Location
Location ANSS
The ANSS event ID is ak0115aiaea0 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0115aiaea0/executive.
2011/04/25 19:29:15 59.062 -152.532 57.4 5 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2011/04/25 19:29:15:0 59.06 -152.53 57.4 5.0 Alaska
Stations used:
AK.BMR AK.BPAW AK.BRLK AK.CAST AK.DHY AK.DIV AK.HOM AK.KLU
AK.KTH AK.SAW AK.SCM AK.SWD AT.OHAK AT.PMR AT.SVW2 AT.TTA
II.KDAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 3.47e+23 dyne-cm
Mw = 4.96
Z = 63 km
Plane Strike Dip Rake
NP1 324 67 153
NP2 65 65 25
Principal Axes:
Axis Value Plunge Azimuth
T 3.47e+23 35 284
N 0.00e+00 55 107
P -3.47e+23 2 15
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.10e+23
Mxy -1.40e+23
Mxz 2.92e+22
Myy 1.98e+23
Myz -1.60e+23
Mzz 1.12e+23
----------- P
--------------- ----
#####-----------------------
#########---------------------
##############--------------------
#################-------------------
####################------------------
######################----------------##
##### ################------------####
###### T #################----------######
###### ##################-------########
############################----##########
##########################################
#########################----###########
#####################---------##########
###############---------------########
-----------------------------#######
-----------------------------#####
---------------------------###
--------------------------##
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
1.12e+23 2.92e+22 1.60e+23
2.92e+22 -3.10e+23 1.40e+23
1.60e+23 1.40e+23 1.98e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110425192915/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 = 65
DIP = 65
RAKE = 25
MW = 4.96
HS = 63.0
The NDK file is 20110425192915.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 2011/04/25 19:29:15:0 59.06 -152.53 57.4 5.0 Alaska
Stations used:
AK.BMR AK.BPAW AK.BRLK AK.CAST AK.DHY AK.DIV AK.HOM AK.KLU
AK.KTH AK.SAW AK.SCM AK.SWD AT.OHAK AT.PMR AT.SVW2 AT.TTA
II.KDAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 3.47e+23 dyne-cm
Mw = 4.96
Z = 63 km
Plane Strike Dip Rake
NP1 324 67 153
NP2 65 65 25
Principal Axes:
Axis Value Plunge Azimuth
T 3.47e+23 35 284
N 0.00e+00 55 107
P -3.47e+23 2 15
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.10e+23
Mxy -1.40e+23
Mxz 2.92e+22
Myy 1.98e+23
Myz -1.60e+23
Mzz 1.12e+23
----------- P
--------------- ----
#####-----------------------
#########---------------------
##############--------------------
#################-------------------
####################------------------
######################----------------##
##### ################------------####
###### T #################----------######
###### ##################-------########
############################----##########
##########################################
#########################----###########
#####################---------##########
###############---------------########
-----------------------------#######
-----------------------------#####
---------------------------###
--------------------------##
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
1.12e+23 2.92e+22 1.60e+23
2.92e+22 -3.10e+23 1.40e+23
1.60e+23 1.40e+23 1.98e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110425192915/index.html
|
USGS/SLU Regional Moment Solution
11/04/25 19:29:15.59
Epicenter: 59.175 -152.834
MW 4.9
USGS/SLU REGIONAL MOMENT TENSOR
Depth 56 No. of sta: 43
Moment Tensor; Scale 10**16 Nm
Mrr= 0.48 Mtt=-1.79
Mpp= 1.31 Mrt= 0.67
Mrp= 0.78 Mtp= 2.14
Principal axes:
T Val= 2.83 Plg=23 Azm=298
N 0.07 66 106
P -2.90 4 206
Best Double Couple:Mo=2.9*10**16
NP1:Strike= 74 Dip=77 Slip= 20
NP2: 340 71 166
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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 0.10 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 320 50 -30 4.01 0.2262
WVFGRD96 1.0 330 70 -15 4.00 0.2270
WVFGRD96 2.0 330 80 -10 4.13 0.2885
WVFGRD96 3.0 320 85 -45 4.24 0.3318
WVFGRD96 4.0 325 90 -35 4.25 0.3588
WVFGRD96 5.0 325 90 -30 4.28 0.3732
WVFGRD96 6.0 325 90 -30 4.30 0.3797
WVFGRD96 7.0 325 90 -25 4.33 0.3827
WVFGRD96 8.0 325 90 -30 4.37 0.3810
WVFGRD96 9.0 300 75 -25 4.39 0.3817
WVFGRD96 10.0 300 75 -25 4.41 0.3797
WVFGRD96 11.0 300 70 -25 4.44 0.3766
WVFGRD96 12.0 300 70 -25 4.45 0.3728
WVFGRD96 13.0 300 70 -25 4.47 0.3671
WVFGRD96 14.0 300 70 -25 4.48 0.3598
WVFGRD96 15.0 300 70 -25 4.49 0.3509
WVFGRD96 16.0 300 70 -25 4.50 0.3403
WVFGRD96 17.0 300 70 -25 4.51 0.3293
WVFGRD96 18.0 240 65 10 4.51 0.3219
WVFGRD96 19.0 240 65 10 4.53 0.3262
WVFGRD96 20.0 240 65 10 4.54 0.3307
WVFGRD96 21.0 235 65 10 4.55 0.3362
WVFGRD96 22.0 235 65 5 4.57 0.3443
WVFGRD96 23.0 235 65 0 4.58 0.3547
WVFGRD96 24.0 235 65 0 4.59 0.3642
WVFGRD96 25.0 235 65 0 4.61 0.3747
WVFGRD96 26.0 240 75 15 4.62 0.3867
WVFGRD96 27.0 240 75 15 4.64 0.4058
WVFGRD96 28.0 240 70 15 4.64 0.4248
WVFGRD96 29.0 240 70 15 4.66 0.4433
WVFGRD96 30.0 240 70 15 4.67 0.4597
WVFGRD96 31.0 240 70 15 4.68 0.4756
WVFGRD96 32.0 240 70 15 4.69 0.4907
WVFGRD96 33.0 240 75 10 4.70 0.5037
WVFGRD96 34.0 240 75 10 4.71 0.5139
WVFGRD96 35.0 240 75 10 4.72 0.5247
WVFGRD96 36.0 240 75 10 4.74 0.5346
WVFGRD96 37.0 240 75 10 4.75 0.5433
WVFGRD96 38.0 60 85 0 4.78 0.5544
WVFGRD96 39.0 60 85 0 4.80 0.5697
WVFGRD96 40.0 65 75 10 4.84 0.5842
WVFGRD96 41.0 65 75 10 4.86 0.5866
WVFGRD96 42.0 65 75 10 4.87 0.5867
WVFGRD96 43.0 65 75 10 4.88 0.5887
WVFGRD96 44.0 65 70 15 4.89 0.5929
WVFGRD96 45.0 65 70 15 4.90 0.5996
WVFGRD96 46.0 65 70 15 4.91 0.6074
WVFGRD96 47.0 65 70 15 4.92 0.6165
WVFGRD96 48.0 65 70 15 4.93 0.6253
WVFGRD96 49.0 65 70 15 4.93 0.6339
WVFGRD96 50.0 65 70 15 4.94 0.6407
WVFGRD96 51.0 65 65 20 4.94 0.6479
WVFGRD96 52.0 65 65 20 4.94 0.6555
WVFGRD96 53.0 65 65 20 4.95 0.6642
WVFGRD96 54.0 65 65 20 4.95 0.6695
WVFGRD96 55.0 65 65 20 4.95 0.6712
WVFGRD96 56.0 65 65 20 4.96 0.6776
WVFGRD96 57.0 65 65 20 4.96 0.6819
WVFGRD96 58.0 65 65 20 4.96 0.6839
WVFGRD96 59.0 65 65 20 4.96 0.6854
WVFGRD96 60.0 65 65 20 4.96 0.6881
WVFGRD96 61.0 65 65 25 4.96 0.6878
WVFGRD96 62.0 65 65 25 4.96 0.6866
WVFGRD96 63.0 65 65 25 4.96 0.6892
WVFGRD96 64.0 65 65 25 4.96 0.6871
WVFGRD96 65.0 65 65 25 4.96 0.6884
WVFGRD96 66.0 65 65 25 4.96 0.6872
WVFGRD96 67.0 65 65 25 4.96 0.6838
WVFGRD96 68.0 65 65 25 4.96 0.6852
WVFGRD96 69.0 65 65 25 4.96 0.6795
WVFGRD96 70.0 65 65 25 4.96 0.6806
WVFGRD96 71.0 65 65 25 4.96 0.6774
WVFGRD96 72.0 65 65 25 4.96 0.6776
WVFGRD96 73.0 65 65 25 4.96 0.6733
WVFGRD96 74.0 65 65 25 4.96 0.6720
WVFGRD96 75.0 65 70 25 4.96 0.6700
WVFGRD96 76.0 65 65 25 4.96 0.6673
WVFGRD96 77.0 65 70 25 4.96 0.6648
WVFGRD96 78.0 65 65 25 4.96 0.6627
WVFGRD96 79.0 65 70 25 4.96 0.6597
WVFGRD96 80.0 65 70 25 4.96 0.6585
WVFGRD96 81.0 65 70 25 4.96 0.6553
WVFGRD96 82.0 65 70 25 4.96 0.6531
WVFGRD96 83.0 65 70 25 4.96 0.6507
WVFGRD96 84.0 65 70 30 4.96 0.6497
WVFGRD96 85.0 65 70 30 4.96 0.6449
WVFGRD96 86.0 65 70 30 4.96 0.6463
WVFGRD96 87.0 65 70 30 4.96 0.6392
WVFGRD96 88.0 65 70 30 4.96 0.6420
WVFGRD96 89.0 65 70 30 4.96 0.6368
WVFGRD96 90.0 65 70 30 4.96 0.6369
WVFGRD96 91.0 65 70 30 4.96 0.6345
WVFGRD96 92.0 65 70 30 4.96 0.6317
WVFGRD96 93.0 65 70 30 4.96 0.6313
WVFGRD96 94.0 65 70 30 4.96 0.6259
WVFGRD96 95.0 65 70 30 4.96 0.6280
WVFGRD96 96.0 65 70 35 4.96 0.6239
WVFGRD96 97.0 65 70 35 4.96 0.6235
WVFGRD96 98.0 65 70 35 4.96 0.6213
WVFGRD96 99.0 65 70 35 4.96 0.6186
The best solution is
WVFGRD96 63.0 65 65 25 4.96 0.6892
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 0.10 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 01:19:45 PM CDT 2024
