Location

2014/04/23 07:57:40 62.831 -150.556 95.5 4.0 Alaska

Arrival Times (from USGS)

Felt Map

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2014/04/23 07:57:40:0 62.83 -150.56 95.5 4.0 Alaska Stations used: AK.DHY AK.GHO AK.KNK AK.KTH AK.MCK AK.PPLA AK.RND AK.SAW AK.SCM AK.SKN AK.TRF AT.PMR Filtering commands used: cut a -30 a 100 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.26e+22 dyne-cm Mw = 4.00 Z = 102 km Plane Strike Dip Rake NP1 160 60 40 NP2 47 56 143 Principal Axes: Axis Value Plunge Azimuth T 1.26e+22 48 15 N 0.00e+00 42 191 P -1.26e+22 2 283 Moment Tensor: (dyne-cm) Component Value Mxx 4.55e+21 Mxy 4.15e+21 Mxz 5.92e+21 Myy -1.16e+22 Myz 2.15e+21 Mzz 7.01e+21 ############## ---################### -----####################### ------######################## --------########## ###########-- ---------########## T ###########--- ----------########## ###########---- ---------#######################------ P ---------#######################------ ---------######################-------- -------------####################--------- -------------###################---------- --------------################------------ --------------#############------------- ---------------###########-------------- --------------########---------------- ---------------####----------------- --------------#------------------- -------#######---------------- ###############------------- ##############-------- ############## Global CMT Convention Moment Tensor: R T P 7.01e+21 5.92e+21 -2.15e+21 5.92e+21 4.55e+21 -4.15e+21 -2.15e+21 -4.15e+21 -1.16e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140423075740/index.html

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is

      STK = 160
      DIP = 60
     RAKE = 40
       MW = 4.00
       HS = 102.0

The NDK file is 20140423075740.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2014/04/23 07:57:40:0 62.83 -150.56 95.5 4.0 Alaska Stations used: AK.DHY AK.GHO AK.KNK AK.KTH AK.MCK AK.PPLA AK.RND AK.SAW AK.SCM AK.SKN AK.TRF AT.PMR Filtering commands used: cut a -30 a 100 rtr taper w 0.1 hp c 0.02 n 3 lp c 0.10 n 3 Best Fitting Double Couple Mo = 1.26e+22 dyne-cm Mw = 4.00 Z = 102 km Plane Strike Dip Rake NP1 160 60 40 NP2 47 56 143 Principal Axes: Axis Value Plunge Azimuth T 1.26e+22 48 15 N 0.00e+00 42 191 P -1.26e+22 2 283 Moment Tensor: (dyne-cm) Component Value Mxx 4.55e+21 Mxy 4.15e+21 Mxz 5.92e+21 Myy -1.16e+22 Myz 2.15e+21 Mzz 7.01e+21 ############## ---################### -----####################### ------######################## --------########## ###########-- ---------########## T ###########--- ----------########## ###########---- ---------#######################------ P ---------#######################------ ---------######################-------- -------------####################--------- -------------###################---------- --------------################------------ --------------#############------------- ---------------###########-------------- --------------########---------------- ---------------####----------------- --------------#------------------- -------#######---------------- ###############------------- ##############-------- ############## Global CMT Convention Moment Tensor: R T P 7.01e+21 5.92e+21 -2.15e+21 5.92e+21 4.55e+21 -4.15e+21 -2.15e+21 -4.15e+21 -1.16e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20140423075740/index.html

Magnitudes

ML Magnitude

(a) ML computed using the IASPEI formula for Horizontal components; (b) 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.

(a) ML computed using the IASPEI formula for Vertical components (research); (b) 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.

Context

The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors).

Waveform Inversion

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.

Location of broadband stations used for waveform inversion

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 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 -30 a 100
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.10 n 3

The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    2.0    15    45   -85   3.18 0.2004
WVFGRD96    4.0   205    65   -65   3.22 0.2170
WVFGRD96    6.0   205    65   -60   3.25 0.2554
WVFGRD96    8.0   205    65   -65   3.35 0.2857
WVFGRD96   10.0    65    60    25   3.36 0.2961
WVFGRD96   12.0    65    60    20   3.41 0.2965
WVFGRD96   14.0    65    60    20   3.44 0.2855
WVFGRD96   16.0    65    55    15   3.47 0.2684
WVFGRD96   18.0    65    45    -5   3.52 0.2490
WVFGRD96   20.0    65    40    -5   3.54 0.2301
WVFGRD96   22.0   340    70    45   3.53 0.2379
WVFGRD96   24.0   335    65    35   3.54 0.2526
WVFGRD96   26.0   335    65    35   3.57 0.2674
WVFGRD96   28.0   330    70    35   3.58 0.2794
WVFGRD96   30.0   335    70    35   3.61 0.2864
WVFGRD96   32.0   330    75    30   3.61 0.2872
WVFGRD96   34.0   330    75    30   3.62 0.2866
WVFGRD96   36.0   335    75    30   3.66 0.2846
WVFGRD96   38.0   335    80    30   3.68 0.2801
WVFGRD96   40.0   340    75    45   3.78 0.2881
WVFGRD96   42.0   335    75    40   3.77 0.2730
WVFGRD96   44.0   145    70   -20   3.77 0.2725
WVFGRD96   46.0   145    70   -20   3.78 0.2730
WVFGRD96   48.0   155    60    30   3.83 0.2734
WVFGRD96   50.0   160    55    40   3.87 0.2848
WVFGRD96   52.0   160    55    40   3.88 0.2980
WVFGRD96   54.0   160    55    40   3.90 0.3117
WVFGRD96   56.0   160    55    40   3.91 0.3235
WVFGRD96   58.0   160    60    40   3.92 0.3360
WVFGRD96   60.0   160    60    35   3.93 0.3521
WVFGRD96   62.0   160    60    35   3.94 0.3667
WVFGRD96   64.0   160    60    35   3.94 0.3802
WVFGRD96   66.0   160    60    35   3.95 0.3912
WVFGRD96   68.0   160    60    35   3.96 0.4014
WVFGRD96   70.0   160    55    35   3.96 0.4099
WVFGRD96   72.0   160    55    35   3.97 0.4175
WVFGRD96   74.0   160    55    35   3.97 0.4237
WVFGRD96   76.0   165    55    50   3.98 0.4297
WVFGRD96   78.0   165    55    45   3.99 0.4364
WVFGRD96   80.0   165    55    45   3.99 0.4417
WVFGRD96   82.0   165    55    45   3.99 0.4458
WVFGRD96   84.0   165    55    45   3.99 0.4491
WVFGRD96   86.0   160    60    45   3.99 0.4507
WVFGRD96   88.0   160    60    45   3.99 0.4537
WVFGRD96   90.0   160    60    45   3.99 0.4553
WVFGRD96   92.0   160    60    45   3.99 0.4567
WVFGRD96   94.0   160    60    45   3.99 0.4567
WVFGRD96   96.0   160    60    45   4.00 0.4571
WVFGRD96   98.0   160    60    40   4.00 0.4581
WVFGRD96  100.0   160    60    40   4.00 0.4598
WVFGRD96  102.0   160    60    40   4.00 0.4602
WVFGRD96  104.0   160    60    40   4.00 0.4597
WVFGRD96  106.0   160    60    40   4.01 0.4589
WVFGRD96  108.0   160    60    40   4.01 0.4592
WVFGRD96  110.0   160    60    40   4.01 0.4593
WVFGRD96  112.0   160    60    35   4.02 0.4596
WVFGRD96  114.0   160    60    35   4.02 0.4596
WVFGRD96  116.0   160    60    35   4.02 0.4593
WVFGRD96  118.0   160    60    35   4.02 0.4583

The best solution is

WVFGRD96  102.0   160    60    40   4.00 0.4602

The mechanism correspond to the best fit is

Figure 1. Waveform inversion focal mechanism

The best fit as a function of depth is given in the following figure:

Figure 2. Depth sensitivity for waveform mechanism

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 and because the velocity model used in the predictions may not be perfect. 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 -30 a 100
rtr
taper w 0.1
hp c 0.02 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.

Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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:

The origin time and epicentral distance are incorrect
The velocity model used for the inversion is incorrect
The velocity model used to define the P-arrival time is not the same as the velocity model used for the waveform inversion (assuming that the initial trace alignment is based on the P arrival time)

Assuming only a mislocation, the time shifts are fit to a functional form:

 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.

Discussion

Acknowledgements

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Iris stations and the Transportable Array of EarthScope.

Velocity Model

The WUS model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

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

Quality Control

Here we tabulate the reasons for not using certain digital data sets

The following stations did not have a valid response files:

Last Changed Mon Dec 7 00:11:28 CST 2015