The ANSS event ID is ak01910ggmj6 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak01910ggmj6/executive.
2019/01/22 04:43:19 58.332 -155.323 121.5 5.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/01/22 04:43:19:0 58.33 -155.32 121.5 5.2 Alaska
Stations used:
AK.BRLK AK.CNP AK.HOM AK.SII AT.CHGN AT.OHAK AV.ACH AV.ILSW
II.KDAK TA.M17K TA.N15K TA.N17K TA.N18K TA.N19K TA.O14K
TA.O16K TA.O18K TA.O19K TA.P18K TA.P19K TA.Q19K TA.Q20K
TA.R18K
Filtering commands used:
cut o DIST/3.4 -50 o DIST/3.4 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 4.73e+23 dyne-cm
Mw = 5.05
Z = 130 km
Plane Strike Dip Rake
NP1 343 71 137
NP2 90 50 25
Principal Axes:
Axis Value Plunge Azimuth
T 4.73e+23 43 299
N 0.00e+00 44 144
P -4.73e+23 13 41
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.97e+23
Mxy -3.28e+23
Mxz 3.47e+22
Myy -2.87e+16
Myz -2.76e+23
Mzz 1.97e+23
##------------
#######---------------
############------------ -
##############----------- P --
#################---------- ----
###################-----------------
#####################-----------------
######## ############-----------------
######## T #############----------------
######### #############-----------------
##########################----------------
-#########################---------------#
--#########################-------------##
---#######################-----------###
------####################--------######
---------#################---#########
-------------------------###########
------------------------##########
----------------------########
---------------------#######
------------------####
-------------#
Global CMT Convention Moment Tensor:
R T P
1.97e+23 3.47e+22 2.76e+23
3.47e+22 -1.97e+23 3.28e+23
2.76e+23 3.28e+23 -2.87e+16
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190122044319/index.html
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STK = 90
DIP = 50
RAKE = 25
MW = 5.05
HS = 130.0
The NDK file is 20190122044319.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/3.4 -50 o DIST/3.4 +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 2.0 350 50 70 4.11 0.1485
WVFGRD96 4.0 335 70 40 4.12 0.1920
WVFGRD96 6.0 335 75 35 4.19 0.2340
WVFGRD96 8.0 335 80 40 4.29 0.2702
WVFGRD96 10.0 335 75 35 4.34 0.3000
WVFGRD96 12.0 335 75 35 4.39 0.3183
WVFGRD96 14.0 335 75 35 4.42 0.3263
WVFGRD96 16.0 335 75 35 4.45 0.3250
WVFGRD96 18.0 335 75 35 4.48 0.3165
WVFGRD96 20.0 335 70 35 4.50 0.3016
WVFGRD96 22.0 250 60 15 4.52 0.3040
WVFGRD96 24.0 250 60 15 4.54 0.3089
WVFGRD96 26.0 250 60 10 4.57 0.3143
WVFGRD96 28.0 250 60 10 4.59 0.3187
WVFGRD96 30.0 250 65 10 4.60 0.3224
WVFGRD96 32.0 250 65 10 4.62 0.3224
WVFGRD96 34.0 250 65 5 4.64 0.3253
WVFGRD96 36.0 250 65 5 4.66 0.3367
WVFGRD96 38.0 250 75 0 4.70 0.3534
WVFGRD96 40.0 245 65 -5 4.77 0.3854
WVFGRD96 42.0 245 70 -5 4.80 0.4021
WVFGRD96 44.0 245 70 -5 4.82 0.4147
WVFGRD96 46.0 245 70 -5 4.84 0.4224
WVFGRD96 48.0 245 70 -5 4.86 0.4270
WVFGRD96 50.0 250 75 -5 4.89 0.4289
WVFGRD96 52.0 250 75 -5 4.90 0.4299
WVFGRD96 54.0 250 75 -5 4.91 0.4296
WVFGRD96 56.0 250 75 -5 4.92 0.4312
WVFGRD96 58.0 245 75 -10 4.91 0.4316
WVFGRD96 60.0 70 80 5 4.90 0.4336
WVFGRD96 62.0 70 75 0 4.90 0.4415
WVFGRD96 64.0 70 75 0 4.90 0.4499
WVFGRD96 66.0 70 70 5 4.90 0.4558
WVFGRD96 68.0 75 70 10 4.92 0.4654
WVFGRD96 70.0 75 70 10 4.93 0.4733
WVFGRD96 72.0 75 65 10 4.92 0.4815
WVFGRD96 74.0 75 65 10 4.93 0.4894
WVFGRD96 76.0 75 65 10 4.93 0.4969
WVFGRD96 78.0 75 65 15 4.93 0.5072
WVFGRD96 80.0 80 60 20 4.95 0.5322
WVFGRD96 82.0 80 60 20 4.96 0.5571
WVFGRD96 84.0 80 60 20 4.97 0.5802
WVFGRD96 86.0 80 60 20 4.97 0.6004
WVFGRD96 88.0 85 55 25 4.98 0.6160
WVFGRD96 90.0 85 55 25 4.99 0.6266
WVFGRD96 92.0 85 55 25 4.99 0.6332
WVFGRD96 94.0 85 55 25 4.99 0.6391
WVFGRD96 96.0 85 55 25 5.00 0.6447
WVFGRD96 98.0 85 55 25 5.00 0.6495
WVFGRD96 100.0 85 55 25 5.00 0.6547
WVFGRD96 102.0 85 55 25 5.00 0.6591
WVFGRD96 104.0 85 55 25 5.01 0.6624
WVFGRD96 106.0 85 55 25 5.01 0.6655
WVFGRD96 108.0 85 55 25 5.01 0.6687
WVFGRD96 110.0 85 55 25 5.01 0.6718
WVFGRD96 112.0 90 50 25 5.03 0.6749
WVFGRD96 114.0 90 50 25 5.03 0.6786
WVFGRD96 116.0 90 50 25 5.03 0.6816
WVFGRD96 118.0 90 50 25 5.03 0.6834
WVFGRD96 120.0 90 50 25 5.04 0.6869
WVFGRD96 122.0 90 50 25 5.04 0.6890
WVFGRD96 124.0 90 50 25 5.04 0.6898
WVFGRD96 126.0 90 50 25 5.04 0.6898
WVFGRD96 128.0 90 50 25 5.05 0.6909
WVFGRD96 130.0 90 50 25 5.05 0.6917
WVFGRD96 132.0 90 50 30 5.04 0.6909
WVFGRD96 134.0 95 50 30 5.06 0.6913
WVFGRD96 136.0 95 45 25 5.07 0.6908
WVFGRD96 138.0 95 50 30 5.06 0.6889
WVFGRD96 140.0 95 50 30 5.07 0.6892
WVFGRD96 142.0 95 45 30 5.06 0.6882
WVFGRD96 144.0 95 45 25 5.07 0.6858
WVFGRD96 146.0 95 45 25 5.08 0.6849
WVFGRD96 148.0 95 45 25 5.08 0.6837
WVFGRD96 150.0 95 45 25 5.08 0.6822
WVFGRD96 152.0 95 45 25 5.08 0.6800
WVFGRD96 154.0 95 45 25 5.08 0.6772
WVFGRD96 156.0 95 50 25 5.09 0.6762
WVFGRD96 158.0 95 50 25 5.09 0.6735
The best solution is
WVFGRD96 130.0 90 50 25 5.05 0.6917
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/3.4 -50 o DIST/3.4 +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