Earthquake terminology and seismic construction

Earthquake terminology

        Epicentre (E)
 ground ------*--------------------
              |  \  seismic waves
              |   \
              |    \
  depth h     |     \
              * Focus (F)

• Focus (hypocentre): the point inside the Earth where the rock first ruptures and energy is released.
• Epicentre: the point on the surface vertically above the focus.
• Focal depth: vertical distance from focus to epicentre. Shallow (0-70 km), intermediate (70-300 km), deep (300-700 km).
• Epicentral distance: distance from the observation station to the epicentre.
• Magnitude: measure of energy released (Richter scale), a single value.
• Intensity: measure of effects and damage at a place (Modified Mercalli scale, I to XII).
• Isoseismal lines: lines joining points of equal intensity.
• Seismograph / seismogram: instrument and record of ground motion.
• Foreshocks and aftershocks: smaller shocks before and after the main shock.
• Seismic waves: P, S, Love and Rayleigh waves.

Points to consider for construction in seismic areas

1. Avoid sites on active faults, soft or loose saturated soils (liquefaction), steep unstable slopes and landslide areas; prefer hard rock or firm soil.
2. Follow the earthquake code (Nepal National Building Code NBC 105 and IS 1893), using the seismic zone factor.
3. Use simple, symmetrical plans and regular elevations, and avoid large overhangs and soft storeys.
4. Provide ductile detailing, tie beams, bands (plinth, lintel, roof) and vertical reinforcement in masonry.
5. Make the foundation uniform and at one level; use a raft or tie beams, and avoid mixed foundation types.
6. Use light roofing and light materials; avoid heavy parapets.
7. Provide adequate separation between adjacent buildings to prevent pounding.
8. Use good quality materials and workmanship, proper mortar and anchorage.
9. For dams and bridges, use flexible joints, adequate freeboard and proper shear keys.
10. Use base isolation or dampers for important structures.