Location
Location ANSS
The ANSS event ID is ak009ambz3p6 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak009ambz3p6/executive.
2009/08/19 18:19:27 61.228 -150.858 66.4 5.1 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2009/08/19 18:19:27:0 61.23 -150.86 66.4 5.1 Alaska
Stations used:
AK.BMR AK.CAST AK.CHUM AK.DIV AK.EYAK AK.MCK AK.PPLA AK.SAW
AK.SSN AK.TRF AT.PMR AT.SVW2 IU.COLA
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 2.82e+23 dyne-cm
Mw = 4.90
Z = 66 km
Plane Strike Dip Rake
NP1 337 83 119
NP2 80 30 15
Principal Axes:
Axis Value Plunge Azimuth
T 2.82e+23 45 276
N 0.00e+00 29 153
P -2.82e+23 31 43
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.08e+23
Mxy -1.17e+23
Mxz -7.69e+22
Myy 4.47e+22
Myz -2.26e+23
Mzz 6.32e+22
--------------
####------------------
########--------------------
##########--------------------
#############------------ ------
###############----------- P -------
#################---------- --------
###################---------------------
####################--------------------
######## ###########-------------------#
######## T ############------------------#
######## ############-----------------##
########################----------------##
########################--------------##
-########################-----------####
--######################----------####
---#####################------######
-----##################----#######
-------#############--########
---------------------#######
-------------------###
--------------
Global CMT Convention Moment Tensor:
R T P
6.32e+22 -7.69e+22 2.26e+23
-7.69e+22 -1.08e+23 1.17e+23
2.26e+23 1.17e+23 4.47e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090819181927/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 = 80
DIP = 30
RAKE = 15
MW = 4.90
HS = 66.0
The NDK file is 20090819181927.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 2009/08/19 18:19:27:0 61.23 -150.86 66.4 5.1 Alaska
Stations used:
AK.BMR AK.CAST AK.CHUM AK.DIV AK.EYAK AK.MCK AK.PPLA AK.SAW
AK.SSN AK.TRF AT.PMR AT.SVW2 IU.COLA
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 2.82e+23 dyne-cm
Mw = 4.90
Z = 66 km
Plane Strike Dip Rake
NP1 337 83 119
NP2 80 30 15
Principal Axes:
Axis Value Plunge Azimuth
T 2.82e+23 45 276
N 0.00e+00 29 153
P -2.82e+23 31 43
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.08e+23
Mxy -1.17e+23
Mxz -7.69e+22
Myy 4.47e+22
Myz -2.26e+23
Mzz 6.32e+22
--------------
####------------------
########--------------------
##########--------------------
#############------------ ------
###############----------- P -------
#################---------- --------
###################---------------------
####################--------------------
######## ###########-------------------#
######## T ############------------------#
######## ############-----------------##
########################----------------##
########################--------------##
-########################-----------####
--######################----------####
---#####################------######
-----##################----#######
-------#############--########
---------------------#######
-------------------###
--------------
Global CMT Convention Moment Tensor:
R T P
6.32e+22 -7.69e+22 2.26e+23
-7.69e+22 -1.08e+23 1.17e+23
2.26e+23 1.17e+23 4.47e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20090819181927/index.html
|
|
Moment tensor inversion summary for event 2009/08/19 18:19
Date: 2009/08/19
Time: 18:19 (UTC)
Region: Cook Inlet Region of Alaska
Mw=5.1
Location:
Lat. 61.2286; Lon. -150.8254; Depth 70 km
(Best-fitting depth from moment tensor inversion)
Solution quality: good;
Number of stations = 7
Best Double Couple:
strike dip rake
Plane 1: 174.0 87.0 -104.0
Plane 2: 72.3 14.3 -12.1
Moment Tensor Parameters:
Mo = 4.23092e+23 dyn-cm
Mxx = -0.19; Mxy = -0.96; Mxz = -0.48
Myy = 0.62; Myz = -4.05; Mzz = -0.43
Principal Axes:
value azimuth plunge
T: 4.22 277.03 40.44
N: 0.01 174.77 14.00
P: -4.23 69.71 46.19
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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:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 320 45 -80 4.13 0.2620
WVFGRD96 4.0 330 75 -65 4.18 0.2239
WVFGRD96 6.0 325 75 -60 4.19 0.2617
WVFGRD96 8.0 325 75 -60 4.26 0.2805
WVFGRD96 10.0 230 35 -25 4.28 0.2987
WVFGRD96 12.0 240 40 0 4.29 0.3179
WVFGRD96 14.0 250 45 30 4.33 0.3392
WVFGRD96 16.0 255 40 30 4.35 0.3560
WVFGRD96 18.0 260 40 30 4.36 0.3700
WVFGRD96 20.0 260 40 30 4.39 0.3822
WVFGRD96 22.0 260 40 25 4.41 0.3917
WVFGRD96 24.0 260 40 25 4.43 0.4003
WVFGRD96 26.0 260 40 25 4.45 0.4071
WVFGRD96 28.0 255 40 20 4.47 0.4118
WVFGRD96 30.0 255 45 20 4.49 0.4142
WVFGRD96 32.0 50 55 0 4.55 0.4347
WVFGRD96 34.0 55 50 0 4.56 0.4602
WVFGRD96 36.0 55 50 0 4.59 0.4824
WVFGRD96 38.0 55 50 0 4.61 0.5016
WVFGRD96 40.0 55 35 0 4.72 0.5168
WVFGRD96 42.0 60 35 5 4.74 0.5430
WVFGRD96 44.0 65 35 5 4.76 0.5646
WVFGRD96 46.0 65 35 5 4.77 0.5818
WVFGRD96 48.0 60 35 5 4.79 0.6014
WVFGRD96 50.0 65 35 5 4.81 0.6240
WVFGRD96 52.0 70 30 5 4.83 0.6449
WVFGRD96 54.0 70 30 10 4.84 0.6640
WVFGRD96 56.0 70 30 10 4.85 0.6787
WVFGRD96 58.0 75 30 10 4.86 0.6915
WVFGRD96 60.0 75 30 10 4.87 0.7008
WVFGRD96 62.0 75 30 10 4.88 0.7073
WVFGRD96 64.0 75 30 15 4.89 0.7085
WVFGRD96 66.0 80 30 15 4.90 0.7102
WVFGRD96 68.0 85 25 5 4.92 0.7085
WVFGRD96 70.0 85 25 5 4.93 0.7048
WVFGRD96 72.0 85 25 5 4.94 0.7014
WVFGRD96 74.0 85 25 5 4.94 0.6939
WVFGRD96 76.0 85 25 5 4.94 0.6858
WVFGRD96 78.0 85 25 5 4.95 0.6759
WVFGRD96 80.0 90 25 5 4.96 0.6650
WVFGRD96 82.0 90 25 5 4.96 0.6542
WVFGRD96 84.0 90 25 5 4.96 0.6429
WVFGRD96 86.0 90 25 5 4.97 0.6304
WVFGRD96 88.0 95 25 10 4.96 0.6171
WVFGRD96 90.0 95 25 10 4.97 0.6037
WVFGRD96 92.0 95 25 10 4.97 0.5898
WVFGRD96 94.0 95 25 10 4.97 0.5766
WVFGRD96 96.0 95 25 10 4.97 0.5629
WVFGRD96 98.0 100 25 15 4.97 0.5503
The best solution is
WVFGRD96 66.0 80 30 15 4.90 0.7102
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
cut o DIST/3.3 -40 o DIST/3.3 +50
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.
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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 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
Last Changed Sun Apr 28 01:08:52 PM CDT 2024
