The ANSS event ID is ak0182ksu6cf and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0182ksu6cf/executive.
2018/02/25 11:32:53 63.215 -150.591 132.8 4.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/02/25 11:32:53:0 63.22 -150.59 132.8 4.2 Alaska
Stations used:
AK.BPAW AK.CAST AK.CUT AK.GHO AK.KNK AK.KTH AK.NEA2 AK.PAX
AK.RND AK.SAW AK.SCM AK.SSN AK.TRF AT.PMR TA.M19K
Filtering commands used:
cut o DIST/3.1 -30 o DIST/3.1 +60
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 2.19e+22 dyne-cm
Mw = 4.16
Z = 138 km
Plane Strike Dip Rake
NP1 35 84 98
NP2 165 10 40
Principal Axes:
Axis Value Plunge Azimuth
T 2.19e+22 51 314
N 0.00e+00 8 215
P -2.19e+22 38 119
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.13e+21
Mxy 1.32e+21
Mxz 1.25e+22
Myy -5.94e+21
Myz -1.70e+22
Mzz 4.81e+21
##############
-#####################
--########################--
-#########################----
-##########################-------
-##########################---------
--######## #############------------
--######### T ############--------------
-########## ###########---------------
--#######################-----------------
--######################------------------
--####################--------------------
--###################---------------------
--#################----------- -------
--###############------------- P -------
--#############-------------- ------
--##########------------------------
--########------------------------
--####------------------------
----------------------------
##--------------------
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Global CMT Convention Moment Tensor:
R T P
4.81e+21 1.25e+22 1.70e+22
1.25e+22 1.13e+21 -1.32e+21
1.70e+22 -1.32e+21 -5.94e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180225113253/index.html
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STK = 165
DIP = 10
RAKE = 40
MW = 4.16
HS = 138.0
The NDK file is 20180225113253.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/3.1 -30 o DIST/3.1 +60 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 30 40 80 3.35 0.2053
WVFGRD96 4.0 40 20 -80 3.52 0.2545
WVFGRD96 6.0 40 25 -80 3.53 0.2781
WVFGRD96 8.0 40 20 -80 3.57 0.2755
WVFGRD96 10.0 185 75 65 3.54 0.2583
WVFGRD96 12.0 180 80 60 3.54 0.2572
WVFGRD96 14.0 180 80 55 3.55 0.2561
WVFGRD96 16.0 180 80 55 3.56 0.2538
WVFGRD96 18.0 175 85 50 3.58 0.2506
WVFGRD96 20.0 175 75 50 3.59 0.2497
WVFGRD96 22.0 180 70 55 3.61 0.2506
WVFGRD96 24.0 180 65 55 3.63 0.2531
WVFGRD96 26.0 185 60 55 3.64 0.2581
WVFGRD96 28.0 110 50 65 3.71 0.2620
WVFGRD96 30.0 115 55 70 3.74 0.2628
WVFGRD96 32.0 170 60 50 3.71 0.2554
WVFGRD96 34.0 170 60 50 3.73 0.2496
WVFGRD96 36.0 165 60 45 3.75 0.2436
WVFGRD96 38.0 165 60 45 3.78 0.2395
WVFGRD96 40.0 45 50 -45 3.85 0.2537
WVFGRD96 42.0 40 50 -45 3.90 0.2619
WVFGRD96 44.0 45 55 -45 3.90 0.2684
WVFGRD96 46.0 45 55 -45 3.92 0.2800
WVFGRD96 48.0 55 65 -55 3.91 0.2941
WVFGRD96 50.0 50 65 -55 3.94 0.3099
WVFGRD96 52.0 50 65 -55 3.96 0.3238
WVFGRD96 54.0 55 70 -50 3.97 0.3366
WVFGRD96 56.0 55 70 -50 3.98 0.3453
WVFGRD96 58.0 55 70 -50 4.00 0.3515
WVFGRD96 60.0 50 70 -50 4.02 0.3564
WVFGRD96 62.0 50 70 -45 4.04 0.3603
WVFGRD96 64.0 50 70 -45 4.05 0.3637
WVFGRD96 66.0 50 70 -45 4.05 0.3646
WVFGRD96 68.0 55 75 -40 4.05 0.3642
WVFGRD96 70.0 55 75 -40 4.05 0.3636
WVFGRD96 72.0 55 75 -40 4.06 0.3617
WVFGRD96 74.0 55 75 -40 4.06 0.3592
WVFGRD96 76.0 55 75 15 4.04 0.3797
WVFGRD96 78.0 140 35 40 4.12 0.4221
WVFGRD96 80.0 140 35 40 4.13 0.4733
WVFGRD96 82.0 140 30 40 4.13 0.5162
WVFGRD96 84.0 140 30 35 4.14 0.5476
WVFGRD96 86.0 140 30 35 4.15 0.5726
WVFGRD96 88.0 140 30 35 4.15 0.5867
WVFGRD96 90.0 135 35 30 4.17 0.5967
WVFGRD96 92.0 135 35 30 4.18 0.6059
WVFGRD96 94.0 135 35 30 4.18 0.6150
WVFGRD96 96.0 135 35 30 4.18 0.6221
WVFGRD96 98.0 135 35 30 4.18 0.6291
WVFGRD96 100.0 135 35 30 4.18 0.6341
WVFGRD96 102.0 135 35 30 4.19 0.6393
WVFGRD96 104.0 135 35 30 4.19 0.6455
WVFGRD96 106.0 135 35 30 4.19 0.6508
WVFGRD96 108.0 135 35 30 4.19 0.6549
WVFGRD96 110.0 135 35 30 4.19 0.6571
WVFGRD96 112.0 135 35 30 4.19 0.6594
WVFGRD96 114.0 175 10 50 4.14 0.6650
WVFGRD96 116.0 170 10 45 4.14 0.6688
WVFGRD96 118.0 170 10 45 4.14 0.6701
WVFGRD96 120.0 170 10 45 4.15 0.6749
WVFGRD96 122.0 170 10 45 4.15 0.6770
WVFGRD96 124.0 170 10 45 4.15 0.6790
WVFGRD96 126.0 165 10 40 4.15 0.6806
WVFGRD96 128.0 170 10 45 4.15 0.6823
WVFGRD96 130.0 165 10 40 4.16 0.6831
WVFGRD96 132.0 165 10 40 4.16 0.6834
WVFGRD96 134.0 165 10 40 4.16 0.6840
WVFGRD96 136.0 165 10 40 4.16 0.6847
WVFGRD96 138.0 165 10 40 4.16 0.6853
WVFGRD96 140.0 165 10 40 4.16 0.6850
WVFGRD96 142.0 165 10 40 4.17 0.6848
WVFGRD96 144.0 165 10 40 4.17 0.6837
WVFGRD96 146.0 165 10 40 4.17 0.6824
WVFGRD96 148.0 165 10 40 4.17 0.6817
The best solution is
WVFGRD96 138.0 165 10 40 4.16 0.6853
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/3.1 -30 o DIST/3.1 +60 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2
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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