The ANSS event ID is ak01560tn6mh and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak01560tn6mh/executive.
2015/05/11 10:48:46 63.757 -149.304 117.8 3.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2015/05/11 10:48:46:0 63.76 -149.30 117.8 3.6 Alaska
Stations used:
AK.CCB AK.KTH AK.MCK AK.NEA2 AK.PPD AK.RND AK.TRF AK.WAT4
AK.WAT5 AK.WRH IM.IL31 IU.COLA TA.POKR 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 = 4.47e+21 dyne-cm
Mw = 3.70
Z = 114 km
Plane Strike Dip Rake
NP1 319 79 134
NP2 60 45 15
Principal Axes:
Axis Value Plunge Azimuth
T 4.47e+21 39 268
N 0.00e+00 43 129
P -4.47e+21 22 17
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.51e+21
Mxy -1.02e+21
Mxz -1.53e+21
Myy 2.35e+21
Myz -2.64e+21
Mzz 1.16e+21
--------------
-------------- -----
----------------- P --------
####-------------- ---------
#########-------------------------
############------------------------
###############----------------------#
##################--------------------##
###################------------------###
######################----------------####
####### ##############-------------#####
####### T ###############-----------######
####### #################--------#######
###########################-----########
############################--##########
###########################--#########
#######################------#######
--################-----------#####
----------------------------##
----------------------------
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
1.16e+21 -1.53e+21 2.64e+21
-1.53e+21 -3.51e+21 1.02e+21
2.64e+21 1.02e+21 2.35e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150511104846/index.html
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STK = 60
DIP = 45
RAKE = 15
MW = 3.70
HS = 114.0
The NDK file is 20150511104846.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 2.0 125 55 -55 2.95 0.3065
WVFGRD96 4.0 130 65 -45 3.01 0.3296
WVFGRD96 6.0 240 50 15 3.05 0.3564
WVFGRD96 8.0 240 50 15 3.12 0.4114
WVFGRD96 10.0 240 55 15 3.16 0.4453
WVFGRD96 12.0 235 55 5 3.18 0.4699
WVFGRD96 14.0 235 60 5 3.21 0.4938
WVFGRD96 16.0 235 65 5 3.23 0.5126
WVFGRD96 18.0 235 65 5 3.25 0.5275
WVFGRD96 20.0 235 70 5 3.27 0.5409
WVFGRD96 22.0 235 70 5 3.29 0.5535
WVFGRD96 24.0 235 70 5 3.31 0.5627
WVFGRD96 26.0 55 65 5 3.33 0.5713
WVFGRD96 28.0 55 65 5 3.34 0.5802
WVFGRD96 30.0 55 60 10 3.36 0.5842
WVFGRD96 32.0 50 65 10 3.37 0.5837
WVFGRD96 34.0 50 65 10 3.39 0.5814
WVFGRD96 36.0 50 65 10 3.40 0.5785
WVFGRD96 38.0 50 65 15 3.43 0.5764
WVFGRD96 40.0 50 55 10 3.49 0.5818
WVFGRD96 42.0 50 55 10 3.51 0.5832
WVFGRD96 44.0 50 55 15 3.53 0.5836
WVFGRD96 46.0 50 60 10 3.53 0.5832
WVFGRD96 48.0 50 60 10 3.54 0.5853
WVFGRD96 50.0 50 60 10 3.55 0.5873
WVFGRD96 52.0 50 60 10 3.56 0.5898
WVFGRD96 54.0 50 60 10 3.57 0.5908
WVFGRD96 56.0 50 60 10 3.58 0.5922
WVFGRD96 58.0 50 60 10 3.58 0.5949
WVFGRD96 60.0 50 60 10 3.59 0.5975
WVFGRD96 62.0 50 55 5 3.59 0.6028
WVFGRD96 64.0 50 55 5 3.60 0.6076
WVFGRD96 66.0 45 55 0 3.61 0.6127
WVFGRD96 68.0 45 55 0 3.61 0.6181
WVFGRD96 70.0 45 55 0 3.62 0.6214
WVFGRD96 72.0 45 50 0 3.62 0.6268
WVFGRD96 74.0 45 50 0 3.63 0.6296
WVFGRD96 76.0 45 50 0 3.63 0.6320
WVFGRD96 78.0 45 50 0 3.64 0.6343
WVFGRD96 80.0 45 50 0 3.64 0.6350
WVFGRD96 82.0 45 50 0 3.65 0.6361
WVFGRD96 84.0 60 45 20 3.65 0.6401
WVFGRD96 86.0 60 45 20 3.66 0.6441
WVFGRD96 88.0 60 45 20 3.66 0.6475
WVFGRD96 90.0 60 45 20 3.66 0.6508
WVFGRD96 92.0 60 45 20 3.67 0.6531
WVFGRD96 94.0 60 45 20 3.67 0.6555
WVFGRD96 96.0 60 45 20 3.67 0.6579
WVFGRD96 98.0 60 45 20 3.68 0.6598
WVFGRD96 100.0 60 45 20 3.68 0.6611
WVFGRD96 102.0 60 45 20 3.68 0.6623
WVFGRD96 104.0 60 45 15 3.69 0.6626
WVFGRD96 106.0 60 45 15 3.69 0.6645
WVFGRD96 108.0 60 45 15 3.70 0.6651
WVFGRD96 110.0 60 45 15 3.70 0.6658
WVFGRD96 112.0 60 45 15 3.70 0.6655
WVFGRD96 114.0 60 45 15 3.70 0.6659
WVFGRD96 116.0 60 45 15 3.71 0.6656
WVFGRD96 118.0 60 45 15 3.71 0.6655
WVFGRD96 120.0 60 45 15 3.71 0.6654
WVFGRD96 122.0 60 45 20 3.72 0.6650
WVFGRD96 124.0 60 45 15 3.72 0.6648
WVFGRD96 126.0 60 45 20 3.72 0.6643
WVFGRD96 128.0 60 45 20 3.73 0.6631
The best solution is
WVFGRD96 114.0 60 45 15 3.70 0.6659
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