The ANSS event ID is ak010835axjw and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak010835axjw/executive.
2010/06/25 04:56:53 61.895 -147.726 33.8 4.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2010/06/25 04:56:53:0 61.90 -147.73 33.8 4.6 Alaska
Stations used:
AK.BMR AK.BPAW AK.BRLK AK.BWN AK.CCB AK.CRQ AK.DIV AK.EYAK
AK.FYU AK.HARP AK.HDA AK.MCK AK.RAG AK.RC01 AK.RIDG AK.RND
AK.SCM AK.SKN AK.SSN AK.SWD AK.TGL AK.TRF AK.WRH AT.PMR
IM.IL31 IU.COLA US.EGAK XF.DOST XF.GRAP XF.KAVU XF.LUPN
XF.STEW XF.TARD XF.TRIP XZ.BAGL XZ.BARK XZ.BARN XZ.BERG
XZ.BGLC XZ.ISLE XZ.KHIT XZ.KULT XZ.MESA XZ.PTPK XZ.RKAV
XZ.VRDI
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 3.94e+22 dyne-cm
Mw = 4.33
Z = 35 km
Plane Strike Dip Rake
NP1 245 80 -70
NP2 0 22 -153
Principal Axes:
Axis Value Plunge Azimuth
T 3.94e+22 32 318
N 0.00e+00 20 61
P -3.94e+22 51 178
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.35e+20
Mxy -1.34e+22
Mxz 3.25e+22
Myy 1.24e+22
Myz -1.26e+22
Mzz -1.26e+22
###########---
##################----
########################----
##########################----
###### #####################----
####### T ######################----
######## #######################-###
#############################------#####
########################------------####
#####################----------------#####
#################--------------------#####
##############-----------------------#####
##########---------------------------#####
######------------------------------####
####-------------------------------#####
#----------------- -------------####
----------------- P ------------####
---------------- -----------####
--------------------------####
------------------------####
------------------####
-----------###
Global CMT Convention Moment Tensor:
R T P
-1.26e+22 3.25e+22 1.26e+22
3.25e+22 2.35e+20 1.34e+22
1.26e+22 1.34e+22 1.24e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100625045653/index.html
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STK = 245
DIP = 80
RAKE = -70
MW = 4.33
HS = 35.0
The NDK file is 20100625045653.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 0.03 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 50 40 85 3.87 0.3284
WVFGRD96 2.0 55 45 90 3.96 0.4013
WVFGRD96 3.0 45 45 85 4.03 0.4103
WVFGRD96 4.0 45 45 85 4.06 0.3502
WVFGRD96 5.0 50 50 -85 4.04 0.2631
WVFGRD96 6.0 190 20 40 3.97 0.2416
WVFGRD96 7.0 60 85 75 3.96 0.2721
WVFGRD96 8.0 60 85 75 4.05 0.2999
WVFGRD96 9.0 60 85 75 4.05 0.3377
WVFGRD96 10.0 240 90 -70 4.06 0.3730
WVFGRD96 11.0 60 90 70 4.07 0.4056
WVFGRD96 12.0 60 90 70 4.08 0.4350
WVFGRD96 13.0 240 85 -70 4.09 0.4622
WVFGRD96 14.0 240 85 -70 4.10 0.4874
WVFGRD96 15.0 245 85 -70 4.12 0.5104
WVFGRD96 16.0 245 85 -70 4.13 0.5319
WVFGRD96 17.0 245 80 -70 4.15 0.5521
WVFGRD96 18.0 245 80 -70 4.16 0.5709
WVFGRD96 19.0 245 80 -70 4.17 0.5878
WVFGRD96 20.0 245 80 -70 4.18 0.6035
WVFGRD96 21.0 245 80 -70 4.20 0.6179
WVFGRD96 22.0 245 80 -70 4.21 0.6317
WVFGRD96 23.0 245 80 -70 4.22 0.6444
WVFGRD96 24.0 245 80 -70 4.23 0.6561
WVFGRD96 25.0 245 80 -70 4.24 0.6667
WVFGRD96 26.0 245 80 -70 4.25 0.6765
WVFGRD96 27.0 245 80 -70 4.26 0.6855
WVFGRD96 28.0 245 80 -70 4.27 0.6934
WVFGRD96 29.0 245 80 -70 4.28 0.7003
WVFGRD96 30.0 245 80 -70 4.29 0.7061
WVFGRD96 31.0 245 80 -70 4.30 0.7111
WVFGRD96 32.0 245 80 -70 4.31 0.7152
WVFGRD96 33.0 245 80 -70 4.32 0.7181
WVFGRD96 34.0 245 80 -70 4.33 0.7197
WVFGRD96 35.0 245 80 -70 4.33 0.7202
WVFGRD96 36.0 245 80 -65 4.35 0.7195
WVFGRD96 37.0 245 80 -65 4.35 0.7178
WVFGRD96 38.0 245 80 -65 4.36 0.7153
WVFGRD96 39.0 245 80 -65 4.36 0.7123
WVFGRD96 40.0 245 80 -75 4.50 0.7087
WVFGRD96 41.0 245 80 -75 4.51 0.7035
WVFGRD96 42.0 245 80 -75 4.52 0.6971
WVFGRD96 43.0 245 80 -75 4.52 0.6907
WVFGRD96 44.0 245 80 -75 4.53 0.6834
WVFGRD96 45.0 245 80 -75 4.54 0.6756
WVFGRD96 46.0 245 80 -75 4.54 0.6671
WVFGRD96 47.0 245 80 -70 4.55 0.6588
WVFGRD96 48.0 245 80 -70 4.56 0.6498
WVFGRD96 49.0 245 80 -70 4.56 0.6400
WVFGRD96 50.0 245 80 -70 4.57 0.6300
WVFGRD96 51.0 245 80 -70 4.58 0.6190
WVFGRD96 52.0 245 80 -70 4.58 0.6078
WVFGRD96 53.0 245 80 -70 4.59 0.5959
WVFGRD96 54.0 245 85 -70 4.58 0.5851
WVFGRD96 55.0 245 85 -70 4.59 0.5738
WVFGRD96 56.0 245 85 -70 4.59 0.5621
WVFGRD96 57.0 245 85 -70 4.60 0.5503
WVFGRD96 58.0 245 85 -70 4.60 0.5381
WVFGRD96 59.0 245 85 -70 4.60 0.5255
The best solution is
WVFGRD96 35.0 245 80 -70 4.33 0.7202
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 0.03 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