Seismicity of Kachchh and Shillong Plateau and their connectivity to plate margins

Abstract

Kachchh and the Shillong plateau are located in the intraplate region but are seismically active almost on par with the plate boundaries. This is an enigma and requires suitable explanation. Bouguer anomaly and isostatic residual anomaly maps of Kachchh and adjoining region in Pakistan derived from satellite gravity data show gravity highs over Kachchh that is connected to the gravity highs of the Western fold belt (WFB) through the NW-SE oriented gravity highs of the Karachi arc. They are part of the western boundary of the Indian plate and are seismically active. Gravity highs in the western part of Kachchh show NW-SE trend related to the Kachchh mainland uplift (KMU) and Kachchh mainland fault (KMF) north of it are indicated by its sharp gradient. Their orientation is similar to that of the Karachi arc. In fact, the KMU may be considered as subsurface basement extension of the Karachi arc in Kachchh. This trend changes to E-W trend in the eastern part of Kachchh where it encounters the NE-SW trend of the Delhi fold belt and Bhuj-Jaisalmer-Ganganagar-Lahore ridge. The latter interacts with the Lahore-Sargodha ridge west of Lahore where several seismic activities have been reported. The epicenter of the Bhuj earthquake of 2001 (M = 7.8) and its aftershocks are located at the intersection of these two prominent regional trends due to the Karachi arc and the KMU on the western side and Delhi fold belt and Bhuj-Jaisalmer-Ganganagar-Lahore ridge on the eastern side. The connection of Kachchh to the western plate boundary through Karachi arc and intersection of the two major regional trends have played important roles in seismicity of Kachchh including Bhuj earthquake of 2001. Modeled crustal structures across the WFB shows ramp type variation in Moho increasing from 36 to 56 km and most of the seismicity of the WFB coincide with this ramp in Moho. Similarly, the Shillong plateau in east India is surrounded by faults and lineaments that connect it to the plate boundaries of the Indian plate towards the east and the north. Notable among them are the Dauki, Dubri, Dudhnoi, Brahmputra and Oldham faults, and Kopili lineament. The Bouguer anomaly map shows gravity highs over the Shillong plateau, and faults are reflected as gradients and trends in the Bouguer anomaly which extend to the eastern and northern boundary of the Indian plate. The gravity gradient related to Dauki, Brahmaputra and Oldham faults towards the south and the north of the plateau, respectively, extend to the Arakan Yoma fold belt (AYFB) that represent eastern boundary of the Indian plate. The gravity trend related to the Kopili lineament extends to the Himalayan fold belt towards the north and Arakan Yoma fold belt towards the east. These connections of the Shillong plateau to plate boundaries may be responsible for seismic activity of the Shillong plateau. Epicenter of the Shillong earthquake of 1897 (M = 8.4) is located close to the junction of the Dudhnoi and Oldham and Brahmaputra faults. Modeled crustal structures across the Shillong plateau show a crustal bulge under the plateau that may be related to the flexure of the Indian plate due to Himalaya and may be an additional factor for the seismicity of the Shillong plateau including the Shillong earthquake of 1897.