Location
Location ANSS
The ANSS event ID is ak01560swtbz and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak01560swtbz/executive.
2015/05/11 09:25:42 63.085 -148.251 67.4 4.5 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2015/05/11 09:25:42:0 63.08 -148.25 67.4 4.5 Alaska
Stations used:
AK.BARN AK.BPAW AK.BRLK AK.BWN AK.CCB AK.CNP AK.CRQ AK.CUT
AK.GHO AK.GLB AK.KLU AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA2
AK.PAX AK.PPD AK.PPLA AK.PWL AK.RND AK.SAW AK.SCM AK.SCRK
AK.SSN AK.TRF AK.WAT3 AK.WAT4 AK.WRH AT.MENT AT.PMR IM.IL31
IU.COLA TA.K27K TA.L27K TA.N25K TA.POKR
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 8.13e+22 dyne-cm
Mw = 4.54
Z = 74 km
Plane Strike Dip Rake
NP1 35 85 175
NP2 125 85 5
Principal Axes:
Axis Value Plunge Azimuth
T 8.13e+22 7 350
N 0.00e+00 83 170
P -8.13e+22 0 80
Moment Tensor: (dyne-cm)
Component Value
Mxx 7.50e+22
Mxy -2.82e+22
Mxz 9.76e+21
Myy -7.62e+22
Myz -1.78e+21
Mzz 1.23e+21
## T #########
###### #############
#########################---
#########################-----
##########################--------
---#######################----------
------####################------------
---------#################--------------
------------#############--------------
---------------#########---------------- P
-----------------######-----------------
--------------------##--------------------
---------------------##-------------------
------------------#######---------------
-----------------##########-------------
---------------##############---------
------------####################----
----------########################
------########################
----########################
######################
##############
Global CMT Convention Moment Tensor:
R T P
1.23e+21 9.76e+21 1.78e+21
9.76e+21 7.50e+22 2.82e+22
1.78e+21 2.82e+22 -7.62e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150511092542/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 = 125
DIP = 85
RAKE = 5
MW = 4.54
HS = 74.0
The NDK file is 20150511092542.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 2015/05/11 09:25:42:0 63.08 -148.25 67.4 4.5 Alaska
Stations used:
AK.BARN AK.BPAW AK.BRLK AK.BWN AK.CCB AK.CNP AK.CRQ AK.CUT
AK.GHO AK.GLB AK.KLU AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA2
AK.PAX AK.PPD AK.PPLA AK.PWL AK.RND AK.SAW AK.SCM AK.SCRK
AK.SSN AK.TRF AK.WAT3 AK.WAT4 AK.WRH AT.MENT AT.PMR IM.IL31
IU.COLA TA.K27K TA.L27K TA.N25K TA.POKR
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 8.13e+22 dyne-cm
Mw = 4.54
Z = 74 km
Plane Strike Dip Rake
NP1 35 85 175
NP2 125 85 5
Principal Axes:
Axis Value Plunge Azimuth
T 8.13e+22 7 350
N 0.00e+00 83 170
P -8.13e+22 0 80
Moment Tensor: (dyne-cm)
Component Value
Mxx 7.50e+22
Mxy -2.82e+22
Mxz 9.76e+21
Myy -7.62e+22
Myz -1.78e+21
Mzz 1.23e+21
## T #########
###### #############
#########################---
#########################-----
##########################--------
---#######################----------
------####################------------
---------#################--------------
------------#############--------------
---------------#########---------------- P
-----------------######-----------------
--------------------##--------------------
---------------------##-------------------
------------------#######---------------
-----------------##########-------------
---------------##############---------
------------####################----
----------########################
------########################
----########################
######################
##############
Global CMT Convention Moment Tensor:
R T P
1.23e+21 9.76e+21 1.78e+21
9.76e+21 7.50e+22 2.82e+22
1.78e+21 2.82e+22 -7.62e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150511092542/index.html
|
Regional Moment Tensor (Mwr)
Moment 8.546e+15 N-m
Magnitude 4.55
Depth 73.0 km
Percent DC 97%
Half Duration –
Catalog AK (ak11589443)
Data Source US2
Contributor US2
Nodal Planes
Plane Strike Dip Rake
NP1 33° 89° 177°
NP2 123° 87° 1°
Principal Axes
Axis Value Plunge Azimuth
T 8.472 3° 348°
N 0.146 87° 197°
P -8.618 1° 78°
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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 o DIST/3.3 -30 o DIST/3.3 +70
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 2.0 125 90 -5 3.68 0.2730
WVFGRD96 4.0 305 80 5 3.77 0.3294
WVFGRD96 6.0 305 85 5 3.83 0.3649
WVFGRD96 8.0 305 80 5 3.89 0.3991
WVFGRD96 10.0 305 85 5 3.93 0.4174
WVFGRD96 12.0 125 90 -10 3.96 0.4292
WVFGRD96 14.0 305 90 10 3.99 0.4450
WVFGRD96 16.0 305 90 5 4.01 0.4637
WVFGRD96 18.0 125 85 -5 4.04 0.4835
WVFGRD96 20.0 305 90 5 4.06 0.5019
WVFGRD96 22.0 125 85 -5 4.08 0.5229
WVFGRD96 24.0 125 85 -5 4.10 0.5396
WVFGRD96 26.0 125 85 -5 4.12 0.5544
WVFGRD96 28.0 125 85 0 4.14 0.5674
WVFGRD96 30.0 125 85 0 4.16 0.5773
WVFGRD96 32.0 125 85 0 4.18 0.5871
WVFGRD96 34.0 125 85 0 4.21 0.5966
WVFGRD96 36.0 125 85 0 4.24 0.6072
WVFGRD96 38.0 125 85 0 4.27 0.6263
WVFGRD96 40.0 125 85 0 4.32 0.6525
WVFGRD96 42.0 125 80 0 4.34 0.6657
WVFGRD96 44.0 125 85 5 4.37 0.6783
WVFGRD96 46.0 125 85 5 4.39 0.6910
WVFGRD96 48.0 125 85 5 4.40 0.7050
WVFGRD96 50.0 125 85 5 4.42 0.7191
WVFGRD96 52.0 125 85 5 4.44 0.7327
WVFGRD96 54.0 125 85 5 4.45 0.7451
WVFGRD96 56.0 125 85 5 4.46 0.7570
WVFGRD96 58.0 125 85 5 4.47 0.7673
WVFGRD96 60.0 125 85 5 4.49 0.7760
WVFGRD96 62.0 125 85 5 4.50 0.7831
WVFGRD96 64.0 125 85 5 4.50 0.7890
WVFGRD96 66.0 125 85 5 4.51 0.7935
WVFGRD96 68.0 125 85 5 4.52 0.7981
WVFGRD96 70.0 125 85 5 4.53 0.7999
WVFGRD96 72.0 125 85 5 4.53 0.8017
WVFGRD96 74.0 125 85 5 4.54 0.8024
WVFGRD96 76.0 125 85 5 4.55 0.8012
WVFGRD96 78.0 125 80 0 4.54 0.8008
WVFGRD96 80.0 125 80 0 4.55 0.7994
WVFGRD96 82.0 125 80 0 4.55 0.7974
WVFGRD96 84.0 125 80 0 4.55 0.7948
WVFGRD96 86.0 125 80 0 4.56 0.7921
WVFGRD96 88.0 125 80 0 4.56 0.7897
WVFGRD96 90.0 125 80 0 4.57 0.7859
WVFGRD96 92.0 125 80 0 4.57 0.7826
WVFGRD96 94.0 125 80 0 4.57 0.7787
WVFGRD96 96.0 125 80 0 4.58 0.7743
WVFGRD96 98.0 125 80 0 4.58 0.7698
The best solution is
WVFGRD96 74.0 125 85 5 4.54 0.8024
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 o DIST/3.3 -30 o DIST/3.3 +70
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 04:32:32 PM CDT 2024
