The ANSS event ID is ak015d51jzsp and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak015d51jzsp/executive.
2015/10/13 03:11:37 62.963 -149.365 86.7 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2015/10/13 03:11:37:0 62.96 -149.37 86.7 4.1 Alaska
Stations used:
AK.BPAW AK.BWN AK.CUT AK.DHY AK.GHO AK.KTH AK.MCK AK.MLY
AK.RND AK.SCM AK.SKN AK.SSN AK.TRF AK.WRH
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 0.10 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 1.50e+22 dyne-cm
Mw = 4.05
Z = 90 km
Plane Strike Dip Rake
NP1 285 90 -45
NP2 15 45 -180
Principal Axes:
Axis Value Plunge Azimuth
T 1.50e+22 30 340
N 0.00e+00 45 105
P -1.50e+22 30 230
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.29e+21
Mxy -9.16e+21
Mxz 1.02e+22
Myy -5.29e+21
Myz 2.74e+21
Mzz 9.25e+14
#############-
###################---
####### #############-----
######## T ##############-----
########## ###############------
#############################-------
##############################--------
################################--------
---#############################--------
-----------######################---------
------------------###############---------
------------------------########----------
-------------------------------#----------
------------------------------#######---
------------------------------##########
------- ------------------##########
------ P -----------------##########
----- ----------------##########
--------------------##########
-----------------###########
------------##########
----##########
Global CMT Convention Moment Tensor:
R T P
9.25e+14 1.02e+22 -2.74e+21
1.02e+22 5.29e+21 9.16e+21
-2.74e+21 9.16e+21 -5.29e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20151013031137/index.html
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STK = 285
DIP = 90
RAKE = -45
MW = 4.05
HS = 90.0
The NDK file is 20151013031137.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 0.10 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 2.0 175 90 0 3.20 0.3620
WVFGRD96 4.0 80 80 -15 3.32 0.4369
WVFGRD96 6.0 90 55 25 3.42 0.4773
WVFGRD96 8.0 75 55 -30 3.48 0.4974
WVFGRD96 10.0 260 70 -25 3.48 0.5109
WVFGRD96 12.0 260 70 -25 3.51 0.5173
WVFGRD96 14.0 80 75 -15 3.52 0.5209
WVFGRD96 16.0 85 75 15 3.55 0.5289
WVFGRD96 18.0 85 70 15 3.58 0.5330
WVFGRD96 20.0 85 70 15 3.60 0.5366
WVFGRD96 22.0 85 70 15 3.63 0.5374
WVFGRD96 24.0 85 80 20 3.64 0.5366
WVFGRD96 26.0 85 85 25 3.66 0.5386
WVFGRD96 28.0 85 85 25 3.68 0.5440
WVFGRD96 30.0 265 85 -30 3.70 0.5522
WVFGRD96 32.0 265 85 -30 3.72 0.5615
WVFGRD96 34.0 265 80 -30 3.73 0.5654
WVFGRD96 36.0 265 85 -25 3.74 0.5624
WVFGRD96 38.0 265 80 -25 3.76 0.5572
WVFGRD96 40.0 90 80 35 3.84 0.5563
WVFGRD96 42.0 90 80 35 3.86 0.5599
WVFGRD96 44.0 90 80 35 3.88 0.5631
WVFGRD96 46.0 95 75 40 3.91 0.5664
WVFGRD96 48.0 95 75 35 3.91 0.5704
WVFGRD96 50.0 95 75 35 3.92 0.5750
WVFGRD96 52.0 95 75 35 3.93 0.5779
WVFGRD96 54.0 90 90 35 3.92 0.5821
WVFGRD96 56.0 90 90 35 3.93 0.5866
WVFGRD96 58.0 270 90 -35 3.93 0.5932
WVFGRD96 60.0 110 65 30 3.98 0.5973
WVFGRD96 62.0 100 85 40 3.96 0.6063
WVFGRD96 64.0 100 85 40 3.97 0.6135
WVFGRD96 66.0 100 90 45 3.98 0.6220
WVFGRD96 68.0 105 85 45 4.00 0.6286
WVFGRD96 70.0 105 85 45 4.00 0.6359
WVFGRD96 72.0 105 85 45 4.01 0.6437
WVFGRD96 74.0 280 90 -45 4.00 0.6461
WVFGRD96 76.0 105 85 45 4.02 0.6543
WVFGRD96 78.0 105 85 45 4.03 0.6577
WVFGRD96 80.0 285 90 -45 4.03 0.6595
WVFGRD96 82.0 285 90 -45 4.03 0.6643
WVFGRD96 84.0 105 90 45 4.04 0.6670
WVFGRD96 86.0 105 90 45 4.04 0.6684
WVFGRD96 88.0 105 90 45 4.05 0.6699
WVFGRD96 90.0 285 90 -45 4.05 0.6707
WVFGRD96 92.0 105 90 40 4.05 0.6700
WVFGRD96 94.0 105 90 40 4.05 0.6699
WVFGRD96 96.0 285 90 -40 4.05 0.6688
WVFGRD96 98.0 285 90 -40 4.06 0.6689
WVFGRD96 100.0 105 90 40 4.06 0.6678
WVFGRD96 102.0 105 90 40 4.07 0.6669
WVFGRD96 104.0 285 90 -40 4.07 0.6658
WVFGRD96 106.0 105 90 40 4.07 0.6640
WVFGRD96 108.0 105 90 35 4.07 0.6627
WVFGRD96 110.0 105 90 35 4.07 0.6613
WVFGRD96 112.0 105 90 35 4.08 0.6585
WVFGRD96 114.0 285 90 -35 4.08 0.6573
WVFGRD96 116.0 105 90 35 4.08 0.6561
WVFGRD96 118.0 285 90 -35 4.09 0.6535
The best solution is
WVFGRD96 90.0 285 90 -45 4.05 0.6707
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 0.10 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