The ANSS event ID is ak0196h8mxap and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0196h8mxap/executive.
2019/05/21 03:12:55 61.466 -149.638 46.9 3.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/05/21 03:12:55:0 61.47 -149.64 46.9 3.6 Alaska
Stations used:
AK.FID AK.FIRE AK.GHO AK.PPLA AK.PWL AK.RC01 AK.SAW AK.SCM
AK.SKN AK.SSN AT.PMR AV.STLK TA.M24K
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 0.10 n 3
Best Fitting Double Couple
Mo = 6.53e+21 dyne-cm
Mw = 3.81
Z = 66 km
Plane Strike Dip Rake
NP1 140 90 -155
NP2 50 65 0
Principal Axes:
Axis Value Plunge Azimuth
T 6.53e+21 17 272
N 0.00e+00 65 140
P -6.53e+21 17 8
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.83e+21
Mxy -1.03e+21
Mxz -1.77e+21
Myy 5.83e+21
Myz -2.11e+21
Mzz 0.00e+00
-------- ---
------------ P -------
#-------------- ----------
###---------------------------
#######--------------------------#
#########------------------------###
############---------------------#####
###############-------------------######
################-----------------#######
## ##############-------------##########
## T ###############-----------###########
## #################-------#############
#######################----###############
########################################
######################---###############
##################--------############
##############-------------#########
#########------------------#######
---------------------------###
---------------------------#
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
0.00e+00 -1.77e+21 2.11e+21
-1.77e+21 -5.83e+21 1.03e+21
2.11e+21 1.03e+21 5.83e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190521031255/index.html
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STK = 50
DIP = 65
RAKE = 0
MW = 3.81
HS = 66.0
The NDK file is 20190521031255.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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 310 85 5 2.79 0.1849
WVFGRD96 2.0 130 90 -5 2.94 0.2537
WVFGRD96 3.0 130 90 -25 3.02 0.2736
WVFGRD96 4.0 315 80 25 3.07 0.2914
WVFGRD96 5.0 310 90 25 3.11 0.3039
WVFGRD96 6.0 130 85 -25 3.14 0.3163
WVFGRD96 7.0 130 85 -20 3.18 0.3288
WVFGRD96 8.0 310 90 25 3.23 0.3406
WVFGRD96 9.0 310 90 25 3.25 0.3483
WVFGRD96 10.0 310 90 20 3.27 0.3515
WVFGRD96 11.0 130 90 -20 3.29 0.3520
WVFGRD96 12.0 130 90 -20 3.31 0.3491
WVFGRD96 13.0 125 80 -20 3.33 0.3453
WVFGRD96 14.0 220 75 -10 3.34 0.3416
WVFGRD96 15.0 220 75 -10 3.35 0.3438
WVFGRD96 16.0 220 75 -10 3.37 0.3449
WVFGRD96 17.0 220 80 -10 3.38 0.3459
WVFGRD96 18.0 235 80 -10 3.38 0.3466
WVFGRD96 19.0 235 80 -10 3.39 0.3478
WVFGRD96 20.0 235 80 -10 3.40 0.3495
WVFGRD96 21.0 60 75 20 3.43 0.3508
WVFGRD96 22.0 60 75 20 3.44 0.3579
WVFGRD96 23.0 60 75 20 3.45 0.3640
WVFGRD96 24.0 60 75 15 3.45 0.3713
WVFGRD96 25.0 60 75 15 3.46 0.3775
WVFGRD96 26.0 60 70 15 3.47 0.3861
WVFGRD96 27.0 55 75 10 3.47 0.3948
WVFGRD96 28.0 55 75 10 3.48 0.4035
WVFGRD96 29.0 55 75 5 3.49 0.4120
WVFGRD96 30.0 55 70 5 3.50 0.4208
WVFGRD96 31.0 55 70 0 3.51 0.4286
WVFGRD96 32.0 55 70 0 3.52 0.4349
WVFGRD96 33.0 55 70 0 3.52 0.4401
WVFGRD96 34.0 55 70 -5 3.53 0.4447
WVFGRD96 35.0 55 70 -5 3.54 0.4475
WVFGRD96 36.0 55 75 0 3.55 0.4492
WVFGRD96 37.0 55 75 0 3.56 0.4506
WVFGRD96 38.0 55 75 5 3.58 0.4562
WVFGRD96 39.0 55 75 5 3.60 0.4631
WVFGRD96 40.0 55 65 10 3.64 0.4746
WVFGRD96 41.0 55 65 10 3.66 0.4762
WVFGRD96 42.0 55 65 5 3.67 0.4777
WVFGRD96 43.0 55 70 5 3.67 0.4786
WVFGRD96 44.0 55 65 5 3.69 0.4793
WVFGRD96 45.0 55 65 5 3.70 0.4803
WVFGRD96 46.0 55 65 5 3.70 0.4823
WVFGRD96 47.0 55 65 5 3.71 0.4841
WVFGRD96 48.0 55 65 5 3.72 0.4856
WVFGRD96 49.0 55 65 5 3.73 0.4882
WVFGRD96 50.0 55 65 5 3.73 0.4884
WVFGRD96 51.0 55 65 5 3.74 0.4918
WVFGRD96 52.0 55 65 5 3.74 0.4932
WVFGRD96 53.0 55 65 5 3.75 0.4949
WVFGRD96 54.0 55 65 5 3.76 0.4954
WVFGRD96 55.0 55 65 5 3.76 0.4965
WVFGRD96 56.0 55 65 5 3.77 0.4978
WVFGRD96 57.0 55 65 5 3.77 0.4976
WVFGRD96 58.0 50 60 0 3.79 0.5004
WVFGRD96 59.0 50 60 0 3.79 0.5005
WVFGRD96 60.0 55 70 0 3.78 0.4991
WVFGRD96 61.0 50 60 0 3.80 0.5010
WVFGRD96 62.0 50 65 0 3.80 0.5011
WVFGRD96 63.0 50 65 0 3.80 0.5013
WVFGRD96 64.0 50 65 0 3.80 0.5009
WVFGRD96 65.0 50 65 0 3.81 0.5007
WVFGRD96 66.0 50 65 0 3.81 0.5013
WVFGRD96 67.0 50 65 0 3.81 0.4996
WVFGRD96 68.0 50 65 0 3.82 0.5005
WVFGRD96 69.0 50 65 0 3.82 0.4999
WVFGRD96 70.0 50 65 0 3.82 0.4983
WVFGRD96 71.0 50 65 0 3.83 0.4985
WVFGRD96 72.0 50 65 0 3.83 0.4964
WVFGRD96 73.0 50 65 0 3.83 0.4965
WVFGRD96 74.0 50 65 0 3.83 0.4954
WVFGRD96 75.0 50 65 0 3.84 0.4937
WVFGRD96 76.0 50 65 0 3.84 0.4932
WVFGRD96 77.0 50 65 0 3.84 0.4915
WVFGRD96 78.0 50 65 0 3.84 0.4914
WVFGRD96 79.0 55 75 -5 3.84 0.4895
The best solution is
WVFGRD96 66.0 50 65 0 3.81 0.5013
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 0.10 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