Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.
The ANSS event ID is ak019452y61d and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019452y61d/executive.
2019/03/31 14:41:36 62.208 -151.247 76.6 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/03/31 14:41:36:0 62.21 -151.25 76.6 4.0 Alaska
Stations used:
AK.BWN AK.CUT AK.GHO AK.GLI AK.KNK AK.KTH AK.MCK AK.NEA2
AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AK.TRF
AT.PMR AV.RDDF AV.SPBG AV.SPCR AV.STLK GS.PR01 GS.PR03
GS.PR04 GS.PR05 PR.CRPR TA.K20K TA.L19K TA.L20K TA.M22K
TA.O22K XV.FAPT XV.FPAP
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +60
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 2.99e+22 dyne-cm
Mw = 4.25
Z = 82 km
Plane Strike Dip Rake
NP1 341 84 104
NP2 95 15 25
Principal Axes:
Axis Value Plunge Azimuth
T 2.99e+22 49 266
N 0.00e+00 14 159
P -2.99e+22 37 59
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.04e+21
Mxy -7.44e+21
Mxz -8.61e+21
Myy -1.26e+21
Myz -2.70e+22
Mzz 6.31e+21
--------------
#####-----------------
#########-------------------
###########-------------------
##############--------------------
################--------------------
##################----------- ------
###################----------- P -------
####################---------- -------
######################--------------------
######### ##########--------------------
-######## T ###########-------------------
-######## ############-----------------#
-######################-----------------
--######################---------------#
--#####################--------------#
--#####################------------#
---###################----------##
---##################-------##
-----###############----####
--------#######---####
--------------
Global CMT Convention Moment Tensor:
R T P
6.31e+21 -8.61e+21 2.70e+22
-8.61e+21 -5.04e+21 7.44e+21
2.70e+22 7.44e+21 -1.26e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190331144136/index.html
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STK = 95
DIP = 15
RAKE = 25
MW = 4.25
HS = 82.0
The NDK file is 20190331144136.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: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
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 +60 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 2.0 155 65 15 3.36 0.2280
WVFGRD96 4.0 155 75 20 3.48 0.2938
WVFGRD96 6.0 155 80 15 3.54 0.3185
WVFGRD96 8.0 155 80 15 3.62 0.3277
WVFGRD96 10.0 330 75 -10 3.66 0.3280
WVFGRD96 12.0 330 80 -5 3.69 0.3261
WVFGRD96 14.0 330 80 -5 3.72 0.3197
WVFGRD96 16.0 325 80 -10 3.73 0.3108
WVFGRD96 18.0 145 75 10 3.74 0.3036
WVFGRD96 20.0 145 75 10 3.76 0.2962
WVFGRD96 22.0 145 75 15 3.77 0.2888
WVFGRD96 24.0 270 80 -10 3.85 0.2875
WVFGRD96 26.0 270 85 -15 3.87 0.3015
WVFGRD96 28.0 265 90 -15 3.90 0.3179
WVFGRD96 30.0 85 80 15 3.91 0.3357
WVFGRD96 32.0 260 90 -15 3.93 0.3475
WVFGRD96 34.0 260 90 -20 3.96 0.3689
WVFGRD96 36.0 80 80 10 3.97 0.3894
WVFGRD96 38.0 80 80 15 4.00 0.4119
WVFGRD96 40.0 80 85 20 4.06 0.4313
WVFGRD96 42.0 85 65 10 4.08 0.4348
WVFGRD96 44.0 85 60 10 4.10 0.4442
WVFGRD96 46.0 85 55 10 4.12 0.4561
WVFGRD96 48.0 85 55 10 4.13 0.4705
WVFGRD96 50.0 80 50 5 4.15 0.4879
WVFGRD96 52.0 80 45 5 4.17 0.5036
WVFGRD96 54.0 80 45 5 4.18 0.5208
WVFGRD96 56.0 80 45 5 4.19 0.5331
WVFGRD96 58.0 80 40 5 4.20 0.5470
WVFGRD96 60.0 85 40 15 4.20 0.5577
WVFGRD96 62.0 85 40 15 4.20 0.5662
WVFGRD96 64.0 90 20 15 4.22 0.5817
WVFGRD96 66.0 90 20 15 4.22 0.5981
WVFGRD96 68.0 95 15 25 4.23 0.6102
WVFGRD96 70.0 95 15 25 4.24 0.6210
WVFGRD96 72.0 95 15 25 4.24 0.6284
WVFGRD96 74.0 95 15 25 4.24 0.6356
WVFGRD96 76.0 95 15 25 4.24 0.6403
WVFGRD96 78.0 95 15 25 4.25 0.6442
WVFGRD96 80.0 95 15 25 4.25 0.6465
WVFGRD96 82.0 95 15 25 4.25 0.6477
WVFGRD96 84.0 100 10 30 4.25 0.6473
WVFGRD96 86.0 100 10 30 4.25 0.6461
WVFGRD96 88.0 100 10 30 4.25 0.6435
WVFGRD96 90.0 105 10 35 4.25 0.6417
WVFGRD96 92.0 105 10 35 4.25 0.6399
WVFGRD96 94.0 105 10 35 4.25 0.6370
WVFGRD96 96.0 105 10 35 4.25 0.6331
WVFGRD96 98.0 105 10 35 4.25 0.6278
WVFGRD96 100.0 105 10 35 4.25 0.6236
WVFGRD96 102.0 105 10 35 4.25 0.6208
WVFGRD96 104.0 105 10 35 4.25 0.6152
WVFGRD96 106.0 110 10 40 4.25 0.6093
WVFGRD96 108.0 110 10 40 4.25 0.6041
WVFGRD96 110.0 110 10 40 4.25 0.5996
WVFGRD96 112.0 110 10 40 4.25 0.5948
WVFGRD96 114.0 115 10 45 4.25 0.5881
WVFGRD96 116.0 115 10 45 4.25 0.5823
WVFGRD96 118.0 115 10 45 4.25 0.5797
WVFGRD96 120.0 120 10 50 4.25 0.5744
WVFGRD96 122.0 120 10 50 4.25 0.5711
WVFGRD96 124.0 120 10 50 4.25 0.5682
WVFGRD96 126.0 125 10 55 4.25 0.5620
WVFGRD96 128.0 130 10 60 4.25 0.5532
WVFGRD96 130.0 140 10 70 4.25 0.5230
WVFGRD96 132.0 140 10 70 4.24 0.4840
WVFGRD96 134.0 140 10 70 4.23 0.4449
WVFGRD96 136.0 145 10 75 4.22 0.4082
WVFGRD96 138.0 130 15 65 4.21 0.3935
WVFGRD96 140.0 130 15 60 4.21 0.3811
WVFGRD96 142.0 140 15 70 4.20 0.3567
WVFGRD96 144.0 140 20 70 4.20 0.3240
WVFGRD96 146.0 150 20 80 4.18 0.2782
WVFGRD96 148.0 140 25 75 4.16 0.2276
The best solution is
WVFGRD96 82.0 95 15 25 4.25 0.6477
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 +60 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