Location
Location ANSS
The ANSS event ID is ak008cyj84rd and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak008cyj84rd/executive.
2008/10/08 09:53:01 60.718 -143.722 8.3 5.2 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2008/10/08 09:53:01:0 60.72 -143.72 8.3 5.2 Alaska
Stations used:
AK.BMR AK.BPAW AK.CAST AK.DIV AK.MCK AK.PAX AK.PPLA AK.RAG
AK.SWD AT.PMR AT.SKAG CN.DAWY CN.DLBC CN.INK CN.PLBC CN.WHY
IU.COLA US.EGAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 7.67e+23 dyne-cm
Mw = 5.19
Z = 17 km
Plane Strike Dip Rake
NP1 299 66 -97
NP2 135 25 -75
Principal Axes:
Axis Value Plunge Azimuth
T 7.67e+23 21 34
N 0.00e+00 6 301
P -7.67e+23 68 195
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.68e+23
Mxy 2.84e+23
Mxz 4.64e+23
Myy 2.00e+23
Myz 2.10e+23
Mzz -5.68e+23
##############
######################
###################### ###
####################### T ####
-######################## ######
-------#############################
##-------------#######################
##-------------------###################
##----------------------################
###-------------------------##############
####---------------------------###########
####-----------------------------#########
#####------------- --------------#######
####------------- P ---------------#####
#####------------ -----------------###
#####--------------------------------#
######------------------------------
#######---------------------------
#######-----------------------
#########------------------#
##############---#####
##############
Global CMT Convention Moment Tensor:
R T P
-5.68e+23 4.64e+23 -2.10e+23
4.64e+23 3.68e+23 -2.84e+23
-2.10e+23 -2.84e+23 2.00e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081008095301/index.html
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Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is
STK = 135
DIP = 25
RAKE = -75
MW = 5.19
HS = 17.0
The NDK file is 20081008095301.ndk
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
| SLU |
AEIC |
USGS/SLU Moment Tensor Solution
ENS 2008/10/08 09:53:01:0 60.72 -143.72 8.3 5.2 Alaska
Stations used:
AK.BMR AK.BPAW AK.CAST AK.DIV AK.MCK AK.PAX AK.PPLA AK.RAG
AK.SWD AT.PMR AT.SKAG CN.DAWY CN.DLBC CN.INK CN.PLBC CN.WHY
IU.COLA US.EGAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 7.67e+23 dyne-cm
Mw = 5.19
Z = 17 km
Plane Strike Dip Rake
NP1 299 66 -97
NP2 135 25 -75
Principal Axes:
Axis Value Plunge Azimuth
T 7.67e+23 21 34
N 0.00e+00 6 301
P -7.67e+23 68 195
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.68e+23
Mxy 2.84e+23
Mxz 4.64e+23
Myy 2.00e+23
Myz 2.10e+23
Mzz -5.68e+23
##############
######################
###################### ###
####################### T ####
-######################## ######
-------#############################
##-------------#######################
##-------------------###################
##----------------------################
###-------------------------##############
####---------------------------###########
####-----------------------------#########
#####------------- --------------#######
####------------- P ---------------#####
#####------------ -----------------###
#####--------------------------------#
######------------------------------
#######---------------------------
#######-----------------------
#########------------------#
##############---#####
##############
Global CMT Convention Moment Tensor:
R T P
-5.68e+23 4.64e+23 -2.10e+23
4.64e+23 3.68e+23 -2.84e+23
-2.10e+23 -2.84e+23 2.00e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081008095301/index.html
|
Moment tensor inversion summary for event 2008/10/08 09:53
Date: 2008/10/08
Time: 09:53 (UTC)
Region: Cape Yakataga Region of Alaska
Mw=5.0
Location:
Lat. 60.6989; Lon. -143.7639; Depth 5 km
(Best-fitting depth from moment tensor inversion)
Solution quality: good;
Number of stations = 8
Best Double Couple:
strike dip rake
Plane 1: 343.8 59.9 -37.3
Plane 2: 94.7 58.4 -144.0
Moment Tensor Parameters:
Mo = 3.50196e+23 dyn-cm
Mxx = 1.38; Mxy = 2.41; Mxz = -1.19
Myy = 0.39; Myz = 1.57; Mzz = -1.78
Principal Axes:
value azimuth plunge
T: 3.35 39.59 0.93
N: 0.30 130.47 43.54
P: -3.65 308.61 46.45
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Magnitudes
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.
ML Magnitude
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.
Context
The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.
Waveform Inversion using wvfgrd96
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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Location of broadband stations used for waveform inversion
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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:
hp c 0.02 n 3
lp c 0.05 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 170 25 -10 5.16 0.3079
WVFGRD96 1.0 170 25 -10 5.18 0.3162
WVFGRD96 2.0 170 35 -15 5.12 0.3302
WVFGRD96 3.0 170 20 -20 5.20 0.3473
WVFGRD96 4.0 170 20 -20 5.19 0.3820
WVFGRD96 5.0 165 20 -30 5.19 0.4185
WVFGRD96 6.0 155 20 -45 5.19 0.4528
WVFGRD96 7.0 145 20 -60 5.19 0.4852
WVFGRD96 8.0 145 25 -60 5.18 0.5117
WVFGRD96 9.0 140 25 -70 5.19 0.5388
WVFGRD96 10.0 135 25 -75 5.21 0.5633
WVFGRD96 11.0 135 25 -75 5.21 0.5772
WVFGRD96 12.0 135 25 -75 5.20 0.5887
WVFGRD96 13.0 135 25 -75 5.20 0.5990
WVFGRD96 14.0 135 25 -75 5.20 0.6014
WVFGRD96 15.0 135 25 -75 5.19 0.6020
WVFGRD96 16.0 130 25 -80 5.19 0.6031
WVFGRD96 17.0 135 25 -75 5.19 0.6034
WVFGRD96 18.0 130 25 -80 5.19 0.6024
WVFGRD96 19.0 130 25 -80 5.19 0.6007
WVFGRD96 20.0 130 25 -80 5.22 0.6011
WVFGRD96 21.0 130 25 -80 5.22 0.5974
WVFGRD96 22.0 130 20 -75 5.22 0.5946
WVFGRD96 23.0 125 20 -85 5.22 0.5915
WVFGRD96 24.0 125 20 -85 5.22 0.5885
WVFGRD96 25.0 125 20 -85 5.22 0.5856
WVFGRD96 26.0 300 70 -90 5.23 0.5817
WVFGRD96 27.0 300 70 -90 5.23 0.5775
WVFGRD96 28.0 300 70 -90 5.23 0.5728
WVFGRD96 29.0 300 70 -90 5.23 0.5683
WVFGRD96 30.0 115 20 -95 5.24 0.5633
WVFGRD96 31.0 300 70 -85 5.25 0.5574
WVFGRD96 32.0 110 20 -100 5.25 0.5524
WVFGRD96 33.0 110 20 -100 5.25 0.5460
WVFGRD96 34.0 300 70 -85 5.26 0.5404
WVFGRD96 35.0 295 75 -85 5.26 0.5347
WVFGRD96 36.0 295 75 -85 5.26 0.5288
WVFGRD96 37.0 295 75 -85 5.26 0.5228
WVFGRD96 38.0 295 75 -85 5.26 0.5165
WVFGRD96 39.0 295 75 -80 5.27 0.5112
WVFGRD96 40.0 300 75 -80 5.40 0.5159
WVFGRD96 41.0 300 75 -80 5.41 0.5138
WVFGRD96 42.0 300 75 -80 5.41 0.5115
WVFGRD96 43.0 300 75 -80 5.42 0.5084
WVFGRD96 44.0 300 75 -80 5.42 0.5049
WVFGRD96 45.0 300 75 -80 5.43 0.5011
WVFGRD96 46.0 300 75 -80 5.43 0.4966
WVFGRD96 47.0 300 75 -80 5.44 0.4922
WVFGRD96 48.0 295 80 -80 5.44 0.4874
WVFGRD96 49.0 295 80 -80 5.44 0.4831
WVFGRD96 50.0 295 80 -80 5.45 0.4785
The best solution is
WVFGRD96 17.0 135 25 -75 5.19 0.6034
The mechanism corresponding to the best fit is
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Figure 1. Waveform inversion focal mechanism
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The best fit as a function of depth is given in the following figure:
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Figure 2. Depth sensitivity for waveform mechanism
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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
hp c 0.02 n 3
lp c 0.05 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.
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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:
- The origin time and epicentral distance are incorrect
- The velocity model used for the inversion is incorrect
- The velocity model used to define the P-arrival time is not the
same as the velocity model used for the waveform inversion
(assuming that the initial trace alignment is based on the
P arrival time)
Assuming only a mislocation, the time shifts are fit to a functional form:
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.
Velocity Model
The CUS.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
CUS Model with Q from simple gamma values
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
1.0000 5.0000 2.8900 2.5000 0.172E-02 0.387E-02 0.00 0.00 1.00 1.00
9.0000 6.1000 3.5200 2.7300 0.160E-02 0.363E-02 0.00 0.00 1.00 1.00
10.0000 6.4000 3.7000 2.8200 0.149E-02 0.336E-02 0.00 0.00 1.00 1.00
20.0000 6.7000 3.8700 2.9020 0.000E-04 0.000E-04 0.00 0.00 1.00 1.00
0.0000 8.1500 4.7000 3.3640 0.194E-02 0.431E-02 0.00 0.00 1.00 1.00
Last Changed Sun Apr 28 01:02:30 PM CDT 2024
