The ANSS event ID is ak019lrs7iu and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019lrs7iu/executive.
2019/01/13 16:45:55 61.299 -150.065 44.8 5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/01/13 16:45:55:0 61.30 -150.07 44.8 5.0 Alaska
Stations used:
AK.BRLK AK.CAST AK.CNP AK.CUT AK.GHO AK.GLB AK.HDA AK.HOM
AK.KNK AK.KTH AK.PAX AK.PWL AK.RND AK.SAW AK.SCM AK.SCRK
AK.SKN AK.SWD AK.WRH AT.MENT AT.PMR AV.ILSW AV.STLK GM.AD09
GM.AD13 IU.COLA TA.I23K TA.J18K TA.K20K TA.M22K TA.M26K
TA.N19K TA.N25K TA.O22K TA.P19K
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 = 2.63e+23 dyne-cm
Mw = 4.88
Z = 46 km
Plane Strike Dip Rake
NP1 200 65 -60
NP2 326 38 -137
Principal Axes:
Axis Value Plunge Azimuth
T 2.63e+23 15 269
N 0.00e+00 27 6
P -2.63e+23 59 153
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.62e+22
Mxy 3.52e+22
Mxz 1.02e+23
Myy 2.31e+23
Myz -1.19e+23
Mzz -1.74e+23
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-####----------#######
###############-############
###############----###########
################-------###########
################----------##########
################-------------#########
################---------------#########
###############-----------------########
###############-------------------########
## ##########--------------------#######
## T #########---------------------#######
## #########----------------------######
############---------- ----------#####
############---------- P ----------#####
###########---------- ----------####
#########-----------------------####
########-----------------------###
#######---------------------##
######--------------------##
###-------------------
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Global CMT Convention Moment Tensor:
R T P
-1.74e+23 1.02e+23 1.19e+23
1.02e+23 -5.62e+22 -3.52e+22
1.19e+23 -3.52e+22 2.31e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190113164555/index.html
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STK = 200
DIP = 65
RAKE = -60
MW = 4.88
HS = 46.0
The NDK file is 20190113164555.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 10 45 90 4.08 0.1781
WVFGRD96 2.0 185 45 85 4.23 0.2464
WVFGRD96 3.0 180 45 80 4.28 0.2445
WVFGRD96 4.0 145 90 -30 4.22 0.2399
WVFGRD96 5.0 300 60 -20 4.26 0.2526
WVFGRD96 6.0 300 55 -20 4.29 0.2678
WVFGRD96 7.0 240 60 25 4.31 0.2856
WVFGRD96 8.0 240 60 35 4.38 0.3059
WVFGRD96 9.0 240 60 30 4.39 0.3235
WVFGRD96 10.0 240 65 30 4.41 0.3379
WVFGRD96 11.0 240 65 30 4.43 0.3496
WVFGRD96 12.0 240 65 30 4.45 0.3597
WVFGRD96 13.0 240 65 30 4.46 0.3673
WVFGRD96 14.0 240 65 30 4.47 0.3727
WVFGRD96 15.0 55 65 25 4.48 0.3813
WVFGRD96 16.0 60 65 30 4.50 0.3910
WVFGRD96 17.0 60 65 30 4.51 0.4009
WVFGRD96 18.0 45 75 35 4.52 0.4106
WVFGRD96 19.0 45 75 35 4.54 0.4202
WVFGRD96 20.0 45 75 35 4.55 0.4292
WVFGRD96 21.0 45 75 40 4.57 0.4374
WVFGRD96 22.0 45 75 40 4.58 0.4473
WVFGRD96 23.0 45 75 40 4.59 0.4563
WVFGRD96 24.0 45 75 40 4.60 0.4644
WVFGRD96 25.0 40 85 40 4.61 0.4718
WVFGRD96 26.0 40 85 40 4.62 0.4807
WVFGRD96 27.0 40 85 40 4.63 0.4892
WVFGRD96 28.0 215 85 -40 4.64 0.4988
WVFGRD96 29.0 35 90 40 4.65 0.5060
WVFGRD96 30.0 215 85 -45 4.66 0.5173
WVFGRD96 31.0 215 80 -45 4.66 0.5266
WVFGRD96 32.0 210 75 -45 4.67 0.5370
WVFGRD96 33.0 210 75 -45 4.68 0.5467
WVFGRD96 34.0 210 70 -45 4.69 0.5561
WVFGRD96 35.0 210 75 -45 4.70 0.5633
WVFGRD96 36.0 210 70 -45 4.71 0.5683
WVFGRD96 37.0 210 70 -45 4.72 0.5725
WVFGRD96 38.0 205 70 -50 4.73 0.5766
WVFGRD96 39.0 205 70 -45 4.74 0.5807
WVFGRD96 40.0 205 70 -60 4.83 0.5820
WVFGRD96 41.0 205 70 -60 4.84 0.5886
WVFGRD96 42.0 205 70 -60 4.85 0.5941
WVFGRD96 43.0 205 70 -60 4.86 0.5974
WVFGRD96 44.0 205 70 -60 4.87 0.5995
WVFGRD96 45.0 200 65 -60 4.87 0.6018
WVFGRD96 46.0 200 65 -60 4.88 0.6026
WVFGRD96 47.0 200 65 -60 4.89 0.6024
WVFGRD96 48.0 200 65 -60 4.89 0.6013
WVFGRD96 49.0 200 65 -60 4.90 0.5988
WVFGRD96 50.0 200 65 -60 4.90 0.5957
WVFGRD96 51.0 200 65 -65 4.91 0.5923
WVFGRD96 52.0 200 65 -65 4.92 0.5879
WVFGRD96 53.0 200 65 -65 4.92 0.5836
WVFGRD96 54.0 200 65 -65 4.92 0.5779
WVFGRD96 55.0 200 65 -65 4.92 0.5719
WVFGRD96 56.0 200 65 -65 4.93 0.5661
WVFGRD96 57.0 200 65 -65 4.93 0.5594
WVFGRD96 58.0 200 65 -65 4.93 0.5522
WVFGRD96 59.0 195 65 -65 4.93 0.5454
The best solution is
WVFGRD96 46.0 200 65 -60 4.88 0.6026
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