The ANSS event ID is ak0159vlh2n6 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0159vlh2n6/executive.
2015/08/03 01:56:10 62.836 -148.974 70.1 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2015/08/03 01:56:10:0 62.84 -148.97 70.1 4.1 Alaska
Stations used:
AK.BPAW AK.BWN AK.CCB AK.CUT AK.GLI AK.HDA AK.KLU AK.KNK
AK.MDM AK.MLY AK.NEA2 AK.PAX AK.PWL AK.RC01 AK.RND AK.SAW
AK.SCM AK.SSN AK.SWD AK.WRH IM.IL31 IU.COLA TA.I23K TA.N25K
TA.TCOL
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 0.07 n 3
Best Fitting Double Couple
Mo = 1.22e+22 dyne-cm
Mw = 3.99
Z = 72 km
Plane Strike Dip Rake
NP1 260 65 -75
NP2 48 29 -119
Principal Axes:
Axis Value Plunge Azimuth
T 1.22e+22 19 339
N 0.00e+00 14 74
P -1.22e+22 67 197
Moment Tensor: (dyne-cm)
Component Value
Mxx 7.75e+21
Mxy -4.22e+21
Mxz 7.67e+21
Myy 1.25e+21
Myz -1.16e+18
Mzz -9.00e+21
##############
### ################
###### T ###################
####### ####################
#################################-
###################################-
####################################--
####################-------------####---
############-------------------------##-
#########-----------------------------####
######--------------------------------####
###----------------------------------#####
#----------------- ----------------#####
----------------- P ---------------#####
----------------- --------------######
--------------------------------######
-----------------------------#######
--------------------------########
---------------------#########
##--------------############
######################
##############
Global CMT Convention Moment Tensor:
R T P
-9.00e+21 7.67e+21 1.16e+18
7.67e+21 7.75e+21 4.22e+21
1.16e+18 4.22e+21 1.25e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150803015610/index.html
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STK = 260
DIP = 65
RAKE = -75
MW = 3.99
HS = 72.0
The NDK file is 20150803015610.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 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 90 50 -75 3.15 0.1518
WVFGRD96 4.0 95 50 -65 3.30 0.1820
WVFGRD96 6.0 100 50 -55 3.33 0.1870
WVFGRD96 8.0 100 50 -60 3.41 0.2140
WVFGRD96 10.0 115 65 -30 3.40 0.1937
WVFGRD96 12.0 250 90 -50 3.32 0.2099
WVFGRD96 14.0 75 80 50 3.35 0.2254
WVFGRD96 16.0 75 80 50 3.37 0.2383
WVFGRD96 18.0 75 80 50 3.40 0.2481
WVFGRD96 20.0 120 60 -20 3.51 0.2560
WVFGRD96 22.0 120 55 -20 3.52 0.2650
WVFGRD96 24.0 300 90 -30 3.57 0.2769
WVFGRD96 26.0 120 90 30 3.59 0.2873
WVFGRD96 28.0 300 90 -30 3.61 0.2951
WVFGRD96 30.0 300 90 -30 3.63 0.3006
WVFGRD96 32.0 300 80 -30 3.64 0.3094
WVFGRD96 34.0 295 75 -35 3.65 0.3248
WVFGRD96 36.0 295 70 -35 3.67 0.3356
WVFGRD96 38.0 295 65 -35 3.69 0.3454
WVFGRD96 40.0 290 60 -45 3.78 0.3675
WVFGRD96 42.0 285 70 -55 3.80 0.3810
WVFGRD96 44.0 285 70 -60 3.82 0.4058
WVFGRD96 46.0 280 70 -65 3.83 0.4397
WVFGRD96 48.0 280 70 -65 3.86 0.4750
WVFGRD96 50.0 280 70 -65 3.88 0.5088
WVFGRD96 52.0 270 65 -70 3.89 0.5422
WVFGRD96 54.0 265 65 -75 3.91 0.5723
WVFGRD96 56.0 265 65 -75 3.92 0.5984
WVFGRD96 58.0 265 65 -75 3.93 0.6197
WVFGRD96 60.0 265 65 -75 3.94 0.6358
WVFGRD96 62.0 260 65 -75 3.95 0.6485
WVFGRD96 64.0 260 65 -75 3.96 0.6593
WVFGRD96 66.0 260 65 -75 3.97 0.6664
WVFGRD96 68.0 260 65 -75 3.97 0.6707
WVFGRD96 70.0 260 65 -75 3.98 0.6726
WVFGRD96 72.0 260 65 -75 3.99 0.6731
WVFGRD96 74.0 260 65 -75 3.99 0.6715
WVFGRD96 76.0 255 65 -75 4.00 0.6693
WVFGRD96 78.0 255 65 -75 4.01 0.6659
WVFGRD96 80.0 255 65 -75 4.01 0.6615
WVFGRD96 82.0 255 65 -75 4.02 0.6555
WVFGRD96 84.0 255 65 -75 4.02 0.6490
WVFGRD96 86.0 255 65 -75 4.02 0.6430
WVFGRD96 88.0 255 65 -75 4.03 0.6364
WVFGRD96 90.0 255 65 -75 4.03 0.6289
WVFGRD96 92.0 255 65 -75 4.03 0.6207
WVFGRD96 94.0 255 65 -75 4.03 0.6121
WVFGRD96 96.0 255 65 -75 4.04 0.6036
WVFGRD96 98.0 255 65 -75 4.04 0.5944
The best solution is
WVFGRD96 72.0 260 65 -75 3.99 0.6731
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 0.07 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