Location
Location ANSS
The ANSS event ID is ak0165dtyd2a and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0165dtyd2a/executive.
2016/04/26 20:28:40 62.739 -149.685 73.3 4.6 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2016/04/26 20:28:40:0 62.74 -149.68 73.3 4.6 Alaska
Stations used:
AK.BPAW AK.BWN AK.CAST AK.CUT AK.DHY AK.FIRE AK.GHO AK.GLI
AK.KNK AK.KTH AK.MCK AK.NEA2 AK.PAX AK.PWL AK.RC01 AK.RIDG
AK.RND AK.SAW AK.SCM AK.TRF AK.WRH AT.MENT AT.PMR IU.COLA
TA.I23K TA.M19K TA.M22K TA.POKR
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 0.10 n 3
Best Fitting Double Couple
Mo = 1.00e+23 dyne-cm
Mw = 4.60
Z = 78 km
Plane Strike Dip Rake
NP1 275 55 65
NP2 134 42 121
Principal Axes:
Axis Value Plunge Azimuth
T 1.00e+23 69 130
N 0.00e+00 20 290
P -1.00e+23 7 23
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.85e+22
Mxy -4.15e+22
Mxz -3.30e+22
Myy -6.66e+21
Myz 2.14e+22
Mzz 8.52e+22
-------------
----------------- P --
-------------------- -----
------------------------------
#---------------------------------
##----------------------------------
###-------#############---------------
####--########################----------
##--##############################------
#-----################################----
------##################################--
-------################ ###############-
--------############### T ################
--------############## ###############
----------##############################
----------############################
------------########################
-------------#####################
---------------###############
----------------------------
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
8.52e+22 -3.30e+22 -2.14e+22
-3.30e+22 -7.85e+22 4.15e+22
-2.14e+22 4.15e+22 -6.66e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160426202840/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 = 275
DIP = 55
RAKE = 65
MW = 4.60
HS = 78.0
The NDK file is 20160426202840.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 2016/04/26 20:28:40:0 62.74 -149.68 73.3 4.6 Alaska
Stations used:
AK.BPAW AK.BWN AK.CAST AK.CUT AK.DHY AK.FIRE AK.GHO AK.GLI
AK.KNK AK.KTH AK.MCK AK.NEA2 AK.PAX AK.PWL AK.RC01 AK.RIDG
AK.RND AK.SAW AK.SCM AK.TRF AK.WRH AT.MENT AT.PMR IU.COLA
TA.I23K TA.M19K TA.M22K TA.POKR
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 0.10 n 3
Best Fitting Double Couple
Mo = 1.00e+23 dyne-cm
Mw = 4.60
Z = 78 km
Plane Strike Dip Rake
NP1 275 55 65
NP2 134 42 121
Principal Axes:
Axis Value Plunge Azimuth
T 1.00e+23 69 130
N 0.00e+00 20 290
P -1.00e+23 7 23
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.85e+22
Mxy -4.15e+22
Mxz -3.30e+22
Myy -6.66e+21
Myz 2.14e+22
Mzz 8.52e+22
-------------
----------------- P --
-------------------- -----
------------------------------
#---------------------------------
##----------------------------------
###-------#############---------------
####--########################----------
##--##############################------
#-----################################----
------##################################--
-------################ ###############-
--------############### T ################
--------############## ###############
----------##############################
----------############################
------------########################
-------------#####################
---------------###############
----------------------------
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
8.52e+22 -3.30e+22 -2.14e+22
-3.30e+22 -7.85e+22 4.15e+22
-2.14e+22 4.15e+22 -6.66e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160426202840/index.html
|
Regional Moment Tensor (Mwr)
Moment 9.370e+15 N-m
Magnitude 4.6 Mwr
Depth 76.0 km
Percent DC 21 %
Half Duration –
Catalog US
Data Source US3
Contributor US3
Nodal Planes
Plane Strike Dip Rake
NP1 280 56 76
NP2 125 36 111
Principal Axes
Axis Value Plunge Azimuth
T 6.499e+15 N-m 74 150
N 4.242e+15 N-m 12 288
P -10.740e+15 N-m 10 20
|
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 0.10 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 130 40 -60 3.81 0.2041
WVFGRD96 4.0 160 75 45 3.84 0.2127
WVFGRD96 6.0 160 70 45 3.90 0.2466
WVFGRD96 8.0 165 70 50 4.00 0.2601
WVFGRD96 10.0 325 80 -50 4.02 0.2658
WVFGRD96 12.0 325 75 -45 4.05 0.2622
WVFGRD96 14.0 325 75 -40 4.08 0.2537
WVFGRD96 16.0 245 50 15 4.11 0.2433
WVFGRD96 18.0 250 50 20 4.14 0.2481
WVFGRD96 20.0 250 55 25 4.17 0.2624
WVFGRD96 22.0 250 55 20 4.20 0.2782
WVFGRD96 24.0 250 55 20 4.22 0.2894
WVFGRD96 26.0 250 55 20 4.24 0.2964
WVFGRD96 28.0 250 55 15 4.26 0.3003
WVFGRD96 30.0 250 60 15 4.27 0.3144
WVFGRD96 32.0 250 60 20 4.28 0.3267
WVFGRD96 34.0 250 60 20 4.30 0.3402
WVFGRD96 36.0 250 60 20 4.32 0.3539
WVFGRD96 38.0 255 60 25 4.35 0.3653
WVFGRD96 40.0 255 50 30 4.43 0.3853
WVFGRD96 42.0 255 50 30 4.45 0.3822
WVFGRD96 44.0 260 50 35 4.47 0.3769
WVFGRD96 46.0 260 50 35 4.49 0.3725
WVFGRD96 48.0 260 60 45 4.50 0.3741
WVFGRD96 50.0 265 60 50 4.52 0.3852
WVFGRD96 52.0 265 60 50 4.53 0.3980
WVFGRD96 54.0 265 60 50 4.54 0.4109
WVFGRD96 56.0 270 60 55 4.55 0.4229
WVFGRD96 58.0 270 60 60 4.56 0.4364
WVFGRD96 60.0 270 60 60 4.57 0.4483
WVFGRD96 62.0 270 60 60 4.57 0.4570
WVFGRD96 64.0 270 60 60 4.58 0.4644
WVFGRD96 66.0 270 60 60 4.58 0.4713
WVFGRD96 68.0 270 60 60 4.59 0.4783
WVFGRD96 70.0 270 60 60 4.59 0.4818
WVFGRD96 72.0 270 60 60 4.60 0.4864
WVFGRD96 74.0 270 60 60 4.60 0.4886
WVFGRD96 76.0 275 55 65 4.60 0.4888
WVFGRD96 78.0 275 55 65 4.60 0.4890
WVFGRD96 80.0 270 60 60 4.61 0.4870
WVFGRD96 82.0 270 60 60 4.61 0.4860
WVFGRD96 84.0 270 60 60 4.61 0.4835
WVFGRD96 86.0 265 60 55 4.61 0.4812
WVFGRD96 88.0 265 60 55 4.61 0.4781
WVFGRD96 90.0 265 60 55 4.62 0.4750
WVFGRD96 92.0 265 65 50 4.63 0.4719
WVFGRD96 94.0 265 65 50 4.63 0.4691
WVFGRD96 96.0 260 70 45 4.64 0.4661
WVFGRD96 98.0 260 65 50 4.63 0.4629
The best solution is
WVFGRD96 78.0 275 55 65 4.60 0.4890
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 0.10 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 04:07:49 PM CDT 2024
