The ANSS event ID is ak0145708jz8 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0145708jz8/executive.
2014/04/23 07:57:40 62.856 -150.548 92.4 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2014/04/23 07:57:40:0 62.86 -150.55 92.4 4.0 Alaska
Stations used:
AK.DHY AK.GHO AK.KNK AK.KTH AK.MCK AK.PPLA AK.RND AK.SAW
AK.SCM AK.SKN AK.TRF AT.PMR
Filtering commands used:
cut a -30 a 100
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.26e+22 dyne-cm
Mw = 4.00
Z = 102 km
Plane Strike Dip Rake
NP1 160 60 40
NP2 47 56 143
Principal Axes:
Axis Value Plunge Azimuth
T 1.26e+22 48 15
N 0.00e+00 42 191
P -1.26e+22 2 283
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.55e+21
Mxy 4.15e+21
Mxz 5.92e+21
Myy -1.16e+22
Myz 2.15e+21
Mzz 7.01e+21
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---------########## T ###########---
----------########## ###########----
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P ---------#######################------
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Global CMT Convention Moment Tensor:
R T P
7.01e+21 5.92e+21 -2.15e+21
5.92e+21 4.55e+21 -4.15e+21
-2.15e+21 -4.15e+21 -1.16e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140423075740/index.html
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STK = 160
DIP = 60
RAKE = 40
MW = 4.00
HS = 102.0
The NDK file is 20140423075740.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 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 15 45 -85 3.18 0.2004
WVFGRD96 4.0 205 65 -65 3.22 0.2170
WVFGRD96 6.0 205 65 -60 3.25 0.2554
WVFGRD96 8.0 205 65 -65 3.35 0.2857
WVFGRD96 10.0 65 60 25 3.36 0.2961
WVFGRD96 12.0 65 60 20 3.41 0.2965
WVFGRD96 14.0 65 60 20 3.44 0.2855
WVFGRD96 16.0 65 55 15 3.47 0.2684
WVFGRD96 18.0 65 45 -5 3.52 0.2490
WVFGRD96 20.0 65 40 -5 3.54 0.2301
WVFGRD96 22.0 340 70 45 3.53 0.2379
WVFGRD96 24.0 335 65 35 3.54 0.2526
WVFGRD96 26.0 335 65 35 3.57 0.2674
WVFGRD96 28.0 330 70 35 3.58 0.2794
WVFGRD96 30.0 335 70 35 3.61 0.2864
WVFGRD96 32.0 330 75 30 3.61 0.2872
WVFGRD96 34.0 330 75 30 3.62 0.2866
WVFGRD96 36.0 335 75 30 3.66 0.2846
WVFGRD96 38.0 335 80 30 3.68 0.2801
WVFGRD96 40.0 340 75 45 3.78 0.2881
WVFGRD96 42.0 335 75 40 3.77 0.2730
WVFGRD96 44.0 145 70 -20 3.77 0.2725
WVFGRD96 46.0 145 70 -20 3.78 0.2730
WVFGRD96 48.0 155 60 30 3.83 0.2734
WVFGRD96 50.0 160 55 40 3.87 0.2848
WVFGRD96 52.0 160 55 40 3.88 0.2980
WVFGRD96 54.0 160 55 40 3.90 0.3117
WVFGRD96 56.0 160 55 40 3.91 0.3235
WVFGRD96 58.0 160 60 40 3.92 0.3360
WVFGRD96 60.0 160 60 35 3.93 0.3521
WVFGRD96 62.0 160 60 35 3.94 0.3667
WVFGRD96 64.0 160 60 35 3.94 0.3802
WVFGRD96 66.0 160 60 35 3.95 0.3912
WVFGRD96 68.0 160 60 35 3.96 0.4014
WVFGRD96 70.0 160 55 35 3.96 0.4099
WVFGRD96 72.0 160 55 35 3.97 0.4175
WVFGRD96 74.0 160 55 35 3.97 0.4237
WVFGRD96 76.0 165 55 50 3.98 0.4297
WVFGRD96 78.0 165 55 45 3.99 0.4364
WVFGRD96 80.0 165 55 45 3.99 0.4417
WVFGRD96 82.0 165 55 45 3.99 0.4458
WVFGRD96 84.0 165 55 45 3.99 0.4491
WVFGRD96 86.0 160 60 45 3.99 0.4507
WVFGRD96 88.0 160 60 45 3.99 0.4537
WVFGRD96 90.0 160 60 45 3.99 0.4553
WVFGRD96 92.0 160 60 45 3.99 0.4567
WVFGRD96 94.0 160 60 45 3.99 0.4567
WVFGRD96 96.0 160 60 45 4.00 0.4571
WVFGRD96 98.0 160 60 40 4.00 0.4581
WVFGRD96 100.0 160 60 40 4.00 0.4598
WVFGRD96 102.0 160 60 40 4.00 0.4602
WVFGRD96 104.0 160 60 40 4.00 0.4597
WVFGRD96 106.0 160 60 40 4.01 0.4589
WVFGRD96 108.0 160 60 40 4.01 0.4592
WVFGRD96 110.0 160 60 40 4.01 0.4593
WVFGRD96 112.0 160 60 35 4.02 0.4596
WVFGRD96 114.0 160 60 35 4.02 0.4596
WVFGRD96 116.0 160 60 35 4.02 0.4593
WVFGRD96 118.0 160 60 35 4.02 0.4583
The best solution is
WVFGRD96 102.0 160 60 40 4.00 0.4602
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 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