Location
Location ANSS
The ANSS event ID is ak0145qswuta and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0145qswuta/executive.
2014/05/05 04:59:44 60.652 -149.560 38.2 4.5 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2014/05/05 04:59:44:0 60.65 -149.56 38.2 4.5 Alaska
Stations used:
AK.BAL AK.BARN AK.BPAW AK.BRLK AK.BWN AK.CCB AK.CNP AK.CRQ
AK.CTG AK.DHY AK.DOT AK.EYAK AK.FID AK.GHO AK.GLB AK.GLI
AK.HDA AK.HIN AK.HOM AK.KNK AK.KTH AK.MCAR AK.MCK AK.MLY
AK.NEA AK.PPLA AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK
AK.TGL AK.TRF AK.WRH AT.MENT AT.PMR IM.IL31 IU.COLA
Filtering commands used:
cut a -30 a 180
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 7.33e+22 dyne-cm
Mw = 4.51
Z = 46 km
Plane Strike Dip Rake
NP1 60 55 -65
NP2 201 42 -121
Principal Axes:
Axis Value Plunge Azimuth
T 7.33e+22 7 132
N 0.00e+00 20 225
P -7.33e+22 69 25
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.48e+22
Mxy -3.97e+22
Mxz -2.86e+22
Myy 3.76e+22
Myz -4.03e+21
Mzz -6.24e+22
##############
############----------
###########-----------------
##########--------------------
##########------------------------
##########--------------------------
##########---------------------------#
##########----------- -------------###
#########------------ P ------------####
#########------------- -----------######
#########--------------------------#######
########--------------------------########
########------------------------##########
#######----------------------###########
#######-------------------##############
######----------------################
######-----------###################
-----######################## ##
----####################### T
----######################
--####################
##############
Global CMT Convention Moment Tensor:
R T P
-6.24e+22 -2.86e+22 4.03e+21
-2.86e+22 2.48e+22 3.97e+22
4.03e+21 3.97e+22 3.76e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140505045944/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 = 60
DIP = 55
RAKE = -65
MW = 4.51
HS = 46.0
The NDK file is 20140505045944.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 2014/05/05 04:59:44:0 60.65 -149.56 38.2 4.5 Alaska
Stations used:
AK.BAL AK.BARN AK.BPAW AK.BRLK AK.BWN AK.CCB AK.CNP AK.CRQ
AK.CTG AK.DHY AK.DOT AK.EYAK AK.FID AK.GHO AK.GLB AK.GLI
AK.HDA AK.HIN AK.HOM AK.KNK AK.KTH AK.MCAR AK.MCK AK.MLY
AK.NEA AK.PPLA AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK
AK.TGL AK.TRF AK.WRH AT.MENT AT.PMR IM.IL31 IU.COLA
Filtering commands used:
cut a -30 a 180
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 7.33e+22 dyne-cm
Mw = 4.51
Z = 46 km
Plane Strike Dip Rake
NP1 60 55 -65
NP2 201 42 -121
Principal Axes:
Axis Value Plunge Azimuth
T 7.33e+22 7 132
N 0.00e+00 20 225
P -7.33e+22 69 25
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.48e+22
Mxy -3.97e+22
Mxz -2.86e+22
Myy 3.76e+22
Myz -4.03e+21
Mzz -6.24e+22
##############
############----------
###########-----------------
##########--------------------
##########------------------------
##########--------------------------
##########---------------------------#
##########----------- -------------###
#########------------ P ------------####
#########------------- -----------######
#########--------------------------#######
########--------------------------########
########------------------------##########
#######----------------------###########
#######-------------------##############
######----------------################
######-----------###################
-----######################## ##
----####################### T
----######################
--####################
##############
Global CMT Convention Moment Tensor:
R T P
-6.24e+22 -2.86e+22 4.03e+21
-2.86e+22 2.48e+22 3.97e+22
4.03e+21 3.97e+22 3.76e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140505045944/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 7.64e+15 N-m
Magnitude 4.5
Percent DC 93%
Depth 48.0 km
Updated 2014-05-05 05:28:41 UTC
Author us
Catalog us
Contributor us
Code us_b000qa69_mwr
Principal Axes
Axis Value Plunge Azimuth
T 7.518 9 134
N 0.244 27 229
P -7.762 62 27
Nodal Planes
Plane Strike Dip Rake
NP1 66 60 -59
NP2 196 43 -131
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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 180
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 2.0 50 45 90 3.92 0.3445
WVFGRD96 4.0 125 40 -90 3.94 0.3147
WVFGRD96 6.0 155 85 15 3.95 0.2983
WVFGRD96 8.0 355 35 25 4.01 0.3007
WVFGRD96 10.0 0 30 30 4.02 0.3348
WVFGRD96 12.0 65 80 -60 4.04 0.3761
WVFGRD96 14.0 65 75 -60 4.06 0.4211
WVFGRD96 16.0 65 70 -60 4.09 0.4634
WVFGRD96 18.0 65 70 -60 4.11 0.5026
WVFGRD96 20.0 65 70 -55 4.14 0.5373
WVFGRD96 22.0 65 70 -55 4.17 0.5690
WVFGRD96 24.0 65 70 -55 4.20 0.5973
WVFGRD96 26.0 65 65 -60 4.21 0.6224
WVFGRD96 28.0 65 65 -60 4.23 0.6451
WVFGRD96 30.0 65 65 -60 4.25 0.6633
WVFGRD96 32.0 65 65 -60 4.27 0.6781
WVFGRD96 34.0 65 60 -60 4.28 0.6928
WVFGRD96 36.0 65 60 -60 4.30 0.7041
WVFGRD96 38.0 65 55 -60 4.32 0.7094
WVFGRD96 40.0 60 55 -65 4.46 0.7353
WVFGRD96 42.0 60 55 -65 4.48 0.7464
WVFGRD96 44.0 60 55 -65 4.49 0.7515
WVFGRD96 46.0 60 55 -65 4.51 0.7520
WVFGRD96 48.0 60 55 -60 4.53 0.7499
WVFGRD96 50.0 60 55 -60 4.54 0.7439
WVFGRD96 52.0 60 55 -60 4.55 0.7347
WVFGRD96 54.0 60 55 -60 4.56 0.7227
WVFGRD96 56.0 65 55 -55 4.56 0.7097
WVFGRD96 58.0 65 55 -55 4.57 0.6947
WVFGRD96 60.0 65 55 -55 4.58 0.6779
WVFGRD96 62.0 65 60 -55 4.58 0.6599
WVFGRD96 64.0 70 60 -50 4.58 0.6440
WVFGRD96 66.0 70 60 -50 4.59 0.6284
WVFGRD96 68.0 70 60 -50 4.59 0.6128
WVFGRD96 70.0 70 60 -50 4.60 0.5968
WVFGRD96 72.0 70 60 -50 4.61 0.5801
WVFGRD96 74.0 75 65 -50 4.60 0.5655
WVFGRD96 76.0 75 65 -45 4.60 0.5522
WVFGRD96 78.0 75 65 -45 4.61 0.5389
WVFGRD96 80.0 70 65 -45 4.62 0.5255
WVFGRD96 82.0 70 65 -45 4.63 0.5129
WVFGRD96 84.0 70 65 -45 4.63 0.4998
WVFGRD96 86.0 70 65 -40 4.64 0.4877
WVFGRD96 88.0 70 70 -45 4.63 0.4795
WVFGRD96 90.0 70 70 -45 4.64 0.4715
WVFGRD96 92.0 70 70 -45 4.65 0.4636
WVFGRD96 94.0 70 70 -40 4.65 0.4559
WVFGRD96 96.0 70 70 -40 4.66 0.4488
WVFGRD96 98.0 70 75 -40 4.65 0.4426
The best solution is
WVFGRD96 46.0 60 55 -65 4.51 0.7520
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 180
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 06:21:08 PM CDT 2024
