The ANSS event ID is ak0153wmqvo7 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0153wmqvo7/executive.
2015/03/26 03:47:03 62.870 -150.766 96.4 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2015/03/26 03:47:03:0 62.87 -150.77 96.4 3.9 Alaska
Stations used:
AK.BPAW AK.CCB AK.GHO AK.KNK AK.KTH AK.MCK AK.NEA2 AK.PAX
AK.PPLA AK.RC01 AK.RND AK.SKN AK.SSN AK.TRF AK.WAT3 AK.WAT4
AK.WAT5 AK.WRH AT.PMR TA.M24K
Filtering commands used:
cut o DIST/3.4 -30 o DIST/3.4 +60
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 = 1.35e+22 dyne-cm
Mw = 4.02
Z = 108 km
Plane Strike Dip Rake
NP1 357 79 107
NP2 120 20 35
Principal Axes:
Axis Value Plunge Azimuth
T 1.35e+22 53 286
N 0.00e+00 16 173
P -1.35e+22 32 73
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.57e+20
Mxy -4.04e+21
Mxz 5.88e+19
Myy -4.52e+21
Myz -1.20e+22
Mzz 4.97e+21
######--------
###########-----------
###############-------------
################--------------
##################----------------
####################----------------
#####################-----------------
-#####################---------- -----
-######### ##########--------- P -----
--######### T ##########--------- ------
--######### ##########------------------
--######################------------------
---#####################------------------
---####################-----------------
----###################-----------------
----##################----------------
----#################---------------
-----###############-------------#
-----#############-----------#
--------########---------###
-------------#########
---------#####
Global CMT Convention Moment Tensor:
R T P
4.97e+21 5.88e+19 1.20e+22
5.88e+19 -4.57e+20 4.04e+21
1.20e+22 4.04e+21 -4.52e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150326034703/index.html
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STK = 120
DIP = 20
RAKE = 35
MW = 4.02
HS = 108.0
The NDK file is 20150326034703.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/3.4 -30 o DIST/3.4 +60 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 10 55 -30 3.23 0.3456
WVFGRD96 4.0 190 65 -40 3.33 0.3867
WVFGRD96 6.0 185 75 -55 3.40 0.4457
WVFGRD96 8.0 185 70 -60 3.47 0.4730
WVFGRD96 10.0 185 70 -50 3.46 0.4707
WVFGRD96 12.0 210 60 45 3.48 0.4760
WVFGRD96 14.0 205 65 40 3.49 0.4757
WVFGRD96 16.0 25 70 35 3.51 0.4750
WVFGRD96 18.0 25 70 35 3.52 0.4667
WVFGRD96 20.0 25 70 35 3.54 0.4566
WVFGRD96 22.0 25 70 40 3.56 0.4438
WVFGRD96 24.0 20 70 40 3.57 0.4317
WVFGRD96 26.0 190 70 -35 3.58 0.4189
WVFGRD96 28.0 190 70 -35 3.59 0.4118
WVFGRD96 30.0 190 70 -35 3.60 0.4083
WVFGRD96 32.0 190 70 -35 3.62 0.4084
WVFGRD96 34.0 190 70 -35 3.64 0.4095
WVFGRD96 36.0 190 70 -35 3.65 0.4101
WVFGRD96 38.0 185 65 -40 3.67 0.4094
WVFGRD96 40.0 180 65 -55 3.77 0.4257
WVFGRD96 42.0 180 60 -55 3.79 0.4138
WVFGRD96 44.0 5 50 -40 3.83 0.4062
WVFGRD96 46.0 10 50 -30 3.83 0.4005
WVFGRD96 48.0 10 50 -30 3.84 0.3979
WVFGRD96 50.0 10 50 -30 3.85 0.3942
WVFGRD96 52.0 190 80 -45 3.81 0.3915
WVFGRD96 54.0 190 80 -45 3.82 0.3927
WVFGRD96 56.0 105 50 20 3.90 0.4027
WVFGRD96 58.0 105 50 20 3.91 0.4162
WVFGRD96 60.0 105 50 20 3.92 0.4278
WVFGRD96 62.0 105 50 20 3.93 0.4357
WVFGRD96 64.0 105 50 15 3.94 0.4437
WVFGRD96 66.0 100 15 0 3.98 0.4613
WVFGRD96 68.0 100 15 0 3.99 0.4856
WVFGRD96 70.0 100 20 5 3.99 0.5074
WVFGRD96 72.0 100 20 5 4.00 0.5271
WVFGRD96 74.0 100 20 5 4.00 0.5431
WVFGRD96 76.0 100 20 5 4.01 0.5581
WVFGRD96 78.0 100 20 5 4.01 0.5703
WVFGRD96 80.0 105 20 15 4.01 0.5805
WVFGRD96 82.0 105 20 15 4.01 0.5898
WVFGRD96 84.0 105 20 20 4.01 0.5979
WVFGRD96 86.0 105 20 20 4.02 0.6041
WVFGRD96 88.0 105 20 20 4.02 0.6105
WVFGRD96 90.0 105 20 20 4.02 0.6161
WVFGRD96 92.0 115 20 25 4.02 0.6206
WVFGRD96 94.0 115 20 25 4.02 0.6248
WVFGRD96 96.0 115 20 25 4.02 0.6283
WVFGRD96 98.0 120 20 30 4.02 0.6304
WVFGRD96 100.0 120 20 30 4.02 0.6328
WVFGRD96 102.0 120 20 30 4.02 0.6356
WVFGRD96 104.0 120 20 30 4.02 0.6372
WVFGRD96 106.0 120 20 30 4.02 0.6374
WVFGRD96 108.0 120 20 35 4.02 0.6379
WVFGRD96 110.0 120 20 35 4.03 0.6370
WVFGRD96 112.0 120 20 35 4.03 0.6353
WVFGRD96 114.0 120 20 35 4.03 0.6339
WVFGRD96 116.0 125 20 35 4.03 0.6320
WVFGRD96 118.0 120 25 30 4.03 0.6313
The best solution is
WVFGRD96 108.0 120 20 35 4.02 0.6379
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/3.4 -30 o DIST/3.4 +60 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