The ANSS event ID is ak0166y1gx2u and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0166y1gx2u/executive.
2016/05/30 19:01:30 59.078 -153.758 102.7 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2016/05/30 19:01:30:0 59.08 -153.76 102.7 4.1 Alaska
Stations used:
AK.BRLK AK.CNP AK.HOM AT.OHAK AV.ILSW II.KDAK TA.N18K
TA.N19K TA.O18K TA.P18K TA.Q19K
Filtering commands used:
cut o DIST/3.2 -30 o DIST/3.2 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.84e+22 dyne-cm
Mw = 4.11
Z = 96 km
Plane Strike Dip Rake
NP1 65 65 60
NP2 299 38 137
Principal Axes:
Axis Value Plunge Azimuth
T 1.84e+22 59 292
N 0.00e+00 27 79
P -1.84e+22 15 176
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.64e+22
Mxy -6.84e+20
Mxz 7.64e+21
Myy 4.21e+21
Myz -7.86e+21
Mzz 1.22e+22
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################################------##
########### ####################--####
############ T ###########################
############ ###################---#####
################################------####
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-------------- -----------
----------- P --------
------- ----
Global CMT Convention Moment Tensor:
R T P
1.22e+22 7.64e+21 7.86e+21
7.64e+21 -1.64e+22 6.84e+20
7.86e+21 6.84e+20 4.21e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160530190130/index.html
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STK = 65
DIP = 65
RAKE = 60
MW = 4.11
HS = 96.0
The NDK file is 20160530190130.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/3.2 -30 o DIST/3.2 +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 2.0 310 90 5 3.14 0.2135
WVFGRD96 4.0 130 70 0 3.28 0.2640
WVFGRD96 6.0 310 85 -30 3.36 0.3000
WVFGRD96 8.0 130 80 25 3.45 0.3356
WVFGRD96 10.0 130 75 25 3.50 0.3556
WVFGRD96 12.0 130 75 20 3.53 0.3561
WVFGRD96 14.0 130 75 20 3.56 0.3445
WVFGRD96 16.0 130 75 25 3.58 0.3268
WVFGRD96 18.0 130 75 25 3.59 0.3031
WVFGRD96 20.0 220 75 15 3.61 0.2925
WVFGRD96 22.0 220 80 15 3.64 0.3094
WVFGRD96 24.0 220 80 15 3.66 0.3260
WVFGRD96 26.0 220 80 10 3.69 0.3413
WVFGRD96 28.0 220 85 10 3.70 0.3535
WVFGRD96 30.0 220 85 10 3.71 0.3607
WVFGRD96 32.0 225 75 10 3.74 0.3680
WVFGRD96 34.0 225 75 10 3.76 0.3779
WVFGRD96 36.0 45 90 10 3.79 0.3860
WVFGRD96 38.0 45 80 10 3.82 0.4138
WVFGRD96 40.0 45 70 15 3.88 0.4498
WVFGRD96 42.0 45 65 15 3.91 0.4578
WVFGRD96 44.0 45 65 15 3.93 0.4624
WVFGRD96 46.0 45 65 15 3.95 0.4665
WVFGRD96 48.0 45 70 20 3.96 0.4733
WVFGRD96 50.0 50 65 35 3.99 0.4846
WVFGRD96 52.0 50 65 35 4.00 0.4980
WVFGRD96 54.0 50 65 35 4.01 0.5100
WVFGRD96 56.0 60 60 50 4.04 0.5259
WVFGRD96 58.0 60 60 50 4.05 0.5452
WVFGRD96 60.0 60 60 50 4.05 0.5622
WVFGRD96 62.0 60 60 50 4.06 0.5744
WVFGRD96 64.0 60 60 50 4.06 0.5870
WVFGRD96 66.0 60 60 50 4.07 0.5984
WVFGRD96 68.0 65 60 55 4.08 0.6066
WVFGRD96 70.0 65 60 55 4.08 0.6175
WVFGRD96 72.0 60 65 55 4.08 0.6250
WVFGRD96 74.0 60 65 55 4.08 0.6342
WVFGRD96 76.0 60 65 55 4.08 0.6400
WVFGRD96 78.0 60 65 55 4.09 0.6449
WVFGRD96 80.0 60 65 55 4.09 0.6512
WVFGRD96 82.0 65 65 60 4.10 0.6556
WVFGRD96 84.0 65 65 60 4.10 0.6602
WVFGRD96 86.0 65 65 60 4.10 0.6623
WVFGRD96 88.0 65 65 60 4.10 0.6675
WVFGRD96 90.0 65 65 60 4.11 0.6702
WVFGRD96 92.0 65 65 60 4.11 0.6712
WVFGRD96 94.0 65 65 60 4.11 0.6722
WVFGRD96 96.0 65 65 60 4.11 0.6728
WVFGRD96 98.0 65 65 60 4.11 0.6723
WVFGRD96 100.0 65 65 60 4.11 0.6723
WVFGRD96 102.0 60 70 65 4.11 0.6713
WVFGRD96 104.0 60 70 65 4.11 0.6712
WVFGRD96 106.0 60 70 65 4.11 0.6713
WVFGRD96 108.0 60 70 65 4.11 0.6702
WVFGRD96 110.0 60 70 65 4.12 0.6701
WVFGRD96 112.0 60 70 65 4.12 0.6706
WVFGRD96 114.0 60 70 65 4.12 0.6693
WVFGRD96 116.0 60 70 60 4.12 0.6674
WVFGRD96 118.0 60 70 60 4.12 0.6665
WVFGRD96 120.0 60 70 60 4.12 0.6660
WVFGRD96 122.0 60 70 60 4.12 0.6636
WVFGRD96 124.0 60 70 60 4.12 0.6595
WVFGRD96 126.0 60 70 60 4.12 0.6578
WVFGRD96 128.0 60 70 60 4.13 0.6543
WVFGRD96 130.0 60 70 60 4.13 0.6501
WVFGRD96 132.0 60 70 60 4.13 0.6491
WVFGRD96 134.0 60 70 60 4.13 0.6459
WVFGRD96 136.0 60 70 60 4.13 0.6416
WVFGRD96 138.0 60 70 60 4.13 0.6393
The best solution is
WVFGRD96 96.0 65 65 60 4.11 0.6728
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/3.2 -30 o DIST/3.2 +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