Non-volcanic tremor at Cascadia Subduction Zone
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Non-volcanic tremor (NVT) is my main focus of research these days. I am working on
non-volcanic tremor at Cascadia Subduction Zone. We installed an 84-element vertical-channel
small-aperture dense seismic array, and recorded the May 2008 ETS event, and a tremor episode two
months earlier. Applying a beamforming technique, we are able to detect up to 4 times more duration
of non-volcnic tremor activity than the detection using a conventional envelope
cross-correlation method [Ghosh et al., submitted]. The movie shows an example of the
beamforming result. 10-15 Hz energy with 5 minutes time window, 2.5 minutes overlap. We map out
the ETS zone with an unprecedented resolution and unveil several distinct patches on the subduction
interface that release much of the tremor moment [Ghosh et al., submitted]. For the first time
in Cascadia, we found clear evidence of short-term tremor migration with migration velocity ranging
from 30 to 350 km/hr [Ghosh et al., in preparation].
Click to see some interesting results of beamforming tremor.
Follow this
to see the local media report on the experiment.
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Triggered tremor at San Andreas Fault near Parkfield
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I am also studying non-volcanic tremor (NVT) at San Andreas Fault (SAF), particularly
those are triggered by large teleseismic earthquakes. We found pulses of tremor activities near Parkfield ignited by
the passing seismic waves from the great 2004 Mw 9.2 Sumatra event [Ghosh et al., in review].
The prolonged shaking reveals the richest, and the most varied observations of dynamically-triggered
tremor to date. Long period Love wave shows the clearest evidence of tremor modulation. Interestingly,
some NVT appear to be associated with the passage of teleseismic P waves, which is unusual and surprising
given the small stress they impart. In this work, we are trying to understand the reaction of NVT to
the dynamic stressing from different body and surface waves. The figure shows seismograms of teleseismic
waves and associated NVT at SAF near Parkfield.
Click to read the official news release issued by the University of Washington on this study.
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Earthquake frequency-magnitude distribution (b-value)
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Understanding the earthquake frequency-magnitude distribution, known as
b-value, is my another area of interest. I studied the spatial distribution of b-value
along the subduction interface of the Middle America Trench (MAT), near Nicoya Peninsula, Costa Rica
[].
We found that spatial b-value mapping can be used to demarcate the zone of strong seismic
coupling along the subduction megathrust. The figure shows the spatial distribution of b
-value along the interface of MAT near Nicoya.
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Earthquake locations and Nicoya catalog
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Earthquake locations are very important for any seismological study. We made a
robust earthquake catalog of Nicoya Peninsula, Costa Rica, that spans from end 1999 to mid 2001
[].
The seismic network, which consists of 40 land and ocean bottom seismometers, is the part of the
project Costa Rica Seismogenic Zone Experiment. We manually picked ~5000 earthquakes and their
associated phase picks, combines them with the picks done by earlier workers, and relocated them
with simul2000 (previously known as Simulps), using a local velocity model made by
DeShon et al., 2007. This catalog, with ~10000 events, is the most comprehensive earthquake
catalog of MAT, near Nicoya Peninsula, till date. The figure shows earthquake locations from the
Nicoya catalog. Red circles represent earthqukes along the subduction interface, and green circles
are the non-interface events.
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Shallow geophysical exploration
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Shallow resistivity survey can be used for groundwater exploration. Though it is
difficult to use in unconsolidated sediment, we successfully used resistivity methods (vertical
electric sounding) for groundwater exploration at and near the beach area at Sagar Island, West
Bengal, India []. Our modeling results suggest a fresh water aquifer of
appreciable thickness at a range of 94 to 174 meters. The figure shows a fence diagram, made from
the resistivity modeling, depicting different lithological layers of the study area.
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I beleive that structural geology is one of the important tools to understand the
tectonism of an area. I studied a structurally complex part of the Sausar Mobile Belt, at Ramtek,
India, that underwent multiple phases of deformation. We found a plunging synform that has the
the evidences of three generations of deformation [Ghosh, 2004]. We also found a linear zone of
brecciated rocks that may represents an ancient fault zone. The figure shows the geometry of the
typical folded structure.
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Last updated by Abhijit Ghosh on May 12th, 2009
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