--- title: "Earthquake occurrence, terminology, causes, effects" subject: "Engineering Geology I" chapter: 2 key: "semester_3/engineering_geology_i:earthquake-occurrence-terminology-causes-effects" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/earthquake-occurrence-terminology-causes-effects site: BCT Engineering --- # Earthquake occurrence, terminology, causes, effects Engineering Geology I · Chapter 2: Basic Reviews of the Earth - Asked in: 2063 Baisakh (old course), 2+3+3 marks - Video: [Earthquakes 101 | National Geographic](https://www.youtube.com/watch?v=_r_nFT2m-Vg) — National Geographic - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/earthquake-occurrence-terminology-causes-effects - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-2 ## Question How does an earthquake occur? Define the different terminologies of earthquake. Describe the causes and effects of earthquake. ## Short answer Earthquakes occur by elastic rebound: accumulated strain energy releases when rocks suddenly slip along a fault. Key terms are focus, epicentre, magnitude, intensity, isoseismal lines, foreshocks/aftershocks. Main causes are tectonic movement, volcanic activity, and human actions; effects include ground shaking, structural collapse, landslides, liquefaction, tsunamis, and fires. ## Answer #### How an earthquake occurs Plate motion slowly loads rocks along a fault. Friction holds the fault, so strain energy is stored in the rock. When the stress exceeds the strength, the rock suddenly breaks and slips (elastic rebound theory), releasing energy as seismic waves that shake the ground. #### Terminology - **Focus (hypocentre)**: point of rupture inside the Earth. - **Epicentre**: point on the surface directly above the focus. - **Focal depth**: distance from focus to epicentre. - **Epicentral distance**: distance from a station to the epicentre. - **Magnitude**: energy released (Richter scale). - **Intensity**: damage and effects at a place (Mercalli scale). - **Isoseismal line**: line joining points of equal intensity. - **Foreshock / aftershock**: smaller shocks before / after the main shock. - **Seismograph**: instrument recording ground motion. ```text Epicentre * | | * Focus ``` #### Causes 1. **Tectonic**: movement of plates and slip on faults (most earthquakes, including Himalayan ones). 2. **Volcanic**: movement of magma and gases. 3. **Isostatic adjustment** and landslides, rockfalls and cave collapse. 4. **Man-made**: reservoir impoundment, deep mining, underground explosions, deep well injection. #### Effects - Ground shaking, cracks and surface rupture. - Collapse of buildings, bridges and dams, with loss of life. - Landslides, rockfall and avalanches in hills. - Liquefaction of saturated sandy soil and ground settlement. - Tsunamis along coasts and seiches in lakes. - Fire, floods from dam failure, and disease. - Change in river courses and groundwater level, and economic loss. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does an earthquake occur according to elastic rebound theory? A. Magma continuously pushes the crust until it cracks B. Seismic waves first form and then cause the fault to move C. Rocks expand when heated and burst D. Plate motion loads a locked fault, strain builds, and the rock suddenly slips when stress exceeds strength Answer: D. Stored strain is released by sudden slip, producing seismic waves. (Key point: Occurrence) 2. Which cause accounts for most earthquakes, including Himalayan ones? A. Volcanic eruptions B. Tectonic: movement of plates and slip on faults C. Reservoir impoundment D. Underground explosions Answer: B. Tectonic activity causes most earthquakes; the others are less frequent. (Key point: Most common cause) 3. Which is a man-made cause of earthquakes? A. Reservoir impoundment or deep well injection B. Plate collision C. Isostatic adjustment D. Magma movement Answer: A. Reservoir loading, deep mining, explosions and injection can trigger earthquakes. (Key point: Man-made causes) 4. What is the effect of liquefaction during an earthquake? A. Dry rock melts into magma B. Seawater floods the coast as a tsunami C. Saturated sandy soil loses strength, causing settlement and ground failure D. Clay swells and gains strength Answer: C. Shaking raises pore pressure in saturated sand, so it behaves like a liquid. 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