The ANSS event ID is ak0179fdtwtr and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0179fdtwtr/executive.
2017/07/24 21:26:40 59.665 -152.511 92.8 3.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/07/24 21:26:40:0 59.67 -152.51 92.8 3.8 Alaska
Stations used:
AK.CNP AK.FIRE AK.HOM AK.RC01 AV.ILSW II.KDAK TA.N19K
TA.O18K TA.P18K TA.P19K TA.Q19K
Filtering commands used:
cut o DIST/3.5 -30 o DIST/3.5 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 7.50e+21 dyne-cm
Mw = 3.85
Z = 100 km
Plane Strike Dip Rake
NP1 215 90 -35
NP2 305 55 -180
Principal Axes:
Axis Value Plunge Azimuth
T 7.50e+21 24 266
N 0.00e+00 55 35
P -7.50e+21 24 164
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.77e+21
Mxy 2.10e+21
Mxz 2.47e+21
Myy 5.77e+21
Myz -3.52e+21
Mzz 3.76e+14
--------------
----------------------
-------------------------###
-------------------------#####
############-------------#########
##################-------###########
######################---#############
#########################-##############
#######################-----############
#######################--------###########
#### ###############-----------#########
#### T #############--------------########
#### ############----------------#######
#################------------------#####
###############---------------------####
#############-----------------------##
###########------------------------#
#########-------------------------
#####------------- ---------
###-------------- P --------
-------------- -----
--------------
Global CMT Convention Moment Tensor:
R T P
3.76e+14 2.47e+21 3.52e+21
2.47e+21 -5.77e+21 -2.10e+21
3.52e+21 -2.10e+21 5.77e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170724212640/index.html
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STK = 215
DIP = 90
RAKE = -35
MW = 3.85
HS = 100.0
The NDK file is 20170724212640.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 -30 o DIST/3.5 +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 2.0 155 30 75 2.93 0.1627
WVFGRD96 4.0 135 30 45 3.00 0.1930
WVFGRD96 6.0 285 40 -30 3.02 0.2108
WVFGRD96 8.0 275 35 -50 3.15 0.2236
WVFGRD96 10.0 260 30 -80 3.23 0.2233
WVFGRD96 12.0 85 55 -80 3.26 0.2192
WVFGRD96 14.0 85 55 -80 3.28 0.2103
WVFGRD96 16.0 85 55 -80 3.30 0.1972
WVFGRD96 18.0 85 55 -80 3.31 0.1805
WVFGRD96 20.0 30 75 -40 3.24 0.1668
WVFGRD96 22.0 250 65 80 3.40 0.1712
WVFGRD96 24.0 245 65 80 3.42 0.1805
WVFGRD96 26.0 250 65 80 3.46 0.1902
WVFGRD96 28.0 250 65 80 3.49 0.1977
WVFGRD96 30.0 245 65 80 3.50 0.2008
WVFGRD96 32.0 245 65 80 3.52 0.1968
WVFGRD96 34.0 245 70 75 3.53 0.1989
WVFGRD96 36.0 245 70 75 3.53 0.2024
WVFGRD96 38.0 240 70 70 3.52 0.2001
WVFGRD96 40.0 45 90 -60 3.52 0.1986
WVFGRD96 42.0 180 40 -70 3.51 0.2261
WVFGRD96 44.0 180 45 -70 3.56 0.2646
WVFGRD96 46.0 180 45 -65 3.59 0.2984
WVFGRD96 48.0 185 55 -65 3.64 0.3283
WVFGRD96 50.0 185 55 -65 3.66 0.3445
WVFGRD96 52.0 190 60 -60 3.67 0.3508
WVFGRD96 54.0 190 60 -60 3.68 0.3568
WVFGRD96 56.0 195 60 -55 3.67 0.3618
WVFGRD96 58.0 195 60 -55 3.68 0.3669
WVFGRD96 60.0 195 60 -55 3.68 0.3741
WVFGRD96 62.0 200 65 -50 3.69 0.3835
WVFGRD96 64.0 200 70 -50 3.72 0.3920
WVFGRD96 66.0 205 75 -45 3.72 0.4016
WVFGRD96 68.0 205 75 -45 3.73 0.4112
WVFGRD96 70.0 205 75 -45 3.74 0.4204
WVFGRD96 72.0 205 75 -45 3.75 0.4289
WVFGRD96 74.0 210 80 -40 3.75 0.4357
WVFGRD96 76.0 210 80 -40 3.76 0.4452
WVFGRD96 78.0 210 85 -40 3.78 0.4512
WVFGRD96 80.0 210 85 -40 3.79 0.4570
WVFGRD96 82.0 210 85 -40 3.80 0.4639
WVFGRD96 84.0 215 90 -40 3.82 0.4684
WVFGRD96 86.0 215 90 -40 3.82 0.4726
WVFGRD96 88.0 215 90 -40 3.83 0.4774
WVFGRD96 90.0 215 90 -35 3.82 0.4795
WVFGRD96 92.0 215 90 -35 3.83 0.4843
WVFGRD96 94.0 215 90 -35 3.83 0.4852
WVFGRD96 96.0 215 90 -35 3.84 0.4881
WVFGRD96 98.0 215 90 -35 3.84 0.4883
WVFGRD96 100.0 215 90 -35 3.85 0.4906
WVFGRD96 102.0 215 90 -35 3.85 0.4889
WVFGRD96 104.0 215 90 -35 3.85 0.4894
WVFGRD96 106.0 215 90 -35 3.86 0.4873
WVFGRD96 108.0 210 85 -30 3.84 0.4871
WVFGRD96 110.0 210 85 -30 3.85 0.4857
WVFGRD96 112.0 210 85 -30 3.85 0.4849
WVFGRD96 114.0 210 85 -30 3.85 0.4833
WVFGRD96 116.0 210 85 -30 3.86 0.4821
WVFGRD96 118.0 210 85 -30 3.86 0.4800
The best solution is
WVFGRD96 100.0 215 90 -35 3.85 0.4906
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 -30 o DIST/3.5 +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