28
HS = 130.0
The NDK file is 20191217021527.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/3.6 -20 o DIST/3.6 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 325 60 -25 3.48 0.4108
WVFGRD96 4.0 325 85 25 3.56 0.4581
WVFGRD96 6.0 340 65 35 3.63 0.4988
WVFGRD96 8.0 345 60 40 3.70 0.5158
WVFGRD96 10.0 330 65 30 3.70 0.5205
WVFGRD96 12.0 330 70 30 3.71 0.5192
WVFGRD96 14.0 325 75 25 3.73 0.5135
WVFGRD96 16.0 140 90 -25 3.75 0.5059
WVFGRD96 18.0 250 75 25 3.78 0.5103
WVFGRD96 20.0 250 75 25 3.81 0.5263
WVFGRD96 22.0 250 75 20 3.84 0.5449
WVFGRD96 24.0 245 75 20 3.86 0.5616
WVFGRD96 26.0 245 75 20 3.88 0.5727
WVFGRD96 28.0 245 80 20 3.90 0.5776
WVFGRD96 30.0 245 80 20 3.92 0.5798
WVFGRD96 32.0 245 80 20 3.93 0.5777
WVFGRD96 34.0 245 80 20 3.95 0.5764
WVFGRD96 36.0 245 80 20 3.98 0.5755
WVFGRD96 38.0 240 70 15 4.00 0.5744
WVFGRD96 40.0 240 65 20 4.06 0.5747
WVFGRD96 42.0 240 65 20 4.09 0.5659
WVFGRD96 44.0 235 70 15 4.10 0.5553
WVFGRD96 46.0 235 65 10 4.11 0.5491
WVFGRD96 48.0 230 70 -10 4.13 0.5479
WVFGRD96 50.0 230 70 -15 4.14 0.5507
WVFGRD96 52.0 230 70 -15 4.15 0.5546
WVFGRD96 54.0 230 70 -15 4.17 0.5606
WVFGRD96 56.0 230 70 -15 4.18 0.5669
WVFGRD96 58.0 230 70 -20 4.19 0.5723
WVFGRD96 60.0 65 70 35 4.19 0.5835
WVFGRD96 62.0 65 70 35 4.20 0.5954
WVFGRD96 64.0 65 70 35 4.20 0.6036
WVFGRD96 66.0 65 65 35 4.20 0.6138
WVFGRD96 68.0 65 65 30 4.20 0.6225
WVFGRD96 70.0 65 65 30 4.20 0.6310
WVFGRD96 72.0 65 65 30 4.21 0.6383
WVFGRD96 74.0 65 65 30 4.21 0.6444
WVFGRD96 76.0 70 65 35 4.22 0.6493
WVFGRD96 78.0 70 65 35 4.22 0.6562
WVFGRD96 80.0 70 65 35 4.22 0.6602
WVFGRD96 82.0 70 65 35 4.23 0.6628
WVFGRD96 84.0 70 65 35 4.23 0.6668
WVFGRD96 86.0 70 65 30 4.22 0.6711
WVFGRD96 88.0 70 65 30 4.23 0.6740
WVFGRD96 90.0 70 65 30 4.23 0.6770
WVFGRD96 92.0 70 65 30 4.23 0.6793
WVFGRD96 94.0 70 65 30 4.23 0.6811
WVFGRD96 96.0 70 65 30 4.24 0.6830
WVFGRD96 98.0 70 65 30 4.24 0.6846
WVFGRD96 100.0 70 65 30 4.24 0.6856
WVFGRD96 102.0 70 65 30 4.24 0.6864
WVFGRD96 104.0 70 65 30 4.25 0.6869
WVFGRD96 106.0 70 65 30 4.25 0.6870
WVFGRD96 108.0 70 65 30 4.25 0.6869
WVFGRD96 110.0 70 65 30 4.25 0.6864
WVFGRD96 112.0 70 65 30 4.25 0.6853
WVFGRD96 114.0 75 60 30 4.26 0.6849
WVFGRD96 116.0 75 60 30 4.26 0.6853
WVFGRD96 118.0 75 60 25 4.26 0.6858
WVFGRD96 120.0 75 60 25 4.26 0.6857
WVFGRD96 122.0 75 60 25 4.27 0.6852
WVFGRD96 124.0 75 60 25 4.27 0.6852
WVFGRD96 126.0 75 60 25 4.27 0.6869
WVFGRD96 128.0 75 60 25 4.28 0.6881
WVFGRD96 130.0 75 60 25 4.28 0.6883
WVFGRD96 132.0 75 55 25 4.27 0.6874
WVFGRD96 134.0 75 55 25 4.28 0.6873
WVFGRD96 136.0 75 55 20 4.28 0.6869
WVFGRD96 138.0 75 55 20 4.28 0.6873
The best solution is
WVFGRD96 130.0 75 60 25 4.28 0.6883
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/3.6 -20 o DIST/3.6 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2
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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