Location
Location ANSS
The ANSS event ID is ak0163bfjfdc and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0163bfjfdc/executive.
2016/03/12 21:57:55 60.261 -152.304 99.8 4.7 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2016/03/12 21:57:55:0 60.26 -152.30 99.8 4.7 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CNP AK.CUT AK.FIRE AK.HOM AK.KNK AK.RC01
AK.SWD AT.PMR AT.SVW2 AV.ILSW TA.L19K TA.M19K TA.M22K
TA.N19K TA.O19K TA.O22K TA.P18K
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 1.41e+23 dyne-cm
Mw = 4.70
Z = 90 km
Plane Strike Dip Rake
NP1 235 75 -20
NP2 330 71 -164
Principal Axes:
Axis Value Plunge Azimuth
T 1.41e+23 3 283
N 0.00e+00 65 20
P -1.41e+23 25 192
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.04e+23
Mxy -5.52e+22
Mxz 5.40e+22
Myy 1.28e+23
Myz 4.14e+21
Mzz -2.42e+22
--------------
##--------------------
#######---------------------
###########-------------------
##############--------------------
#################------------#######
###################------#############
####################-#################
T ##################---#################
###############-------#################
################----------################
##############-------------###############
###########-----------------##############
#########-------------------############
#######---------------------############
####------------------------##########
##-------------------------#########
---------------------------#######
---------- ------------#####
--------- P ------------####
------ ------------#
--------------
Global CMT Convention Moment Tensor:
R T P
-2.42e+22 5.40e+22 -4.14e+21
5.40e+22 -1.04e+23 5.52e+22
-4.14e+21 5.52e+22 1.28e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160312215755/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 = 235
DIP = 75
RAKE = -20
MW = 4.70
HS = 90.0
The NDK file is 20160312215755.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 2016/03/12 21:57:55:0 60.26 -152.30 99.8 4.7 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CNP AK.CUT AK.FIRE AK.HOM AK.KNK AK.RC01
AK.SWD AT.PMR AT.SVW2 AV.ILSW TA.L19K TA.M19K TA.M22K
TA.N19K TA.O19K TA.O22K TA.P18K
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 1.41e+23 dyne-cm
Mw = 4.70
Z = 90 km
Plane Strike Dip Rake
NP1 235 75 -20
NP2 330 71 -164
Principal Axes:
Axis Value Plunge Azimuth
T 1.41e+23 3 283
N 0.00e+00 65 20
P -1.41e+23 25 192
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.04e+23
Mxy -5.52e+22
Mxz 5.40e+22
Myy 1.28e+23
Myz 4.14e+21
Mzz -2.42e+22
--------------
##--------------------
#######---------------------
###########-------------------
##############--------------------
#################------------#######
###################------#############
####################-#################
T ##################---#################
###############-------#################
################----------################
##############-------------###############
###########-----------------##############
#########-------------------############
#######---------------------############
####------------------------##########
##-------------------------#########
---------------------------#######
---------- ------------#####
--------- P ------------####
------ ------------#
--------------
Global CMT Convention Moment Tensor:
R T P
-2.42e+22 5.40e+22 -4.14e+21
5.40e+22 -1.04e+23 5.52e+22
-4.14e+21 5.52e+22 1.28e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160312215755/index.html
|
Regional Moment Tensor (Mwr)
Moment 1.328e+16 N-m
Magnitude 4.68
Depth 92.0 km
Percent DC 64%
Half Duration –
Catalog US (us10004x3k)
Data Source US3
Contributor US3
Nodal Planes
Plane Strike Dip Rake
NP1 235 74 -9
NP2 327 81 -164
Principal Axes
Axis Value Plunge Azimuth
T 1.182 5 100
N 0.255 72 355
P -1.437 18 192
|
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 0.03 n 3
lp c 0.07 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 55 75 15 3.81 0.2638
WVFGRD96 4.0 50 90 -10 3.88 0.3245
WVFGRD96 6.0 55 90 -15 3.96 0.3637
WVFGRD96 8.0 50 85 -15 4.02 0.3965
WVFGRD96 10.0 50 85 -15 4.06 0.4146
WVFGRD96 12.0 50 80 -10 4.09 0.4237
WVFGRD96 14.0 50 85 -10 4.12 0.4264
WVFGRD96 16.0 50 85 -10 4.14 0.4265
WVFGRD96 18.0 50 85 -10 4.16 0.4240
WVFGRD96 20.0 230 90 10 4.18 0.4215
WVFGRD96 22.0 50 90 -10 4.20 0.4210
WVFGRD96 24.0 50 90 -10 4.21 0.4218
WVFGRD96 26.0 230 90 10 4.23 0.4244
WVFGRD96 28.0 50 90 -5 4.25 0.4294
WVFGRD96 30.0 230 90 5 4.26 0.4351
WVFGRD96 32.0 230 90 0 4.28 0.4408
WVFGRD96 34.0 50 90 5 4.30 0.4456
WVFGRD96 36.0 50 90 5 4.33 0.4496
WVFGRD96 38.0 230 85 -5 4.36 0.4544
WVFGRD96 40.0 230 85 -10 4.40 0.4634
WVFGRD96 42.0 230 85 -10 4.42 0.4689
WVFGRD96 44.0 230 85 -15 4.44 0.4746
WVFGRD96 46.0 230 80 -15 4.47 0.4795
WVFGRD96 48.0 230 80 -20 4.49 0.4860
WVFGRD96 50.0 230 80 -20 4.51 0.4934
WVFGRD96 52.0 230 80 -20 4.52 0.5005
WVFGRD96 54.0 230 75 -25 4.55 0.5095
WVFGRD96 56.0 230 75 -20 4.55 0.5187
WVFGRD96 58.0 230 70 -25 4.59 0.5286
WVFGRD96 60.0 230 70 -25 4.60 0.5375
WVFGRD96 62.0 230 70 -25 4.61 0.5477
WVFGRD96 64.0 230 70 -25 4.62 0.5563
WVFGRD96 66.0 230 70 -25 4.63 0.5637
WVFGRD96 68.0 230 70 -25 4.64 0.5712
WVFGRD96 70.0 230 70 -20 4.64 0.5784
WVFGRD96 72.0 235 75 -25 4.65 0.5842
WVFGRD96 74.0 235 75 -25 4.66 0.5895
WVFGRD96 76.0 235 75 -20 4.66 0.5946
WVFGRD96 78.0 235 75 -20 4.66 0.5987
WVFGRD96 80.0 235 75 -20 4.67 0.6021
WVFGRD96 82.0 235 75 -20 4.68 0.6047
WVFGRD96 84.0 235 75 -20 4.68 0.6069
WVFGRD96 86.0 235 75 -20 4.69 0.6075
WVFGRD96 88.0 235 75 -20 4.69 0.6087
WVFGRD96 90.0 235 75 -20 4.70 0.6087
WVFGRD96 92.0 235 75 -20 4.70 0.6078
WVFGRD96 94.0 235 75 -15 4.70 0.6066
WVFGRD96 96.0 235 75 -15 4.71 0.6054
WVFGRD96 98.0 235 75 -15 4.71 0.6046
WVFGRD96 100.0 235 80 -15 4.70 0.6039
WVFGRD96 102.0 235 80 -15 4.70 0.6024
WVFGRD96 104.0 235 80 -15 4.71 0.6004
WVFGRD96 106.0 60 80 45 4.72 0.5998
WVFGRD96 108.0 60 80 45 4.72 0.6012
The best solution is
WVFGRD96 90.0 235 75 -20 4.70 0.6087
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 0.03 n 3
lp c 0.07 n 3
|
|
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 02:51:32 PM CDT 2024
