The ANSS event ID is ak01929bgwg5 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak01929bgwg5/executive.
2019/02/18 17:02:45 61.468 -149.961 37.6 4.4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/02/18 17:02:45:0 61.47 -149.96 37.6 4.4 Alaska
Stations used:
AK.CAST AK.FIRE AK.GHO AK.KNK AK.PPLA AK.PWL AK.RC01 AK.RND
AK.SAW AK.SCM AK.SLK AK.SWD AT.PMR AV.ILS AV.ILSW AV.RDDF
AV.RDT AV.SPBG AV.SPCR AV.SPU AV.STLK GM.AD11 TA.M23K
TA.M24K TA.O22K
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 = 4.68e+22 dyne-cm
Mw = 4.38
Z = 49 km
Plane Strike Dip Rake
NP1 205 70 -92
NP2 30 20 -85
Principal Axes:
Axis Value Plunge Azimuth
T 4.68e+22 25 296
N 0.00e+00 2 205
P -4.68e+22 65 112
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.28e+21
Mxy -1.23e+22
Mxz 1.45e+22
Myy 2.37e+22
Myz -3.28e+22
Mzz -3.00e+22
##############
##################----
###################--------#
##################-----------#
##################--------------##
##################----------------##
### ############------------------##
#### T ###########-------------------###
#### ##########--------------------###
#################---------------------####
################----------------------####
###############----------- ---------####
##############------------ P ---------####
#############------------ --------####
############------------------------####
###########-----------------------####
#########----------------------#####
########---------------------#####
######-------------------#####
#####-----------------######
##-------------#######
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Global CMT Convention Moment Tensor:
R T P
-3.00e+22 1.45e+22 3.28e+22
1.45e+22 6.28e+21 1.23e+22
3.28e+22 1.23e+22 2.37e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190218170245/index.html
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STK = 30
DIP = 20
RAKE = -85
MW = 4.38
HS = 49.0
The NDK file is 20190218170245.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 1.0 135 85 15 3.47 0.1597
WVFGRD96 2.0 90 40 -90 3.75 0.2290
WVFGRD96 3.0 135 80 25 3.68 0.2424
WVFGRD96 4.0 135 75 25 3.72 0.2556
WVFGRD96 5.0 135 80 25 3.74 0.2608
WVFGRD96 6.0 135 80 25 3.77 0.2634
WVFGRD96 7.0 45 60 -10 3.78 0.2772
WVFGRD96 8.0 50 55 5 3.82 0.2934
WVFGRD96 9.0 55 55 15 3.84 0.3075
WVFGRD96 10.0 55 55 15 3.86 0.3201
WVFGRD96 11.0 55 60 20 3.89 0.3321
WVFGRD96 12.0 60 60 30 3.91 0.3430
WVFGRD96 13.0 60 60 25 3.92 0.3516
WVFGRD96 14.0 60 60 25 3.93 0.3587
WVFGRD96 15.0 60 60 25 3.95 0.3642
WVFGRD96 16.0 80 60 -25 3.95 0.3723
WVFGRD96 17.0 75 55 -25 3.96 0.3814
WVFGRD96 18.0 75 55 -25 3.97 0.3897
WVFGRD96 19.0 75 55 -25 3.98 0.3967
WVFGRD96 20.0 75 55 -25 4.00 0.4029
WVFGRD96 21.0 75 50 -20 4.01 0.4087
WVFGRD96 22.0 80 50 -10 4.02 0.4157
WVFGRD96 23.0 80 50 -10 4.03 0.4222
WVFGRD96 24.0 80 50 -10 4.04 0.4289
WVFGRD96 25.0 80 45 -10 4.05 0.4344
WVFGRD96 26.0 75 45 -20 4.06 0.4418
WVFGRD96 27.0 75 45 -20 4.07 0.4480
WVFGRD96 28.0 75 45 -25 4.09 0.4559
WVFGRD96 29.0 75 45 -25 4.10 0.4632
WVFGRD96 30.0 70 40 -35 4.12 0.4712
WVFGRD96 31.0 70 40 -35 4.13 0.4799
WVFGRD96 32.0 65 35 -45 4.14 0.4883
WVFGRD96 33.0 65 35 -45 4.15 0.4965
WVFGRD96 34.0 60 30 -50 4.16 0.5040
WVFGRD96 35.0 60 30 -55 4.18 0.5123
WVFGRD96 36.0 60 30 -55 4.19 0.5190
WVFGRD96 37.0 50 25 -65 4.20 0.5245
WVFGRD96 38.0 50 25 -65 4.20 0.5303
WVFGRD96 39.0 50 25 -70 4.22 0.5367
WVFGRD96 40.0 40 20 -75 4.33 0.5316
WVFGRD96 41.0 35 20 -80 4.34 0.5348
WVFGRD96 42.0 35 20 -80 4.34 0.5378
WVFGRD96 43.0 205 70 -95 4.35 0.5401
WVFGRD96 44.0 30 20 -85 4.36 0.5432
WVFGRD96 45.0 30 20 -85 4.36 0.5445
WVFGRD96 46.0 30 20 -85 4.37 0.5463
WVFGRD96 47.0 205 70 -90 4.37 0.5465
WVFGRD96 48.0 205 70 -90 4.38 0.5465
WVFGRD96 49.0 30 20 -85 4.38 0.5465
WVFGRD96 50.0 30 20 -85 4.39 0.5459
WVFGRD96 51.0 30 20 -85 4.39 0.5445
WVFGRD96 52.0 30 20 -85 4.39 0.5431
WVFGRD96 53.0 25 20 -85 4.39 0.5407
WVFGRD96 54.0 25 20 -85 4.39 0.5387
WVFGRD96 55.0 25 20 -85 4.39 0.5363
WVFGRD96 56.0 25 20 -85 4.40 0.5327
WVFGRD96 57.0 25 20 -85 4.40 0.5297
WVFGRD96 58.0 25 20 -85 4.40 0.5262
WVFGRD96 59.0 25 20 -85 4.40 0.5219
The best solution is
WVFGRD96 49.0 30 20 -85 4.38 0.5465
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