Risk Reduction

Seismic risk and earthquake probability

There are a number of strategies which can be used to reduce earthquake risk. Mostly collapsing buildings kill people, not the earthquakes themselves. The death toll in two earthquakes in 1988-89, (Armenia and San Francisco), contrasted principally as a result of the differing building practices in the two regions. In Armenia, of the 25,000 people killed, 6,000 were students and teachers who died when their schools collapsed. San Francisco has strict earthquake-proofing building codes which are enforced. Although there were 60 deaths, these occurred when a double-decked bridge collapsed, crushing people in their vehicles on the lower deck (UNESCO 1991) and not through collapsing buildings. Reducing earthquake risk therefore involves both estimating the probability associated with the likelihood of earthquakes of a certain size occurring at the location of interest and developing codes for engineering practice when constructing buildings on areas of appreciable risk, based on estimates of ground acceleration.

The United Nations declared the decade 1990 to 2000 to be an International Decade for Natural Disaster Reduction (IDNDR). Programmes operating in countries involved in IDNDR included the following elements:
(Hays et al. 1991)

• hazard and risk assessment

• prediction and warning of events

• mitigation

• preparedness for emergency response

• preparedness for recovery and reconstruction

• awareness and education

• learning from disasters

• international collaboration

Geologists and seismologists contribute to preparing hazard and risk maps, and translate this information for use by land use planners, emergency managers, architects and engineers for coordinated planning, design and construction practices. Hazard maps are prepared at regional (1:100,000, 1:250,000) and urban scales (1:5,000, 1:25,000).

Regional scale hazard assessment establishes the physical parameters of the region that are required for determining the hazards associated with earthquakes, e.g. ground-shaking, ground failure, surface-fault rupture and tsunami run up. The following tasks are all important elements of regional earthquake hazard assessment:

• Catalogue the prehistoric and historic earthquake activity in the region, (neotectonics) including case histories which can be used to determine earthquake-recurrence intervals over thousands of years.

• Prepare a seismotectonic map, showing the location of active faults and seismogenic features, together with their correlation with known seismicity.

• Prepare a map showing the seismogenic zones, with magnitudes of the maximum earthquake and earthquake frequency for each zone.

• Prepare a map of tsunami zones and their correlation with historic (and, if possible, prehistoric) tsunami.

• Prepare maps indicating slope-stability and potential for ground liquefaction.

• Specify regional spreading and absorption of the earthquake shock waves, which will depend on regional geology, and their uncertainties.

• Prepare probabilistic ground-shaking-hazard maps which indicate the probability of movements based on bedrock peak acceleration and velocity measured during previous events.

Seismic hazard assessment on an urban scale (i.e. seismic zonation) combines the seismotectonic and physical data acquired for a regional study with more detailed site-specific data acquired in an urban area. The following tasks are elements of an urban study:

• Compile existing and new geologic, geophysical and geotechnical data to determine the physical properties and response to earthquake-triggered shaking of soil and bedrock.

• Prepare ground-shaking maps for soil and bedrock, showing the amplitude and frequency responses of the materials.

• Prepare maps showing the likelihood of earthquake-triggered surface-fault rupture and tsunami inundation.

• Prepare a map showing the potential for earthquake-triggered liquefaction.

• Prepare a map showing potential for earthquake-triggered landslides.

Regional and global programmes have been designed to provide opportunities for reducing earthquake risks, e.g. Cities At Risk (an IDNDR programme making cities safer ...before disaster strikes). The goal of these programmes is to make the urban centres of the world safer from natural hazards, through coordinated societal, scientific and technical actions including:

• Avoidance of natural hazard. Planners should avoid locating structures on soils or land fill with similar resonance to the structure itself; they should avoid constructing buildings so close to one another that they may touch as they sway during an earthquake; they should avoid constructing in areas susceptible to liquefaction and landsliding, or in locations liable to be inundated by tsunami, of by flooding from dam failure.

• Wise use of the land. Planners should expect earthquakes to occur in places where they have happened in the past, and any new construction in these areas should be designed accordingly, using the past activity as a guide to developing building codes etc. Planners should also expect that earthquake effects such as liquefaction, landsliding etc. will occur in susceptible foundation materials, and plan accordingly.

• Emergency preparedness and disaster recovery planning. Emergency managers should accept that earthquakes will strike without warning at the worst time possible. The initial ground shaking effects of an earthquake will be compounded by fires and flooding etc. which will all serve to complicate the emergency response. The oldest and most densely populated areas of the city will be the worst affected and movement to and from damaged areas will be hampered due to damaged transport systems and other lifelines. Communications may be disrupted for hours to weeks. Emergency responses will be complicated too by the varied emotional and social impacts of the earthquake.

Disaster-recovery should be based on the following assumptions (1) political pressure to restore services will be high, (2) damage assessment will be a high priority, but there may be a short fall in people qualified to make these judgements, (3) designating unsafe versus safe areas will be complicated by emotional or political pressures, (4) restoring communications and returning the city to order will be complicated while search and rescue operations are ongoing and (5) rebuilding to meet new earthquake building codes will be problematical, and improved building codes should be put in place in advance.

• Reduction of vulnerability. This element is particularly important for those countries where the capital city (including financial, cultural and administrative centres etc.) is situated in an active earthquake zone, e.g. many Mediterranean countries, Japan and New Zealand. It is very important that, in order to reduce the vulnerability of the whole country (including areas that might otherwise be unaffected by the earthquake), these institutions be protected by reinforcing important buildings and structures. On a smaller scale, the vulnerability of people living close to structures which, if damaged, may be hazardous to their health (e.g. nuclear power plants, chemical plants etc.), must be respected by reinforcing these structures.

• Adoption and enforcement of building and zoning regulations

• Coordinated planning, design and construction practices

• Modification of the characteristics of ground shaking and ground failure

• Prediction and warning.