Bhaskararao ILLA
Institute: Tata Institute of Fundamental Research, Mumbai, India & CSIR-National Geophysical Research Institute, Hyderabad, India
Title: Visiting Postdoctoral Researcher
Country/Region: India
Period: 2023/04/02-2023/06/30
Theme: Precise location of earthquakes along the subduction zone in Japan by improving the detection of the P and S arrivals on Distributed Acoustic Sensing (DAS) records
Host: Masanao SHINOHARA
Introduction: I pursued my PhD on “Uppermost Mantle Seismic Velocity and Anisotropy Structure beneath the Indian Shield and Adjacent Regions” at the CSIR-National Geophysical Research Institute (CSIR-NGRI). The Pn and Sn waves are refracted twice from the uppermost mantle and eventually recorded as first arrivals over a regional distance range, which are useful for constraining regional tectonic models based on velocity, anisotropy, and Vp/Vs ratios. I have written regional body (Pn and Sn) wave tomographic codes in an anisotropic medium using Python and Shell scripting. The major outcomes from these studies are: the distinct nature of the Pn and SKS anisotropic directions below the Indian shield reveals that the lithospheric mantle is still preserving the past remnant anisotropy; the east-west trending fast directions in the Tibetan region support the eastward flow that turns southward in the eastern syntaxis and flows parallel to the Burmese arc; the presence of basaltic pillows related to the Deccan volcanism in central India is revealed by the faster Pn and Sn velocities; in the oceanic part of the Indian plate (i.e., Bay of Bengal), the 90° E ridge distinctly separates the high and low Vp/Vs anomalies, which reveal the partial melts and water content in the uppermost mantle in the east and an presence of orthopyroxene-rich mantle or eclogite in the western part of this ridge.

In order to understand the origin and rupture propagation characteristics of the 2004 Andaman-Sumatra mega-thrust earthquake (Mw 9.1-9.3, hypocentre-30 km) and the nature of the subducting plates, I have recently studied the anisotropic Pn wave tomography in the Andaman-Sumatra and produced very intriguing results. The clear evidence of the uppermost mantle structural heterogeneity between the Andaman and north Sumatra segments is well correlated with the subducting oceanic slab age and spreading rate.

As a part of my postdoctoral research, I am working on the topic “upscaling the elastic wave equation using renormalization group theory”.

I will collaborate with Prof. Masanao Shinohara to carry out research on the topic “precise location of earthquakes along the subduction zone in Japan by improving the detection of the P and S arrivals on Distributed Acoustic Sensing (DAS) records” as a visiting researcher at the ERI.

Research Report:
3D Subsurface Imaging Using Off Sanriku Ocean-bottom
Distributed Acoustic Sensing (DAS) Cable Data, Japan
Bhaskararao Illa, PhD (
Postdoctoral Researcher (Tata Institute of Fundamental Research-Mumbai, India)
Short-term Visiting Researcher (1st April 2023 to 30th June 2023)
Host: Prof. Masanao Shinohara
Division: Center for Geophysical Observation and Instrumentation
Earthquake Research Institute, Tokyo University
Marine sediment characterization holds significant importance for various applications in
geotechnical engineering, seismic hazard assessment, and hydrocarbon exploration. Additionally,
understanding the oceanic lithosphere structure in subduction settings is crucial for comprehending
earthquake genesis and geodynamics. However, precise seismic subsurface imaging traditionally
relies on costly active seismic surveys conducted during offshore campaigns, which are not only
expensive but also limited in coverage. This limitation hinders our ability to comprehensively
characterize marine sedimentary layers and deeper structures.
Many disciplines of earth research stand to benefit from the ability to utilize pre-existing
communications fiber-optic networks as seismic arrays, providing tens of thousands of sensing
locations in areas where seismologists have traditionally lacked access. Moreover, considering that
the ocean covers 70% of the planet's surface, and seismometer coverage is limited to a few
permanent ocean bottom seismometers, this idle and accessible infrastructure can be leveraged by
DAS to fill many significant gaps in ocean-basin seismic coverage. The seismic tsunami
observation system, deployed off the Sanriku region by the Earthquake Research Institute at the
University of Tokyo in 1996, employs optical fiber for data transmission and incorporates
Distributed Acoustic Sensing (DAS) technology with observations commencing in February 2019
(Shinohara et al., 2019 and 2022).
Recent advancements offer promising avenues for cost-effective solutions using Distributed
Acoustic Sensing (DAS) recordings. Spica et al. (2020) and Fukushima et al. (2022) demonstrated
the potential of ambient noise imaging for estimating 2D Vs structures in marine sediments.
However, their studies were constrained to shallow depths and a limited 50 km cable length. To
overcome these limitations, we carried out experiments utilizing ocean-bottom dark fibers for highresolution
3D subsurface imaging using 3D finite-element travel-time tomography.
The DAS is an emerging technology where standard fiber-optic cables used for
telecommunications are repurposed as a long series of single-component, in-line strain or strain-rate
sensors with sensing point separations as small as 1 m or less. The DAS approach uses a laser
interrogator unit at one end of the fiber that employs short pulses to illuminate the fiber and then
performs high-rate optical interferometry of the Rayleigh backscattered light. The backscattered
photons return to the interrogator unit at a rate that is proportional to how far they have traveled
along the linear fiber. Distributed optic sensors use optical time-domain reflectometry (OTDR)- a
series of pulses are transmitted into the fibre by an interrogator and the backscattered signal is
detected, amplified and digitised. The retrieved phase shift is quasi- linearly proportional to the
change in strain.
View All
Fiscal Year: 2023