The ANSS event ID is ak024aiz7bgz and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak024aiz7bgz/executive.
2024/08/16 15:34:26 61.199 -149.663 38.7 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2024/08/16 15:34:26:0 61.20 -149.66 38.7 3.7 Alaska
Stations used:
AK.BAE AK.FID AK.FIRE AK.GHO AK.GLI AK.KNK AK.L22K AK.RC01
AK.SAW AK.SLK AT.PMR
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +30
rtr
taper w 0.1
hp c 0.05 n 3
lp c 0.15 n 3
Best Fitting Double Couple
Mo = 7.50e+21 dyne-cm
Mw = 3.85
Z = 49 km
Plane Strike Dip Rake
NP1 10 50 -80
NP2 175 41 -102
Principal Axes:
Axis Value Plunge Azimuth
T 7.50e+21 5 93
N 0.00e+00 8 184
P -7.50e+21 81 333
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.22e+20
Mxy -3.06e+20
Mxz -1.05e+21
Myy 7.39e+21
Myz 1.12e+21
Mzz -7.27e+21
###---------##
#####------------#####
######---------------#######
######-----------------#######
#######-------------------########
#######---------------------########
########---------------------#########
########----------------------##########
########--------- ----------##########
#########--------- P ----------###########
#########--------- ----------########
#########----------------------######## T
#########----------------------########
########---------------------###########
#########--------------------###########
#########------------------###########
########-----------------###########
########---------------###########
########------------##########
########---------###########
#######-----##########
##############
Global CMT Convention Moment Tensor:
R T P
-7.27e+21 -1.05e+21 -1.12e+21
-1.05e+21 -1.22e+20 3.06e+20
-1.12e+21 3.06e+20 7.39e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240816153426/index.html
|
STK = 10
DIP = 50
RAKE = -80
MW = 3.85
HS = 49.0
The NDK file is 20240816153426.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/3.3 -30 o DIST/3.3 +30 rtr taper w 0.1 hp c 0.05 n 3 lp c 0.15 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 150 35 -95 2.81 0.1469
WVFGRD96 2.0 170 35 -85 3.01 0.1738
WVFGRD96 3.0 70 5 -20 3.10 0.2051
WVFGRD96 4.0 55 10 -45 3.16 0.2681
WVFGRD96 5.0 75 10 -25 3.19 0.3074
WVFGRD96 6.0 80 10 -20 3.22 0.3307
WVFGRD96 7.0 80 15 -25 3.25 0.3453
WVFGRD96 8.0 75 10 -30 3.35 0.3512
WVFGRD96 9.0 75 10 -30 3.38 0.3615
WVFGRD96 10.0 75 15 -35 3.42 0.3639
WVFGRD96 11.0 145 15 40 3.42 0.3621
WVFGRD96 12.0 150 15 45 3.44 0.3614
WVFGRD96 13.0 160 20 60 3.44 0.3601
WVFGRD96 14.0 160 20 60 3.46 0.3548
WVFGRD96 15.0 145 20 40 3.48 0.3467
WVFGRD96 16.0 150 20 45 3.50 0.3403
WVFGRD96 17.0 155 20 55 3.50 0.3307
WVFGRD96 18.0 220 80 80 3.64 0.3299
WVFGRD96 19.0 220 80 80 3.66 0.3391
WVFGRD96 20.0 220 80 80 3.67 0.3448
WVFGRD96 21.0 210 80 75 3.66 0.3491
WVFGRD96 22.0 210 80 80 3.68 0.3493
WVFGRD96 23.0 210 80 80 3.68 0.3461
WVFGRD96 24.0 210 80 80 3.69 0.3395
WVFGRD96 25.0 200 80 75 3.66 0.3370
WVFGRD96 26.0 195 70 -80 3.61 0.3317
WVFGRD96 27.0 190 65 -80 3.60 0.3468
WVFGRD96 28.0 195 70 -80 3.63 0.3616
WVFGRD96 29.0 0 30 -90 3.61 0.3725
WVFGRD96 30.0 190 60 -80 3.62 0.3852
WVFGRD96 31.0 195 50 -75 3.62 0.4098
WVFGRD96 32.0 195 50 -75 3.63 0.4285
WVFGRD96 33.0 190 45 -80 3.64 0.4415
WVFGRD96 34.0 180 40 -90 3.64 0.4591
WVFGRD96 35.0 185 45 -85 3.64 0.4712
WVFGRD96 36.0 180 45 -90 3.64 0.4782
WVFGRD96 37.0 5 50 -85 3.65 0.4841
WVFGRD96 38.0 10 50 -80 3.67 0.4998
WVFGRD96 39.0 5 45 -85 3.68 0.5175
WVFGRD96 40.0 180 40 -95 3.75 0.5188
WVFGRD96 41.0 5 50 -85 3.77 0.5352
WVFGRD96 42.0 10 55 -80 3.80 0.5376
WVFGRD96 43.0 10 55 -80 3.81 0.5487
WVFGRD96 44.0 10 50 -80 3.82 0.5498
WVFGRD96 45.0 10 55 -80 3.83 0.5502
WVFGRD96 46.0 10 50 -80 3.84 0.5564
WVFGRD96 47.0 10 50 -80 3.84 0.5563
WVFGRD96 48.0 10 50 -80 3.85 0.5519
WVFGRD96 49.0 10 50 -80 3.85 0.5568
WVFGRD96 50.0 15 50 -75 3.87 0.5557
WVFGRD96 51.0 15 50 -75 3.87 0.5474
WVFGRD96 52.0 10 50 -80 3.86 0.5518
WVFGRD96 53.0 15 50 -75 3.88 0.5509
WVFGRD96 54.0 15 50 -75 3.88 0.5446
WVFGRD96 55.0 15 50 -75 3.88 0.5450
WVFGRD96 56.0 15 50 -75 3.89 0.5441
WVFGRD96 57.0 15 50 -75 3.89 0.5394
WVFGRD96 58.0 15 50 -75 3.89 0.5308
WVFGRD96 59.0 15 50 -75 3.89 0.5320
The best solution is
WVFGRD96 49.0 10 50 -80 3.85 0.5568
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/3.3 -30 o DIST/3.3 +30 rtr taper w 0.1 hp c 0.05 n 3 lp c 0.15 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