The ANSS event ID is ak019ds7srl5 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019ds7srl5/executive.
2019/10/27 04:32:27 62.880 -148.302 74.9 4.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/10/27 04:32:27:0 62.88 -148.30 74.9 4.5 Alaska
Stations used:
AK.BMR AK.BPAW AK.CCB AK.CUT AK.DHY AK.DIV AK.GHO AK.KLU
AK.KNK AK.KTH AK.MCK AK.NEA2 AK.PAX AK.PPLA AK.RC01 AK.RIDG
AK.RND AK.SAW AK.SCM AK.TRF AK.WRH AT.PMR IM.IL31 IU.COLA
TA.M24K
Filtering commands used:
cut o DIST/3.3 -50 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 5.56e+22 dyne-cm
Mw = 4.43
Z = 80 km
Plane Strike Dip Rake
NP1 223 82 -114
NP2 115 25 -20
Principal Axes:
Axis Value Plunge Azimuth
T 5.56e+22 33 333
N 0.00e+00 23 227
P -5.56e+22 48 108
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.89e+22
Mxy -8.61e+21
Mxz 3.11e+22
Myy -1.43e+22
Myz -3.77e+22
Mzz -1.46e+22
##############
######################
############################
####### #################---
######### T ###############-------
########## #############----------
########################--------------
-######################-----------------
-#####################------------------
--###################---------------------
--#################-----------------------
---##############------------ ----------
---#############------------- P ----------
---##########--------------- ---------
----########----------------------------
-----####-----------------------------
-----##-----------------------------
-----#---------------------------#
-######---------------------##
###########-----------######
######################
##############
Global CMT Convention Moment Tensor:
R T P
-1.46e+22 3.11e+22 3.77e+22
3.11e+22 2.89e+22 8.61e+21
3.77e+22 8.61e+21 -1.43e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191027043227/index.html
|
STK = 115
DIP = 25
RAKE = -20
MW = 4.43
HS = 80.0
The NDK file is 20191027043227.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 -50 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 40.0 140 35 25 4.26 0.3832
WVFGRD96 42.0 120 30 15 4.31 0.3942
WVFGRD96 44.0 115 30 10 4.33 0.4160
WVFGRD96 46.0 125 30 15 4.34 0.4408
WVFGRD96 48.0 125 30 10 4.34 0.4719
WVFGRD96 50.0 125 30 10 4.36 0.5035
WVFGRD96 52.0 125 30 5 4.36 0.5356
WVFGRD96 54.0 120 30 0 4.37 0.5662
WVFGRD96 56.0 120 25 -5 4.38 0.5942
WVFGRD96 58.0 120 25 -5 4.39 0.6193
WVFGRD96 60.0 120 25 -5 4.40 0.6414
WVFGRD96 62.0 120 25 -5 4.40 0.6584
WVFGRD96 64.0 120 25 -5 4.41 0.6741
WVFGRD96 66.0 120 25 -10 4.41 0.6864
WVFGRD96 68.0 120 25 -10 4.42 0.6963
WVFGRD96 70.0 115 25 -15 4.42 0.7031
WVFGRD96 72.0 115 25 -15 4.42 0.7102
WVFGRD96 74.0 115 25 -15 4.42 0.7131
WVFGRD96 76.0 115 25 -20 4.43 0.7163
WVFGRD96 78.0 115 25 -20 4.43 0.7167
WVFGRD96 80.0 115 25 -20 4.43 0.7174
WVFGRD96 82.0 115 25 -20 4.43 0.7149
WVFGRD96 84.0 110 25 -25 4.43 0.7129
WVFGRD96 86.0 110 25 -25 4.44 0.7096
WVFGRD96 88.0 110 25 -30 4.44 0.7051
WVFGRD96 90.0 110 25 -30 4.44 0.7008
WVFGRD96 92.0 110 25 -30 4.44 0.6952
WVFGRD96 94.0 110 25 -30 4.44 0.6892
WVFGRD96 96.0 110 25 -30 4.44 0.6816
WVFGRD96 98.0 105 25 -35 4.44 0.6754
WVFGRD96 100.0 105 25 -35 4.45 0.6685
WVFGRD96 102.0 105 25 -35 4.45 0.6611
WVFGRD96 104.0 105 25 -40 4.45 0.6540
WVFGRD96 106.0 105 25 -40 4.45 0.6462
WVFGRD96 108.0 105 25 -40 4.45 0.6379
WVFGRD96 110.0 100 25 -45 4.45 0.6299
WVFGRD96 112.0 100 25 -45 4.45 0.6217
WVFGRD96 114.0 100 25 -45 4.46 0.6132
WVFGRD96 116.0 100 25 -45 4.46 0.6050
WVFGRD96 118.0 100 25 -45 4.46 0.5969
The best solution is
WVFGRD96 80.0 115 25 -20 4.43 0.7174
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 -50 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 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