Impact of Earthquakes

Damage from the 19 February 1990 Weber earthquake. This building in the main street of Dannevirke was condemned, and new retail premises were built. V.E. Neall

The impact of a large earthquake on land and buildings is usually very obvious. There are consistent relationships between the amount of damage and the properties of both buildings and the ground on which they are sited.

During an earthquake the ground surface moves in all directions. The most damaging effects on buildings are usually the horizontal movements because most structures are designed to withstand vertical gravity loads.

Ambrose and Vergun (1990) identify the general effects of earthquakes as:

1. Direct movement of structures. The structure is attached to the ground; when the ground moves, so does the structure. This destabilises the structure by shaking.

2. Ground surface faults. When the fault movement propagates to the surface during an earthquake, cracking and slumping occurs, which can damage roads and railways, fence lines and structures. If the ground surface settles during an earthquake, it may become flooded. The earthquake may trigger landslides. In sandy soils the ground may flow like a liquid (liquefaction) and buildings may sink below ground level.

3. Tsunami. Ground movements caused by earthquakes can set up large waves on overlying bodies of water. These waves may travel hundreds of kilometres, at hundreds of kilometres an hour and may reach heights up to 40 m.

4. Earthquakes can damage dams, trigger small scale tsunamis in reservoirs and may sever pipelines, all of which will cause flooding, fire, gas explosions etc.

Landsliding created by 19 February 1990 Weber earthquake, damming a stream and breaking a hillside with future failure planes. Photo from Dannevirke-Weber Road. V.E. Neall

Kiwifruit orchard that subsided 1.5 m, near Thornton, due to faulting during the 1987 Edgecumbe earthquake. This lowered part of the orchard below the water table and drowned the kiwifruit. V.E. Neall

Structures will be shaken in all directions during an earthquake. The force effect caused by this motion is generally directly proportional to the dead weight of the structure, which also bears on the dynamic response of the structure. Two other factors that will influence the structure's response are:

1. The structure's resonance (determined by the mass, stiffness and size of the structure; Fig. 14).

Figure 14

Figure 15.

2. The structures efficiency in energy absorption (determined by the elasticity of the structure, stiffness of supports, number of separate moving parts etc.).

The relationship between the resonance of the earthquake and the structure is a major concern. This relationship is shown as spectrum curves (Fig. 14). Earthquakes have their major direct force effect on structures with short resonance (e.g. small or squat buildings, structures with stiff lateral resistive systems). Resonance of tall towers and multi-storey buildings may be so long that different parts of the building move in opposite directions simultaneously (Fig. 15)

All structures have a natural resonance and if a building has a frequency similar to that of the seismic waves then the movement of the building is amplified by resonance and severe damage may result. Different parts of a single building may have different frequency responses, leading to differential damage, e.g. collapse of a tower while a low storey part of the same building is undamaged.

Figure 16.

Earthquakes behave differently depending on the type of bedrock or sediments they pass through. Figure 16 shows recording of horizontal ground shaking for a variety of ground conditions. There is a general relationship between earthquake intensity and acceleration for different bedrock conditions. Significantly, these bedrock conditions represent the foundation conditions for structures. Foundation material comprising soil or landfill is likely to produce 3 times the ground acceleration than firm bedrock (Fig. 17; Rahn 1986). The 1775 Lisbon, Portugal earthquake destroyed the half of the city which was underlain by soft Tertiary strata.

Figure 17.

Weak earthquake tremors can be amplified as they pass through water-logged muds, or clays which contain large amounts of water in their mineral structure. An earthquake in Mexico on 19-20 September 1985 (epicentre offshore, 400 km southwest of Mexico City), killed nearly 20,000 people in Mexico City. This city is built on montmorillonitic clays, which can increase their water content by 350 - 400 %. These clays can behave as if they are fluids. Near to the epicentre, which was sparsely populated, damage was minimal. Major populations centres close to the epicentre were badly affected, but damage decreased sharply away from the epicentre.

At Mexico City the peak acceleration of the earthquake corresponded to an estimated return period of 50 years. However, as the earthquake shock wave passed thorough the clays beneath the city they were amplified 6 times. Surface waves, just like ocean waves, crossed back and forth across the basin for nearly three minutes. The wavelengths of these waves (50 m) closely matched the dimensions of the bases of many of the city's buildings. Peak accelerations for the surface seismic waves matched those for the buildings which resonated, amplifying the shaking of the buildings sufficiently to cause them to collapse.

Liquefaction

Liquefaction occurs when shear waves distort the structure of soils, causing a soil to suddenly collapse. Soil comprises grains which are in contact with one another. Below the water table the spaces (voids) between these grains are filled with water. As long as the grains stay in contact the soil will retain its strength. However, if pressure exceeding the weight of the soil is put upon the water in the voids the soil dilates, and the soil turns into a slurry which behaves like a fluid and has virtually no bearing strength. This process can happen very quickly and structures built on top of soil which behaves in this way will sink into the slurry.

Liquefaction can be triggered by earthquakes. Shear waves disrupt and reorganise the soil particles which decreases void space and thus increases pore pressure with pressure increasing with each passing shock. The size of the grains in the soil is an important factor. Fine-grained soils, with a large proportion of silt and clay, are less likely to liquefy because the capillary water tension between gains is high. Fine- to medium-grained sands do not allow rapid draining of pore water but have void spaces large enough to make capillary cohesion irrelevant. These sand sizes are common in recently deposited marine and river sediments, and so liquefaction commonly occurs in these materials during earthquakes (Bryant 1991, 203-206).

Liquefaction is represented by lateral spreads (movement of blocks of soil over a liquefied layer beneath the surface), flow failures (fluidised soils moving down slope as a slurry) and loss of bearing strength. During the 1906 San Francisco earthquake, liquefaction spreads in estuarine muds and landfill near the harbour broke water mains, hampering fire fighting. Liquefaction was also a problem during the 27 March 1964 Alaskan earthquake where nearly every bridge in the affected area was destroyed by lateral spreads. On 16 June, 1964 a major earthquake struck 60 km north of Niigata city, Japan. The earthquake produced maximum ground accelerations of 1.58 cm s^-2 (i.e. about MM II, not excessive). The earthquake, however, liquefied river sediments on Shinano River, across which the city had expanded, and large apartment blocks sank into the liquefied sediments.

Although liquefaction generally occurs in water-saturated soils, it can also involve air. During the 16 December 1920 Kansu earthquake, 200,000 people were buried by landslides triggered by liquefaction of fine-grained loess. The shear strength of the loess was exceeded by the earthquake effects but low permeability of the fine-grained soil prevented air from escaping from the loess, which then liquefied.

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Tsunami

A tsunami is a series of large ocean waves generated by impulses from geophysical events, such as earthquakes, landslides and volcanic eruptions, on the ocean bottom or along the coastline. The word Tsunami was coined by the Japanese and translates literally to "great wave in harbour".

The most common event that generates tsunamis is submarine faulting which causes part of the ocean floor to be vertically displaced. Only a small number of submarine earthquakes result in tsunamis. Most have a small spatial effect or are too low in magnitude to cause vertical displacement of the ocean floor. Generally, only those earthquakes with a Richter magnitude > 6.6, and shallow focal depths (< 50 km) are likely to be accompanied by tsunamis. The abrupt submarine faulting and vertical displacement of the sea floor causes rapid upward movement of the overlying water column, forcing it into a wave-like turbulence, which is expressed as a series of seismic sea waves. In the ocean these are barely detectable and difficult to tell from normal swells. They have low wave heights (~ 1 m) and long wavelengths (100 km) and travel at speeds up to 1000 km/hr. A tsunami may comprise numerous waves, which may arrive at the shoreline minutes or hours apart.

The April 1946 Alaskan earthquake triggered a tsunami which damaged coastal areas in the Aleutian Islands, and also crossed the Pacific Ocean to strike Hilo Bay in Hawaii. A 30-m high wave, travelling at approximately 780 km/hr, killed five people. At Hilo Bay tsunami waves crested more than 19 m above normal water level, and waves swept more than 600 m inland. Some 500 homes and businesses were destroyed and thousands damaged; 96 people were killed. Hawaii was also affected by tsunamis following the Richter magnitude 8.6, 22 May 1960 Chilean earthquake, which generated a Pacific-wide tsunami.

Tsunamis and their effects are fully explored in another module.

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