The ANSS event ID is ak0117rtyygs and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0117rtyygs/executive.
2011/06/18 20:40:29 62.081 -148.264 29.0 4.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2011/06/18 20:40:29:0 62.08 -148.26 29.0 4.5 Alaska
Stations used:
AK.BAL AK.BMR AK.CRQ AK.DIV AK.EYAK AK.KLU AK.MCK AK.MDM
AK.MLY AK.PAX AK.PPLA AK.RAG AK.SAW AK.SCM AK.SWD AK.TRF
AT.PMR AT.SVW2 IU.COLA
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 1.72e+22 dyne-cm
Mw = 4.09
Z = 46 km
Plane Strike Dip Rake
NP1 70 50 -60
NP2 208 48 -121
Principal Axes:
Axis Value Plunge Azimuth
T 1.72e+22 1 139
N 0.00e+00 23 230
P -1.72e+22 67 47
Moment Tensor: (dyne-cm)
Component Value
Mxx 8.71e+21
Mxy -9.75e+21
Mxz -4.32e+21
Myy 5.94e+21
Myz -4.30e+21
Mzz -1.47e+22
##############
################------
###############-------------
#############-----------------
#############---------------------
############------------------------
############--------------------------
############----------- -------------#
###########------------ P -------------#
###########------------- ------------###
##########----------------------------####
#########----------------------------#####
#########--------------------------#######
########------------------------########
#######----------------------###########
-#####-------------------#############
----##-------------#################
-----#############################
---########################
---####################### T
-#####################
##############
Global CMT Convention Moment Tensor:
R T P
-1.47e+22 -4.32e+21 4.30e+21
-4.32e+21 8.71e+21 9.75e+21
4.30e+21 9.75e+21 5.94e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110618204029/index.html
|
STK = 70
DIP = 50
RAKE = -60
MW = 4.09
HS = 46.0
The NDK file is 20110618204029.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.
 |
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.
|
|
|
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:
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 0.5 225 45 90 3.37 0.2269
WVFGRD96 1.0 45 45 90 3.41 0.2303
WVFGRD96 2.0 45 45 90 3.52 0.2918
WVFGRD96 3.0 210 45 80 3.61 0.3036
WVFGRD96 4.0 340 90 -5 3.60 0.3076
WVFGRD96 5.0 165 85 0 3.63 0.3042
WVFGRD96 6.0 165 80 -5 3.66 0.2959
WVFGRD96 7.0 165 80 -5 3.68 0.2901
WVFGRD96 8.0 165 75 -10 3.71 0.2869
WVFGRD96 9.0 80 85 -35 3.70 0.2870
WVFGRD96 10.0 90 90 -45 3.68 0.2887
WVFGRD96 11.0 275 80 50 3.69 0.3024
WVFGRD96 12.0 275 80 50 3.71 0.3176
WVFGRD96 13.0 275 80 50 3.72 0.3310
WVFGRD96 14.0 275 80 50 3.73 0.3440
WVFGRD96 15.0 90 90 -50 3.73 0.3573
WVFGRD96 16.0 90 90 -50 3.74 0.3717
WVFGRD96 17.0 270 90 50 3.75 0.3851
WVFGRD96 18.0 85 80 -55 3.74 0.3975
WVFGRD96 19.0 85 80 -50 3.76 0.4116
WVFGRD96 20.0 85 80 -50 3.77 0.4257
WVFGRD96 21.0 85 80 -50 3.79 0.4373
WVFGRD96 22.0 85 75 -55 3.79 0.4503
WVFGRD96 23.0 85 75 -50 3.80 0.4625
WVFGRD96 24.0 80 70 -55 3.80 0.4756
WVFGRD96 25.0 80 70 -55 3.81 0.4880
WVFGRD96 26.0 80 70 -50 3.83 0.4998
WVFGRD96 27.0 80 65 -55 3.83 0.5119
WVFGRD96 28.0 80 65 -55 3.84 0.5238
WVFGRD96 29.0 80 65 -55 3.85 0.5350
WVFGRD96 30.0 80 65 -50 3.86 0.5453
WVFGRD96 31.0 80 65 -50 3.87 0.5547
WVFGRD96 32.0 80 60 -55 3.87 0.5633
WVFGRD96 33.0 80 60 -55 3.88 0.5715
WVFGRD96 34.0 80 60 -55 3.89 0.5790
WVFGRD96 35.0 80 60 -50 3.90 0.5854
WVFGRD96 36.0 75 55 -55 3.91 0.5911
WVFGRD96 37.0 75 55 -55 3.92 0.5971
WVFGRD96 38.0 75 55 -55 3.93 0.6004
WVFGRD96 39.0 70 50 -60 3.95 0.6014
WVFGRD96 40.0 70 55 -60 4.05 0.5948
WVFGRD96 41.0 70 50 -60 4.06 0.6010
WVFGRD96 42.0 70 50 -60 4.06 0.6059
WVFGRD96 43.0 70 50 -60 4.07 0.6104
WVFGRD96 44.0 70 50 -60 4.08 0.6126
WVFGRD96 45.0 70 50 -60 4.09 0.6139
WVFGRD96 46.0 70 50 -60 4.09 0.6139
WVFGRD96 47.0 70 50 -60 4.10 0.6126
WVFGRD96 48.0 70 50 -60 4.10 0.6104
WVFGRD96 49.0 65 45 -65 4.11 0.6079
WVFGRD96 50.0 65 45 -65 4.12 0.6043
WVFGRD96 51.0 65 45 -65 4.12 0.6004
WVFGRD96 52.0 70 45 -60 4.12 0.5955
WVFGRD96 53.0 70 45 -60 4.12 0.5902
WVFGRD96 54.0 70 45 -60 4.13 0.5849
WVFGRD96 55.0 70 45 -60 4.13 0.5784
WVFGRD96 56.0 70 45 -60 4.13 0.5708
WVFGRD96 57.0 70 45 -60 4.14 0.5633
WVFGRD96 58.0 70 45 -60 4.14 0.5547
WVFGRD96 59.0 70 45 -60 4.14 0.5458
The best solution is
WVFGRD96 46.0 70 50 -60 4.09 0.6139
The mechanism corresponding to the best fit is
|
|
|
The best fit as a function of depth is given in the following figure:
|
|
|
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
hp c 0.02 n 3 lp c 0.06 n 3
|
| 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. |
|
| 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