The ANSS event ID is ak015b9bmkyd and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak015b9bmkyd/executive.
2015/09/02 08:33:06 63.287 -151.379 14.9 4.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2015/09/02 08:33:06:0 63.29 -151.38 14.9 4.2 Alaska
Stations used:
AK.BERG AK.BMR AK.BPAW AK.BWN AK.CCB AK.COLD AK.CRQ AK.CUT
AK.DIV AK.DOT AK.EYAK AK.FID AK.FIRE AK.FYU AK.GLB AK.GLI
AK.HDA AK.HIN AK.HOM AK.KAI AK.KLU AK.KNK AK.MCAR AK.MCK
AK.MDM AK.NEA2 AK.PAX AK.PPD AK.PPLA AK.PWL AK.RAG AK.RC01
AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.SSN AK.SWD
AK.TGL AK.TRF AK.VRDI AK.WAX AK.WRH AT.MENT AT.MID AT.PMR
AT.SVW2 AT.TTA IM.IL31 IU.COLA TA.H23K TA.H24K TA.I21K
TA.I23K TA.J20K TA.K20K TA.L19K TA.L26K TA.L27K TA.M19K
TA.M20K TA.M24K TA.M26K TA.M27K TA.N18K TA.N19K TA.N25K
TA.O18K TA.O19K TA.O22K TA.P19K TA.POKR TA.Q19K TA.TCOL
US.EGAK
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 1.60e+22 dyne-cm
Mw = 4.07
Z = 18 km
Plane Strike Dip Rake
NP1 56 66 97
NP2 220 25 75
Principal Axes:
Axis Value Plunge Azimuth
T 1.60e+22 68 340
N 0.00e+00 6 234
P -1.60e+22 21 141
Moment Tensor: (dyne-cm)
Component Value
Mxx -6.63e+21
Mxy 6.15e+21
Mxz 9.28e+21
Myy -5.23e+21
Myz -5.21e+21
Mzz 1.19e+22
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-----------#########--
---------###################
-------#######################
-------###########################
------#############################-
------#############################---
-----############ ###############-----
-----############ T ##############------
-----############# ############---------
----############################----------
----##########################------------
----########################--------------
---#####################----------------
---##################-------------------
--###############---------------------
--#########-------------------------
#------------------------ ------
----------------------- P ----
#--------------------- ---
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Global CMT Convention Moment Tensor:
R T P
1.19e+22 9.28e+21 5.21e+21
9.28e+21 -6.63e+21 -6.15e+21
5.21e+21 -6.15e+21 -5.23e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150902083306/index.html
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STK = 220
DIP = 25
RAKE = 75
MW = 4.07
HS = 18.0
The NDK file is 20150902083306.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 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 65 45 -90 3.69 0.2578
WVFGRD96 2.0 245 45 -90 3.81 0.3199
WVFGRD96 3.0 65 50 -85 3.82 0.2265
WVFGRD96 4.0 235 15 -90 3.84 0.2196
WVFGRD96 5.0 230 15 -95 3.85 0.2619
WVFGRD96 6.0 230 10 -95 3.85 0.2990
WVFGRD96 7.0 230 10 -95 3.85 0.3277
WVFGRD96 8.0 55 80 -90 3.94 0.3507
WVFGRD96 9.0 215 15 70 3.95 0.3849
WVFGRD96 10.0 220 20 75 3.98 0.4223
WVFGRD96 11.0 220 20 75 3.99 0.4558
WVFGRD96 12.0 220 20 75 4.00 0.4822
WVFGRD96 13.0 220 25 75 4.02 0.5040
WVFGRD96 14.0 220 25 75 4.03 0.5207
WVFGRD96 15.0 220 25 75 4.04 0.5324
WVFGRD96 16.0 220 25 75 4.05 0.5401
WVFGRD96 17.0 220 25 75 4.06 0.5441
WVFGRD96 18.0 220 25 75 4.07 0.5448
WVFGRD96 19.0 220 25 75 4.08 0.5430
WVFGRD96 20.0 220 25 75 4.09 0.5389
WVFGRD96 21.0 215 25 70 4.10 0.5326
WVFGRD96 22.0 215 25 70 4.11 0.5254
WVFGRD96 23.0 215 25 70 4.12 0.5168
WVFGRD96 24.0 215 25 70 4.13 0.5069
WVFGRD96 25.0 210 25 65 4.13 0.4960
WVFGRD96 26.0 210 25 65 4.14 0.4843
WVFGRD96 27.0 210 25 65 4.15 0.4716
WVFGRD96 28.0 210 25 65 4.15 0.4582
WVFGRD96 29.0 210 25 65 4.16 0.4439
The best solution is
WVFGRD96 18.0 220 25 75 4.07 0.5448
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 +70 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 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