Location
Location ANSS
The ANSS event ID is ak013bpso0xd and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak013bpso0xd/executive.
2013/09/12 04:41:02 59.774 -152.831 11.3 4.1 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2013/09/12 04:41:02:0 59.77 -152.83 11.3 4.1 Alaska
Stations used:
AK.BAL AK.BARN AK.BPAW AK.BRLK AK.BWN AK.CAST AK.CCB AK.CNP
AK.CRQ AK.EYAK AK.FID AK.GHO AK.GLI AK.HIN AK.KIAG AK.KNK
AK.KTH AK.MCK AK.MLY AK.NICH AK.PAX AK.PPLA AK.PTPK AK.RC01
AK.SAW AK.SCM AK.SGA AK.SKN AK.SLK AK.SWD AK.TGL AK.WAX
AT.SVW2 II.KDAK IM.IL31 TA.HDA TA.POKR TA.TCOL
Filtering commands used:
cut a -30 a 210
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 1.60e+22 dyne-cm
Mw = 4.07
Z = 19 km
Plane Strike Dip Rake
NP1 205 81 -150
NP2 110 60 -10
Principal Axes:
Axis Value Plunge Azimuth
T 1.60e+22 14 334
N 0.00e+00 59 219
P -1.60e+22 27 72
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.09e+22
Mxy -9.70e+21
Mxz 1.39e+21
Myy -8.51e+21
Myz -7.89e+21
Mzz -2.41e+21
##############
# ###############---
#### T #############--------
##### ############----------
#####################-------------
#####################---------------
#####################-----------------
-####################------------ ----
--##################------------- P ----
----################-------------- -----
------#############-----------------------
-------###########------------------------
----------#######-------------------------
------------####------------------------
---------------#------------------------
-------------#######----------------##
-----------#########################
----------########################
-------#######################
------######################
--####################
##############
Global CMT Convention Moment Tensor:
R T P
-2.41e+21 1.39e+21 7.89e+21
1.39e+21 1.09e+22 9.70e+21
7.89e+21 9.70e+21 -8.51e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130912044102/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 = 110
DIP = 60
RAKE = -10
MW = 4.07
HS = 19.0
The NDK file is 20130912044102.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 |
USGSMT |
USGS/SLU Moment Tensor Solution
ENS 2013/09/12 04:41:02:0 59.77 -152.83 11.3 4.1 Alaska
Stations used:
AK.BAL AK.BARN AK.BPAW AK.BRLK AK.BWN AK.CAST AK.CCB AK.CNP
AK.CRQ AK.EYAK AK.FID AK.GHO AK.GLI AK.HIN AK.KIAG AK.KNK
AK.KTH AK.MCK AK.MLY AK.NICH AK.PAX AK.PPLA AK.PTPK AK.RC01
AK.SAW AK.SCM AK.SGA AK.SKN AK.SLK AK.SWD AK.TGL AK.WAX
AT.SVW2 II.KDAK IM.IL31 TA.HDA TA.POKR TA.TCOL
Filtering commands used:
cut a -30 a 210
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 1.60e+22 dyne-cm
Mw = 4.07
Z = 19 km
Plane Strike Dip Rake
NP1 205 81 -150
NP2 110 60 -10
Principal Axes:
Axis Value Plunge Azimuth
T 1.60e+22 14 334
N 0.00e+00 59 219
P -1.60e+22 27 72
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.09e+22
Mxy -9.70e+21
Mxz 1.39e+21
Myy -8.51e+21
Myz -7.89e+21
Mzz -2.41e+21
##############
# ###############---
#### T #############--------
##### ############----------
#####################-------------
#####################---------------
#####################-----------------
-####################------------ ----
--##################------------- P ----
----################-------------- -----
------#############-----------------------
-------###########------------------------
----------#######-------------------------
------------####------------------------
---------------#------------------------
-------------#######----------------##
-----------#########################
----------########################
-------#######################
------######################
--####################
##############
Global CMT Convention Moment Tensor:
R T P
-2.41e+21 1.39e+21 7.89e+21
1.39e+21 1.09e+22 9.70e+21
7.89e+21 9.70e+21 -8.51e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130912044102/index.html
|
Regional Moment Tensor (Mwr)
Moment magnitude derived from a moment tensor inversion of complete waveforms at regional distances (less than ~8 degrees), generally used for the analysis of small to moderate size earthquakes (typically Mw 3.5-6.0) crust or upper mantle earthquakes.
Moment
2.05e+15 N-m
Magnitude
4.1
Percent DC
87%
Depth
19.0 km
Updated
2013-09-12 14:09:35 UTC
Author
neic
Catalog
ak
Contributor
us
Code
ak10804220-neic-mwr
Principal Axes
Axis Value Plunge Azimuth
T 2.117 24 72
N -0.135 65 242
P -1.981 4 340
Nodal Planes
Plane Strike Dip Rake
NP1 209 76 20
NP2 114 70 165
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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 a -30 a 210
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 225 45 -80 3.72 0.3115
WVFGRD96 2.0 55 45 -95 3.86 0.3956
WVFGRD96 3.0 25 70 -35 3.84 0.3896
WVFGRD96 4.0 30 80 -30 3.87 0.4038
WVFGRD96 5.0 30 85 -30 3.90 0.4223
WVFGRD96 6.0 30 85 -30 3.92 0.4426
WVFGRD96 7.0 210 90 25 3.93 0.4634
WVFGRD96 8.0 30 90 -35 3.98 0.4837
WVFGRD96 9.0 30 90 -30 3.99 0.4969
WVFGRD96 10.0 105 50 -15 3.97 0.5158
WVFGRD96 11.0 105 50 -15 3.98 0.5297
WVFGRD96 12.0 110 55 -15 4.01 0.5419
WVFGRD96 13.0 110 55 -10 4.01 0.5513
WVFGRD96 14.0 110 55 -10 4.02 0.5582
WVFGRD96 15.0 110 55 -10 4.03 0.5631
WVFGRD96 16.0 110 60 -15 4.05 0.5678
WVFGRD96 17.0 110 60 -15 4.06 0.5711
WVFGRD96 18.0 110 60 -15 4.07 0.5725
WVFGRD96 19.0 110 60 -10 4.07 0.5730
WVFGRD96 20.0 115 60 10 4.08 0.5724
WVFGRD96 21.0 115 60 10 4.09 0.5700
WVFGRD96 22.0 115 60 10 4.10 0.5682
WVFGRD96 23.0 115 60 10 4.10 0.5648
WVFGRD96 24.0 115 60 10 4.11 0.5593
WVFGRD96 25.0 115 60 15 4.11 0.5542
WVFGRD96 26.0 115 60 15 4.12 0.5471
WVFGRD96 27.0 115 60 15 4.12 0.5390
WVFGRD96 28.0 115 60 15 4.13 0.5311
WVFGRD96 29.0 115 60 15 4.13 0.5197
WVFGRD96 30.0 115 60 20 4.13 0.5106
WVFGRD96 31.0 115 65 20 4.15 0.5011
WVFGRD96 32.0 115 65 20 4.15 0.4891
WVFGRD96 33.0 115 65 20 4.16 0.4797
WVFGRD96 34.0 115 65 25 4.16 0.4699
WVFGRD96 35.0 115 65 25 4.16 0.4617
WVFGRD96 36.0 115 65 25 4.17 0.4544
WVFGRD96 37.0 115 65 25 4.18 0.4468
WVFGRD96 38.0 115 65 25 4.19 0.4408
WVFGRD96 39.0 115 65 25 4.20 0.4358
WVFGRD96 40.0 110 60 -25 4.30 0.4396
WVFGRD96 41.0 110 60 -25 4.30 0.4390
WVFGRD96 42.0 110 60 -25 4.31 0.4384
WVFGRD96 43.0 110 60 -25 4.32 0.4379
WVFGRD96 44.0 110 60 -25 4.33 0.4371
WVFGRD96 45.0 110 60 -20 4.33 0.4365
WVFGRD96 46.0 110 65 -25 4.34 0.4358
WVFGRD96 47.0 110 65 -25 4.35 0.4351
WVFGRD96 48.0 110 65 -25 4.35 0.4341
WVFGRD96 49.0 110 65 -20 4.35 0.4326
The best solution is
WVFGRD96 19.0 110 60 -10 4.07 0.5730
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 a -30 a 210
rtr
taper w 0.1
hp c 0.02 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.
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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 Fri Apr 26 08:33:01 PM CDT 2024
