Earthquake Hazard Mitigation

Earthquakes are unlike most other natural disasters in one crucial way: they strike without warning, and they destroy the very infrastructure — roads, bridges, communication lines — that would be needed to deliver relief. Because it is physically impossible to prevent an earthquake from occurring, the only sensible strategy is to shift focus from cure to preparedness and mitigation. The book lays out four concrete measures in this direction.

First, establish earthquake monitoring centres, also called seismological centres, for continuous observation and rapid dissemination of information to people living in vulnerable areas. The use of the Global Positioning System (GPS) is highlighted as a particularly valuable tool here, since it can track the movement of tectonic plates and thereby give some advance sense of accumulating strain.

Second, prepare a vulnerability map of the country. This map must be paired with active dissemination of risk information among the people, along with education about the ways and means of minimising the adverse impacts of disasters. In other words, awareness is treated as a mitigation tool in its own right.

Third, modify house types and building designs in vulnerable areas, and actively discourage the construction of high-rise buildings, large industrial establishments, and big urban centres in such zones. This is a planning-level intervention that reduces the exposure of people and property to seismic risk.

Fourth, make it mandatory to adopt earthquake-resistant designs and to use light materials in all major construction activities in vulnerable areas. This is the regulatory backbone of seismic safety.

Beyond these human-centred measures, the book stresses that earthquakes have serious and far-reaching environmental consequences. Surface seismic waves produce a range of effects that the book summarises in a table, organised by what is affected — the ground, manmade structures, and water.

The most dramatic of these environmental consequences is the tsunami, which the book treats as a distinct hazard arising from earthquakes and volcanic eruptions. When the sea-floor moves abruptly, it displaces ocean water suddenly, producing high vertical waves called tsunamis, or harbour waves, also known as seismic sea waves. Normally, the seismic disturbance causes only one instantaneous vertical wave, but after that initial disturbance a series of afterwaves is created in the water, oscillating between high crest and low trough as the water level tries to restore itself.

The behaviour of a tsunami depends fundamentally on water depth. The speed of the wave in the ocean depends on the depth of the water — it is greater in shallow water than in the deep ocean. This has a counter-intuitive consequence: the impact of a tsunami is less over the open ocean and far more severe near the coast, where it causes large-scale devastation. A ship at sea is barely affected by a tsunami, and detecting a tsunami in the deeper parts of the sea is difficult. This is because over deep water the tsunami has a very long wavelength and a limited wave height — it raises a ship only a metre or two, with each rise and fall taking several minutes. But when a tsunami enters shallow water, its wavelength gets reduced while its period remains unchanged, and this increases the wave height. Sometimes this height can reach 15 metres or more, causing large-scale destruction along the shores. For this reason, tsunamis are also called shallow water waves. …