Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.
The ANSS event ID is ak01934n5mur and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak01934n5mur/executive.
2019/03/09 11:11:00 55.276 -134.909 28.2 4.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/03/09 11:11:00:0 55.28 -134.91 28.2 4.8 Alaska
Stations used:
AK.BESE AK.JIS AT.CRAG AT.SIT AT.SKAG CN.BBB CN.BNAB
CN.BUTB CN.DLBC CN.GRNB CN.HYT CN.KITB CN.MOBC CN.NDB
CN.PCLB CN.PLBC CN.WHY CN.YUK6 TA.O30N TA.P30M TA.P32M
TA.P33M TA.Q32M TA.R31K TA.R32K TA.R33M TA.S31K TA.S32K
TA.S34M TA.T33K TA.T35M TA.U33K TA.U35K TA.V35K US.WRAK
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 0.07 n 3
Best Fitting Double Couple
Mo = 1.27e+23 dyne-cm
Mw = 4.67
Z = 24 km
Plane Strike Dip Rake
NP1 295 60 40
NP2 182 56 143
Principal Axes:
Axis Value Plunge Azimuth
T 1.27e+23 48 150
N 0.00e+00 42 326
P -1.27e+23 2 58
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.49e+21
Mxy -8.15e+22
Mxz -5.77e+22
Myy -7.74e+22
Myz 2.69e+22
Mzz 7.09e+22
######--------
########--------------
##########------------------
##########--------------------
###########----------------------
#####----##----------------------- P
------------#######----------------
------------############----------------
------------###############-------------
-------------##################-----------
------------#####################---------
------------#######################-------
------------#########################-----
------------#########################---
------------########### ############--
-----------########### T #############
-----------########## ############
----------########################
---------#####################
---------###################
-------###############
-----#########
Global CMT Convention Moment Tensor:
R T P
7.09e+22 -5.77e+22 -2.69e+22
-5.77e+22 6.49e+21 8.15e+22
-2.69e+22 8.15e+22 -7.74e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190309111100/index.html
|
STK = 295
DIP = 60
RAKE = 40
MW = 4.67
HS = 24.0
The NDK file is 20190309111100.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: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
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 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 130 40 85 4.40 0.4745
WVFGRD96 2.0 145 45 90 4.48 0.4973
WVFGRD96 3.0 100 55 -20 4.48 0.4510
WVFGRD96 4.0 100 55 -20 4.48 0.4491
WVFGRD96 5.0 100 50 -15 4.49 0.4513
WVFGRD96 6.0 100 45 -10 4.49 0.4611
WVFGRD96 7.0 100 45 -10 4.49 0.4765
WVFGRD96 8.0 100 40 -5 4.49 0.4951
WVFGRD96 9.0 105 40 10 4.50 0.5139
WVFGRD96 10.0 90 40 -25 4.54 0.5338
WVFGRD96 11.0 90 45 -30 4.55 0.5582
WVFGRD96 12.0 90 45 -30 4.56 0.5814
WVFGRD96 13.0 90 50 -35 4.57 0.6023
WVFGRD96 14.0 90 50 -35 4.58 0.6199
WVFGRD96 15.0 90 50 -35 4.59 0.6347
WVFGRD96 16.0 300 60 50 4.58 0.6471
WVFGRD96 17.0 300 60 50 4.59 0.6612
WVFGRD96 18.0 300 60 50 4.60 0.6727
WVFGRD96 19.0 300 60 45 4.61 0.6820
WVFGRD96 20.0 300 60 50 4.64 0.6925
WVFGRD96 21.0 300 60 45 4.65 0.6988
WVFGRD96 22.0 300 60 45 4.66 0.7030
WVFGRD96 23.0 295 60 40 4.67 0.7054
WVFGRD96 24.0 295 60 40 4.67 0.7059
WVFGRD96 25.0 295 60 40 4.68 0.7045
WVFGRD96 26.0 295 60 40 4.69 0.7012
WVFGRD96 27.0 295 60 40 4.69 0.6962
WVFGRD96 28.0 295 60 40 4.70 0.6900
WVFGRD96 29.0 295 60 40 4.71 0.6823
WVFGRD96 30.0 295 60 40 4.71 0.6732
WVFGRD96 31.0 295 55 40 4.72 0.6638
WVFGRD96 32.0 295 55 35 4.73 0.6539
WVFGRD96 33.0 295 55 35 4.74 0.6428
WVFGRD96 34.0 295 55 35 4.74 0.6313
WVFGRD96 35.0 295 55 35 4.75 0.6193
WVFGRD96 36.0 295 55 35 4.76 0.6071
WVFGRD96 37.0 290 55 35 4.76 0.5950
WVFGRD96 38.0 290 55 35 4.78 0.5829
WVFGRD96 39.0 290 55 35 4.79 0.5704
WVFGRD96 40.0 295 50 35 4.86 0.5291
WVFGRD96 41.0 295 50 35 4.87 0.5180
WVFGRD96 42.0 295 50 35 4.87 0.5051
WVFGRD96 43.0 295 50 35 4.88 0.4909
WVFGRD96 44.0 290 50 35 4.88 0.4760
WVFGRD96 45.0 290 50 30 4.88 0.4610
WVFGRD96 46.0 290 50 30 4.89 0.4457
WVFGRD96 47.0 290 50 30 4.89 0.4299
WVFGRD96 48.0 70 45 -45 4.90 0.4167
WVFGRD96 49.0 70 45 -45 4.91 0.4077
WVFGRD96 50.0 70 45 -40 4.91 0.3988
The best solution is
WVFGRD96 24.0 295 60 40 4.67 0.7059
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 0.07 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 CUS.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 CUS Model with Q from simple gamma values 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 1.0000 5.0000 2.8900 2.5000 0.172E-02 0.387E-02 0.00 0.00 1.00 1.00 9.0000 6.1000 3.5200 2.7300 0.160E-02 0.363E-02 0.00 0.00 1.00 1.00 10.0000 6.4000 3.7000 2.8200 0.149E-02 0.336E-02 0.00 0.00 1.00 1.00 20.0000 6.7000 3.8700 2.9020 0.000E-04 0.000E-04 0.00 0.00 1.00 1.00 0.0000 8.1500 4.7000 3.3640 0.194E-02 0.431E-02 0.00 0.00 1.00 1.00