Location
Location ANSS
The ANSS event ID is ak0144vn0ngp and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0144vn0ngp/executive.
2014/04/16 20:24:24 62.894 -149.912 83.0 5.1 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2014/04/16 20:24:24:0 62.89 -149.91 83.0 5.1 Alaska
Stations used:
AK.BARN AK.BPAW AK.BRLK AK.BWN AK.CCB AK.COLD AK.CRQ AK.CTG
AK.DHY AK.FID AK.FYU AK.GHO AK.GLB AK.GLI AK.HDA AK.HOM
AK.KNK AK.KTH AK.MCAR AK.MCK AK.MDM AK.MLY AK.NEA AK.PPD
AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AK.SWD
AK.TGL AK.TRF AK.WRH AT.MID AT.PMR AT.SVW2 AV.RED CN.DAWY
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 = 3.24e+23 dyne-cm
Mw = 4.94
Z = 86 km
Plane Strike Dip Rake
NP1 100 85 25
NP2 8 65 174
Principal Axes:
Axis Value Plunge Azimuth
T 3.24e+23 21 327
N 0.00e+00 65 111
P -3.24e+23 14 231
Moment Tensor: (dyne-cm)
Component Value
Mxx 7.69e+22
Mxy -2.79e+23
Mxz 1.37e+23
Myy -1.01e+23
Myz -1.79e+21
Mzz 2.37e+22
###########---
###############-------
### #############---------
#### T ##############---------
###### ##############-----------
########################------------
##########################------------
###########################-------------
###########################-------------
---#########################--------------
----------#################---------------
-------------------########---------------
---------------------------####-----------
--------------------------##############
-------------------------###############
------------------------##############
--- ----------------##############
-- P ---------------##############
--------------#############
---------------#############
-----------###########
-----#########
Global CMT Convention Moment Tensor:
R T P
2.37e+22 1.37e+23 1.79e+21
1.37e+23 7.69e+22 2.79e+23
1.79e+21 2.79e+23 -1.01e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140416202424/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 = 100
DIP = 85
RAKE = 25
MW = 4.94
HS = 86.0
The NDK file is 20140416202424.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 2014/04/16 20:24:24:0 62.89 -149.91 83.0 5.1 Alaska
Stations used:
AK.BARN AK.BPAW AK.BRLK AK.BWN AK.CCB AK.COLD AK.CRQ AK.CTG
AK.DHY AK.FID AK.FYU AK.GHO AK.GLB AK.GLI AK.HDA AK.HOM
AK.KNK AK.KTH AK.MCAR AK.MCK AK.MDM AK.MLY AK.NEA AK.PPD
AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AK.SWD
AK.TGL AK.TRF AK.WRH AT.MID AT.PMR AT.SVW2 AV.RED CN.DAWY
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 = 3.24e+23 dyne-cm
Mw = 4.94
Z = 86 km
Plane Strike Dip Rake
NP1 100 85 25
NP2 8 65 174
Principal Axes:
Axis Value Plunge Azimuth
T 3.24e+23 21 327
N 0.00e+00 65 111
P -3.24e+23 14 231
Moment Tensor: (dyne-cm)
Component Value
Mxx 7.69e+22
Mxy -2.79e+23
Mxz 1.37e+23
Myy -1.01e+23
Myz -1.79e+21
Mzz 2.37e+22
###########---
###############-------
### #############---------
#### T ##############---------
###### ##############-----------
########################------------
##########################------------
###########################-------------
###########################-------------
---#########################--------------
----------#################---------------
-------------------########---------------
---------------------------####-----------
--------------------------##############
-------------------------###############
------------------------##############
--- ----------------##############
-- P ---------------##############
--------------#############
---------------#############
-----------###########
-----#########
Global CMT Convention Moment Tensor:
R T P
2.37e+22 1.37e+23 1.79e+21
1.37e+23 7.69e+22 2.79e+23
1.79e+21 2.79e+23 -1.01e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140416202424/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
3.62e+16 N-m
Magnitude
5.0
Percent DC
87%
Depth
85.0 km
Updated
2014-04-16 21:06:10 UTC
Author
us
Catalog
us
Contributor
us
Code
us_b000pn4i_mwr
Principal Axes
Axis Value Plunge Azimuth
T 3.733 25 328
N -0.227 63 126
P -3.506 9 234
Nodal Planes
Plane Strike Dip Rake
NP1 103 79 25
NP2 8 65 168
|
|
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.
|
|
Location of broadband stations used for waveform inversion
|
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 5 85 -20 4.11 0.2424
WVFGRD96 4.0 185 90 15 4.19 0.2761
WVFGRD96 6.0 5 85 -10 4.24 0.2867
WVFGRD96 8.0 185 90 20 4.30 0.2922
WVFGRD96 10.0 5 80 -25 4.33 0.2983
WVFGRD96 12.0 5 80 -25 4.36 0.3012
WVFGRD96 14.0 5 85 -25 4.37 0.3017
WVFGRD96 16.0 95 85 20 4.37 0.3066
WVFGRD96 18.0 275 90 -20 4.39 0.3157
WVFGRD96 20.0 95 85 20 4.41 0.3276
WVFGRD96 22.0 95 85 20 4.44 0.3405
WVFGRD96 24.0 95 85 20 4.46 0.3531
WVFGRD96 26.0 95 85 15 4.48 0.3645
WVFGRD96 28.0 95 85 15 4.50 0.3753
WVFGRD96 30.0 100 80 15 4.53 0.3838
WVFGRD96 32.0 100 80 20 4.54 0.3924
WVFGRD96 34.0 100 80 20 4.56 0.4021
WVFGRD96 36.0 100 75 15 4.60 0.4112
WVFGRD96 38.0 100 75 15 4.63 0.4218
WVFGRD96 40.0 100 70 20 4.70 0.4371
WVFGRD96 42.0 100 70 20 4.72 0.4497
WVFGRD96 44.0 100 70 20 4.74 0.4621
WVFGRD96 46.0 100 70 20 4.76 0.4743
WVFGRD96 48.0 100 70 15 4.78 0.4874
WVFGRD96 50.0 100 70 15 4.80 0.5015
WVFGRD96 52.0 100 70 15 4.81 0.5163
WVFGRD96 54.0 100 75 20 4.82 0.5332
WVFGRD96 56.0 100 75 20 4.83 0.5504
WVFGRD96 58.0 100 75 20 4.85 0.5680
WVFGRD96 60.0 100 75 20 4.86 0.5861
WVFGRD96 62.0 100 75 20 4.87 0.6026
WVFGRD96 64.0 100 75 20 4.88 0.6179
WVFGRD96 66.0 100 75 20 4.89 0.6314
WVFGRD96 68.0 105 75 25 4.90 0.6442
WVFGRD96 70.0 105 75 25 4.91 0.6562
WVFGRD96 72.0 105 75 25 4.92 0.6661
WVFGRD96 74.0 105 75 25 4.93 0.6744
WVFGRD96 76.0 105 75 25 4.94 0.6810
WVFGRD96 78.0 100 80 25 4.93 0.6857
WVFGRD96 80.0 100 80 25 4.94 0.6892
WVFGRD96 82.0 100 80 25 4.94 0.6916
WVFGRD96 84.0 100 80 25 4.95 0.6922
WVFGRD96 86.0 100 85 25 4.94 0.6932
WVFGRD96 88.0 100 85 25 4.95 0.6929
WVFGRD96 90.0 100 85 25 4.95 0.6915
WVFGRD96 92.0 100 85 25 4.96 0.6888
WVFGRD96 94.0 100 85 25 4.96 0.6855
WVFGRD96 96.0 280 90 -25 4.95 0.6736
WVFGRD96 98.0 280 90 -25 4.96 0.6703
WVFGRD96 100.0 280 90 -25 4.96 0.6666
The best solution is
WVFGRD96 86.0 100 85 25 4.94 0.6932
The mechanism corresponding to the best fit is
|
|
Figure 1. Waveform inversion focal mechanism
|
The best fit as a function of depth is given in the following figure:
|
|
Figure 2. Depth sensitivity for waveform mechanism
|
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
|
|
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.
|
|
|
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.
|
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 05:45:01 PM CDT 2024
