Location
Location ANSS
The ANSS event ID is ak016anz0nt3 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak016anz0nt3/executive.
2016/08/19 17:36:28 61.600 -146.336 23.9 4.3 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2016/08/19 17:36:28:0 61.60 -146.34 23.9 4.3 Alaska
Stations used:
AK.BERG AK.BMR AK.CUT AK.DHY AK.DIV AK.EYAK AK.FID AK.GHO
AK.GLI AK.HIN AK.HMT AK.KLU AK.KNK AK.MCAR AK.PAX AK.PWL
AK.RAG AK.RC01 AK.SAW AK.SCM AK.TGL AT.PMR TA.M24K TA.M26K
TA.N25K
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 0.08 n 3
Best Fitting Double Couple
Mo = 3.55e+22 dyne-cm
Mw = 4.30
Z = 41 km
Plane Strike Dip Rake
NP1 50 70 -70
NP2 183 28 -133
Principal Axes:
Axis Value Plunge Azimuth
T 3.55e+22 22 125
N 0.00e+00 19 223
P -3.55e+22 60 349
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.35e+21
Mxy -1.25e+22
Mxz -2.22e+22
Myy 2.01e+22
Myz 1.32e+22
Mzz -2.14e+22
####----------
#####-----------------
######----------------------
#####-------------------------
######---------------------------#
#####---------- ---------------###
######---------- P --------------#####
######----------- -------------#######
#####--------------------------#########
######-------------------------###########
######-----------------------#############
######----------------------##############
######--------------------################
#####-----------------##################
######--------------####################
#####-----------############### ####
#####-------################## T ###
#####--###################### ##
----##########################
-----#######################
----##################
---###########
Global CMT Convention Moment Tensor:
R T P
-2.14e+22 -2.22e+22 -1.32e+22
-2.22e+22 1.35e+21 1.25e+22
-1.32e+22 1.25e+22 2.01e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160819173628/index.html
|
Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is
STK = 50
DIP = 70
RAKE = -70
MW = 4.30
HS = 41.0
The NDK file is 20160819173628.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 2016/08/19 17:36:28:0 61.60 -146.34 23.9 4.3 Alaska
Stations used:
AK.BERG AK.BMR AK.CUT AK.DHY AK.DIV AK.EYAK AK.FID AK.GHO
AK.GLI AK.HIN AK.HMT AK.KLU AK.KNK AK.MCAR AK.PAX AK.PWL
AK.RAG AK.RC01 AK.SAW AK.SCM AK.TGL AT.PMR TA.M24K TA.M26K
TA.N25K
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 0.08 n 3
Best Fitting Double Couple
Mo = 3.55e+22 dyne-cm
Mw = 4.30
Z = 41 km
Plane Strike Dip Rake
NP1 50 70 -70
NP2 183 28 -133
Principal Axes:
Axis Value Plunge Azimuth
T 3.55e+22 22 125
N 0.00e+00 19 223
P -3.55e+22 60 349
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.35e+21
Mxy -1.25e+22
Mxz -2.22e+22
Myy 2.01e+22
Myz 1.32e+22
Mzz -2.14e+22
####----------
#####-----------------
######----------------------
#####-------------------------
######---------------------------#
#####---------- ---------------###
######---------- P --------------#####
######----------- -------------#######
#####--------------------------#########
######-------------------------###########
######-----------------------#############
######----------------------##############
######--------------------################
#####-----------------##################
######--------------####################
#####-----------############### ####
#####-------################## T ###
#####--###################### ##
----##########################
-----#######################
----##################
---###########
Global CMT Convention Moment Tensor:
R T P
-2.14e+22 -2.22e+22 -1.32e+22
-2.22e+22 1.35e+21 1.25e+22
-1.32e+22 1.25e+22 2.01e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160819173628/index.html
|
Regional Moment Tensor (Mwr)
Moment 4.051e+15 N-m
Magnitude 4.3 Mwr
Depth 43.0 km
Percent DC 82 %
Half Duration –
Catalog US
Data Source US3
Contributor US3
Nodal Planes
Plane Strike Dip Rake
NP1 47 64 -84
NP2 213 27 -103
Principal Axes
Axis Value Plunge Azimuth
T 4.232e+15 N-m 19 132
N -0.391e+15 N-m 6 224
P -3.841e+15 N-m 70 330
|
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 0.08 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 220 45 90 3.50 0.1923
WVFGRD96 2.0 35 45 -95 3.64 0.2681
WVFGRD96 3.0 30 45 -100 3.71 0.2834
WVFGRD96 4.0 225 50 -80 3.73 0.2707
WVFGRD96 5.0 250 55 -35 3.69 0.2641
WVFGRD96 6.0 210 80 55 3.70 0.2895
WVFGRD96 7.0 215 75 55 3.72 0.3115
WVFGRD96 8.0 225 75 65 3.81 0.3319
WVFGRD96 9.0 220 75 60 3.81 0.3495
WVFGRD96 10.0 220 75 60 3.82 0.3649
WVFGRD96 11.0 220 75 60 3.84 0.3769
WVFGRD96 12.0 215 75 60 3.84 0.3874
WVFGRD96 13.0 215 75 60 3.85 0.3963
WVFGRD96 14.0 300 40 20 3.87 0.4048
WVFGRD96 15.0 300 40 20 3.88 0.4134
WVFGRD96 16.0 300 40 20 3.89 0.4215
WVFGRD96 17.0 300 40 20 3.91 0.4289
WVFGRD96 18.0 280 35 -25 3.92 0.4409
WVFGRD96 19.0 275 35 -30 3.93 0.4501
WVFGRD96 20.0 270 35 -30 3.94 0.4591
WVFGRD96 21.0 280 35 -25 3.96 0.4671
WVFGRD96 22.0 270 30 -30 3.97 0.4771
WVFGRD96 23.0 270 30 -30 3.98 0.4862
WVFGRD96 24.0 265 30 -40 4.00 0.4956
WVFGRD96 25.0 265 30 -40 4.01 0.5044
WVFGRD96 26.0 65 75 -60 4.05 0.5220
WVFGRD96 27.0 65 75 -55 4.07 0.5361
WVFGRD96 28.0 60 70 -60 4.08 0.5494
WVFGRD96 29.0 60 70 -60 4.09 0.5630
WVFGRD96 30.0 60 70 -60 4.10 0.5751
WVFGRD96 31.0 60 70 -60 4.11 0.5868
WVFGRD96 32.0 60 70 -60 4.12 0.5968
WVFGRD96 33.0 60 70 -60 4.13 0.6056
WVFGRD96 34.0 60 70 -60 4.14 0.6113
WVFGRD96 35.0 60 70 -55 4.15 0.6162
WVFGRD96 36.0 60 70 -55 4.16 0.6187
WVFGRD96 37.0 55 70 -65 4.16 0.6198
WVFGRD96 38.0 55 70 -65 4.17 0.6227
WVFGRD96 39.0 55 70 -65 4.17 0.6241
WVFGRD96 40.0 50 70 -70 4.29 0.6453
WVFGRD96 41.0 50 70 -70 4.30 0.6454
WVFGRD96 42.0 50 70 -70 4.31 0.6432
WVFGRD96 43.0 50 70 -70 4.32 0.6417
WVFGRD96 44.0 50 70 -70 4.32 0.6385
WVFGRD96 45.0 45 65 -75 4.33 0.6350
WVFGRD96 46.0 45 65 -75 4.34 0.6337
WVFGRD96 47.0 45 65 -75 4.34 0.6313
WVFGRD96 48.0 55 70 -60 4.35 0.6287
WVFGRD96 49.0 45 65 -75 4.35 0.6269
WVFGRD96 50.0 55 70 -60 4.36 0.6244
WVFGRD96 51.0 50 70 -65 4.37 0.6213
WVFGRD96 52.0 50 70 -65 4.37 0.6207
WVFGRD96 53.0 50 70 -65 4.38 0.6185
WVFGRD96 54.0 45 65 -75 4.38 0.6179
WVFGRD96 55.0 45 65 -75 4.38 0.6162
WVFGRD96 56.0 45 65 -75 4.39 0.6142
WVFGRD96 57.0 45 65 -75 4.39 0.6114
WVFGRD96 58.0 45 65 -75 4.39 0.6085
WVFGRD96 59.0 45 65 -75 4.39 0.6052
WVFGRD96 60.0 45 65 -75 4.40 0.6010
WVFGRD96 61.0 45 65 -75 4.40 0.5964
WVFGRD96 62.0 45 65 -75 4.40 0.5918
WVFGRD96 63.0 45 65 -75 4.40 0.5864
WVFGRD96 64.0 45 65 -75 4.40 0.5808
WVFGRD96 65.0 45 65 -75 4.40 0.5739
WVFGRD96 66.0 45 65 -75 4.40 0.5678
WVFGRD96 67.0 50 70 -70 4.41 0.5606
WVFGRD96 68.0 50 70 -70 4.41 0.5540
WVFGRD96 69.0 45 70 -80 4.41 0.5477
WVFGRD96 70.0 45 75 -80 4.42 0.5425
WVFGRD96 71.0 45 75 -80 4.42 0.5397
WVFGRD96 72.0 45 75 -80 4.42 0.5354
WVFGRD96 73.0 45 75 -80 4.42 0.5322
WVFGRD96 74.0 45 75 -80 4.42 0.5285
WVFGRD96 75.0 45 75 -80 4.42 0.5241
WVFGRD96 76.0 260 20 -45 4.42 0.5212
WVFGRD96 77.0 260 20 -45 4.43 0.5182
WVFGRD96 78.0 260 20 -45 4.43 0.5156
WVFGRD96 79.0 260 20 -45 4.43 0.5106
WVFGRD96 80.0 265 25 -40 4.43 0.5047
WVFGRD96 81.0 265 25 -40 4.43 0.5002
WVFGRD96 82.0 265 25 -40 4.43 0.4931
WVFGRD96 83.0 265 25 -40 4.43 0.4856
WVFGRD96 84.0 265 25 -40 4.43 0.4776
WVFGRD96 85.0 260 25 -45 4.43 0.4689
WVFGRD96 86.0 260 25 -45 4.43 0.4602
WVFGRD96 87.0 265 25 -35 4.43 0.4528
WVFGRD96 88.0 270 25 -30 4.43 0.4502
WVFGRD96 89.0 270 25 -30 4.43 0.4477
WVFGRD96 90.0 270 25 -30 4.43 0.4464
WVFGRD96 91.0 270 25 -30 4.43 0.4441
WVFGRD96 92.0 270 25 -30 4.44 0.4404
WVFGRD96 93.0 270 25 -30 4.44 0.4366
WVFGRD96 94.0 275 25 -25 4.44 0.4321
WVFGRD96 95.0 275 25 -25 4.44 0.4256
WVFGRD96 96.0 280 25 -25 4.44 0.4194
WVFGRD96 97.0 285 25 -20 4.44 0.4127
WVFGRD96 98.0 285 25 -20 4.44 0.4046
WVFGRD96 99.0 290 25 -15 4.44 0.3963
The best solution is
WVFGRD96 41.0 50 70 -70 4.30 0.6454
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 0.08 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 08:05:50 PM CDT 2024
