Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.
The ANSS event ID is ak016b9dz3vp and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak016b9dz3vp/executive.
2016/09/01 12:27:41 61.299 -152.165 131.7 4.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2016/09/01 12:27:41:0 61.30 -152.16 131.7 4.5 Alaska
Stations used:
AK.BRSE AK.CHUM AK.SLK AK.WAT1 AK.WAT6 AK.WAT7 AV.AUJA
AV.NCT AV.RDDF AV.RDSO AV.RDWB AV.RED AV.SPNN TA.K24K
TA.L20K TA.M23K TA.N16K TA.N20K TA.O16K TA.O17K TA.O18K
TA.O20K TA.P16K TA.Q16K TA.Q20K XV.FAPT XV.FPAP XV.FTGH
Filtering commands used:
cut a -20 a 100
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 5.96e+22 dyne-cm
Mw = 4.45
Z = 128 km
Plane Strike Dip Rake
NP1 307 51 124
NP2 80 50 55
Principal Axes:
Axis Value Plunge Azimuth
T 5.96e+22 64 283
N 0.00e+00 26 104
P -5.96e+22 1 14
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.56e+22
Mxy -1.64e+22
Mxz 4.53e+21
Myy 7.50e+21
Myz -2.31e+22
Mzz 4.81e+22
----------- P
--------------- ----
----------------------------
------------------------------
###############-------------------
####################----------------
########################--------------
############################------------
#############################-----------
############# ################---------#
############# T #################-------##
############# ###################----###
####################################-#####
-#################################-#####
---############################-----####
-----#####################---------###
-----------#######-----------------#
----------------------------------
------------------------------
----------------------------
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
4.81e+22 4.53e+21 2.31e+22
4.53e+21 -5.56e+22 1.64e+22
2.31e+22 1.64e+22 7.50e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160901122741/index.html
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STK = 80
DIP = 50
RAKE = 55
MW = 4.45
HS = 128.0
The NDK file is 20160901122741.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: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
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 a -20 a 100 rtr taper w 0.1 hp c 0.02 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 95 55 -85 3.59 0.1614
WVFGRD96 4.0 335 30 -15 3.67 0.1614
WVFGRD96 6.0 335 35 -10 3.70 0.1913
WVFGRD96 8.0 340 35 0 3.78 0.1974
WVFGRD96 10.0 -5 35 30 3.82 0.2035
WVFGRD96 12.0 195 40 -35 3.82 0.2149
WVFGRD96 14.0 190 40 -35 3.86 0.2315
WVFGRD96 16.0 190 40 -35 3.90 0.2399
WVFGRD96 18.0 195 40 -25 3.92 0.2405
WVFGRD96 20.0 195 35 -25 3.93 0.2365
WVFGRD96 22.0 205 35 -10 3.95 0.2288
WVFGRD96 24.0 210 35 0 3.97 0.2197
WVFGRD96 26.0 215 35 10 3.98 0.2098
WVFGRD96 28.0 200 35 -15 4.03 0.2015
WVFGRD96 30.0 205 35 -5 4.04 0.1954
WVFGRD96 32.0 210 35 0 4.05 0.1901
WVFGRD96 34.0 210 35 0 4.06 0.1878
WVFGRD96 36.0 250 80 15 4.11 0.1876
WVFGRD96 38.0 65 50 50 4.06 0.1946
WVFGRD96 40.0 155 70 -30 4.20 0.2152
WVFGRD96 42.0 155 70 -30 4.24 0.2217
WVFGRD96 44.0 155 70 -30 4.26 0.2255
WVFGRD96 46.0 150 65 -40 4.26 0.2304
WVFGRD96 48.0 145 60 -45 4.27 0.2335
WVFGRD96 50.0 145 60 -40 4.29 0.2344
WVFGRD96 52.0 90 65 70 4.26 0.2379
WVFGRD96 54.0 280 25 105 4.27 0.2528
WVFGRD96 56.0 90 65 70 4.29 0.2760
WVFGRD96 58.0 245 35 55 4.31 0.2949
WVFGRD96 60.0 245 35 55 4.32 0.3140
WVFGRD96 62.0 90 65 70 4.33 0.3344
WVFGRD96 64.0 85 65 65 4.35 0.3516
WVFGRD96 66.0 85 65 65 4.36 0.3686
WVFGRD96 68.0 85 65 65 4.36 0.3836
WVFGRD96 70.0 85 65 65 4.37 0.3973
WVFGRD96 72.0 85 65 65 4.38 0.4092
WVFGRD96 74.0 85 65 65 4.38 0.4202
WVFGRD96 76.0 80 65 60 4.40 0.4310
WVFGRD96 78.0 80 65 60 4.40 0.4402
WVFGRD96 80.0 80 65 60 4.41 0.4491
WVFGRD96 82.0 80 65 60 4.41 0.4564
WVFGRD96 84.0 80 60 60 4.41 0.4636
WVFGRD96 86.0 80 60 60 4.41 0.4706
WVFGRD96 88.0 80 60 60 4.41 0.4772
WVFGRD96 90.0 80 60 60 4.42 0.4829
WVFGRD96 92.0 80 60 55 4.43 0.4876
WVFGRD96 94.0 80 60 55 4.43 0.4918
WVFGRD96 96.0 80 55 60 4.42 0.4957
WVFGRD96 98.0 80 55 60 4.42 0.5000
WVFGRD96 100.0 80 55 60 4.42 0.5032
WVFGRD96 102.0 80 55 60 4.43 0.5057
WVFGRD96 104.0 80 55 60 4.43 0.5086
WVFGRD96 106.0 80 55 60 4.43 0.5116
WVFGRD96 108.0 80 55 60 4.43 0.5146
WVFGRD96 110.0 80 55 55 4.44 0.5165
WVFGRD96 112.0 75 55 55 4.44 0.5174
WVFGRD96 114.0 75 55 55 4.45 0.5188
WVFGRD96 116.0 75 55 55 4.45 0.5202
WVFGRD96 118.0 75 55 55 4.45 0.5219
WVFGRD96 120.0 75 55 55 4.45 0.5234
WVFGRD96 122.0 80 50 55 4.45 0.5234
WVFGRD96 124.0 80 50 55 4.45 0.5237
WVFGRD96 126.0 80 50 55 4.45 0.5251
WVFGRD96 128.0 80 50 55 4.45 0.5253
WVFGRD96 130.0 80 50 55 4.45 0.5243
WVFGRD96 132.0 75 50 50 4.46 0.5248
WVFGRD96 134.0 75 50 50 4.46 0.5241
WVFGRD96 136.0 75 50 50 4.46 0.5238
WVFGRD96 138.0 75 50 50 4.47 0.5232
WVFGRD96 140.0 75 50 50 4.47 0.5217
WVFGRD96 142.0 75 50 50 4.47 0.5213
WVFGRD96 144.0 75 50 50 4.47 0.5194
WVFGRD96 146.0 75 50 50 4.47 0.5184
WVFGRD96 148.0 75 50 50 4.47 0.5170
The best solution is
WVFGRD96 128.0 80 50 55 4.45 0.5253
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 a -20 a 100 rtr taper w 0.1 hp c 0.02 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