The ANSS event ID is ak014am75wqb and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak014am75wqb/executive.
2014/08/19 10:11:22 60.028 -153.091 131.9 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2014/08/19 10:11:22:0 60.03 -153.09 131.9 4.1 Alaska
Stations used:
AK.BRLK AK.CNP AK.FID AK.GHO AK.GLI AK.KNK AK.SAW AK.SCM
AK.SKN AK.SSN AK.SWD AT.OHAK AT.PMR AT.TTA II.KDAK
Filtering commands used:
cut a -30 a 110
rtr
taper w 0.1
hp c 0.04 n 3
lp c 0.09 n 3
Best Fitting Double Couple
Mo = 1.60e+22 dyne-cm
Mw = 4.07
Z = 136 km
Plane Strike Dip Rake
NP1 306 60 145
NP2 55 60 35
Principal Axes:
Axis Value Plunge Azimuth
T 1.60e+22 45 270
N 0.00e+00 45 91
P -1.60e+22 0 0
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.60e+22
Mxy -1.48e+20
Mxz 4.78e+14
Myy 8.07e+21
Myz -8.02e+21
Mzz 7.96e+21
------ P -----
---------- ---------
----------------------------
------------------------------
#########-------------------------
###############--------------------#
####################---------------###
########################-----------#####
##########################---------#####
######## ##################-----########
######## T ####################--#########
######## ####################--#########
##############################----########
##########################--------######
########################-----------#####
####################---------------###
###############--------------------#
########--------------------------
------------------------------
----------------------------
----------------------
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Global CMT Convention Moment Tensor:
R T P
7.96e+21 4.78e+14 8.02e+21
4.78e+14 -1.60e+22 1.48e+20
8.02e+21 1.48e+20 8.07e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140819101122/index.html
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STK = 55
DIP = 60
RAKE = 35
MW = 4.07
HS = 136.0
The NDK file is 20140819101122.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 a -30 a 110 rtr taper w 0.1 hp c 0.04 n 3 lp c 0.09 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 120 55 -40 3.16 0.1513
WVFGRD96 4.0 115 65 -40 3.23 0.1898
WVFGRD96 6.0 120 75 -25 3.26 0.2138
WVFGRD96 8.0 120 70 -25 3.34 0.2314
WVFGRD96 10.0 120 70 -20 3.38 0.2389
WVFGRD96 12.0 125 75 -20 3.42 0.2413
WVFGRD96 14.0 125 75 -20 3.44 0.2391
WVFGRD96 16.0 125 75 -15 3.47 0.2356
WVFGRD96 18.0 125 75 -15 3.49 0.2256
WVFGRD96 20.0 135 80 -15 3.52 0.2108
WVFGRD96 22.0 40 80 30 3.53 0.2122
WVFGRD96 24.0 40 80 30 3.56 0.2178
WVFGRD96 26.0 215 70 30 3.58 0.2244
WVFGRD96 28.0 215 75 30 3.61 0.2390
WVFGRD96 30.0 215 75 30 3.63 0.2496
WVFGRD96 32.0 220 70 30 3.66 0.2578
WVFGRD96 34.0 215 70 30 3.68 0.2608
WVFGRD96 36.0 220 65 35 3.71 0.2679
WVFGRD96 38.0 220 65 30 3.73 0.2746
WVFGRD96 40.0 225 65 40 3.83 0.2893
WVFGRD96 42.0 225 60 40 3.86 0.2878
WVFGRD96 44.0 225 60 40 3.88 0.2855
WVFGRD96 46.0 225 60 40 3.89 0.2822
WVFGRD96 48.0 225 60 40 3.91 0.2817
WVFGRD96 50.0 230 60 40 3.93 0.2820
WVFGRD96 52.0 230 60 40 3.94 0.2843
WVFGRD96 54.0 35 50 0 3.93 0.2952
WVFGRD96 56.0 40 55 10 3.93 0.3058
WVFGRD96 58.0 45 55 5 3.95 0.3169
WVFGRD96 60.0 50 60 10 3.97 0.3289
WVFGRD96 62.0 50 60 10 3.97 0.3429
WVFGRD96 64.0 50 60 10 3.98 0.3534
WVFGRD96 66.0 50 60 10 3.99 0.3648
WVFGRD96 68.0 55 65 15 4.01 0.3737
WVFGRD96 70.0 55 65 15 4.02 0.3847
WVFGRD96 72.0 55 65 15 4.02 0.3923
WVFGRD96 74.0 55 65 15 4.02 0.4016
WVFGRD96 76.0 55 70 20 4.03 0.4101
WVFGRD96 78.0 55 70 20 4.04 0.4172
WVFGRD96 80.0 55 70 20 4.04 0.4246
WVFGRD96 82.0 55 70 20 4.04 0.4312
WVFGRD96 84.0 55 70 20 4.04 0.4366
WVFGRD96 86.0 55 70 25 4.05 0.4408
WVFGRD96 88.0 55 70 25 4.05 0.4461
WVFGRD96 90.0 55 70 25 4.05 0.4520
WVFGRD96 92.0 55 70 25 4.05 0.4578
WVFGRD96 94.0 55 70 25 4.06 0.4628
WVFGRD96 96.0 55 70 25 4.06 0.4678
WVFGRD96 98.0 55 65 25 4.04 0.4724
WVFGRD96 100.0 55 65 25 4.05 0.4777
WVFGRD96 102.0 55 65 25 4.05 0.4818
WVFGRD96 104.0 55 65 30 4.05 0.4876
WVFGRD96 106.0 55 65 30 4.05 0.4924
WVFGRD96 108.0 55 65 30 4.06 0.4970
WVFGRD96 110.0 55 65 30 4.06 0.5014
WVFGRD96 112.0 55 65 30 4.06 0.5042
WVFGRD96 114.0 55 65 35 4.06 0.5070
WVFGRD96 116.0 55 65 35 4.06 0.5095
WVFGRD96 118.0 55 65 35 4.07 0.5113
WVFGRD96 120.0 55 65 35 4.07 0.5116
WVFGRD96 122.0 55 65 35 4.07 0.5140
WVFGRD96 124.0 55 60 35 4.06 0.5158
WVFGRD96 126.0 55 60 35 4.06 0.5166
WVFGRD96 128.0 55 60 35 4.06 0.5166
WVFGRD96 130.0 55 60 35 4.06 0.5161
WVFGRD96 132.0 55 60 35 4.06 0.5162
WVFGRD96 134.0 55 60 35 4.06 0.5169
WVFGRD96 136.0 55 60 35 4.07 0.5174
WVFGRD96 138.0 55 60 40 4.07 0.5165
WVFGRD96 140.0 55 60 40 4.07 0.5145
WVFGRD96 142.0 55 60 40 4.07 0.5146
WVFGRD96 144.0 55 60 40 4.07 0.5140
WVFGRD96 146.0 55 60 40 4.07 0.5122
WVFGRD96 148.0 55 60 40 4.07 0.5104
The best solution is
WVFGRD96 136.0 55 60 35 4.07 0.5174
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 -30 a 110 rtr taper w 0.1 hp c 0.04 n 3 lp c 0.09 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