Seismic Threats in India: Probability Estimates for the Next 100 Years

Title:

Varying Rate of Occurrence of Earthquakes in India and Its Implication on Probabilities of Occurrence

Authors and Publication Details:

Authors: Priyanka Sharma, Roopesh Kumar, U.C. Naithani Published in: Journal of Mountain Research, Vol. 18(1), 2023, Pages 127-139 DOI: 10.51220/jmr.v18i1.13

Objective and Background

The study investigates India's seismicity trends and evaluates the probability of earthquake occurrences across different seismogenic zones using return period estimations. The research analyzes earthquake data from historical records up to 2020 and employs the Gutenberg-Richter (GR) relationship to estimate earthquake recurrence intervals. The primary goal is to assess the likelihood of future earthquakes in various regions of India, which is crucial for seismic hazard assessment and earthquake engineering.

Introduction

India is highly vulnerable to earthquakes, with 59% of its landmass categorized under moderate to high seismic risk as per IS 1893:2002. Over the past 15 years, India has experienced over 10 major earthquakes, resulting in 20,000+ casualties and extensive economic losses.

Historically, India has faced several catastrophic earthquakes, particularly in the Himalayan region, including:

• 1897 Shillong (M8.7)
• 1905 Kangra (M8.0)
• 1934 Bihar-Nepal (M8.3)
• 1950 Assam-Tibet (M8.6)

While inter-plate boundary regions (e.g., Himalayas, Andaman-Nicobar Islands) are well-documented for seismic activity, intra-plate regions in peninsular India have also experienced destructive earthquakes, prompting updates in India's seismic zoning map.

This study seeks to:

1. Analyze seismic trends across India’s 24 seismogenic zones.
2. Estimate earthquake return periods for different magnitudes using the Gutenberg-Richter (GR) model.
3. Compute probability estimates for earthquakes occurring in the next 100 to 10,000 years using a Poisson model.

Methodology

The study employs seismic hazard assessment techniques based on the Gutenberg-Richter (GR) relationship, defined as:

log10[N(>M)]=a?bM\log_{10}[N(>M)] = a - bMlog10[N(>M)]=a?bM

Where:

• N(>M) = Number of earthquakes above magnitude M
• a = Seismic activity parameter
• b = Tendency for small vs. large earthquakes

Key Steps in Analysis:

1. Data Collection: Historical earthquake records up to 2020. Data sources include India Meteorological Department (IMD), USGS, ISC, and other global seismological agencies.
2. Division into 24 Seismogenic Zones: Zones categorized based on tectonic characteristics (Himalayan region, Peninsular India, Andaman-Nicobar Islands).
3. GR Parameter Estimation: a-values (1.47 - 7.05) and b-values (0.54 - 1.17) calculated for each zone. Higher b-values indicate a larger frequency of small earthquakes, while lower values suggest higher probabilities for large earthquakes.
4. Return Period Computation: Estimation of recurrence intervals for M6, M7, and M8 earthquakes. Zones 17 and 11 have the shortest return periods, implying high seismic activity. Zones 6 and 12 exhibit the longest return periods for M8, indicating lower probabilities for high-magnitude events.
5. Probability Estimations using Poisson Model: Earthquake occurrence probabilities computed for 100, 225, 475, 2475, 5000, and 10,000-year return periods. Poissonian probability model assumes earthquakes occur randomly over time.

Key Findings

1. Seismic Activity Variation: Zones 17 and 11 (northeastern India) show shortest return periods and highest seismic activity. Zones 6 and 12 (peninsular India) show longest return periods for major earthquakes.
2. Return Periods for Major Earthquakes: M6 earthquakes: Return period ranges from 1 year (Zone 24) to 150 years (Zone 17). M7 earthquakes: Return period ranges from 14 years (Zone 24) to 1024 years (Zone 11). M8 earthquakes: Return period varies from 144 years (Zone 19) to 15,489 years (Zone 7).
3. Probability of Earthquake Occurrence: Zone 24 has the highest probability of M6+ earthquakes occurring within 100 years. Zone 17 has the lowest probability for large earthquakes within the next 100 years.
4. Implications for Seismic Hazard Assessment: Zones with short return periods require enhanced earthquake preparedness. Himalayan belt and northeast India remain the most seismically active regions.

Conclusion

The study provides a comprehensive earthquake recurrence analysis across 24 seismogenic zones in India. The findings highlight regional variations in seismic activity and provide probability estimates for future earthquakes.

Key insights include:

1. High seismicity in the Himalayan and northeastern regions.
2. Long return periods for M8+ earthquakes in the Deccan region.
3. Need for updated seismic hazard maps for improved earthquake resilience.

The results are valuable for engineers, policymakers, and disaster management authorities to refine earthquake preparedness strategies.

Future Work and Applications

1. Refinement of GR parameters using advanced statistical models.
2. Integration of machine learning techniques for seismic hazard predictions.
3. Validation of findings using real-time seismograph networks.
4. Improvement of seismic design codes in India based on return period data.
5. Expansion of research to include ground motion modeling for earthquake impact assessments.

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