Location
Location ANSS
The ANSS event ID is ak008cbfae65 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak008cbfae65/executive.
2008/09/24 12:19:52 63.413 -150.060 6.5 3.5 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2008/09/24 12:19:52:0 63.41 -150.06 6.5 3.5 Alaska
Stations used:
AK.BPAW AK.COLD AK.MCK AK.PAX AK.PPLA AT.PMR AT.SVW2
IU.COLA US.EGAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 1.97e+22 dyne-cm
Mw = 4.13
Z = 9 km
Plane Strike Dip Rake
NP1 210 55 45
NP2 90 55 135
Principal Axes:
Axis Value Plunge Azimuth
T 1.97e+22 55 60
N 0.00e+00 35 240
P -1.97e+22 0 330
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.32e+22
Mxy 1.14e+22
Mxz 4.54e+21
Myy 6.47e+19
Myz 8.13e+21
Mzz 1.31e+22
--------------
P -----------------###
-- ------------###########
----------------##############
---------------###################
---------------#####################
--------------########################
--------------############ ###########
------------############## T ###########
------------############### ############
------------##############################
#----------##############################-
###-------#############################---
####-----###########################----
################################--------
#######------############-------------
######------------------------------
#####-----------------------------
###---------------------------
###-------------------------
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
1.31e+22 4.54e+21 -8.13e+21
4.54e+21 -1.32e+22 -1.14e+22
-8.13e+21 -1.14e+22 6.47e+19
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080924121952/index.html
|
Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is
STK = 210
DIP = 55
RAKE = 45
MW = 4.13
HS = 9.0
The NDK file is 20080924121952.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 2008/09/24 12:19:52:0 63.41 -150.06 6.5 3.5 Alaska
Stations used:
AK.BPAW AK.COLD AK.MCK AK.PAX AK.PPLA AT.PMR AT.SVW2
IU.COLA US.EGAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 1.97e+22 dyne-cm
Mw = 4.13
Z = 9 km
Plane Strike Dip Rake
NP1 210 55 45
NP2 90 55 135
Principal Axes:
Axis Value Plunge Azimuth
T 1.97e+22 55 60
N 0.00e+00 35 240
P -1.97e+22 0 330
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.32e+22
Mxy 1.14e+22
Mxz 4.54e+21
Myy 6.47e+19
Myz 8.13e+21
Mzz 1.31e+22
--------------
P -----------------###
-- ------------###########
----------------##############
---------------###################
---------------#####################
--------------########################
--------------############ ###########
------------############## T ###########
------------############### ############
------------##############################
#----------##############################-
###-------#############################---
####-----###########################----
################################--------
#######------############-------------
######------------------------------
#####-----------------------------
###---------------------------
###-------------------------
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
1.31e+22 4.54e+21 -8.13e+21
4.54e+21 -1.32e+22 -1.14e+22
-8.13e+21 -1.14e+22 6.47e+19
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080924121952/index.html
|
|
Moment tensor inversion summary for event 2008/09/24 12:19
This is a fully automatic solution. It has not yet been reviewed by a seismologist.2008/09/24 12:19
Date 2008/09/24
Region: Central Region of Alaska
Mw=4.1
Centroid Location:
Time 12:19; Lat. 63.46N; Lon. 209.83W; Depth 10 km
Best Double Couple:
Plane 1: strike = 211; dip = 69; rake = 40
Plane 2: strike = 104; dip = 53; rake = 154
Moment Tensor:
Mo = 1.42074e+22 dyn-cm
Mxx = -109.997; Mxy = 83.630; Mxz = -12.066
Myy = 43.179; Myz = 64.976; Mzz = 66.818
Principal Axes:
T: value = 80.000; azimuth = 334; plunge = 10
N: value = 84.000; azimuth = 73; plunge = 43
P: value = 78.000; azimuth = 233; plunge = 46
|
|
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.
|
|
Location of broadband stations used for waveform inversion
|
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 0.5 185 35 -15 4.02 0.4421
WVFGRD96 1.0 185 30 -15 4.07 0.4476
WVFGRD96 2.0 190 35 -15 4.07 0.4494
WVFGRD96 3.0 195 35 -10 4.08 0.4550
WVFGRD96 4.0 200 40 5 4.07 0.4714
WVFGRD96 5.0 205 50 25 4.08 0.5027
WVFGRD96 6.0 205 55 35 4.10 0.5451
WVFGRD96 7.0 205 55 35 4.11 0.5766
WVFGRD96 8.0 210 55 40 4.12 0.5908
WVFGRD96 9.0 210 55 45 4.13 0.5953
WVFGRD96 10.0 210 55 40 4.14 0.5938
WVFGRD96 11.0 210 55 35 4.13 0.5839
WVFGRD96 12.0 205 60 35 4.13 0.5710
WVFGRD96 13.0 205 60 35 4.12 0.5559
WVFGRD96 14.0 205 60 35 4.12 0.5384
WVFGRD96 15.0 205 60 30 4.11 0.5192
WVFGRD96 16.0 205 60 30 4.11 0.5017
WVFGRD96 17.0 205 60 30 4.11 0.4844
WVFGRD96 18.0 200 70 40 4.11 0.4736
WVFGRD96 19.0 200 70 40 4.11 0.4659
WVFGRD96 20.0 200 70 40 4.13 0.4552
WVFGRD96 21.0 200 70 40 4.13 0.4462
WVFGRD96 22.0 200 70 40 4.14 0.4363
WVFGRD96 23.0 200 75 40 4.14 0.4264
WVFGRD96 24.0 200 75 40 4.14 0.4169
WVFGRD96 25.0 200 75 40 4.14 0.4071
WVFGRD96 26.0 195 80 40 4.14 0.3971
WVFGRD96 27.0 195 80 40 4.14 0.3881
WVFGRD96 28.0 195 80 40 4.15 0.3788
WVFGRD96 29.0 195 80 35 4.15 0.3702
WVFGRD96 30.0 195 80 35 4.15 0.3616
WVFGRD96 31.0 195 85 35 4.16 0.3536
WVFGRD96 32.0 195 85 35 4.16 0.3459
WVFGRD96 33.0 195 85 35 4.17 0.3379
WVFGRD96 34.0 195 85 35 4.17 0.3300
WVFGRD96 35.0 195 85 30 4.18 0.3223
WVFGRD96 36.0 195 85 30 4.18 0.3151
WVFGRD96 37.0 10 90 -30 4.19 0.3047
WVFGRD96 38.0 195 80 40 4.17 0.3016
WVFGRD96 39.0 195 80 40 4.17 0.2960
WVFGRD96 40.0 195 80 45 4.26 0.2890
WVFGRD96 41.0 95 60 -20 4.26 0.2889
WVFGRD96 42.0 95 60 -20 4.27 0.2876
WVFGRD96 43.0 95 60 -15 4.27 0.2863
WVFGRD96 44.0 95 60 -15 4.28 0.2847
WVFGRD96 45.0 95 60 -15 4.28 0.2834
WVFGRD96 46.0 95 65 -15 4.28 0.2819
WVFGRD96 47.0 95 65 -15 4.29 0.2811
WVFGRD96 48.0 95 65 -15 4.29 0.2803
WVFGRD96 49.0 95 65 -15 4.30 0.2786
WVFGRD96 50.0 95 65 -15 4.30 0.2783
The best solution is
WVFGRD96 9.0 210 55 45 4.13 0.5953
The mechanism corresponding to the best fit is
|
|
Figure 1. Waveform inversion focal mechanism
|
The best fit as a function of depth is given in the following figure:
|
|
Figure 2. Depth sensitivity for waveform mechanism
|
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
|
|
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:
- 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 01:02:27 PM CDT 2024
