The ANSS event ID is ak015vldo42 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak015vldo42/executive.
2015/01/19 10:36:11 62.188 -150.572 9.5 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2015/01/19 10:36:11:0 62.19 -150.57 9.5 4.0 Alaska
Stations used:
AK.BAL AK.BPAW AK.BRLK AK.BWN AK.CCB AK.CRQ AK.EYAK AK.FID
AK.FYU AK.GHO AK.GLB AK.GLI AK.HDA AK.HIN AK.KLU AK.KNK
AK.KTH AK.MDM AK.PAX AK.PPD AK.PPLA AK.RIDG AK.RND AK.SAW
AK.SCM AK.SKN AK.SSN AK.SWD AK.TRF AK.VRDI AK.WRH AT.MENT
AT.PMR AT.TTA IM.IL31 IU.COLA TA.I23K TA.M24K TA.N25K
TA.POKR US.EGAK
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 = 9.55e+21 dyne-cm
Mw = 3.92
Z = 17 km
Plane Strike Dip Rake
NP1 70 73 148
NP2 170 60 20
Principal Axes:
Axis Value Plunge Azimuth
T 9.55e+21 34 26
N 0.00e+00 54 224
P -9.55e+21 8 122
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.57e+21
Mxy 6.82e+21
Mxz 4.70e+21
Myy -5.40e+21
Myz 8.29e+20
Mzz 2.83e+21
---###########
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--------############ #######
----------############ T #########
----------############# ##########
-----------###########################
------------###########################-
------------#########################---
-------------#######################------
-------------####################---------
-------------#################------------
--------------############----------------
-------------#######--------------------
----------------------------------------
#############--------------------- -
#############-------------------- P
#############-------------------
############------------------
############----------------
###########-----------
#########-----
Global CMT Convention Moment Tensor:
R T P
2.83e+21 4.70e+21 -8.29e+20
4.70e+21 2.57e+21 -6.82e+21
-8.29e+20 -6.82e+21 -5.40e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150119103611/index.html
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STK = 170
DIP = 60
RAKE = 20
MW = 3.92
HS = 17.0
The NDK file is 20150119103611.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 1.0 45 45 -90 3.51 0.2818
WVFGRD96 2.0 225 40 -90 3.62 0.3462
WVFGRD96 3.0 340 80 -10 3.60 0.3460
WVFGRD96 4.0 340 70 -5 3.65 0.3485
WVFGRD96 5.0 160 50 -15 3.69 0.3723
WVFGRD96 6.0 165 45 -10 3.71 0.4067
WVFGRD96 7.0 165 50 -5 3.73 0.4414
WVFGRD96 8.0 165 45 -5 3.79 0.4731
WVFGRD96 9.0 165 45 0 3.81 0.5038
WVFGRD96 10.0 165 50 0 3.82 0.5316
WVFGRD96 11.0 170 50 15 3.84 0.5586
WVFGRD96 12.0 170 50 20 3.86 0.5835
WVFGRD96 13.0 170 55 20 3.88 0.6047
WVFGRD96 14.0 170 55 20 3.89 0.6199
WVFGRD96 15.0 170 55 20 3.90 0.6300
WVFGRD96 16.0 170 55 20 3.91 0.6359
WVFGRD96 17.0 170 60 20 3.92 0.6389
WVFGRD96 18.0 170 60 20 3.93 0.6385
WVFGRD96 19.0 170 60 20 3.94 0.6351
WVFGRD96 20.0 170 60 20 3.95 0.6298
WVFGRD96 21.0 170 60 20 3.96 0.6232
WVFGRD96 22.0 170 60 15 3.96 0.6145
WVFGRD96 23.0 170 60 20 3.97 0.6050
WVFGRD96 24.0 170 60 15 3.97 0.5949
WVFGRD96 25.0 170 60 15 3.98 0.5838
WVFGRD96 26.0 170 60 15 3.98 0.5720
WVFGRD96 27.0 170 60 15 3.99 0.5594
WVFGRD96 28.0 170 60 15 3.99 0.5468
WVFGRD96 29.0 170 60 20 4.00 0.5336
The best solution is
WVFGRD96 17.0 170 60 20 3.92 0.6389
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