The ANSS event ID is ak016bjapymo and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak016bjapymo/executive.
2016/09/07 11:47:04 59.726 -153.142 104.2 4.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2016/09/07 11:47:04:0 59.73 -153.14 104.2 4.2 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CNP AK.CUT AK.HOM AK.PWL AK.RC01 AK.SKN
AT.OHAK AT.PMR AT.SVW2 II.KDAK TA.L19K TA.M22K TA.N18K
TA.N19K TA.O18K TA.O19K TA.O22K TA.P19K TA.Q19K
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 2.34e+22 dyne-cm
Mw = 4.18
Z = 114 km
Plane Strike Dip Rake
NP1 314 64 146
NP2 60 60 30
Principal Axes:
Axis Value Plunge Azimuth
T 2.34e+22 41 275
N 0.00e+00 49 101
P -2.34e+22 3 8
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.28e+22
Mxy -4.40e+21
Mxz 1.75e+15
Myy 1.27e+22
Myz -1.17e+22
Mzz 1.02e+22
-------- P ---
------------ -------
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################--------------------
###################-----------------##
#######################--------------###
#########################-----------####
####### #################---------######
####### T ###################-----########
####### ####################--##########
##############################--##########
###########################-----########
########################---------#######
###################--------------#####
#############-------------------####
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Global CMT Convention Moment Tensor:
R T P
1.02e+22 1.75e+15 1.17e+22
1.75e+15 -2.28e+22 4.40e+21
1.17e+22 4.40e+21 1.27e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160907114704/index.html
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STK = 60
DIP = 60
RAKE = 30
MW = 4.18
HS = 114.0
The NDK file is 20160907114704.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/3.3 -30 o DIST/3.3 +70 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 2.0 330 55 25 3.04 0.0896
WVFGRD96 4.0 330 55 25 3.16 0.1207
WVFGRD96 6.0 325 60 20 3.23 0.1486
WVFGRD96 8.0 325 55 20 3.32 0.1703
WVFGRD96 10.0 325 60 20 3.38 0.1861
WVFGRD96 12.0 325 60 15 3.42 0.1910
WVFGRD96 14.0 325 60 15 3.45 0.1863
WVFGRD96 16.0 325 60 10 3.47 0.1764
WVFGRD96 18.0 325 55 5 3.49 0.1618
WVFGRD96 20.0 240 80 0 3.54 0.1681
WVFGRD96 22.0 240 85 -5 3.57 0.1812
WVFGRD96 24.0 235 85 -5 3.59 0.1910
WVFGRD96 26.0 235 85 -5 3.61 0.1968
WVFGRD96 28.0 55 90 0 3.62 0.2010
WVFGRD96 30.0 55 90 0 3.64 0.2026
WVFGRD96 32.0 50 85 0 3.64 0.2030
WVFGRD96 34.0 50 85 -5 3.66 0.2013
WVFGRD96 36.0 50 80 -5 3.67 0.1966
WVFGRD96 38.0 235 90 5 3.71 0.1936
WVFGRD96 40.0 50 80 -15 3.75 0.1971
WVFGRD96 42.0 45 70 -25 3.79 0.1983
WVFGRD96 44.0 45 65 -20 3.81 0.1991
WVFGRD96 46.0 45 65 -20 3.84 0.2014
WVFGRD96 48.0 50 65 -15 3.86 0.2047
WVFGRD96 50.0 50 65 -15 3.88 0.2105
WVFGRD96 52.0 60 80 15 3.90 0.2245
WVFGRD96 54.0 55 70 25 3.92 0.2453
WVFGRD96 56.0 60 65 30 3.95 0.2722
WVFGRD96 58.0 60 65 30 3.98 0.2949
WVFGRD96 60.0 60 65 30 3.99 0.3115
WVFGRD96 62.0 60 65 30 4.01 0.3264
WVFGRD96 64.0 60 65 30 4.02 0.3406
WVFGRD96 66.0 60 65 30 4.03 0.3550
WVFGRD96 68.0 60 65 30 4.04 0.3695
WVFGRD96 70.0 60 60 30 4.06 0.3829
WVFGRD96 72.0 60 60 30 4.08 0.3971
WVFGRD96 74.0 60 60 30 4.08 0.4098
WVFGRD96 76.0 60 60 30 4.09 0.4210
WVFGRD96 78.0 60 60 30 4.10 0.4326
WVFGRD96 80.0 60 60 35 4.11 0.4422
WVFGRD96 82.0 60 60 35 4.11 0.4552
WVFGRD96 84.0 60 60 35 4.12 0.4641
WVFGRD96 86.0 60 60 35 4.12 0.4743
WVFGRD96 88.0 60 60 35 4.13 0.4826
WVFGRD96 90.0 60 60 35 4.13 0.4893
WVFGRD96 92.0 60 60 35 4.14 0.4974
WVFGRD96 94.0 60 60 35 4.14 0.5033
WVFGRD96 96.0 60 60 35 4.15 0.5081
WVFGRD96 98.0 60 60 35 4.15 0.5140
WVFGRD96 100.0 60 60 30 4.16 0.5180
WVFGRD96 102.0 60 60 30 4.16 0.5218
WVFGRD96 104.0 60 60 30 4.16 0.5239
WVFGRD96 106.0 60 60 30 4.17 0.5276
WVFGRD96 108.0 60 60 30 4.17 0.5306
WVFGRD96 110.0 60 60 30 4.17 0.5328
WVFGRD96 112.0 60 60 30 4.18 0.5342
WVFGRD96 114.0 60 60 30 4.18 0.5348
WVFGRD96 116.0 60 60 30 4.18 0.5344
WVFGRD96 118.0 60 60 30 4.18 0.5340
The best solution is
WVFGRD96 114.0 60 60 30 4.18 0.5348
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/3.3 -30 o DIST/3.3 +70 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