The ANSS event ID is ak0172b22lil and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0172b22lil/executive.
2017/02/19 22:17:29 59.731 -153.139 103.0 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/02/19 22:17:29:0 59.73 -153.14 103.0 4.1 Alaska
Stations used:
AK.CAPN AK.CNP AK.RC01 AK.SKN AK.SSN AT.SVW2 AV.ILSW
TA.M19K TA.O19K TA.O22K TA.P18K TA.P19K TA.Q19K
Filtering commands used:
cut o DIST/3.5 -40 o DIST/3.5 +60
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.55e+22 dyne-cm
Mw = 4.06
Z = 120 km
Plane Strike Dip Rake
NP1 308 71 159
NP2 45 70 20
Principal Axes:
Axis Value Plunge Azimuth
T 1.55e+22 28 266
N 0.00e+00 62 88
P -1.55e+22 1 357
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.54e+22
Mxy 1.70e+21
Mxz -6.50e+20
Myy 1.20e+22
Myz -6.39e+21
Mzz 3.41e+21
---- P -------
-------- -----------
----------------------------
------------------------------
#####--------------------------###
###########--------------------#####
###############-----------------######
###################-------------########
######################--------##########
#########################-----############
#### ####################--#############
#### T ####################--#############
#### ##################------###########
#######################---------########
#####################------------#######
#################----------------#####
##############-------------------###
##########-----------------------#
###---------------------------
----------------------------
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
3.41e+21 -6.50e+20 6.39e+21
-6.50e+20 -1.54e+22 -1.70e+21
6.39e+21 -1.70e+21 1.20e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170219221729/index.html
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STK = 45
DIP = 70
RAKE = 20
MW = 4.06
HS = 120.0
The NDK file is 20170219221729.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/3.5 -40 o DIST/3.5 +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 2.0 225 70 -20 3.03 0.1106
WVFGRD96 4.0 220 55 -25 3.17 0.1311
WVFGRD96 6.0 220 55 -15 3.22 0.1521
WVFGRD96 8.0 325 70 35 3.32 0.1637
WVFGRD96 10.0 320 65 25 3.35 0.1734
WVFGRD96 12.0 320 60 15 3.39 0.1732
WVFGRD96 14.0 315 55 -10 3.41 0.1699
WVFGRD96 16.0 315 55 -10 3.43 0.1598
WVFGRD96 18.0 315 50 -10 3.43 0.1469
WVFGRD96 20.0 315 50 -10 3.44 0.1337
WVFGRD96 22.0 315 50 -10 3.45 0.1212
WVFGRD96 24.0 50 80 -5 3.48 0.1160
WVFGRD96 26.0 50 80 -5 3.51 0.1269
WVFGRD96 28.0 50 85 -5 3.54 0.1398
WVFGRD96 30.0 230 90 10 3.56 0.1537
WVFGRD96 32.0 50 85 -10 3.59 0.1645
WVFGRD96 34.0 50 85 -10 3.61 0.1708
WVFGRD96 36.0 50 85 -10 3.64 0.1729
WVFGRD96 38.0 230 90 5 3.67 0.1750
WVFGRD96 40.0 50 90 -5 3.73 0.1788
WVFGRD96 42.0 50 90 -5 3.75 0.1777
WVFGRD96 44.0 230 90 5 3.78 0.1760
WVFGRD96 46.0 230 80 -5 3.80 0.1746
WVFGRD96 48.0 230 80 -5 3.82 0.1761
WVFGRD96 50.0 230 80 -5 3.84 0.1784
WVFGRD96 52.0 230 80 -5 3.85 0.1820
WVFGRD96 54.0 230 80 -5 3.87 0.1904
WVFGRD96 56.0 50 80 25 3.89 0.2142
WVFGRD96 58.0 50 80 25 3.91 0.2383
WVFGRD96 60.0 50 75 25 3.92 0.2551
WVFGRD96 62.0 50 75 25 3.94 0.2660
WVFGRD96 64.0 50 75 25 3.95 0.2758
WVFGRD96 66.0 50 75 25 3.96 0.2859
WVFGRD96 68.0 50 75 25 3.97 0.2953
WVFGRD96 70.0 50 75 25 3.98 0.3035
WVFGRD96 72.0 50 75 25 3.98 0.3125
WVFGRD96 74.0 50 75 25 3.99 0.3193
WVFGRD96 76.0 50 75 25 4.00 0.3256
WVFGRD96 78.0 50 75 30 4.00 0.3323
WVFGRD96 80.0 50 75 25 4.01 0.3357
WVFGRD96 82.0 50 70 30 4.01 0.3415
WVFGRD96 84.0 50 70 30 4.01 0.3451
WVFGRD96 86.0 50 70 30 4.01 0.3496
WVFGRD96 88.0 50 65 30 4.01 0.3540
WVFGRD96 90.0 50 65 30 4.02 0.3565
WVFGRD96 92.0 50 65 30 4.02 0.3612
WVFGRD96 94.0 50 65 30 4.02 0.3643
WVFGRD96 96.0 50 65 30 4.02 0.3653
WVFGRD96 98.0 50 65 30 4.03 0.3676
WVFGRD96 100.0 45 70 25 4.04 0.3694
WVFGRD96 102.0 45 70 25 4.04 0.3706
WVFGRD96 104.0 45 70 25 4.05 0.3715
WVFGRD96 106.0 45 70 25 4.05 0.3723
WVFGRD96 108.0 45 70 25 4.05 0.3744
WVFGRD96 110.0 45 70 25 4.05 0.3758
WVFGRD96 112.0 45 70 25 4.05 0.3766
WVFGRD96 114.0 45 70 25 4.06 0.3765
WVFGRD96 116.0 45 70 25 4.06 0.3769
WVFGRD96 118.0 45 70 20 4.06 0.3773
WVFGRD96 120.0 45 70 20 4.06 0.3778
WVFGRD96 122.0 45 70 20 4.07 0.3776
WVFGRD96 124.0 45 70 20 4.07 0.3771
WVFGRD96 126.0 45 70 20 4.07 0.3773
WVFGRD96 128.0 45 70 20 4.07 0.3772
WVFGRD96 130.0 45 70 20 4.07 0.3765
WVFGRD96 132.0 55 65 25 4.05 0.3769
WVFGRD96 134.0 55 65 25 4.05 0.3777
WVFGRD96 136.0 55 65 25 4.05 0.3774
WVFGRD96 138.0 55 65 25 4.06 0.3774
The best solution is
WVFGRD96 120.0 45 70 20 4.06 0.3778
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/3.5 -40 o DIST/3.5 +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