The ANSS event ID is ak0245p867wh and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0245p867wh/executive.
2024/05/03 09:29:32 59.977 -152.749 88.7 4.3 Alaska
USGS/SLU Moment Tensor Solution
ENS 2024/05/03 09:29:32:0 59.98 -152.75 88.7 4.3 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CUT AK.FIRE AK.GHO AK.HOM AK.L17K
AK.L19K AK.M19K AK.N18K AK.O18K AK.O19K AK.P16K AK.P17K
AK.RC01 AK.RND AK.SAW AK.SLK AK.SWD AT.PMR AT.TTA AV.ACH
AV.PLBL AV.STLK II.KDAK
Filtering commands used:
cut o DIST/3.8 -20 o DIST/3.8 +60
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 5.01e+22 dyne-cm
Mw = 4.40
Z = 98 km
Plane Strike Dip Rake
NP1 50 70 25
NP2 311 67 158
Principal Axes:
Axis Value Plunge Azimuth
T 5.01e+22 32 271
N 0.00e+00 58 86
P -5.01e+22 2 180
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.00e+22
Mxy -7.08e+20
Mxz 2.44e+21
Myy 3.64e+22
Myz -2.23e+22
Mzz 1.36e+22
--------------
----------------------
----------------------------
------------------------------
#########------------------------#
##############-------------------###
##################---------------#####
#####################------------#######
########################-------#########
##### ###################----###########
##### T ##################################
##### ####################--############
##########################------##########
######################----------########
####################-------------#######
################-----------------#####
############---------------------###
#######-------------------------##
------------------------------
----------------------------
---------- ---------
------ P -----
Global CMT Convention Moment Tensor:
R T P
1.36e+22 2.44e+21 2.23e+22
2.44e+21 -5.00e+22 7.08e+20
2.23e+22 7.08e+20 3.64e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240503092932/index.html
|
STK = 50
DIP = 70
RAKE = 25
MW = 4.40
HS = 98.0
The NDK file is 20240503092932.ndk The waveform inversion is preferred.
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.
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.
 |
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.
|
|
|
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/3.8 -20 o DIST/3.8 +60 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 310 65 -30 3.52 0.2407
WVFGRD96 4.0 315 85 -10 3.57 0.2830
WVFGRD96 6.0 315 85 -10 3.64 0.2961
WVFGRD96 8.0 315 85 -10 3.71 0.2978
WVFGRD96 10.0 135 90 10 3.75 0.2848
WVFGRD96 12.0 135 90 15 3.78 0.2667
WVFGRD96 14.0 220 80 0 3.79 0.2511
WVFGRD96 16.0 220 80 0 3.82 0.2512
WVFGRD96 18.0 220 80 0 3.85 0.2534
WVFGRD96 20.0 220 80 0 3.88 0.2578
WVFGRD96 22.0 220 80 0 3.90 0.2651
WVFGRD96 24.0 40 80 0 3.92 0.2778
WVFGRD96 26.0 40 80 0 3.95 0.2935
WVFGRD96 28.0 40 80 0 3.97 0.3101
WVFGRD96 30.0 40 80 0 3.99 0.3263
WVFGRD96 32.0 40 85 -10 4.00 0.3396
WVFGRD96 34.0 40 85 -10 4.02 0.3520
WVFGRD96 36.0 40 85 -10 4.04 0.3575
WVFGRD96 38.0 45 85 -5 4.08 0.3653
WVFGRD96 40.0 45 90 -10 4.13 0.3742
WVFGRD96 42.0 45 90 -10 4.16 0.3780
WVFGRD96 44.0 45 85 -5 4.18 0.3806
WVFGRD96 46.0 45 80 5 4.20 0.3838
WVFGRD96 48.0 45 75 5 4.22 0.3885
WVFGRD96 50.0 45 75 5 4.24 0.3969
WVFGRD96 52.0 50 75 10 4.27 0.4064
WVFGRD96 54.0 50 75 10 4.28 0.4161
WVFGRD96 56.0 50 75 10 4.29 0.4277
WVFGRD96 58.0 50 75 15 4.30 0.4389
WVFGRD96 60.0 50 75 15 4.31 0.4498
WVFGRD96 62.0 50 75 15 4.32 0.4608
WVFGRD96 64.0 50 75 15 4.33 0.4701
WVFGRD96 66.0 50 75 20 4.34 0.4799
WVFGRD96 68.0 50 70 20 4.34 0.4891
WVFGRD96 70.0 50 70 20 4.35 0.4975
WVFGRD96 72.0 50 70 20 4.35 0.5052
WVFGRD96 74.0 50 70 20 4.36 0.5125
WVFGRD96 76.0 50 70 25 4.37 0.5175
WVFGRD96 78.0 50 70 25 4.37 0.5258
WVFGRD96 80.0 50 70 25 4.38 0.5324
WVFGRD96 82.0 50 70 25 4.38 0.5366
WVFGRD96 84.0 50 70 25 4.38 0.5403
WVFGRD96 86.0 50 70 25 4.39 0.5445
WVFGRD96 88.0 50 70 25 4.39 0.5467
WVFGRD96 90.0 50 70 25 4.39 0.5502
WVFGRD96 92.0 50 70 25 4.40 0.5512
WVFGRD96 94.0 50 70 25 4.40 0.5511
WVFGRD96 96.0 50 70 25 4.40 0.5510
WVFGRD96 98.0 50 70 25 4.40 0.5530
WVFGRD96 100.0 50 70 25 4.41 0.5528
WVFGRD96 102.0 50 70 25 4.41 0.5515
WVFGRD96 104.0 50 70 25 4.41 0.5517
WVFGRD96 106.0 50 70 25 4.41 0.5496
WVFGRD96 108.0 50 70 25 4.42 0.5473
WVFGRD96 110.0 50 70 25 4.42 0.5460
WVFGRD96 112.0 50 70 25 4.42 0.5425
WVFGRD96 114.0 50 65 25 4.42 0.5402
WVFGRD96 116.0 50 65 25 4.42 0.5373
WVFGRD96 118.0 50 65 25 4.43 0.5350
The best solution is
WVFGRD96 98.0 50 70 25 4.40 0.5530
The mechanism corresponding to the best fit is
|
|
|
The best fit as a function of depth is given in the following figure:
|
|
|
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/3.8 -20 o DIST/3.8 +60 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. |
|
| 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:
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.
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