The ANSS event ID is ak0137l2je73 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0137l2je73/executive.
2013/06/14 11:53:01 60.892 -149.960 36.5 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2013/06/14 11:53:01:0 60.89 -149.96 36.5 3.7 Alaska
Stations used:
AK.BAL AK.BRLK AK.CAST AK.CNP AK.GLI AK.KTH AK.RC01 AK.SCM
AK.SSN AK.TRF AT.MENT AT.PMR AT.SVW2 AT.TTA IU.COLA
Filtering commands used:
hp c 0.02 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 6.53e+21 dyne-cm
Mw = 3.81
Z = 42 km
Plane Strike Dip Rake
NP1 220 55 -80
NP2 23 36 -104
Principal Axes:
Axis Value Plunge Azimuth
T 6.53e+21 9 303
N 0.00e+00 8 34
P -6.53e+21 77 164
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.58e+21
Mxy -2.81e+21
Mxz 1.91e+21
Myy 4.46e+21
Myz -1.27e+21
Mzz -6.04e+21
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#####################-
#######################--###
###################--------###
##############-------------####
# T ############---------------#####
## ##########------------------#####
##############--------------------######
############----------------------######
############-----------------------#######
###########------------------------#######
##########---------- ------------#######
#########----------- P -----------########
#######------------ ----------########
#######-------------------------########
#####-------------------------########
####-----------------------#########
###----------------------#########
#--------------------#########
#-----------------##########
-----------###########
##############
Global CMT Convention Moment Tensor:
R T P
-6.04e+21 1.91e+21 1.27e+21
1.91e+21 1.58e+21 2.81e+21
1.27e+21 2.81e+21 4.46e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130614115301/index.html
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STK = 220
DIP = 55
RAKE = -80
MW = 3.81
HS = 42.0
The NDK file is 20130614115301.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:
hp c 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 70 45 -90 3.06 0.2517
WVFGRD96 1.0 285 85 0 3.08 0.2093
WVFGRD96 2.0 255 45 -90 3.20 0.2875
WVFGRD96 3.0 290 75 10 3.25 0.2488
WVFGRD96 4.0 300 55 25 3.30 0.2467
WVFGRD96 5.0 145 30 -15 3.26 0.2770
WVFGRD96 6.0 140 30 -25 3.27 0.3128
WVFGRD96 7.0 135 30 -30 3.28 0.3337
WVFGRD96 8.0 125 25 -35 3.35 0.3431
WVFGRD96 9.0 125 25 -35 3.34 0.3474
WVFGRD96 10.0 135 30 -30 3.33 0.3490
WVFGRD96 11.0 135 30 -30 3.33 0.3498
WVFGRD96 12.0 135 30 -30 3.33 0.3511
WVFGRD96 13.0 135 30 -30 3.34 0.3519
WVFGRD96 14.0 240 80 -60 3.36 0.3533
WVFGRD96 15.0 240 80 -60 3.37 0.3573
WVFGRD96 16.0 245 85 -55 3.38 0.3615
WVFGRD96 17.0 245 85 -55 3.39 0.3651
WVFGRD96 18.0 245 85 -55 3.40 0.3678
WVFGRD96 19.0 240 65 -50 3.43 0.3717
WVFGRD96 20.0 240 65 -50 3.44 0.3781
WVFGRD96 21.0 240 65 -50 3.46 0.3839
WVFGRD96 22.0 240 65 -50 3.47 0.3901
WVFGRD96 23.0 235 60 -55 3.48 0.3973
WVFGRD96 24.0 230 55 -55 3.49 0.4044
WVFGRD96 25.0 230 55 -55 3.50 0.4127
WVFGRD96 26.0 230 55 -55 3.51 0.4224
WVFGRD96 27.0 225 55 -60 3.53 0.4335
WVFGRD96 28.0 225 55 -65 3.54 0.4460
WVFGRD96 29.0 225 55 -65 3.55 0.4608
WVFGRD96 30.0 225 55 -65 3.56 0.4771
WVFGRD96 31.0 225 55 -65 3.58 0.4942
WVFGRD96 32.0 230 55 -65 3.59 0.5130
WVFGRD96 33.0 225 55 -70 3.61 0.5298
WVFGRD96 34.0 225 55 -70 3.62 0.5439
WVFGRD96 35.0 225 55 -70 3.63 0.5539
WVFGRD96 36.0 225 55 -70 3.64 0.5602
WVFGRD96 37.0 220 50 -75 3.65 0.5652
WVFGRD96 38.0 220 50 -75 3.67 0.5699
WVFGRD96 39.0 215 50 -80 3.69 0.5733
WVFGRD96 40.0 220 55 -80 3.79 0.5746
WVFGRD96 41.0 220 55 -80 3.80 0.5797
WVFGRD96 42.0 220 55 -80 3.81 0.5805
WVFGRD96 43.0 215 50 -85 3.82 0.5780
WVFGRD96 44.0 215 50 -85 3.83 0.5773
WVFGRD96 45.0 215 50 -85 3.84 0.5731
WVFGRD96 46.0 215 50 -85 3.84 0.5688
WVFGRD96 47.0 25 40 -95 3.85 0.5626
WVFGRD96 48.0 25 40 -95 3.85 0.5545
WVFGRD96 49.0 30 40 -90 3.86 0.5461
The best solution is
WVFGRD96 42.0 220 55 -80 3.81 0.5805
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
hp c 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2
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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