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Fault structures illuminated by the Aftershocks of the 2015 Mw 7.8 Gorkha Earthquake in Nepal as captured by a Local Dense seismic network
As a result of the 2015 Mw 7.8 Gorkha earthquake, more than 9,000 people were killed from a combination of infrastructure failure and triggered landslides. The earthquake produced 8m of peak co-seismic slip as the fault ruptured 130 km east under densely populated cities, such as Kathmandu. To understand earthquake dynamics in this part of Himalaya and better prepare for the next destructive event, it is imperative to study the earthquake activity in detail to improve our understanding of the source and structural complexities. In rapid response to the Gorkha Event, multiple institutions developed and deployed a dense, 45- station, seismic network (“NAMASTE”) from June 2015 to May 2016. Composed of a mix of broadband, short-period, and strong motion sensors, with an average spacing of ~20 km, NAMASTE captured the dynamic sequence of aftershock behavior by blanketing the 27, 650 km2 rupture area.
Focal mechanisms of VLFEs from the 2014 ETS event in Cascadia
We find very low frequency earthquakes (VLFEs) in Cascadia during the 2014 episodic tremor and slip event under Washington and Vancouver Island using a grid search centroid moment tensor inversion method. The very low frequency earthquakes occur when and where there is no strong tremor detected, contrasting with previous findings where tremor and very low frequency earthquakes have a clear spatiotemporal relationship. This challenges our current understanding of the dynamic relationship between the different types of slow earthquakes and their relationship to slow slip.
Using Global Arrays to Study the Earthquake Rupture Process and Detect Hidden Earthquakes
We use the back-projection method to study the rupture process of large earthquakes. We use global arrays individually and then combine them to improve the resolution by increasing the azimuth coverage. With higher resolution, we are able to detect and locate hidden earthquakes that are not recorded in current global catalogs.
Non-volcanic Tremors (NVTs) and Low Frequency Earthquakes (LFEs) in the Alaska-Aleutian Subduction Zone
To study the tremor and LFEs in the Alaska-Aleutian subduction zone, we deployed three mini arrays in the Unalaska island. Using the beam-backprojection method, we detect continuous tremor activities and observe different migration patterns both along-strike and dip directions, with various velocities. LFEs and tremors match well with each other spatio-temporally.
Non-volcanic tremor in Cascadia subduction zone using multiple seismic arrays (Array of Arrays)
Non-volcanic tremor is my main focus of research these days. We installed 8 small-aperture seismic arrays in Cascadia, northern Washington, over the migration path of tremor during an episodic tremor and slip (ETS) event. The map shows the location of the arrays (red squares). The arrays recorded seismic data for more than a year, which includes an ETS in August 2010. One of the goals of this experiment is to determine the depth of tremor in this region. I have developed a technique (multi beam backprojection) to image tremor source with high resolution. Initial results shows that the majority of the tremor is near the plate interface, and aligns parallel to it. This is a research very much in progress, and more results to come soon.
Non-volcanic Tremor in Cascadia using a solo seismic array
The experiment leading to the 'Array of Arrays' involves a single, dense, small-aperture seismic array we installed in the same area that we later reoccupied for the multiple arrays experiment (see above). I developed a technique to detect and locate tremor [Ghosh et al., 2009]. Using this technique, I demonstarte that tremor behavior during an ETS in Cascadia is more complex than previously thought, and changes dramatically over the observation time scale. I find that tremor in Cascadia migrates in different direction with different velocities indicating a complex interaction between geologic structure, high fluid pressure, and stress transfer [Ghosh et al., 2009, Ghosh et al., 2010a, Ghosh et al., 2010b]. Thses migration patterns give new insight to the evolution of slip during an ETS, and help better understand the underlying physics.
Triggered tremor at San Andreas Fault near Parkfield
I also study tremor at San Andreas Fault (SAF), particularly those are triggered by large teleseismic earthquakes. I examine pulses of tremor activities near Parkfield ignited by the passing seismic waves from the great 2004 Mw 9.2 Sumatra event [Ghosh et al., 2009]. The prolonged shaking reveals very rich, and varied observations of dynamically-triggered tremor. Long period Love waves show the clearest evidence of tremor modulation. Based on the relationship between induced stress and tremor modulation, I show that tremor is consistent with shaer slip on the fault plane. Interestingly, some tremor bursts 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, I presented a detail analyses of tremor response due to the dynamic stressing from various body and surface waves. The figure shows seismograms of teleseismic waves and associated tremor at SAF near Parkfield.
Earthquake frequency-magnitude distribution (b-value)
Understanding the earthquake frequency-magnitude distribution, known as b-value, is my another area of interest. I study the spatial distribution of b-value along the subduction interface of the Middle America Trench (MAT), near Nicoya Peninsula, Costa Rica [Ghosh et al., 2008]. Based on the spatial variation of b-value and geodetically determined interseismic locking, I demonstrate that spatial b-value mapping has potential to be used as a proxy for seismic coupling along the subduction fault. The figure shows the spatial distribution of b -value along the interface of MAT near Nicoya.
Earthquake locations and Nicoya catalog
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 [Ghosh et al., 2008]. 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.
Shallow geophysical exploration
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 [Majumdar et al., 2006]. 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.
I beleive that structural geology is one of the important tools to understand the tectonism of an area. I study a structurally complex part of the Sausar Mobile Belt, 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.