The ANSS event ID is ak0169k0j7gg and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0169k0j7gg/executive.
2016/07/26 01:26:06 61.831 -151.737 108.6 3.9 Hawaii
USGS/SLU Moment Tensor Solution
ENS 2016/07/26 01:26:06:0 61.83 -151.74 108.6 3.9 Hawaii
Stations used:
AK.DHY AK.FIRE AK.MCK AK.PPLA AK.RC01 AK.RND AK.SAW AK.SKN
AT.PMR TA.K20K TA.L19K TA.M22K TA.N18K TA.N19K TA.O18K
Filtering commands used:
cut a -20 a 80
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 7.76e+21 dyne-cm
Mw = 3.86
Z = 112 km
Plane Strike Dip Rake
NP1 75 60 55
NP2 309 45 135
Principal Axes:
Axis Value Plunge Azimuth
T 7.76e+21 59 293
N 0.00e+00 30 94
P -7.76e+21 9 189
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.07e+21
Mxy -1.96e+21
Mxz 2.49e+21
Myy 1.56e+21
Myz -2.97e+21
Mzz 5.51e+21
--------------
----------------------
----------------------------
-###########------------------
###################---------------
#######################-------------
###########################-----------
##############################----------
########### #################---------
############ T ###################-----###
############ ####################---####
##########################################
#################################---######
#############################-------####
--#######################-----------####
-------#########-------------------###
----------------------------------##
---------------------------------#
------------------------------
----------------------------
------ -------------
-- P ---------
Global CMT Convention Moment Tensor:
R T P
5.51e+21 2.49e+21 2.97e+21
2.49e+21 -7.07e+21 1.96e+21
2.97e+21 1.96e+21 1.56e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160726012606/index.html
|
STK = 75
DIP = 60
RAKE = 55
MW = 3.86
HS = 112.0
The NDK file is 20160726012606.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 a -20 a 80 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 75 50 95 3.02 0.2537
WVFGRD96 4.0 40 75 45 3.07 0.2903
WVFGRD96 6.0 210 85 -40 3.10 0.3319
WVFGRD96 8.0 205 75 -45 3.17 0.3466
WVFGRD96 10.0 205 75 -40 3.17 0.3508
WVFGRD96 12.0 200 70 -40 3.19 0.3540
WVFGRD96 14.0 200 70 -35 3.21 0.3551
WVFGRD96 16.0 200 70 -35 3.23 0.3570
WVFGRD96 18.0 200 70 -30 3.25 0.3540
WVFGRD96 20.0 205 75 -30 3.27 0.3492
WVFGRD96 22.0 205 75 -30 3.29 0.3435
WVFGRD96 24.0 200 65 -35 3.30 0.3402
WVFGRD96 26.0 75 70 45 3.40 0.3412
WVFGRD96 28.0 200 65 -35 3.34 0.3405
WVFGRD96 30.0 70 75 40 3.42 0.3393
WVFGRD96 32.0 70 80 30 3.45 0.3433
WVFGRD96 34.0 70 80 25 3.47 0.3512
WVFGRD96 36.0 70 75 20 3.49 0.3563
WVFGRD96 38.0 70 75 20 3.51 0.3581
WVFGRD96 40.0 70 65 15 3.59 0.3524
WVFGRD96 42.0 70 70 25 3.59 0.3528
WVFGRD96 44.0 70 75 30 3.61 0.3587
WVFGRD96 46.0 65 85 30 3.62 0.3683
WVFGRD96 48.0 65 85 30 3.64 0.3768
WVFGRD96 50.0 65 85 30 3.65 0.3840
WVFGRD96 52.0 65 85 30 3.66 0.3915
WVFGRD96 54.0 65 80 30 3.68 0.3970
WVFGRD96 56.0 65 80 30 3.69 0.4032
WVFGRD96 58.0 65 80 30 3.70 0.4082
WVFGRD96 60.0 70 70 25 3.72 0.4114
WVFGRD96 62.0 70 70 25 3.73 0.4156
WVFGRD96 64.0 70 70 25 3.74 0.4193
WVFGRD96 66.0 70 70 25 3.75 0.4213
WVFGRD96 68.0 70 70 25 3.75 0.4233
WVFGRD96 70.0 70 70 25 3.76 0.4243
WVFGRD96 72.0 75 60 35 3.77 0.4271
WVFGRD96 74.0 75 60 40 3.77 0.4334
WVFGRD96 76.0 75 60 40 3.78 0.4390
WVFGRD96 78.0 80 55 45 3.79 0.4441
WVFGRD96 80.0 80 55 45 3.79 0.4487
WVFGRD96 82.0 80 55 45 3.80 0.4535
WVFGRD96 84.0 80 55 45 3.81 0.4576
WVFGRD96 86.0 80 55 45 3.81 0.4608
WVFGRD96 88.0 80 55 45 3.82 0.4634
WVFGRD96 90.0 75 60 50 3.81 0.4657
WVFGRD96 92.0 75 60 50 3.82 0.4666
WVFGRD96 94.0 75 60 55 3.82 0.4679
WVFGRD96 96.0 75 60 55 3.82 0.4701
WVFGRD96 98.0 75 60 55 3.83 0.4707
WVFGRD96 100.0 75 60 55 3.83 0.4719
WVFGRD96 102.0 75 60 55 3.84 0.4723
WVFGRD96 104.0 75 60 55 3.84 0.4729
WVFGRD96 106.0 75 60 55 3.85 0.4740
WVFGRD96 108.0 75 60 55 3.85 0.4740
WVFGRD96 110.0 75 60 55 3.85 0.4739
WVFGRD96 112.0 75 60 55 3.86 0.4747
WVFGRD96 114.0 75 60 55 3.86 0.4740
WVFGRD96 116.0 75 60 55 3.87 0.4736
WVFGRD96 118.0 75 60 60 3.86 0.4734
WVFGRD96 120.0 75 60 60 3.87 0.4722
WVFGRD96 122.0 75 60 60 3.87 0.4721
WVFGRD96 124.0 75 60 60 3.87 0.4706
WVFGRD96 126.0 75 60 60 3.88 0.4687
WVFGRD96 128.0 75 60 60 3.88 0.4674
The best solution is
WVFGRD96 112.0 75 60 55 3.86 0.4747
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 a -20 a 80 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2
|
| 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