The ANSS event ID is ak012c1bnwdr and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak012c1bnwdr/executive.
2012/09/18 01:44:49 56.937 -154.142 38.6 5.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2012/09/18 01:44:49:0 56.94 -154.14 38.6 5.2 Alaska
Stations used:
AK.BMR AK.BPAW AK.BRLK AK.CAST AK.CNP AK.DHY AK.DIV AK.EYAK
AK.FALS AK.FID AK.GLI AK.HOM AK.KLU AK.KNK AK.KTH AK.MCK
AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.TRF
AK.UNV AT.AKUT AT.CHGN AT.OHAK AT.SDPT AT.SVW2 II.KDAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.04 n 3
Best Fitting Double Couple
Mo = 5.25e+23 dyne-cm
Mw = 5.08
Z = 42 km
Plane Strike Dip Rake
NP1 142 86 -150
NP2 50 60 -5
Principal Axes:
Axis Value Plunge Azimuth
T 5.25e+23 17 272
N 0.00e+00 60 150
P -5.25e+23 24 10
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.23e+23
Mxy -9.81e+22
Mxz -1.86e+23
Myy 4.62e+23
Myz -1.86e+23
Mzz -3.96e+22
--------------
------------ -------
##------------- P ----------
####------------ -----------
#######--------------------------#
#########-------------------------##
############----------------------####
##############--------------------######
###############------------------#######
## #############---------------#########
## T ##############-------------##########
## ###############-----------###########
######################-------#############
######################----##############
########################################
#####################---##############
#################--------###########
###########--------------#########
-------------------------#####
--------------------------##
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
-3.96e+22 -1.86e+23 1.86e+23
-1.86e+23 -4.23e+23 9.81e+22
1.86e+23 9.81e+22 4.62e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120918014449/index.html
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STK = 50
DIP = 60
RAKE = -5
MW = 5.08
HS = 42.0
The NDK file is 20120918014449.ndk The waveform inversion is preferred.
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.
USGS/SLU Moment Tensor Solution
ENS 2012/09/18 01:44:49:0 56.94 -154.14 38.6 5.2 Alaska
Stations used:
AK.BMR AK.BPAW AK.BRLK AK.CAST AK.CNP AK.DHY AK.DIV AK.EYAK
AK.FALS AK.FID AK.GLI AK.HOM AK.KLU AK.KNK AK.KTH AK.MCK
AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.TRF
AK.UNV AT.AKUT AT.CHGN AT.OHAK AT.SDPT AT.SVW2 II.KDAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.04 n 3
Best Fitting Double Couple
Mo = 5.25e+23 dyne-cm
Mw = 5.08
Z = 42 km
Plane Strike Dip Rake
NP1 142 86 -150
NP2 50 60 -5
Principal Axes:
Axis Value Plunge Azimuth
T 5.25e+23 17 272
N 0.00e+00 60 150
P -5.25e+23 24 10
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.23e+23
Mxy -9.81e+22
Mxz -1.86e+23
Myy 4.62e+23
Myz -1.86e+23
Mzz -3.96e+22
--------------
------------ -------
##------------- P ----------
####------------ -----------
#######--------------------------#
#########-------------------------##
############----------------------####
##############--------------------######
###############------------------#######
## #############---------------#########
## T ##############-------------##########
## ###############-----------###########
######################-------#############
######################----##############
########################################
#####################---##############
#################--------###########
###########--------------#########
-------------------------#####
--------------------------##
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
-3.96e+22 -1.86e+23 1.86e+23
-1.86e+23 -4.23e+23 9.81e+22
1.86e+23 9.81e+22 4.62e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120918014449/index.html
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Global CMT Project Moment Tensor Solution
September 18, 2012, KODIAK ISLAND REGION, ALASKA, MW=5.2
Howard Koss
CENTROID-MOMENT-TENSOR SOLUTION
GCMT EVENT: C201209180144A
DATA: II IU CU MN LD G IC DK GE
L.P.BODY WAVES: 72S, 104C, T= 40
SURFACE WAVES: 115S, 185C, T= 50
TIMESTAMP: Q-20120918053847
CENTROID LOCATION:
ORIGIN TIME: 01:44:49.8 0.2
LAT:56.96N 0.02;LON:154.30W 0.02
DEP: 30.7 1.0;TRIANG HDUR: 1.0
MOMENT TENSOR: SCALE 10**23 D-CM
RR=-2.770 0.174; TT=-3.040 0.159
PP= 5.810 0.145; RT= 3.640 0.233
RP= 4.830 0.218; TP= 2.600 0.103
PRINCIPAL AXES:
1.(T) VAL= 9.233;PLG=26;AZM=290
2.(N) -2.440; 29; 35
3.(P) -6.793; 50; 165
BEST DBLE.COUPLE:M0= 8.01*10**23
NP1: STRIKE=335;DIP=32;SLIP=-154
NP2: STRIKE=223;DIP=77;SLIP= -60
-----------
###########--------
################------#
####################-######
###################----######
## #############-------######
## T ###########----------#####
### #########-------------#####
##############--------------#####
############----------------#####
###########-----------------#####
########-------------------####
#######--------- --------####
#####---------- P --------###
###----------- -------###
---------------------##
------------------#
-----------
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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.
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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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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 0.04 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 190 45 80 4.62 0.3181
WVFGRD96 1.0 105 90 5 4.50 0.3342
WVFGRD96 2.0 190 45 90 4.69 0.3974
WVFGRD96 3.0 115 65 30 4.62 0.4228
WVFGRD96 4.0 290 60 20 4.66 0.4384
WVFGRD96 5.0 290 65 20 4.68 0.4418
WVFGRD96 6.0 105 75 10 4.69 0.4389
WVFGRD96 7.0 285 75 10 4.71 0.4335
WVFGRD96 8.0 65 55 10 4.75 0.4346
WVFGRD96 9.0 65 55 15 4.76 0.4565
WVFGRD96 10.0 60 55 10 4.76 0.4739
WVFGRD96 11.0 60 55 10 4.77 0.4903
WVFGRD96 12.0 60 55 10 4.78 0.5054
WVFGRD96 13.0 60 55 15 4.78 0.5190
WVFGRD96 14.0 60 60 10 4.80 0.5314
WVFGRD96 15.0 60 60 10 4.80 0.5437
WVFGRD96 16.0 55 60 5 4.80 0.5557
WVFGRD96 17.0 55 60 5 4.81 0.5685
WVFGRD96 18.0 55 60 5 4.82 0.5793
WVFGRD96 19.0 55 60 5 4.82 0.5892
WVFGRD96 20.0 55 60 5 4.83 0.5993
WVFGRD96 21.0 50 60 0 4.84 0.6085
WVFGRD96 22.0 50 60 -5 4.85 0.6188
WVFGRD96 23.0 50 60 -5 4.86 0.6280
WVFGRD96 24.0 50 60 -5 4.87 0.6365
WVFGRD96 25.0 50 60 -5 4.87 0.6450
WVFGRD96 26.0 50 60 -5 4.88 0.6519
WVFGRD96 27.0 50 60 -5 4.89 0.6584
WVFGRD96 28.0 50 60 -5 4.90 0.6648
WVFGRD96 29.0 50 60 -10 4.91 0.6697
WVFGRD96 30.0 50 65 -5 4.93 0.6748
WVFGRD96 31.0 50 65 -5 4.93 0.6802
WVFGRD96 32.0 50 65 0 4.94 0.6846
WVFGRD96 33.0 50 65 0 4.95 0.6892
WVFGRD96 34.0 50 65 0 4.95 0.6930
WVFGRD96 35.0 50 65 0 4.96 0.6965
WVFGRD96 36.0 50 70 10 4.98 0.7001
WVFGRD96 37.0 50 70 10 4.99 0.7062
WVFGRD96 38.0 50 70 10 5.00 0.7123
WVFGRD96 39.0 50 70 10 5.01 0.7181
WVFGRD96 40.0 50 60 -5 5.06 0.7188
WVFGRD96 41.0 50 60 -5 5.07 0.7199
WVFGRD96 42.0 50 60 -5 5.08 0.7200
WVFGRD96 43.0 50 60 -5 5.08 0.7193
WVFGRD96 44.0 50 60 -5 5.09 0.7180
WVFGRD96 45.0 50 60 -5 5.09 0.7160
WVFGRD96 46.0 50 60 -5 5.10 0.7135
WVFGRD96 47.0 50 60 -5 5.10 0.7104
WVFGRD96 48.0 50 65 0 5.11 0.7075
WVFGRD96 49.0 50 65 5 5.11 0.7048
The best solution is
WVFGRD96 42.0 50 60 -5 5.08 0.7200
The mechanism corresponding to the best fit is
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The best fit as a function of depth is given in the following figure:
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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 0.04 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. |
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