Location
Location ANSS
The ANSS event ID is ak0117oi3hnt and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0117oi3hnt/executive.
2011/06/16 19:06:05 60.765 -151.076 58.9 5.1 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2011/06/16 19:06:05:0 60.76 -151.08 58.9 5.1 Alaska
Stations used:
AK.BPAW AK.BRLK AK.BWN AK.CNP AK.DIV AK.HOM AK.KLU AK.KTH
AK.MCK AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AK.SWD
AT.MENT AT.PMR II.KDAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 3.85e+23 dyne-cm
Mw = 4.99
Z = 74 km
Plane Strike Dip Rake
NP1 45 75 20
NP2 310 71 164
Principal Axes:
Axis Value Plunge Azimuth
T 3.85e+23 25 268
N 0.00e+00 65 80
P -3.85e+23 3 177
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.82e+23
Mxy 3.29e+22
Mxz 1.44e+22
Myy 3.16e+23
Myz -1.47e+23
Mzz 6.58e+22
--------------
----------------------
----------------------------
-----------------------------#
######-------------------------###
############-------------------#####
################---------------#######
####################----------##########
######################-------###########
#########################----#############
#### ###################################
#### T ##################----#############
#### #################-------###########
#####################----------#########
###################--------------#######
###############------------------#####
############---------------------###
########-------------------------#
###---------------------------
----------------------------
----------- --------
------- P ----
Global CMT Convention Moment Tensor:
R T P
6.58e+22 1.44e+22 1.47e+23
1.44e+22 -3.82e+23 -3.29e+22
1.47e+23 -3.29e+22 3.16e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110616190605/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 = 45
DIP = 75
RAKE = 20
MW = 4.99
HS = 74.0
The NDK file is 20110616190605.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 2011/06/16 19:06:05:0 60.76 -151.08 58.9 5.1 Alaska
Stations used:
AK.BPAW AK.BRLK AK.BWN AK.CNP AK.DIV AK.HOM AK.KLU AK.KTH
AK.MCK AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AK.SWD
AT.MENT AT.PMR II.KDAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.05 n 3
Best Fitting Double Couple
Mo = 3.85e+23 dyne-cm
Mw = 4.99
Z = 74 km
Plane Strike Dip Rake
NP1 45 75 20
NP2 310 71 164
Principal Axes:
Axis Value Plunge Azimuth
T 3.85e+23 25 268
N 0.00e+00 65 80
P -3.85e+23 3 177
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.82e+23
Mxy 3.29e+22
Mxz 1.44e+22
Myy 3.16e+23
Myz -1.47e+23
Mzz 6.58e+22
--------------
----------------------
----------------------------
-----------------------------#
######-------------------------###
############-------------------#####
################---------------#######
####################----------##########
######################-------###########
#########################----#############
#### ###################################
#### T ##################----#############
#### #################-------###########
#####################----------#########
###################--------------#######
###############------------------#####
############---------------------###
########-------------------------#
###---------------------------
----------------------------
----------- --------
------- P ----
Global CMT Convention Moment Tensor:
R T P
6.58e+22 1.44e+22 1.47e+23
1.44e+22 -3.82e+23 -3.29e+22
1.47e+23 -3.29e+22 3.16e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110616190605/index.html
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USGS/SLU Regional Moment Solution
11/06/16 19:06:05.42
Epicenter: 60.807 -151.216
MW 5.0
USGS/SLU REGIONAL MOMENT TENSOR
Depth 66 No. of sta: 58
Moment Tensor; Scale 10**16 Nm
Mrr= 0.17 Mtt=-3.62
Mpp= 3.45 Mrt= 0.40
Mrp= 1.31 Mtp=-0.15
Principal axes:
T Val= 3.91 Plg=19 Azm=270
N -0.23 69 69
P -3.68 7 177
Best Double Couple:Mo=3.8*10**16
NP1:Strike= 45 Dip=81 Slip= 19
NP2: 312 71 171
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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 0.5 315 70 15 4.16 0.2135
WVFGRD96 1.0 310 85 0 4.17 0.2327
WVFGRD96 2.0 315 75 15 4.28 0.2936
WVFGRD96 3.0 130 90 0 4.31 0.3231
WVFGRD96 4.0 310 85 0 4.34 0.3426
WVFGRD96 5.0 220 80 15 4.38 0.3572
WVFGRD96 6.0 220 80 15 4.41 0.3817
WVFGRD96 7.0 35 90 -5 4.44 0.4075
WVFGRD96 8.0 220 85 15 4.47 0.4351
WVFGRD96 9.0 220 85 15 4.49 0.4546
WVFGRD96 10.0 220 85 15 4.51 0.4696
WVFGRD96 11.0 40 90 -10 4.52 0.4766
WVFGRD96 12.0 35 90 -15 4.54 0.4838
WVFGRD96 13.0 35 90 -15 4.55 0.4888
WVFGRD96 14.0 35 90 -10 4.56 0.4933
WVFGRD96 15.0 35 90 -10 4.57 0.4974
WVFGRD96 16.0 35 90 -15 4.58 0.5003
WVFGRD96 17.0 35 90 -10 4.58 0.5031
WVFGRD96 18.0 35 90 -10 4.59 0.5060
WVFGRD96 19.0 35 90 -10 4.60 0.5088
WVFGRD96 20.0 35 90 -10 4.61 0.5121
WVFGRD96 21.0 35 90 -10 4.61 0.5162
WVFGRD96 22.0 35 90 -10 4.62 0.5192
WVFGRD96 23.0 35 90 -10 4.63 0.5219
WVFGRD96 24.0 215 90 10 4.64 0.5242
WVFGRD96 25.0 35 90 -10 4.64 0.5267
WVFGRD96 26.0 40 90 10 4.64 0.5291
WVFGRD96 27.0 40 90 10 4.65 0.5327
WVFGRD96 28.0 40 90 10 4.66 0.5370
WVFGRD96 29.0 215 85 -15 4.67 0.5433
WVFGRD96 30.0 215 85 -15 4.67 0.5485
WVFGRD96 31.0 215 85 -15 4.68 0.5533
WVFGRD96 32.0 215 85 -15 4.69 0.5583
WVFGRD96 33.0 40 85 10 4.71 0.5669
WVFGRD96 34.0 40 85 10 4.72 0.5748
WVFGRD96 35.0 220 90 -10 4.73 0.5779
WVFGRD96 36.0 40 85 10 4.74 0.5916
WVFGRD96 37.0 220 90 -10 4.76 0.5931
WVFGRD96 38.0 40 85 10 4.77 0.6092
WVFGRD96 39.0 40 85 10 4.79 0.6184
WVFGRD96 40.0 40 80 20 4.82 0.6262
WVFGRD96 41.0 40 80 15 4.83 0.6302
WVFGRD96 42.0 40 80 15 4.83 0.6343
WVFGRD96 43.0 40 80 15 4.84 0.6382
WVFGRD96 44.0 40 80 15 4.85 0.6422
WVFGRD96 45.0 40 80 15 4.86 0.6460
WVFGRD96 46.0 40 80 15 4.86 0.6497
WVFGRD96 47.0 40 80 15 4.87 0.6536
WVFGRD96 48.0 40 80 15 4.88 0.6576
WVFGRD96 49.0 40 80 15 4.88 0.6618
WVFGRD96 50.0 40 80 15 4.89 0.6656
WVFGRD96 51.0 40 80 15 4.90 0.6690
WVFGRD96 52.0 40 80 15 4.90 0.6729
WVFGRD96 53.0 40 80 15 4.91 0.6771
WVFGRD96 54.0 40 80 15 4.91 0.6803
WVFGRD96 55.0 40 80 15 4.92 0.6830
WVFGRD96 56.0 40 80 15 4.93 0.6868
WVFGRD96 57.0 40 80 15 4.93 0.6900
WVFGRD96 58.0 40 80 15 4.93 0.6921
WVFGRD96 59.0 40 80 15 4.94 0.6953
WVFGRD96 60.0 40 80 20 4.94 0.6978
WVFGRD96 61.0 45 75 20 4.95 0.6994
WVFGRD96 62.0 45 75 20 4.95 0.7026
WVFGRD96 63.0 45 75 20 4.95 0.7044
WVFGRD96 64.0 45 75 20 4.96 0.7063
WVFGRD96 65.0 45 75 20 4.96 0.7085
WVFGRD96 66.0 45 75 20 4.96 0.7091
WVFGRD96 67.0 45 75 20 4.97 0.7113
WVFGRD96 68.0 45 75 20 4.97 0.7118
WVFGRD96 69.0 45 75 20 4.97 0.7131
WVFGRD96 70.0 45 75 20 4.98 0.7140
WVFGRD96 71.0 45 75 20 4.98 0.7143
WVFGRD96 72.0 45 75 20 4.98 0.7150
WVFGRD96 73.0 45 75 20 4.98 0.7145
WVFGRD96 74.0 45 75 20 4.99 0.7151
WVFGRD96 75.0 45 75 20 4.99 0.7144
WVFGRD96 76.0 45 75 20 4.99 0.7150
WVFGRD96 77.0 45 75 20 4.99 0.7138
WVFGRD96 78.0 45 75 20 5.00 0.7139
WVFGRD96 79.0 45 75 20 5.00 0.7127
WVFGRD96 80.0 45 75 20 5.00 0.7128
WVFGRD96 81.0 45 75 20 5.00 0.7111
WVFGRD96 82.0 45 75 20 5.00 0.7106
WVFGRD96 83.0 45 75 20 5.01 0.7094
WVFGRD96 84.0 45 75 20 5.01 0.7083
WVFGRD96 85.0 45 75 20 5.01 0.7067
WVFGRD96 86.0 45 75 20 5.01 0.7056
WVFGRD96 87.0 45 75 20 5.01 0.7043
WVFGRD96 88.0 45 75 20 5.01 0.7018
WVFGRD96 89.0 45 75 20 5.02 0.7012
WVFGRD96 90.0 40 80 20 5.02 0.6987
WVFGRD96 91.0 40 80 20 5.02 0.6978
WVFGRD96 92.0 40 80 20 5.02 0.6963
WVFGRD96 93.0 40 80 20 5.02 0.6944
WVFGRD96 94.0 40 80 25 5.02 0.6932
WVFGRD96 95.0 40 80 25 5.02 0.6911
WVFGRD96 96.0 40 80 25 5.02 0.6900
WVFGRD96 97.0 40 80 25 5.02 0.6883
WVFGRD96 98.0 40 80 25 5.02 0.6867
WVFGRD96 99.0 40 80 25 5.02 0.6847
The best solution is
WVFGRD96 74.0 45 75 20 4.99 0.7151
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 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 Sat Apr 27 02:19:33 PM CDT 2024
