The ANSS event ID is ak014c9ou8yz and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak014c9ou8yz/executive.
2014/09/24 07:30:56 61.353 -146.778 27.4 4.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2014/09/24 07:30:56:0 61.35 -146.78 27.4 4.5 Alaska
Stations used:
AK.BAL AK.BARN AK.BPAW AK.BRLK AK.BWN AK.CCB AK.CTG AK.DHY
AK.DOT AK.EYAK AK.FID AK.GHO AK.GLB AK.GLI AK.HDA AK.HIN
AK.HMT AK.ISLE AK.KLU AK.KNK AK.KTH AK.MCAR AK.MCK AK.MDM
AK.MESA AK.PAX AK.PIN AK.PPLA AK.RAG AK.RIDG AK.RND AK.SAW
AK.SCM AK.SKN AK.SUCK AK.VRDI AK.WAX AK.WRH AK.YAH AT.PMR
AT.SVW2 AT.TTA IM.IL31 IU.COLA TA.M24K US.EGAK
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 5.19e+22 dyne-cm
Mw = 4.41
Z = 44 km
Plane Strike Dip Rake
NP1 201 60 -87
NP2 15 30 -95
Principal Axes:
Axis Value Plunge Azimuth
T 5.19e+22 15 289
N 0.00e+00 2 19
P -5.19e+22 75 118
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.13e+21
Mxy -1.31e+22
Mxz 1.05e+22
Myy 4.06e+22
Myz -2.39e+22
Mzz -4.48e+22
#############-
###############-----##
###############---------####
###############------------###
###############---------------####
###############-----------------####
# ###########------------------#####
## T ##########--------------------#####
## #########---------------------#####
##############----------------------######
#############-----------------------######
#############----------- ---------######
############------------ P --------#######
###########------------ --------######
###########----------------------#######
##########---------------------#######
#########--------------------#######
########-------------------#######
######-----------------#######
#####---------------########
###------------#######
------########
Global CMT Convention Moment Tensor:
R T P
-4.48e+22 1.05e+22 2.39e+22
1.05e+22 4.13e+21 1.31e+22
2.39e+22 1.31e+22 4.06e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140924073056/index.html
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STK = 15
DIP = 30
RAKE = -95
MW = 4.41
HS = 44.0
The NDK file is 20140924073056.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 +50 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.06 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 40 45 90 3.73 0.3170
WVFGRD96 2.0 215 45 85 3.83 0.3922
WVFGRD96 3.0 215 45 90 3.90 0.3890
WVFGRD96 4.0 30 50 -90 3.92 0.3304
WVFGRD96 5.0 205 40 -95 3.92 0.2773
WVFGRD96 6.0 45 75 -35 3.88 0.2724
WVFGRD96 7.0 50 90 -40 3.88 0.2766
WVFGRD96 8.0 50 90 -50 3.93 0.2835
WVFGRD96 9.0 235 80 55 3.93 0.2948
WVFGRD96 10.0 235 80 55 3.94 0.3083
WVFGRD96 11.0 235 80 55 3.94 0.3207
WVFGRD96 12.0 230 85 60 3.94 0.3322
WVFGRD96 13.0 45 90 -60 3.94 0.3455
WVFGRD96 14.0 45 90 -60 3.95 0.3589
WVFGRD96 15.0 45 90 65 3.95 0.3720
WVFGRD96 16.0 45 90 65 3.96 0.3880
WVFGRD96 17.0 220 85 -70 3.97 0.4062
WVFGRD96 18.0 70 20 -50 4.00 0.4231
WVFGRD96 19.0 65 20 -55 4.01 0.4433
WVFGRD96 20.0 65 20 -55 4.02 0.4631
WVFGRD96 21.0 60 20 -60 4.04 0.4804
WVFGRD96 22.0 60 20 -60 4.06 0.4997
WVFGRD96 23.0 55 20 -65 4.07 0.5183
WVFGRD96 24.0 50 20 -70 4.08 0.5369
WVFGRD96 25.0 60 25 -60 4.10 0.5554
WVFGRD96 26.0 55 25 -65 4.11 0.5737
WVFGRD96 27.0 50 25 -70 4.12 0.5910
WVFGRD96 28.0 210 65 -95 4.13 0.6067
WVFGRD96 29.0 210 65 -95 4.14 0.6233
WVFGRD96 30.0 215 65 -80 4.15 0.6411
WVFGRD96 31.0 215 65 -80 4.16 0.6575
WVFGRD96 32.0 215 65 -80 4.17 0.6726
WVFGRD96 33.0 215 65 -80 4.18 0.6864
WVFGRD96 34.0 215 65 -80 4.19 0.6992
WVFGRD96 35.0 210 60 -80 4.21 0.7119
WVFGRD96 36.0 210 60 -80 4.22 0.7234
WVFGRD96 37.0 210 60 -80 4.23 0.7325
WVFGRD96 38.0 205 60 -85 4.24 0.7403
WVFGRD96 39.0 205 60 -85 4.26 0.7462
WVFGRD96 40.0 205 60 -85 4.38 0.7336
WVFGRD96 41.0 205 60 -85 4.38 0.7416
WVFGRD96 42.0 15 30 -100 4.39 0.7464
WVFGRD96 43.0 15 30 -100 4.40 0.7487
WVFGRD96 44.0 15 30 -95 4.41 0.7488
WVFGRD96 45.0 15 30 -95 4.41 0.7481
WVFGRD96 46.0 15 30 -95 4.42 0.7449
WVFGRD96 47.0 15 30 -95 4.42 0.7398
WVFGRD96 48.0 205 60 -85 4.42 0.7332
WVFGRD96 49.0 205 60 -85 4.42 0.7246
WVFGRD96 50.0 205 60 -85 4.43 0.7151
WVFGRD96 51.0 200 60 -90 4.43 0.7044
WVFGRD96 52.0 15 30 -95 4.43 0.6932
WVFGRD96 53.0 205 55 -85 4.44 0.6813
WVFGRD96 54.0 15 35 -95 4.44 0.6699
WVFGRD96 55.0 15 35 -95 4.44 0.6576
WVFGRD96 56.0 15 35 -95 4.44 0.6442
WVFGRD96 57.0 15 35 -95 4.44 0.6306
WVFGRD96 58.0 15 35 -95 4.44 0.6162
WVFGRD96 59.0 15 35 -95 4.44 0.6019
The best solution is
WVFGRD96 44.0 15 30 -95 4.41 0.7488
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 +50 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.06 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