Location

Location ANSS

2019/07/28 04:47:43 59.999 -152.659 102.4 3.8 Alaska

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2019/07/28 04:47:43:0 60.00 -152.66 102.4 3.8 Alaska Stations used: AK.CAPN AK.CNP AK.HOM AK.RC01 AK.SKN AK.SLK AK.SSN AK.SWD AT.PMR AV.ILSW AV.SPU II.KDAK TA.L19K TA.M19K TA.M20K TA.M22K TA.N18K TA.N19K TA.O18K TA.O22K TA.P18K TA.P19K TA.Q19K TA.Q20K 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 = 1.26e+22 dyne-cm Mw = 4.00 Z = 102 km Plane Strike Dip Rake NP1 65 65 30 NP2 321 63 152 Principal Axes: Axis Value Plunge Azimuth T 1.26e+22 38 284 N 0.00e+00 52 101 P -1.26e+22 1 193 Moment Tensor: (dyne-cm) Component Value Mxx -1.15e+22 Mxy -4.50e+21 Mxz 1.72e+21 Myy 6.71e+21 Myz -5.89e+21 Mzz 4.82e+21 -------------- ---------------------- ####------------------------ #########--------------------- ##############-------------------- ##################------------------ #####################----------------- #######################---------------## ###### ################-----------#### ####### T #################---------###### ####### ###################-----######## ##############################--########## #############################--########### #########################------######### #####################-----------######## ###############----------------####### -------------------------------##### ------------------------------#### ----------------------------## ---------------------------# ----- -------------- - P ---------- Global CMT Convention Moment Tensor: R T P 4.82e+21 1.72e+21 5.89e+21 1.72e+21 -1.15e+22 4.50e+21 5.89e+21 4.50e+21 6.71e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190728044743/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 = 65
      DIP = 65
     RAKE = 30
       MW = 4.00
       HS = 102.0

The NDK file is 20190728044743.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others

SLU

USGS/SLU Moment Tensor Solution ENS 2019/07/28 04:47:43:0 60.00 -152.66 102.4 3.8 Alaska Stations used: AK.CAPN AK.CNP AK.HOM AK.RC01 AK.SKN AK.SLK AK.SSN AK.SWD AT.PMR AV.ILSW AV.SPU II.KDAK TA.L19K TA.M19K TA.M20K TA.M22K TA.N18K TA.N19K TA.O18K TA.O22K TA.P18K TA.P19K TA.Q19K TA.Q20K 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 = 1.26e+22 dyne-cm Mw = 4.00 Z = 102 km Plane Strike Dip Rake NP1 65 65 30 NP2 321 63 152 Principal Axes: Axis Value Plunge Azimuth T 1.26e+22 38 284 N 0.00e+00 52 101 P -1.26e+22 1 193 Moment Tensor: (dyne-cm) Component Value Mxx -1.15e+22 Mxy -4.50e+21 Mxz 1.72e+21 Myy 6.71e+21 Myz -5.89e+21 Mzz 4.82e+21 -------------- ---------------------- ####------------------------ #########--------------------- ##############-------------------- ##################------------------ #####################----------------- #######################---------------## ###### ################-----------#### ####### T #################---------###### ####### ###################-----######## ##############################--########## #############################--########### #########################------######### #####################-----------######## ###############----------------####### -------------------------------##### ------------------------------#### ----------------------------## ---------------------------# ----- -------------- - P ---------- Global CMT Convention Moment Tensor: R T P 4.82e+21 1.72e+21 5.89e+21 1.72e+21 -1.15e+22 4.50e+21 5.89e+21 4.50e+21 6.71e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190728044743/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 using wvfgrd96

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 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

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   300    50   -55   3.12 0.2105
WVFGRD96    4.0   140    90    30   3.12 0.2411
WVFGRD96    6.0   320    90   -30   3.19 0.2615
WVFGRD96    8.0   320    85   -35   3.28 0.2732
WVFGRD96   10.0   325    90   -30   3.32 0.2724
WVFGRD96   12.0   325    90   -30   3.36 0.2648
WVFGRD96   14.0   325    90   -30   3.38 0.2505
WVFGRD96   16.0   325    90   -25   3.40 0.2311
WVFGRD96   18.0    50    70    15   3.42 0.2138
WVFGRD96   20.0    50    70    10   3.44 0.2136
WVFGRD96   22.0   220    60   -30   3.47 0.2194
WVFGRD96   24.0   220    60   -30   3.50 0.2317
WVFGRD96   26.0   225    65   -25   3.53 0.2497
WVFGRD96   28.0   225    65   -20   3.55 0.2694
WVFGRD96   30.0   225    65   -20   3.57 0.2862
WVFGRD96   32.0   225    65   -20   3.59 0.2991
WVFGRD96   34.0   235    85   -15   3.61 0.3067
WVFGRD96   36.0   235    90   -15   3.64 0.3127
WVFGRD96   38.0    55    90    15   3.67 0.3171
WVFGRD96   40.0    60    80    25   3.74 0.3250
WVFGRD96   42.0    60    85    30   3.79 0.3356
WVFGRD96   44.0    60    85    30   3.82 0.3483
WVFGRD96   46.0    60    85    30   3.84 0.3643
WVFGRD96   48.0    65    80    25   3.87 0.3800
WVFGRD96   50.0    65    80    25   3.89 0.3975
WVFGRD96   52.0    65    75    25   3.90 0.4145
WVFGRD96   54.0    65    70    25   3.90 0.4309
WVFGRD96   56.0    65    70    30   3.92 0.4489
WVFGRD96   58.0    65    70    30   3.93 0.4677
WVFGRD96   60.0    65    70    30   3.94 0.4849
WVFGRD96   62.0    65    70    30   3.95 0.5001
WVFGRD96   64.0    65    70    30   3.96 0.5145
WVFGRD96   66.0    65    70    30   3.96 0.5270
WVFGRD96   68.0    65    70    30   3.97 0.5371
WVFGRD96   70.0    65    70    30   3.97 0.5474
WVFGRD96   72.0    65    70    30   3.98 0.5551
WVFGRD96   74.0    65    70    30   3.98 0.5607
WVFGRD96   76.0    60    70    30   3.97 0.5679
WVFGRD96   78.0    60    70    30   3.97 0.5737
WVFGRD96   80.0    60    70    30   3.98 0.5776
WVFGRD96   82.0    60    70    30   3.98 0.5827
WVFGRD96   84.0    60    70    30   3.98 0.5854
WVFGRD96   86.0    60    70    30   3.98 0.5879
WVFGRD96   88.0    60    70    30   3.99 0.5890
WVFGRD96   90.0    60    70    30   3.99 0.5896
WVFGRD96   92.0    60    70    30   3.99 0.5894
WVFGRD96   94.0    60    70    30   3.99 0.5895
WVFGRD96   96.0    60    70    30   3.99 0.5905
WVFGRD96   98.0    60    70    30   4.00 0.5901
WVFGRD96  100.0    65    65    30   4.00 0.5907
WVFGRD96  102.0    65    65    30   4.00 0.5909
WVFGRD96  104.0    60    65    25   3.99 0.5902
WVFGRD96  106.0    60    65    25   4.00 0.5888
WVFGRD96  108.0    60    65    25   4.00 0.5883
WVFGRD96  110.0    60    65    25   4.00 0.5864
WVFGRD96  112.0    60    65    25   4.00 0.5867
WVFGRD96  114.0    60    65    25   4.01 0.5855
WVFGRD96  116.0    60    65    25   4.01 0.5849
WVFGRD96  118.0    65    60    25   4.01 0.5839
WVFGRD96  120.0    65    60    25   4.01 0.5826
WVFGRD96  122.0    65    60    25   4.02 0.5810
WVFGRD96  124.0    65    60    25   4.02 0.5780
WVFGRD96  126.0    65    60    25   4.02 0.5764
WVFGRD96  128.0    60    65    20   4.02 0.5745
WVFGRD96  130.0    60    65    20   4.02 0.5738
WVFGRD96  132.0    60    65    20   4.03 0.5721
WVFGRD96  134.0    60    65    20   4.03 0.5700
WVFGRD96  136.0    60    65    20   4.03 0.5660
WVFGRD96  138.0    60    65    20   4.03 0.5655
WVFGRD96  140.0    60    65    20   4.03 0.5634
WVFGRD96  142.0    60    65    20   4.04 0.5608
WVFGRD96  144.0    60    65    20   4.04 0.5571
WVFGRD96  146.0    65    60    20   4.04 0.5559
WVFGRD96  148.0    65    60    20   4.04 0.5542

The best solution is

WVFGRD96  102.0    65    65    30   4.00 0.5909

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 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.

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 Bureau of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Oklahoma Geological Survey, TexNet, the Iris stations, the Transportable Array of EarthScope and other networks.

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 Sun Jul 28 07:12:07 CDT 2019