The ANSS event ID is ak01912e2f51 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak01912e2f51/executive.
2019/01/23 21:34:23 63.237 -150.573 129.1 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/01/23 21:34:23:0 63.24 -150.57 129.1 3.9 Alaska
Stations used:
AK.BPAW AK.CAST AK.CUT AK.KTH AK.MCK AK.RND AK.SKN AK.SSN
AK.TRF AK.WRH AT.TTA TA.H21K TA.J19K TA.J20K TA.J25K
TA.K20K TA.M22K
Filtering commands used:
cut o DIST/3.4 -40 o DIST/3.4 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 2.11e+22 dyne-cm
Mw = 4.15
Z = 136 km
Plane Strike Dip Rake
NP1 204 83 -103
NP2 85 15 -30
Principal Axes:
Axis Value Plunge Azimuth
T 2.11e+22 36 306
N 0.00e+00 13 206
P -2.11e+22 51 100
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.42e+21
Mxy -5.12e+21
Mxz 7.58e+21
Myy 8.63e+20
Myz -1.84e+22
Mzz -5.28e+21
##############
##################----
####################--------
####################----------
#####################-------------
###### ############---------------
####### T ###########-----------------
######## ##########-------------------
####################--------------------
####################----------------------
###################---------- ----------
###################---------- P ---------#
-#################----------- ---------#
################-----------------------#
-##############-----------------------##
-#############----------------------##
-###########----------------------##
--########---------------------###
--######-------------------###
----##-----------------#####
---####-------########
##############
Global CMT Convention Moment Tensor:
R T P
-5.28e+21 7.58e+21 1.84e+22
7.58e+21 4.42e+21 5.12e+21
1.84e+22 5.12e+21 8.63e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190123213423/index.html
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STK = 85
DIP = 15
RAKE = -30
MW = 4.15
HS = 136.0
The NDK file is 20190123213423.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 -40 o DIST/3.4 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 35 45 75 3.28 0.1547
WVFGRD96 4.0 185 55 30 3.28 0.1621
WVFGRD96 6.0 185 65 25 3.31 0.1820
WVFGRD96 8.0 185 70 30 3.38 0.1972
WVFGRD96 10.0 350 75 -40 3.42 0.2096
WVFGRD96 12.0 350 70 -40 3.46 0.2205
WVFGRD96 14.0 355 70 -35 3.49 0.2266
WVFGRD96 16.0 -5 70 -35 3.52 0.2283
WVFGRD96 18.0 -5 70 -30 3.54 0.2264
WVFGRD96 20.0 5 75 -25 3.57 0.2226
WVFGRD96 22.0 5 70 -25 3.59 0.2187
WVFGRD96 24.0 5 70 -20 3.61 0.2111
WVFGRD96 26.0 100 70 30 3.61 0.2012
WVFGRD96 28.0 105 70 25 3.64 0.2016
WVFGRD96 30.0 105 70 20 3.66 0.2021
WVFGRD96 32.0 105 75 15 3.68 0.2025
WVFGRD96 34.0 105 75 15 3.70 0.2023
WVFGRD96 36.0 105 80 10 3.72 0.2030
WVFGRD96 38.0 105 80 10 3.75 0.2043
WVFGRD96 40.0 105 75 20 3.79 0.2060
WVFGRD96 42.0 105 80 15 3.81 0.2083
WVFGRD96 44.0 105 85 10 3.83 0.2123
WVFGRD96 46.0 285 90 -5 3.85 0.2173
WVFGRD96 48.0 285 80 10 3.86 0.2267
WVFGRD96 50.0 285 80 10 3.88 0.2372
WVFGRD96 52.0 285 80 10 3.90 0.2470
WVFGRD96 54.0 285 80 15 3.91 0.2564
WVFGRD96 56.0 285 80 15 3.93 0.2647
WVFGRD96 58.0 285 80 15 3.94 0.2707
WVFGRD96 60.0 285 80 15 3.95 0.2762
WVFGRD96 62.0 285 80 15 3.96 0.2817
WVFGRD96 64.0 285 80 15 3.97 0.2877
WVFGRD96 66.0 285 80 15 3.98 0.2941
WVFGRD96 68.0 285 80 15 3.99 0.3044
WVFGRD96 70.0 285 85 15 4.01 0.3141
WVFGRD96 72.0 285 85 15 4.02 0.3245
WVFGRD96 74.0 105 85 -10 4.03 0.3332
WVFGRD96 76.0 100 80 -15 4.02 0.3445
WVFGRD96 78.0 100 65 -10 4.02 0.3547
WVFGRD96 80.0 120 35 10 4.03 0.3787
WVFGRD96 82.0 115 30 5 4.04 0.4174
WVFGRD96 84.0 115 30 5 4.06 0.4536
WVFGRD96 86.0 115 25 5 4.07 0.4840
WVFGRD96 88.0 115 25 5 4.08 0.5073
WVFGRD96 90.0 110 20 0 4.08 0.5195
WVFGRD96 92.0 110 20 0 4.09 0.5309
WVFGRD96 94.0 110 20 0 4.09 0.5402
WVFGRD96 96.0 110 20 0 4.10 0.5482
WVFGRD96 98.0 105 15 -5 4.10 0.5576
WVFGRD96 100.0 105 15 -5 4.11 0.5651
WVFGRD96 102.0 100 15 -10 4.11 0.5727
WVFGRD96 104.0 100 15 -10 4.11 0.5802
WVFGRD96 106.0 100 15 -10 4.12 0.5865
WVFGRD96 108.0 85 15 -25 4.12 0.5927
WVFGRD96 110.0 70 10 -40 4.12 0.6005
WVFGRD96 112.0 70 10 -40 4.13 0.6083
WVFGRD96 114.0 75 10 -35 4.13 0.6144
WVFGRD96 116.0 75 10 -35 4.13 0.6193
WVFGRD96 118.0 75 10 -35 4.13 0.6245
WVFGRD96 120.0 75 10 -35 4.14 0.6289
WVFGRD96 122.0 75 10 -35 4.14 0.6324
WVFGRD96 124.0 75 10 -35 4.14 0.6365
WVFGRD96 126.0 75 10 -35 4.14 0.6380
WVFGRD96 128.0 85 15 -25 4.14 0.6399
WVFGRD96 130.0 85 15 -25 4.14 0.6428
WVFGRD96 132.0 85 15 -25 4.14 0.6436
WVFGRD96 134.0 85 15 -30 4.15 0.6432
WVFGRD96 136.0 85 15 -30 4.15 0.6454
WVFGRD96 138.0 85 15 -25 4.15 0.6453
WVFGRD96 140.0 85 15 -30 4.15 0.6440
WVFGRD96 142.0 85 15 -30 4.15 0.6448
WVFGRD96 144.0 85 15 -30 4.15 0.6444
WVFGRD96 146.0 85 15 -30 4.15 0.6423
WVFGRD96 148.0 85 15 -30 4.16 0.6419
WVFGRD96 150.0 85 15 -30 4.16 0.6403
WVFGRD96 152.0 85 15 -30 4.16 0.6380
WVFGRD96 154.0 85 15 -30 4.16 0.6366
WVFGRD96 156.0 85 15 -30 4.16 0.6353
WVFGRD96 158.0 85 15 -30 4.16 0.6327
The best solution is
WVFGRD96 136.0 85 15 -30 4.15 0.6454
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 -40 o DIST/3.4 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 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