The ANSS event ID is ak0104gorpek and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0104gorpek/executive.
2010/04/07 16:19:15 61.580 -149.652 35.3 4.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2010/04/07 16:19:15:0 61.58 -149.65 35.3 4.6 Alaska
Stations used:
AK.BRLK AK.BWN AK.CHUM AK.CNP AK.DDM AK.DIV AK.EYAK AK.HOM
AK.KLU AK.KTH AK.PAX AK.PPLA AK.RC01 AK.RND AK.SCM AK.SCRK
AK.SSN AK.TRF AT.PMR AT.SVW2 AV.AUL AV.SPBG XZ.NICH 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 = 7.33e+22 dyne-cm
Mw = 4.51
Z = 44 km
Plane Strike Dip Rake
NP1 210 75 -75
NP2 344 21 -134
Principal Axes:
Axis Value Plunge Azimuth
T 7.33e+22 28 288
N 0.00e+00 14 26
P -7.33e+22 57 140
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.02e+21
Mxy -6.17e+21
Mxz 3.49e+22
Myy 4.24e+22
Myz -5.06e+22
Mzz -3.54e+22
########------
#################----#
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####################------####
#####################--------#####
####################-----------#####
####################--------------####
#### #############---------------#####
#### T ###########------------------####
##### ##########-------------------#####
#################---------------------####
################----------------------####
###############-----------------------####
#############----------- ----------###
#############----------- P ---------####
###########------------ ---------###
#########------------------------###
########-----------------------###
#####-----------------------##
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#-------------------##
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Global CMT Convention Moment Tensor:
R T P
-3.54e+22 3.49e+22 5.06e+22
3.49e+22 -7.02e+21 6.17e+21
5.06e+22 6.17e+21 4.24e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100407161915/index.html
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STK = 210
DIP = 75
RAKE = -75
MW = 4.51
HS = 44.0
The NDK file is 20100407161915.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 1.0 155 45 85 3.77 0.2137
WVFGRD96 2.0 155 40 90 3.87 0.2635
WVFGRD96 3.0 150 35 80 3.93 0.2480
WVFGRD96 4.0 135 35 50 3.94 0.2286
WVFGRD96 5.0 125 40 30 3.93 0.2265
WVFGRD96 6.0 105 30 -5 3.94 0.2399
WVFGRD96 7.0 105 30 -5 3.95 0.2632
WVFGRD96 8.0 105 25 -5 4.03 0.2797
WVFGRD96 9.0 105 25 -5 4.03 0.3016
WVFGRD96 10.0 105 30 -5 4.04 0.3210
WVFGRD96 11.0 105 30 -5 4.05 0.3382
WVFGRD96 12.0 105 30 -5 4.06 0.3531
WVFGRD96 13.0 105 30 -5 4.06 0.3662
WVFGRD96 14.0 105 30 -5 4.07 0.3776
WVFGRD96 15.0 105 30 -5 4.08 0.3878
WVFGRD96 16.0 105 30 -5 4.09 0.3971
WVFGRD96 17.0 105 30 -5 4.09 0.4055
WVFGRD96 18.0 105 30 -5 4.10 0.4126
WVFGRD96 19.0 100 30 -15 4.12 0.4197
WVFGRD96 20.0 95 30 -20 4.13 0.4270
WVFGRD96 21.0 95 30 -20 4.14 0.4332
WVFGRD96 22.0 70 25 -50 4.17 0.4424
WVFGRD96 23.0 55 25 -65 4.19 0.4537
WVFGRD96 24.0 50 25 -70 4.21 0.4656
WVFGRD96 25.0 50 25 -70 4.22 0.4771
WVFGRD96 26.0 40 20 -80 4.23 0.4884
WVFGRD96 27.0 40 20 -80 4.25 0.4996
WVFGRD96 28.0 35 20 -85 4.26 0.5095
WVFGRD96 29.0 35 20 -85 4.27 0.5171
WVFGRD96 30.0 25 20 -90 4.28 0.5246
WVFGRD96 31.0 20 20 -95 4.29 0.5301
WVFGRD96 32.0 210 70 -80 4.31 0.5342
WVFGRD96 33.0 210 70 -80 4.31 0.5407
WVFGRD96 34.0 205 70 -90 4.31 0.5453
WVFGRD96 35.0 205 70 -85 4.32 0.5533
WVFGRD96 36.0 205 70 -85 4.32 0.5598
WVFGRD96 37.0 205 70 -85 4.33 0.5650
WVFGRD96 38.0 205 70 -80 4.34 0.5690
WVFGRD96 39.0 205 70 -80 4.34 0.5725
WVFGRD96 40.0 5 20 -110 4.47 0.5704
WVFGRD96 41.0 5 20 -110 4.48 0.5732
WVFGRD96 42.0 210 75 -80 4.49 0.5784
WVFGRD96 43.0 210 75 -80 4.50 0.5793
WVFGRD96 44.0 210 75 -75 4.51 0.5798
WVFGRD96 45.0 210 75 -75 4.51 0.5785
WVFGRD96 46.0 210 75 -75 4.52 0.5773
WVFGRD96 47.0 210 75 -75 4.52 0.5747
WVFGRD96 48.0 210 75 -75 4.53 0.5720
WVFGRD96 49.0 210 80 -75 4.53 0.5694
WVFGRD96 50.0 210 80 -75 4.53 0.5664
WVFGRD96 51.0 210 80 -70 4.54 0.5632
WVFGRD96 52.0 210 80 -70 4.55 0.5597
WVFGRD96 53.0 210 80 -70 4.55 0.5560
WVFGRD96 54.0 210 80 -70 4.55 0.5511
WVFGRD96 55.0 210 80 -70 4.56 0.5461
WVFGRD96 56.0 210 80 -70 4.56 0.5404
WVFGRD96 57.0 210 80 -70 4.56 0.5337
WVFGRD96 58.0 210 80 -70 4.56 0.5276
WVFGRD96 59.0 210 80 -70 4.57 0.5206
The best solution is
WVFGRD96 44.0 210 75 -75 4.51 0.5798
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