The ANSS event ID is ak0152krol27 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0152krol27/executive.
2015/02/25 09:38:48 63.194 -150.437 122.0 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2015/02/25 09:38:48:0 63.19 -150.44 122.0 4.1 Alaska
Stations used:
AK.BPAW AK.BWN AK.CCB AK.HDA AK.KTH AK.MDM AK.PAX AK.RND
AK.SAW AK.TRF AK.WRH AT.MENT AT.SVW2 AT.TTA IU.COLA TA.I23K
TA.M24K
Filtering commands used:
cut o DIST/3.3 -50 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 2.19e+22 dyne-cm
Mw = 4.16
Z = 132 km
Plane Strike Dip Rake
NP1 350 85 70
NP2 247 21 166
Principal Axes:
Axis Value Plunge Azimuth
T 2.19e+22 46 239
N 0.00e+00 20 352
P -2.19e+22 37 98
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.44e+21
Mxy 6.39e+21
Mxz -4.16e+21
Myy -6.01e+21
Myz -1.98e+22
Mzz 3.57e+21
----##########
--------##############
--------####------------####
------#######---------------##
-----###########-----------------#
----#############-------------------
----###############-------------------
---#################--------------------
--##################--------------------
---###################--------------------
--####################----------- ------
--####################----------- P ------
-######### ##########---------- ------
######### T ##########------------------
-######## ##########------------------
######################----------------
#####################---------------
####################--------------
##################------------
#################-----------
###############-------
###########---
Global CMT Convention Moment Tensor:
R T P
3.57e+21 -4.16e+21 1.98e+22
-4.16e+21 2.44e+21 -6.39e+21
1.98e+22 -6.39e+21 -6.01e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150225093848/index.html
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STK = 350
DIP = 85
RAKE = 70
MW = 4.16
HS = 132.0
The NDK file is 20150225093848.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 -50 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.05 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 20 45 -75 3.43 0.1655
WVFGRD96 4.0 80 45 -20 3.40 0.1812
WVFGRD96 6.0 80 40 -15 3.46 0.1977
WVFGRD96 8.0 70 30 -30 3.57 0.2150
WVFGRD96 10.0 75 30 -20 3.60 0.2304
WVFGRD96 12.0 80 30 -15 3.62 0.2432
WVFGRD96 14.0 85 35 -5 3.62 0.2525
WVFGRD96 16.0 90 35 0 3.65 0.2585
WVFGRD96 18.0 90 35 0 3.67 0.2595
WVFGRD96 20.0 90 40 0 3.67 0.2568
WVFGRD96 22.0 100 40 20 3.68 0.2512
WVFGRD96 24.0 100 40 15 3.70 0.2446
WVFGRD96 26.0 5 80 -40 3.71 0.2493
WVFGRD96 28.0 195 75 50 3.74 0.2492
WVFGRD96 30.0 5 80 -45 3.76 0.2543
WVFGRD96 32.0 5 80 -40 3.77 0.2565
WVFGRD96 34.0 5 80 -40 3.78 0.2576
WVFGRD96 36.0 5 80 -35 3.79 0.2598
WVFGRD96 38.0 5 80 -30 3.81 0.2614
WVFGRD96 40.0 5 85 -35 3.87 0.2642
WVFGRD96 42.0 5 85 -30 3.87 0.2647
WVFGRD96 44.0 10 80 -40 3.94 0.2653
WVFGRD96 46.0 5 60 -15 3.88 0.2759
WVFGRD96 48.0 5 65 -10 3.88 0.2866
WVFGRD96 50.0 5 65 -10 3.90 0.2975
WVFGRD96 52.0 5 65 -5 3.90 0.3100
WVFGRD96 54.0 5 70 10 3.89 0.3238
WVFGRD96 56.0 5 70 15 3.90 0.3477
WVFGRD96 58.0 5 75 25 3.92 0.3782
WVFGRD96 60.0 5 75 25 3.94 0.4106
WVFGRD96 62.0 5 80 35 3.96 0.4464
WVFGRD96 64.0 5 80 40 3.98 0.4853
WVFGRD96 66.0 -5 90 40 3.99 0.5255
WVFGRD96 68.0 0 90 40 4.01 0.5658
WVFGRD96 70.0 0 90 40 4.03 0.6036
WVFGRD96 72.0 0 90 45 4.04 0.6396
WVFGRD96 74.0 0 90 50 4.06 0.6689
WVFGRD96 76.0 0 90 50 4.06 0.6881
WVFGRD96 78.0 175 90 -55 4.06 0.6979
WVFGRD96 80.0 -5 90 55 4.07 0.7066
WVFGRD96 82.0 -5 90 55 4.07 0.7140
WVFGRD96 84.0 -5 90 60 4.08 0.7231
WVFGRD96 86.0 -5 90 60 4.09 0.7302
WVFGRD96 88.0 -5 90 60 4.09 0.7385
WVFGRD96 90.0 175 90 -60 4.10 0.7446
WVFGRD96 92.0 175 90 -60 4.10 0.7512
WVFGRD96 94.0 355 90 60 4.10 0.7569
WVFGRD96 96.0 175 90 -60 4.11 0.7616
WVFGRD96 98.0 175 90 -60 4.11 0.7666
WVFGRD96 100.0 175 90 -65 4.12 0.7698
WVFGRD96 102.0 175 90 -65 4.12 0.7745
WVFGRD96 104.0 170 90 -65 4.12 0.7778
WVFGRD96 106.0 170 90 -65 4.12 0.7820
WVFGRD96 100.0 175 90 -65 4.12 0.7698
WVFGRD96 110.0 355 85 65 4.13 0.7885
WVFGRD96 112.0 170 90 -70 4.14 0.7915
WVFGRD96 114.0 170 90 -70 4.14 0.7942
WVFGRD96 116.0 170 90 -70 4.14 0.7970
WVFGRD96 118.0 170 90 -70 4.14 0.7986
WVFGRD96 120.0 170 90 -70 4.15 0.7997
WVFGRD96 122.0 170 90 -70 4.15 0.8000
WVFGRD96 124.0 170 90 -70 4.15 0.8019
WVFGRD96 126.0 350 85 70 4.15 0.8042
WVFGRD96 128.0 350 85 70 4.16 0.8060
WVFGRD96 130.0 170 90 -70 4.16 0.8023
WVFGRD96 132.0 350 85 70 4.16 0.8063
WVFGRD96 134.0 165 90 -75 4.17 0.8034
WVFGRD96 136.0 165 90 -75 4.17 0.8014
WVFGRD96 138.0 350 85 70 4.17 0.8054
WVFGRD96 140.0 350 85 70 4.17 0.8046
WVFGRD96 142.0 350 85 70 4.17 0.8025
WVFGRD96 144.0 350 85 70 4.18 0.8012
WVFGRD96 146.0 345 85 75 4.19 0.8006
WVFGRD96 148.0 345 85 75 4.19 0.7996
The best solution is
WVFGRD96 132.0 350 85 70 4.16 0.8063
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 -50 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.05 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