Rajesh R. Nair

Subsurface Images Shed Light on Past Tsunamis in India

The 2004 Indian Ocean tsunami caused massive devastation and left a lasting impact along many of the major coastal regions in South Asia, including the coast of Tamil Nadu, a state in the southeastern tip of India. Following the event, sand deposits draped the low-lying areas and buried the muddy sediments of the coastal plain [Babu et al., 2007; Srinivasalu et al., 2007]. In addition, erosional features related to the tsunami, such as channels and scarps, have been observed along many parts of the coast (Figure 1a). This tsunami, along with a recorded history of intense monsoons, has highlighted the need for focused research on the role of extreme events in shaping the geological character of Indias coastal plains.

Uplift of an Underground Tank in Northern Malabar Region, India

An underground reinforced concrete tank was constructed for a project in the southwest region of India. The tank was 90 m x 35 m in plan and 7.3 m deep resting on partly filled-up and partly native soil. During the peak monsoon, a sudden uplifting of the base slab by about 300 mm and subsequent failure of the foundation raft and a partition wall was observed. Laboratory testing was executed and hydrogeological survey was carried out using ground penetrating radar, seismic refraction and infiltrometer testing, and an analytical study was carried out to identify the root cause of the tank uplifting. Based on this study, it was observed that the uplifting and structural failure was essentially due to the peculiar land terrain and soil properties and the development of excess hydraulic head below the bottom of the tank. After considering different options, the rectification measures were carried out by provision of dewatering wells along the tank periphery to release the excess hydrostatic pressure and stabilize the foundation raft. The structural repair of the top of the foundation raft and partition wall was carried out to strengthen the reinforced concrete members. The rectification measures worked well to increase the structural stability of the tank and to prevent build-up of excess hydrostatic pressure preventing uplift and subsequent damage in the future.

Assessment of earthquake early warning potential from KiK-Net seismograms of 2000–2011 north and east Japan events

An earthquake early warning alert system with a high degree of accuracy and precision and measured in seconds is a necessity to save many lives in Japan, one of the most tectonically active countries of the world. In this paper, we estimate the potential value for early warning of crustal and intraslab earthquakes recorded at the KiK-Net network. The work reported here is similar to that of the early warning analysis system of California and uses new comprehensive data from northern and eastern Japan (Hokkaido and Tohoko). An attempt was also made to analyze whether the slopes of the regression lines obtained for Californian events (all are crustal events) are similar to those obtained for Hokkaido and Tohoko crustal events. We also analyzed whether there was any difference between the slopes obtained from Japanese crustal and intraslab earthquakes. The poor correlation of the higher magnitude (>5.5) with predominant period implied quite a limited predictability of predominant frequency. This was evident from the scattered plot obtained for higher magnitudes. These limitations led us to use a discrete wavelet transform-automated algorithm based on the bi-orthogonal construction with two and four vanishing moments for the primal and dual wavelets.

Forensic applications including mapping and classification of tsunami sand deposits and sand dunes with ground penetrating radar

The coastal region experiences erosional, transportational and depositional events during mega events such as tsunami. The onshore deposits are significant in determining the past events of high-energy events in the coastal region. The Optically Simulated Luminescence (OSL) dating of the sediments by combining Ground Penetrating Radar (GPR) records will reveal the past tsunami events. OSL dates obtained on the sediments, trenched manually will indicate the depositional events having a characteristic recurrence interval and also the presence of heavy mineral concentration (HMCs). These characteristic layers can be identified using the radar signatures. GPR is a high-resolution technique of imaging shallow soil and ground structures using electromagnetic waves in the frequency band of 100–400 MHz, with penetration depths of 30–40 m. This paper highlights the application of radar technique as a forensic tool combined with the analysis of soil samples for the study of tsunamis and sand dunes in India.