The ANSS event ID is ak019azmohto and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019azmohto/executive.
2019/08/27 22:23:39 63.238 -149.852 95.8 3.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/08/27 22:23:39:0 63.24 -149.85 95.8 3.5 Alaska
Stations used:
AK.CUT AK.GHO AK.HDA AK.MCK AK.PPLA AK.RND AK.SCM AK.SSN
AK.TRF AT.PMR TA.M20K TA.M22K
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 4.62e+21 dyne-cm
Mw = 3.71
Z = 112 km
Plane Strike Dip Rake
NP1 60 75 25
NP2 323 66 164
Principal Axes:
Axis Value Plunge Azimuth
T 4.62e+21 28 283
N 0.00e+00 61 89
P -4.62e+21 6 190
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.24e+21
Mxy -1.60e+21
Mxz 9.23e+20
Myy 3.26e+21
Myz -1.79e+21
Mzz 9.77e+20
--------------
----------------------
####------------------------
#########---------------------
##############--------------------
#################-------------------
####################----------------##
#######################------------#####
#### #################---------#######
##### T ###################-----##########
##### ####################-#############
###########################--#############
########################------############
####################----------##########
################---------------#########
##########--------------------########
##----------------------------######
-----------------------------#####
---------------------------###
---------------------------#
------ -------------
-- P ---------
Global CMT Convention Moment Tensor:
R T P
9.77e+20 9.23e+20 1.79e+21
9.23e+20 -4.24e+21 1.60e+21
1.79e+21 1.60e+21 3.26e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190827222339/index.html
|
STK = 60
DIP = 75
RAKE = 25
MW = 3.71
HS = 112.0
The NDK file is 20190827222339.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 -40 o DIST/3.3 +50 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 2.0 130 50 -50 2.78 0.1968
WVFGRD96 4.0 330 75 25 2.80 0.2134
WVFGRD96 6.0 150 70 20 2.88 0.2347
WVFGRD96 8.0 330 75 30 2.96 0.2460
WVFGRD96 10.0 330 70 30 3.01 0.2462
WVFGRD96 12.0 330 70 30 3.04 0.2365
WVFGRD96 14.0 5 75 -30 3.21 0.2369
WVFGRD96 16.0 10 75 -25 3.23 0.2352
WVFGRD96 18.0 10 75 -20 3.25 0.2307
WVFGRD96 20.0 55 55 15 3.16 0.2320
WVFGRD96 22.0 55 55 15 3.19 0.2395
WVFGRD96 24.0 55 55 20 3.22 0.2505
WVFGRD96 26.0 55 55 20 3.24 0.2651
WVFGRD96 28.0 60 50 20 3.28 0.2831
WVFGRD96 30.0 60 50 20 3.30 0.3032
WVFGRD96 32.0 60 50 20 3.33 0.3237
WVFGRD96 34.0 60 55 20 3.34 0.3474
WVFGRD96 36.0 60 55 20 3.36 0.3725
WVFGRD96 38.0 60 60 20 3.38 0.3890
WVFGRD96 40.0 65 55 25 3.45 0.4006
WVFGRD96 42.0 60 60 20 3.46 0.4084
WVFGRD96 44.0 60 55 25 3.50 0.4222
WVFGRD96 46.0 60 60 25 3.51 0.4379
WVFGRD96 48.0 65 60 40 3.52 0.4538
WVFGRD96 50.0 65 65 45 3.53 0.4755
WVFGRD96 52.0 65 65 45 3.54 0.4916
WVFGRD96 54.0 65 65 45 3.55 0.5037
WVFGRD96 56.0 65 65 40 3.56 0.5133
WVFGRD96 58.0 65 65 40 3.56 0.5224
WVFGRD96 60.0 65 65 40 3.57 0.5325
WVFGRD96 62.0 60 70 35 3.57 0.5413
WVFGRD96 64.0 60 70 35 3.58 0.5482
WVFGRD96 66.0 60 70 30 3.59 0.5551
WVFGRD96 68.0 60 70 30 3.60 0.5597
WVFGRD96 70.0 60 70 30 3.60 0.5650
WVFGRD96 72.0 60 70 30 3.61 0.5703
WVFGRD96 74.0 60 70 30 3.61 0.5754
WVFGRD96 76.0 60 70 30 3.62 0.5780
WVFGRD96 78.0 60 70 30 3.62 0.5811
WVFGRD96 80.0 60 70 25 3.64 0.5850
WVFGRD96 82.0 60 70 25 3.64 0.5866
WVFGRD96 84.0 60 70 25 3.65 0.5900
WVFGRD96 86.0 60 75 25 3.65 0.5946
WVFGRD96 88.0 60 75 25 3.65 0.5953
WVFGRD96 90.0 60 75 25 3.66 0.5982
WVFGRD96 92.0 60 75 25 3.66 0.5988
WVFGRD96 94.0 60 75 25 3.67 0.6022
WVFGRD96 96.0 60 75 25 3.67 0.6030
WVFGRD96 98.0 60 75 25 3.68 0.6019
WVFGRD96 100.0 60 75 25 3.68 0.6048
WVFGRD96 102.0 60 75 25 3.69 0.6046
WVFGRD96 104.0 60 75 25 3.69 0.6038
WVFGRD96 106.0 60 75 25 3.70 0.6050
WVFGRD96 108.0 60 75 25 3.70 0.6059
WVFGRD96 110.0 60 75 25 3.71 0.6036
WVFGRD96 112.0 60 75 25 3.71 0.6062
WVFGRD96 114.0 60 75 25 3.71 0.6036
WVFGRD96 116.0 60 75 25 3.72 0.6050
WVFGRD96 118.0 60 75 25 3.72 0.6037
WVFGRD96 120.0 60 75 25 3.73 0.6019
WVFGRD96 122.0 60 75 25 3.73 0.6030
WVFGRD96 124.0 60 75 25 3.73 0.5987
WVFGRD96 126.0 60 80 25 3.73 0.6004
WVFGRD96 128.0 65 75 25 3.74 0.5952
WVFGRD96 130.0 65 75 25 3.74 0.5970
WVFGRD96 132.0 65 75 25 3.75 0.5938
WVFGRD96 134.0 65 75 25 3.75 0.5949
WVFGRD96 136.0 65 75 25 3.75 0.5923
WVFGRD96 138.0 65 75 25 3.76 0.5909
WVFGRD96 140.0 65 75 25 3.76 0.5891
WVFGRD96 142.0 65 75 25 3.76 0.5877
WVFGRD96 144.0 65 75 25 3.77 0.5851
WVFGRD96 146.0 65 75 25 3.77 0.5847
WVFGRD96 148.0 65 75 25 3.77 0.5795
The best solution is
WVFGRD96 112.0 60 75 25 3.71 0.6062
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 -40 o DIST/3.3 +50 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