The ANSS event ID is ak017akg929d and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak017akg929d/executive.
2017/08/18 04:44:36 59.002 -136.394 11.6 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/08/18 04:44:36:0 59.00 -136.39 11.6 4.0 Alaska
Stations used:
AK.BARN AK.BCP AK.BESE AK.CTG AK.GRNC AK.JIS AK.LOGN
AK.TABL AT.SIT AT.SKAG AT.YKU2 CN.HYT CN.WHY TA.P29M
TA.P33M
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.04 n 3
lp c 0.12 n 3
Best Fitting Double Couple
Mo = 1.35e+22 dyne-cm
Mw = 4.02
Z = 21 km
Plane Strike Dip Rake
NP1 185 70 80
NP2 32 22 116
Principal Axes:
Axis Value Plunge Azimuth
T 1.35e+22 64 79
N 0.00e+00 9 188
P -1.35e+22 24 283
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.47e+20
Mxy 2.91e+21
Mxz -8.88e+19
Myy -8.09e+21
Myz 1.02e+22
Mzz 8.54e+21
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--- ---------######################---
---- P ---------########## #########----
---- ---------########## T #########----
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Global CMT Convention Moment Tensor:
R T P
8.54e+21 -8.88e+19 -1.02e+22
-8.88e+19 -4.47e+20 -2.91e+21
-1.02e+22 -2.91e+21 -8.09e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170818044436/index.html
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STK = 185
DIP = 70
RAKE = 80
MW = 4.02
HS = 21.0
The NDK file is 20170818044436.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 +40 rtr taper w 0.1 hp c 0.04 n 3 lp c 0.12 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 0 50 95 3.73 0.4043
WVFGRD96 2.0 5 50 95 3.85 0.3980
WVFGRD96 3.0 175 55 35 3.82 0.3176
WVFGRD96 4.0 260 45 15 3.75 0.2994
WVFGRD96 5.0 290 35 15 3.72 0.3325
WVFGRD96 6.0 295 30 20 3.72 0.3696
WVFGRD96 7.0 300 30 30 3.74 0.4042
WVFGRD96 8.0 310 25 40 3.74 0.4359
WVFGRD96 9.0 310 25 40 3.76 0.4636
WVFGRD96 10.0 310 25 40 3.81 0.4887
WVFGRD96 11.0 310 30 35 3.83 0.5132
WVFGRD96 12.0 320 25 45 3.84 0.5360
WVFGRD96 13.0 320 25 45 3.86 0.5560
WVFGRD96 14.0 320 25 50 3.88 0.5730
WVFGRD96 15.0 340 20 65 3.89 0.5893
WVFGRD96 16.0 340 20 65 3.91 0.6029
WVFGRD96 17.0 185 70 100 3.93 0.6129
WVFGRD96 18.0 185 70 85 3.95 0.6224
WVFGRD96 19.0 185 70 80 3.97 0.6288
WVFGRD96 20.0 185 70 80 4.01 0.6333
WVFGRD96 21.0 185 70 80 4.02 0.6336
WVFGRD96 22.0 185 70 80 4.04 0.6307
WVFGRD96 23.0 185 70 75 4.05 0.6240
WVFGRD96 24.0 185 70 75 4.06 0.6140
WVFGRD96 25.0 180 75 65 4.09 0.6002
WVFGRD96 26.0 180 75 70 4.09 0.5841
WVFGRD96 27.0 180 75 65 4.10 0.5646
WVFGRD96 28.0 180 75 65 4.11 0.5434
WVFGRD96 29.0 180 75 65 4.11 0.5201
The best solution is
WVFGRD96 21.0 185 70 80 4.02 0.6336
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 +40 rtr taper w 0.1 hp c 0.04 n 3 lp c 0.12 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 CUS.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 CUS Model with Q from simple gamma values 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 1.0000 5.0000 2.8900 2.5000 0.172E-02 0.387E-02 0.00 0.00 1.00 1.00 9.0000 6.1000 3.5200 2.7300 0.160E-02 0.363E-02 0.00 0.00 1.00 1.00 10.0000 6.4000 3.7000 2.8200 0.149E-02 0.336E-02 0.00 0.00 1.00 1.00 20.0000 6.7000 3.8700 2.9020 0.000E-04 0.000E-04 0.00 0.00 1.00 1.00 0.0000 8.1500 4.7000 3.3640 0.194E-02 0.431E-02 0.00 0.00 1.00 1.00