The ANSS event ID is ak016dnda5em and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak016dnda5em/executive.
2016/10/23 11:20:10 61.919 -151.748 99.7 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2016/10/23 11:20:10:0 61.92 -151.75 99.7 4.1 Alaska
Stations used:
AK.CAPN AK.CAST AK.DHY AK.KTH AK.MCK AK.RC01 AK.RND AK.SKN
AK.SSN AK.TRF AV.ILSW TA.L19K TA.M19K TA.M22K TA.N18K
TA.N19K TA.O18K TA.O19K TA.O22K
Filtering commands used:
cut o DIST/3.9 -40 o DIST/3.9 +40
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 2.60e+22 dyne-cm
Mw = 4.21
Z = 134 km
Plane Strike Dip Rake
NP1 350 88 115
NP2 85 25 5
Principal Axes:
Axis Value Plunge Azimuth
T 2.60e+22 42 284
N 0.00e+00 25 169
P -2.60e+22 38 58
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.62e+21
Mxy -1.06e+22
Mxz -3.50e+21
Myy 1.89e+21
Myz -2.33e+22
Mzz 1.74e+21
####----------
########--------------
############----------------
#############-----------------
###############-------------------
#################-------------------
##################---------- -------
###################---------- P --------
####### ##########--------- --------
######## T ##########---------------------
######## ##########---------------------
######################-------------------#
-#####################------------------##
#####################------------------#
--####################---------------###
--###################--------------###
---#################------------####
----###############----------#####
-----#############------######
------------###--###########
--------------########
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Global CMT Convention Moment Tensor:
R T P
1.74e+21 -3.50e+21 2.33e+22
-3.50e+21 -3.62e+21 1.06e+22
2.33e+22 1.06e+22 1.89e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20161023112010/index.html
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STK = 85
DIP = 25
RAKE = 5
MW = 4.21
HS = 134.0
The NDK file is 20161023112010.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/3.9 -40 o DIST/3.9 +40 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 2.0 160 90 0 3.25 0.2692
WVFGRD96 4.0 160 80 -10 3.37 0.3151
WVFGRD96 6.0 160 80 -5 3.44 0.3306
WVFGRD96 8.0 160 80 -5 3.51 0.3345
WVFGRD96 10.0 160 80 -5 3.55 0.3190
WVFGRD96 12.0 160 80 -5 3.58 0.2942
WVFGRD96 14.0 160 80 -5 3.60 0.2669
WVFGRD96 16.0 245 75 5 3.59 0.2457
WVFGRD96 18.0 245 75 5 3.62 0.2504
WVFGRD96 20.0 245 75 5 3.66 0.2618
WVFGRD96 22.0 245 75 5 3.69 0.2789
WVFGRD96 24.0 250 80 5 3.74 0.2979
WVFGRD96 26.0 250 80 5 3.76 0.3173
WVFGRD96 28.0 245 80 0 3.76 0.3341
WVFGRD96 30.0 245 80 0 3.78 0.3500
WVFGRD96 32.0 245 80 5 3.80 0.3663
WVFGRD96 34.0 245 80 5 3.82 0.3849
WVFGRD96 36.0 245 80 5 3.85 0.3980
WVFGRD96 38.0 250 85 5 3.90 0.4112
WVFGRD96 40.0 70 90 0 3.95 0.4266
WVFGRD96 42.0 250 85 5 3.98 0.4314
WVFGRD96 44.0 250 80 5 4.01 0.4376
WVFGRD96 46.0 250 85 5 4.03 0.4406
WVFGRD96 48.0 250 85 5 4.04 0.4438
WVFGRD96 50.0 70 90 -5 4.06 0.4471
WVFGRD96 52.0 70 80 -5 4.06 0.4559
WVFGRD96 54.0 70 80 -5 4.07 0.4636
WVFGRD96 56.0 70 75 -5 4.08 0.4726
WVFGRD96 58.0 70 75 0 4.08 0.4813
WVFGRD96 60.0 70 75 0 4.09 0.4879
WVFGRD96 62.0 70 75 0 4.09 0.4923
WVFGRD96 64.0 70 70 0 4.09 0.4978
WVFGRD96 66.0 70 70 0 4.09 0.5030
WVFGRD96 68.0 70 70 0 4.09 0.5060
WVFGRD96 70.0 70 65 0 4.09 0.5116
WVFGRD96 72.0 70 65 5 4.10 0.5135
WVFGRD96 74.0 70 65 5 4.10 0.5180
WVFGRD96 76.0 70 60 5 4.10 0.5189
WVFGRD96 78.0 70 60 5 4.10 0.5243
WVFGRD96 80.0 70 60 10 4.10 0.5271
WVFGRD96 82.0 70 55 10 4.10 0.5304
WVFGRD96 84.0 70 55 10 4.10 0.5342
WVFGRD96 86.0 70 55 10 4.11 0.5357
WVFGRD96 88.0 70 55 10 4.11 0.5350
WVFGRD96 90.0 70 50 5 4.11 0.5380
WVFGRD96 92.0 70 50 5 4.11 0.5419
WVFGRD96 94.0 70 50 5 4.11 0.5434
WVFGRD96 96.0 70 50 5 4.11 0.5433
WVFGRD96 98.0 75 40 5 4.13 0.5447
WVFGRD96 100.0 75 40 5 4.13 0.5490
WVFGRD96 102.0 75 40 5 4.13 0.5509
WVFGRD96 104.0 75 40 5 4.14 0.5523
WVFGRD96 106.0 75 40 5 4.14 0.5563
WVFGRD96 108.0 80 30 5 4.16 0.5581
WVFGRD96 110.0 80 30 5 4.17 0.5609
WVFGRD96 112.0 80 30 5 4.17 0.5651
WVFGRD96 114.0 80 30 5 4.17 0.5659
WVFGRD96 116.0 80 30 5 4.17 0.5695
WVFGRD96 118.0 80 30 5 4.17 0.5712
WVFGRD96 120.0 85 25 5 4.20 0.5716
WVFGRD96 122.0 85 25 5 4.20 0.5747
WVFGRD96 124.0 85 25 5 4.20 0.5730
WVFGRD96 126.0 85 25 5 4.20 0.5757
WVFGRD96 128.0 85 25 5 4.20 0.5745
WVFGRD96 130.0 85 25 5 4.21 0.5761
WVFGRD96 132.0 85 25 5 4.21 0.5762
WVFGRD96 134.0 85 25 5 4.21 0.5766
WVFGRD96 136.0 85 25 5 4.21 0.5761
WVFGRD96 138.0 85 25 5 4.21 0.5746
The best solution is
WVFGRD96 134.0 85 25 5 4.21 0.5766
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/3.9 -40 o DIST/3.9 +40 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