Geophysical Research Letters
Supporting Information for
The 2 March 2016 Wharton Basin MW 7.8 earthquake:
High stress drop north-south strike-slip rupture in the
diffuse oceanic deformation zone between the Indian and
Australian Plates
Thorne Lay1, Lingling Ye2, Charles J. Ammon3, Audrey Dunham3, Keith D.
Koper4
1Department
of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, California,
Laboratory, California Institute of Technology, Pasadena, California, USA, 3Department of
Geosciences, Pennsylvania State University, University Park, Pennsylvania, USA, 4Department of
Geology and Geophysics, University of Utah, Salt Lake City, Utah, USA
2Seismological
Correspondence to: Thorne Lay, [email protected]
Contents of this file
Figures S1 to S8
Table S1
Animations M1 to M2
Introduction
Supporting information includes 8 figures, 1 table, and 2 animations.
1
Figure S1. Maps of seismicity from 1976 to 2016 from the NEIC catalog (left) and GCMT solutions (right) for the deformation zone
between the Indian and Australian plates and the Sumatran subduction zone. Symbols are color-coded for source depth, with radius
scaled proportional to MW. Note the widespread intraplate seismicity, comprised primarily of strike slip and thrusting events.
1
Figure S2. The array response functions [Xu et al., 2009] of the European and Australia station configurations
for a P wave with a period of 1 s. Also known as point spread functions, these images show the distorting
effects of the finite and discrete wavefield sampling achieved by the arrays. The response of an ideal array
would be a 2D delta function at the epicenter. The smearing and sidelobes are mitigated by Nth root stacking,
but are not completely eliminated. In addition to the array response, the distance range of the network
influences the streaking effects in the back-projections, as apparent in Movie S1.
1
Figure S3. Broadband (50 to 200 s period) surface wave peak amplitudes for waveforms equalized to a
propagation distance of 90° for the 2 March 2016 mainshock. Love wave (G1) and vertical component
Rayleigh wave (R1) amplitudes are shown in rose diagrams (left) and as functions of azimuth (right). The
symbols are color-coded by epicentral distance of the recording prior to distance equalization. The solid lines
indicate the azimuthal radiation patterns expected for the double-couple focal mechanism of the mainshock
with arbitrary amplitude scales.
2
Figure S4. Love wave (G1) relative source time functions (RSTFs) obtained by iterative time-domain
deconvolution with positivity constraint of the 2 March 2016 mainshock recordings by the corresponding station
recordings for the 3 March 2016 EGF event (Table 1). A Gaussian filter with width parameter of 0.2 is applied
to each deconvolution. The data are aligned with respect to directivity parameter,  = cos(–r)/c, where is
the station azimuth, r is the rupture direction, and c is a reference phase velocity (assumed to be 4.0 km/s),
defined for the two possible fault plane orientations with rupture azimuths of (a) 5° and (b) 95°. The data are
aligned on the hypocentral reference time for the mainshock. Reference lines at 30 s duration are provided to
help assess the duration variability.
3
Figure S5. SH body wave relative source time functions (RSTFs) obtained by iterative time-domain
deconvolution with positivity constraint of the 2 March 2016 mainshock recordings by the corresponding station
recordings for the 3 March 2016 EGF event (Table 1). A Gaussian filter with width parameter of 0.5 is applied
to each deconvolution. The data are aligned with respect to directivity parameter,  = cos(–r)/c, where is
the station azimuth, r is the rupture direction, and c is a reference phase velocity (assumed to be 10.0 km/s),
defined for the two possible fault plane orientations with rupture azimuths of (a) 5° and (b) 95°. The data are
aligned on the hypocentral reference time for the mainshock.
4
Figure S6. Observed SH body wave relative source time functions (RSTFs) at different azimuths obtained by
deconvolving the corresponding EGF signals are compared with predicted source time functions from the
finite-fault source models that use varying rupture expansion speeds from 1.5 to 3.0 km/s and a strike of 5°.
The overall RSTF duration is less than 30 s at all azimuths, but the high apparent velocity (~10 km/s) of the SH
waves results in little sensitivity to rupture speed and spatial extent.
5
Figure S7. Comparison of observed (black lines) and computed (red lines) P-wave and SH-wave broadband
ground displacement waveforms for the finite-fault model in Figure 5 with strike of 5° and rupture speed of 2.0
km/s. The azimuth and distance of each station are shown below the station name. The blue numbers indicate
the peak-to-peak amplitude of the observed signals in microns.
6
Figure S8. The stress change distribution on the fault model shown in Figure 5. The stress change at the midpoint of each subfault is computed for the entire distribution of slip over the model surface. The shear stress
amplitude and direction at each subfault are sown by the color scale and length and angle the vectors in each
subfault. The stress drops calculated by trimming off those subfaults that have a seismic moment less than
15% of the peak subfault moment and using the remaining average slip and residual fault area, 0.15 is 15
MPa. The slip-weighted stress drop measured from the variable stress change distribution, E is 20 MPa.
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Table S1
Source velocity model used in finite-fault inversions
P-Velocity
km/s
1.5
5.0
6.6
7.1
7.7
S-Velocity
km/s
0
2.5
3.65
3.9
4.5
Density
kg/m3
1000
2600
2900
3050
3300
Thickness
km
5.0
1.7
2.3
2.5
halfspace
8
Animations
Movie M1. Back-projections of 0.5-2.0 Hz P wave signals from large regional networks of broadband stations
in Europe and Australia.
Movie M2. Rupture animation for slip history (top) cumulative slip (bottom) for the preferred finite-fault rupture
model with a strike of 5° and rupture expansion speed of 2.0 km/s.
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