Seismology of Nepal: An Overview

Lok Bijaya Adhikari

Contents

• Introduction

• Geology and tectonics

• National Seismological Network

• Seismicity

• Introduction

• Geology and tectonics

• National Seismological Network

• Seismicity

Introduction• Nepal is a mountainous country within Latitude

260 22’ to 300 27’E and Longitude 800 4’ to 880 12’E. 83% of the area is covered by mountain peaks.

• Remaining 17% is covered by alluvial plains called ‘Terai plains’.

• The total area is 147181 sq.km. • Altitude varies from 50 m to 8848 m. (msl)

• The population of the country is about 25 million.

• Climate varies from subtropical to alpine type.

• Rich in biodiversity. Many types of plants, birds and wild animals.

• Nepal is country of Mt. Everest and birth place of Buddha.

Geology and Tectonics• Nepal lies within 2400 km long active Himalayan

mountain belt with 800km coverage.• Himalaya mountain is the product of continuous

collision between Indian and Eurassian plate since early Tertiary.

• Himalaya is divided into four main tectonic region

- Higher Himalaya

- Lesser Himalaya

- Sub- Himalaya

- Indogangetic alluvial plains.

• Three major longitudinal faults divide Himalaya in 4

major tectonics zones.

- Main Central Thrust (MCT).

- Main Boundary Thrust(MBT).

- Himalayan Frontal Thrust (HFT)

• These are formed due to underthrusting of Indian plates during different epochs.

• Gangetic plain- Pleistocene to recent alluvial deposits over 1000m thick.

• Sub-Himalaya – Consists of about 5 km thick Neogene molasse sediments of Siwaliks overthrust by older Lesser Himalayan rocks along MBT.

• Lesser Himalaya- Consists of intensely folded metasedimentary rocks between MBT and MCT.

• Higher Himalaya- thick sequence of higher Himalayan crystalline and overlying Tethyan sedimentary rocks.

National Seismological Centre• The centre was established in first in 1978 with a

vertical component seismometer at the periphery of Kathmandu valley.

• By 1999 the total number of vertical component short period seismic stations increased to 21, Plus two strong motion seismograph in the Kathmandu valley.

• Among them signals from 9 seismic station are recorded in digital form at autonomous Regional Seismological Center, Birendranagar.

• This year one BB STS2 is installed in RSC and one Long Period seismometer is installed in NSC.

National Seismic Network

Seismic Vault

Seismic Station

Seismic Data Acquisition System in Surkhet

Record of Local Earthquake

Seismicity• West Nepal seems more active than the east.• The major cause of seismicity in the Himalaya

is continuous collision between Indian and Eurassian plates.

• The seismicity in Nepal Himalaya is concentrated E-W along the narrow belt between MBT and MCT.

• The depth of most of the earthquake lies between 10-20 km.

Epicenter Map of Nepal Himalaya (1994-2005)

• In the last 100 years 4 major earthquakes accommodate the slip of Indian plate towards Tibet , but in the area between 1905 and 1934 earthquake there is no large earthquake since1255 earthquake(??). So this area of seismic gap is seems to be most vulnerable area for the large future earthquake.

Historical Earthquakes of Nepal

Date Magnitude (Intensity, MMI)

Death Houses destroyed

1255 (IX-X) King was also killed 1/3 of kingdom of Nepal Perished

1408 Innumerable people killed

All big temples were collapsed

1833 7.8 (VIII-IX) 500

1934 8.2 (IX-X) 10500 80000

1980 6.5 (VIII- 178 40000

1988 6.7 ( VIII-IX) 721 100000

Geology of Nepal Himalaya

• Building damaged in 1988 Udayapur earthquake.