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20. Causes of Earthquake

Earthquakes result from various geophysical, geological, and anthropogenic causes, impacting civil engineering designs for resilient structures. They can be classified into different types such as tectonic, volcanic, and induced seismicity, each with unique characteristics. The chapter describes the mechanisms behind earthquakes, their classification, and predicting factors, emphasizing the importance of understanding these aspects for infrastructure resilience.

Sections

Causes of Earthquake

This section explores the diverse causes of earthquakes, including tectonic, volcanic, and human-induced factors.

20 Section Overview

Start current section content and materials

20.1 Classification of Earthquakes Based on Causes

Earthquakes can be classified based on their causes, including tectonic, volcanic, and anthropogenic factors.

20.2 Tectonic Earthquakes

Tectonic earthquakes are the most common and destructive type of earthquake caused by the movement of the Earth's lithospheric plates.

20.2.1 Plate Tectonics Theory

Plate tectonics theory explains how the movement of Earth's lithospheric plates causes tectonic earthquakes, due to their interaction at boundaries.

20.2.2 Types of Plate Boundaries

This section discusses different types of tectonic plate boundaries and their associated geological features.

20.2.3 Faulting and Elastic Rebound Theory

Faults are fractures in the Earth's crust where rocks move, and the Elastic Rebound Theory describes how accumulated stress leads to earthquakes.

20.3 Volcanic Earthquakes

Volcanic earthquakes are linked to volcanic activity, arising from magma movement and pressure changes within the Earth's crust.

20.4 Collapse Earthquakes

Collapse earthquakes are minor seismic events that occur due to the failure of underground structures such as mines or caves.

20.5 Explosion-induced Earthquakes

Explosion-induced earthquakes are man-made seismic events caused by activities such as nuclear detonations and large-scale chemical explosions.

20.6 Reservoir-Induced Seismicity (RIS)

Reservoir-Induced Seismicity (RIS) refers to earthquakes triggered by the filling of large reservoirs behind dams, increasing stress on underlying faults.

20.6.1 Concept of RIS

Reservoir-induced seismicity (RIS) refers to earthquakes triggered by the filling of large reservoirs behind dams, due to increased stress on underlying faults.

20.6.2 Mechanism

Reservoir-induced seismicity occurs when the filling of large reservoirs increases stress on geological faults, potentially triggering earthquakes.

20.6.3 Notable Examples

This section highlights notable examples of reservoir-induced seismicity, focusing on significant events and their impacts.

20.7 Isostatic Adjustment Earthquakes

Isostatic adjustment earthquakes occur as the Earth's crust adjusts due to the addition or removal of surface loads, especially in post-glacial regions.

20.8 Induced Seismicity Due to Human Activities

This section explains how human activities, particularly industrial practices, contribute to induced seismicity through mechanisms such as deep well injection, hydraulic fracturing, and mining.

20.8.1 Deep Well Injection

Deep well injection involves the disposal of industrial fluids and wastewater into deep wells, which can alter subsurface stress and lead to seismic activity.

20.8.2 Hydraulic Fracturing (Fracking)

Hydraulic fracturing, commonly known as fracking, involves the high-pressure injection of fluid to fracture shale rocks, which can inadvertently trigger seismic events.

20.8.3 Mining and Quarrying

Mining and quarrying activities can cause small earthquakes through blasting operations and excavation, significantly altering the stress in the Earth's crust.

20.8.4 Geothermal and Oil Extraction

This section discusses how geothermal and oil extraction activities can lead to induced seismicity by altering subsurface pressure and causing ground subsidence.

20.9 Seismic Gaps and Earthquake Prediction

Seismic gaps are segments of active faults that have not experienced slip in a significant time, indicating potential earthquake hotspots.

20.9.1 Seismic Gaps

Seismic gaps are segments of active faults that have not experienced significant earthquakes for an unusually long time, presenting a potential risk for future seismic activity.

20.9.2 Earthquake Precursors

This section discusses various precursors that may indicate an impending earthquake, including foreshocks and geological changes.

20.10 Role of Geological Structures

This section emphasizes the critical role of geological structures, such as faults and folds, in influencing seismic activity and energy release during earthquakes.

20.10.1 Faults

Faults are fractures in the Earth's crust where rocks move relative to each other under various stress conditions.

20.10.2 Folds, Joints, and Rock Strength

This section discusses how geological structures, such as folds and joints, and the strength of rocks influence the storage and release of seismic energy.

20.11 Earthquake Belts and Zones

The section discusses significant earthquake belts and zones around the world, highlighting their causes and characteristics.

20.11.1 Circum-Pacific Belt (Ring of Fire)

The Circum-Pacific Belt, also known as the Ring of Fire, is the most seismically active region on Earth, encompassing various countries and geological features.

20.11.2 Alpine-Himalayan Belt

The Alpine-Himalayan Belt is a seismically active region formed by the collision of the Indian and Eurasian tectonic plates, resulting in significant earthquakes.

20.11.3 Mid-Oceanic Ridges

Mid-oceanic ridges are divergent boundaries where tectonic plates move apart, resulting in seismic activity due to the creation of new oceanic crust.

20.11.4 Intraplate Earthquakes

Intraplate earthquakes occur within tectonic plates rather than at their boundaries, typically resulting from stress accumulation.

20.12 Earthquake Magnitude and Energy Release

This section discusses the different magnitude scales used to measure earthquakes and the relationship between magnitude and the energy released during seismic events.

20.12.1 Magnitude Scales

Magnitude scales measure the energy released by earthquakes, providing standardized quantification of seismic activity.

20.12.2 Energy Released by Earthquakes

This section discusses the calculation of energy released by earthquakes using the Richter scale.

20.13 Seismic Waves and Their Propagation

This section covers the types of seismic waves produced during earthquakes and how these waves propagate through different materials.

20.13.1 Types of Seismic Waves

Seismic waves are classified into body waves and surface waves, each with distinct characteristics and modes of propagation.

20.13.2 Wave Propagation and Attenuation

This section explains how seismic waves propagate through different media and the factors that lead to energy attenuation.

20.14 Depth of Focus

Depth of focus refers to the varying depths at which earthquakes occur, significantly impacting their destructiveness and surface shaking.

20.14.1 Classification Based on Focal Depth

This section classifies earthquakes based on their focal depth into three categories: shallow, intermediate, and deep focus.

20.15 Microseismicity and Background Seismic Noise

Microseismicity involves small earthquakes that are not typically felt but play a crucial role in monitoring seismic activity, while background seismic noise consists of continuous vibrations from various natural and human sources.

20.16 Paleoseismology and Historical Seismicity

This section introduces paleoseismology, the study of prehistoric earthquakes, and discusses the importance of historical records in understanding seismic activity.

20.16.1 Paleoseismology

Paleoseismology is the study of prehistoric earthquakes through geological analysis, aiding in understanding the frequency and impact of past seismic events.

20.16.2 Historical Records

Historical records provide evidence of past earthquakes, aiding in the understanding of seismic activity in regions with limited modern data.

20.17 Triggering Mechanisms of Earthquakes

This section examines the various natural and anthropogenic mechanisms that can trigger earthquakes.

20.17.1 Natural Triggers

Natural triggers of earthquakes include processes like stress transfer from nearby seismic events, landslides, and even solar tidal forces.

20.17.2 Anthropogenic Triggers

This section explores human-induced factors that can trigger earthquakes, highlighting various activities such as reservoir filling and nuclear testing.

20.18 Earthquake Clustering and Swarm Activity

This section discusses the phenomena of earthquake clustering and swarm activity, exploring concepts like aftershocks and foreshocks.

20.18.1 Aftershocks

Aftershocks are smaller earthquakes that follow a major seismic event, often occurring over an extended period.

20.18.2 Foreshocks

Foreshocks are smaller seismic events that occur prior to a larger earthquake, known as the mainshock.

20.18.3 Earthquake Swarms

Earthquake swarms are clusters of small-to-moderate earthquakes occurring in a short period, typically without a significant mainshock.

20.19 Earthquake Cycle and Recurrence

The earthquake cycle describes the phases of stress accumulation and release along faults, while the recurrence interval indicates the average time between significant earthquakes.

20.19.1 Earthquake Cycle

The earthquake cycle describes the phases of stress accumulation, release, and reaccumulation along faults, which are critical for understanding seismic activity.

20.19.2 Recurrence Interval

The recurrence interval is the average time between significant earthquakes on a fault segment, vital for hazard assessments.

20.20 Influence of Geological and Geomorphological Features

Geological and geomorphological features significantly influence seismic activity by affecting ground shaking and behavior during earthquakes.

Learning Objectives

  • Earthquakes can be classified into tectonic, volcanic, collapse, and induced types based on their origin.

  • The movement of lithospheric plates causes tectonic earthquakes, which are the most destructive.

  • Earthquake prediction remains unreliable, but certain precursors like foreshocks and changes in groundwater levels can provide insights.

Key Concepts

Tectonic Earthquakes

Earthquakes caused by the movement of the Earth's lithospheric plates.

Elastic Rebound Theory

Describes how stress builds in rocks and is released as seismic energy during an earthquake.

Seismic Gaps

Segments of active faults that have not experienced a significant earthquake in a long time, indicating areas of potential future quakes.

Induced Seismicity

Earthquakes triggered by human activities, such as deep well injection or hydraulic fracturing.

Practice Exercises

Total Questions

2

Estimated Time

4 min

Passing Score

70%

Instructions

  • Read each question carefully
  • You can use hints if you need help
  • Complete all questions before submitting
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