The ANSS event ID is ak018vt8d37 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018vt8d37/executive.
2018/01/19 23:55:05 63.977 -148.990 129.3 4.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/01/19 23:55:05:0 63.98 -148.99 129.3 4.6 Alaska
Stations used:
AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV AK.GHO
AK.HDA AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PAX
AK.PPD AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN
AK.TRF AK.WRH AT.MENT AT.PMR IM.IL31 IU.COLA TA.H21K
TA.I20K TA.I21K TA.I23K TA.I26K TA.J20K TA.J25K TA.J26L
TA.L26K TA.L27K TA.POKR
Filtering commands used:
cut o DIST/3.7 -30 o DIST/3.7 +60
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.00e+23 dyne-cm
Mw = 4.60
Z = 136 km
Plane Strike Dip Rake
NP1 38 85 155
NP2 130 65 5
Principal Axes:
Axis Value Plunge Azimuth
T 1.00e+23 21 351
N 0.00e+00 65 208
P -1.00e+23 14 87
Moment Tensor: (dyne-cm)
Component Value
Mxx 8.50e+22
Mxy -1.90e+22
Mxz 3.14e+22
Myy -9.17e+22
Myz -2.87e+22
Mzz 6.68e+21
##############
####### ############
########## T #############--
########### ############----
-##########################-------
---########################---------
-----######################-----------
------####################--------------
-------##################---------------
----------###############------------- -
-----------############--------------- P -
-------------#########---------------- -
---------------#####----------------------
----------------##----------------------
----------------##----------------------
--------------######------------------
-----------###########--------------
--------#################---------
-----#########################
--##########################
######################
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Global CMT Convention Moment Tensor:
R T P
6.68e+21 3.14e+22 2.87e+22
3.14e+22 8.50e+22 1.90e+22
2.87e+22 1.90e+22 -9.17e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180119235505/index.html
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STK = 130
DIP = 65
RAKE = 5
MW = 4.60
HS = 136.0
The NDK file is 20180119235505.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/3.7 -30 o DIST/3.7 +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 45 65 -20 3.53 0.1902
WVFGRD96 4.0 45 60 -15 3.62 0.2194
WVFGRD96 6.0 50 65 -10 3.69 0.2465
WVFGRD96 8.0 50 65 -15 3.78 0.2676
WVFGRD96 10.0 55 70 15 3.82 0.2713
WVFGRD96 12.0 50 70 10 3.85 0.2664
WVFGRD96 14.0 50 75 10 3.88 0.2543
WVFGRD96 16.0 140 75 20 3.90 0.2440
WVFGRD96 18.0 140 75 20 3.93 0.2451
WVFGRD96 20.0 140 75 20 3.96 0.2465
WVFGRD96 22.0 140 80 20 3.98 0.2491
WVFGRD96 24.0 140 80 20 4.00 0.2512
WVFGRD96 26.0 140 80 20 4.02 0.2514
WVFGRD96 28.0 140 80 20 4.03 0.2482
WVFGRD96 30.0 140 80 25 4.04 0.2422
WVFGRD96 32.0 140 80 25 4.04 0.2337
WVFGRD96 34.0 140 75 20 4.05 0.2256
WVFGRD96 36.0 140 75 20 4.06 0.2201
WVFGRD96 38.0 140 75 20 4.09 0.2202
WVFGRD96 40.0 145 55 20 4.17 0.2312
WVFGRD96 42.0 140 65 20 4.19 0.2354
WVFGRD96 44.0 140 70 20 4.20 0.2408
WVFGRD96 46.0 140 70 15 4.22 0.2475
WVFGRD96 48.0 140 70 20 4.25 0.2561
WVFGRD96 50.0 140 70 20 4.26 0.2663
WVFGRD96 52.0 140 70 15 4.28 0.2777
WVFGRD96 54.0 140 70 15 4.29 0.2914
WVFGRD96 56.0 140 65 15 4.32 0.3099
WVFGRD96 58.0 140 65 15 4.33 0.3317
WVFGRD96 60.0 140 60 15 4.36 0.3561
WVFGRD96 62.0 140 60 15 4.38 0.3828
WVFGRD96 64.0 140 60 15 4.39 0.4103
WVFGRD96 66.0 140 60 15 4.41 0.4356
WVFGRD96 68.0 140 60 15 4.42 0.4582
WVFGRD96 70.0 140 55 15 4.44 0.4741
WVFGRD96 72.0 140 55 15 4.45 0.4874
WVFGRD96 74.0 135 55 10 4.46 0.5066
WVFGRD96 76.0 135 55 10 4.47 0.5301
WVFGRD96 78.0 135 55 10 4.48 0.5550
WVFGRD96 80.0 135 55 10 4.49 0.5744
WVFGRD96 82.0 135 55 10 4.50 0.5865
WVFGRD96 84.0 135 55 10 4.50 0.5939
WVFGRD96 86.0 135 55 10 4.51 0.5999
WVFGRD96 88.0 135 55 10 4.51 0.6052
WVFGRD96 90.0 135 55 10 4.52 0.6105
WVFGRD96 92.0 135 55 10 4.52 0.6167
WVFGRD96 94.0 135 55 10 4.53 0.6222
WVFGRD96 96.0 135 55 10 4.53 0.6266
WVFGRD96 98.0 135 55 10 4.54 0.6304
WVFGRD96 100.0 135 55 10 4.54 0.6334
WVFGRD96 102.0 135 60 10 4.54 0.6356
WVFGRD96 104.0 135 60 10 4.54 0.6400
WVFGRD96 106.0 130 60 5 4.55 0.6430
WVFGRD96 108.0 130 60 5 4.55 0.6454
WVFGRD96 110.0 130 60 5 4.55 0.6488
WVFGRD96 112.0 130 60 5 4.56 0.6528
WVFGRD96 114.0 130 60 5 4.56 0.6550
WVFGRD96 116.0 130 60 5 4.57 0.6560
WVFGRD96 118.0 130 60 5 4.57 0.6588
WVFGRD96 120.0 130 60 5 4.58 0.6602
WVFGRD96 122.0 130 60 5 4.58 0.6607
WVFGRD96 124.0 130 60 5 4.58 0.6625
WVFGRD96 126.0 130 60 5 4.59 0.6613
WVFGRD96 128.0 130 60 5 4.59 0.6616
WVFGRD96 130.0 130 65 5 4.59 0.6627
WVFGRD96 132.0 130 65 5 4.59 0.6619
WVFGRD96 134.0 130 65 5 4.59 0.6625
WVFGRD96 136.0 130 65 5 4.60 0.6633
WVFGRD96 138.0 130 65 0 4.60 0.6626
WVFGRD96 140.0 130 65 0 4.60 0.6623
WVFGRD96 142.0 130 65 0 4.61 0.6602
WVFGRD96 144.0 130 65 5 4.61 0.6604
WVFGRD96 146.0 130 65 0 4.61 0.6577
WVFGRD96 148.0 130 65 0 4.62 0.6568
The best solution is
WVFGRD96 136.0 130 65 5 4.60 0.6633
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/3.7 -30 o DIST/3.7 +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