190 REPORTS AND COMMENT

The Chile earthquake of - March 1985

Edmund Booth

Ove Arup & Partners 13 Fitzroy Street London W1P 6BQ, U.K.

I spent just over a week in the damaged area, six days after the earthquake took place. I was one of two members from the U.K.-based group EEFIT (Earthquake Engin- eering Field Investigation Team) which is studying the earthquake. Our interest was primarily to see how man- made structures had withstood the event, so that lessons could be learnt about the features which appeared to have been favourable, and those which were unsuccessful. Both EEFIT (Booth, 1985; Taylor and Booth, 1985) and a number of other international observers have either already published their findings on the seismological and engineer- ing aspects of the earthquake, or are about to do so (EERI, 1985, in press; Dowrick, 1985; Conner, 1985; EQE, 1985). In this article, therefore, I have only given a brief review of these features. My main concern has been to give a personal view of the human consequences of the event, and how the authorities and the people living in the affected areas reacted.

SEISMOLOGICAL ASPECTS INTRODUCTION

Chile’s position, crowded between the Pacific and the Andes, means that no part of it is more than 300 km from one of the most active earthquake belts in the world - the “subduction zone” where the Nazca plate pushes under- neath the South American plate. The results of this compressive movement can be seen in the buckling of the South American continent to form the Andes, and of course in the release of the resulting stresses by frequent minor earthquakes and regular major ones. Table 1 shows earthquakes during the past thirty years with a magnitude greater than 5.5 (approximately the threshold at which significant damage would be expected) within 200 km of Santiago. During this period, there was on average almost one such event per year and Chile as a whole has suffered three great earthquakes this century with magnitude exceeding 8. The situation in Chile is thus very different froni some other seismically active parts of the world. Parts of the eastern United States (for example) are also subject to very severe earthquakes, but only at a frequency of once in every three or more generations. Chileans, on the other hand, have constant reminders of the geological instability of their country, and this was almost certainly a major factor in the success of the Chileans in coping with their most recent major earthquake.

The 1985 event took place at 7.47 p.m. local time on Sunday 3rd March. It was centred in the Pacific some 25 km offshore, about 120 km due west of Santiago (see Fig. 1). Its magnitude (a logarithmic measure of the energy release) was 7.8, which makes it one of the larger events worldwide this century. Mercifully, less than 200 people were killed, though 2,000 were injured and an estimated 1,OOO,ooO people were made homeless. The damaged area was very large, extending as far south as Curico, a town 200 km from the epicentre, and Santiago, 120 km away, was also severely affected. The total cost of damage has been estimated at around U.S.$l billion.

Preliminary analysis of the records has indicated that the earthquake was a multiple event, with the major shock of surface wave magnitude Ms = 7.8, immediately preceded and followed by a number of other smaller events. The main

Table 1. Earthquakes since 1955 with magnitude greater than 5.5 within 200 km of Santiago

Date Magnitude @?b)

1955 1958 1965 1966 1967 1970 1971 1971 1971 1972 1973 1975 1978 1979 1979 1 980 1980 1981 1981 1983 1984 1984 1985

6.8 6.7 6.4 5.6 5.7 5.5 5.5 6.5 5.7 5.5 5.8 5.6 5.8 6.0 5.5 5.5 5.6 5.7 6.2 5.9 5.5 6.3 6.9

Source: ISC, Newbury.

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Epice (M,=

100 Km 200 KJn

Fig. 1. Map of epicentral region (preliminary and incomplete Modified Mercalli intensities are indicated in

brackets).

earthquake was a shallow one, centred 25 km offshore, and was generated by thrusting on a reverse fault running in a north - south direction. The magnitude of Ms 7.8 would be consistent with a fault-break length of some 170 km. The earthquake produced a high content of long period motions.

Two features make this a particularly interesting event to study. First, at least twenty high-quality strong ground motion recordings (traces of the ground accelerations in three dimensions produced by the earthquake) were obtained from sites well distributed over the affected area. Such recordings provide essential information for studying both the energy and frequency content of the earthquake, and also the attenuation of the ground motions with distance from the source. Records as comprehensive as these are rare for a major event outside California and Japan. Second, the epicentral area has a large number of modern engineered structures and a well developed infrastructure. Data on their performance in a major event are still needed. Moreover, engineered structures can give much better information on the spread of damage in the epicentral region, than can traditional housing, which may collapse even in relatively weak shocks.

Preliminary conclusions regarding the spread of damage, based both on the strong motion recordings and the observed distribution of damage, are that the maximum effects nearest the epicentre were somewhat lower than expected, but the damage spread much farther than would normally be expected for what appears to be a relatively shallow event. Figure 1 gives a preliminary (and incomplete) indication of the distribution of Modified Mercalli intensity (a measure of observed damage in man-made structures,

based on eye-witness reports rather than instruments). Based primarily on Californian and Japanese data, a maximum intensity on land of X or XI would not be unusual for such an event so close inshore. However, an intensity of only VI I I to 1X was actually experienced. Conversely, the intensity of VI to VII observed at Curico is unusually large at an epicentral distance of 200 km.

These are initial observations but if confirmed, may lead to a re-evaluation of the seismic hazard in regions where the circumstances of this event apply. Some interesting conclusions concerning “microzonation,” or the effect of very local conditions on seismic hazard, may also emerge. There was evidence of locally increased damage at the tops of hill ridges and also in alluvial basins. These effects have been observed before but are incompletely understood. Liquefaction is another local phenomenon, involving soil and foundation failures, on which more field data are needed. Liquefaction almost certainly played a part in the failure of a wharf at San Antonio (Fig. 2) and possibly also of a nearby bridge (Fig. 9) and was observed elsewhere, so knowledge of this effect should be improved.

ENGINEERING ASPECTS

Chile has a highly developed capability in civil engineering, and its earthquake design code, though perhaps fifteen years behind the latest Californian practice, is well founded. Overall, engineered structures rode the earthquake well though there were isolated and spectacular failures. Damage to unreinforced brick and block buildings was much more Comprehensive, though wooden housing

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Fig. 2. Failed wharf at San Antonio.

generally performed well. All the housing stock over fifteen years old had been through previous damaging earthquakes and damage from these events, sometimes inadequately repaired, had clearly weakened some structures.

In San Antonio, the major engineered building casualty was a recently completed five-storey concrete hospital (Fig. 31, which suffered extensive damage but no significant structural collapse. Many engineered buildings in San Antonio, however, suffered little or no structural damage. In Vina del Mar, just north of Valparaiso, two fifteen storey tower blocks dating from the 1960s were badly damaged

Fig. 3. Claudio Vicuna hospital, San Antonio.

(Fig. 4) though, again instances of damage to such structures were fairly isolated in both Vina and Valparaiso. In Santiago, there was one instance of total collapse (Fig. 5) and another large housing estate was badly damaged (Fig. 6). I n both cases, the structures dated from the 1960s. A new housing estate of three-storey, partially reinforced brick houses was also badly damaged (Fig. 7). Elsewhere in Santiago, non-structural damage was quite widespread in engineered buildings though serious failure was rare.

Damage to adobe (mud brick) houses was widespread over the entire epicentral region and many religious

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construction and inadequate repair of previous earthquake damage. This event is unlikely to alter radically our perception of how to design against earthquakes; it will however underscore and refine the lessons learnt from previous disasters.

THE HUMAN TOLL

The death toll - less than 200 killed at the time of the earthquake - was low for such a large event especially in the developing world. One reason may have been the timing -at 7.45 p.m. on a fine autumn Sunday, many people were outside, taking their traditional Latin promenade, and so were not trapped inside the many one- and two-storey masonry or adobe (mud brick) houses that collapsed. Another important reason must have been the fact that Chile expects and is prepared for earthquakes. The high level of engineering within the country meant, as reported above, that few major buildings or other structures collapsed, and services vital to health after the event -

Fig. 4. Failed shear wall in Hangarosh building in Vina del Mar.

buildings in unreinforced blockwork, including the fine Basilica del Salvador in Santiago, were severely affected. One and two-storey wooden houses with corrugated iron roofs, being light and well tied together, survived very well, though there were instances of rotton timber frames with heavy masonry infill leading to partial collapse.

Water and sewage pipe lines were major casualties in the epicentral regions, though the damage appears to have been in dd pipes without positive connections. Other “lifelines” - roads, electricity, telephone networks - generally fared well and were mainly functioning normally one week after the earthquake.

As in previous earthquakes, it appeared that well designed reinforced buildings stood an infinitely better chance of surviving than those not conforming to modern principles of earthquake resistant design. (Gere and Shah, 1984, outline these principles for the layman in a recent, excellent, publication.) Certain features characterized the engineered buildings that suffered: non-symmetrical dis- position of the earthquake resisting elements of the structure, poor detailing of the reinforcing steel, faults in Fig. 5. Villa Olympia housing estate, Santiago.

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water, sanitation, power, communications and hospitals - were generally quickly restored in the places where they were affected. Nevertheless, that the human toll was high is apparent from the figure of one million made homeless and the estimated total financial loss of $1 billion.

The poorest section of society tends to own single storey wooden houses, which generally escaped with remarkably little damage (Fig. 8). It was the unreinforced block and brick housing, owned by a slightly more prosperous section, that suffered most and although there were notable instances of failures in reinforced low cost housing, it was the failure of traditional housing that caused the majority of the homelessness. Olsen has written (EERI, 1985) that the final toll in human life and health is likely to be much greater than the initial figures suggest. The effect of the onset of winter rains and chill on a large population living in makeshift housing may have a devastating impact at a time when the initial crisis and consequent national and international concern and aid have faded away.

Fig. 7. Partially reinforced brickwork of three storey housing in Santiago.

CLEARANCE AND RECONSTRUCTION

Fig. 6. Villa Portales housing estate, Santiago.

Notwithstanding the longer-term effects of the earth- quake discussed in the previous paragraph, what happens in the immediate aftermath of an earthquake is crucial, and quick and effective actions are needed to save lives. We were in the country during the second week after the event and were most impressed by what we saw. Taylor (in press) has compared from personal knowledge the state of reconstruc- tion in Italy three months after a major earthquake in 1980 with that in Chile and found the Chilean response further advanced after two weeks. At an international level, aid came from South and North America; the U.K. contribution of E0.25 million, though a small proportion of the total, should nevertheless also be mentioned.

Response at a national government level was subject to all sorts of rumours, and we were not in a position to form a coherent picture. The only tangible results we could see were what the army was doing. Squads of soldiers clearing up rubble were a common sight, and the army was quick to reinstate a temporary pedestrian and vehicular crossing

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Fig. 8. Undamaged wooden house, San Antonio.

over the river Maipo, south of San Antonio, where a major bridge had been destroyed by the earthquake (Fig. 9). Thirteen other major road bridges had been made impassable by the earthquake, but temporary repairs had enabled traffic through in most places and road communications were generally quickly restored. The low water levels at river crossings at the end of the summer season may have helped in this respect.

We had more first hand knowledge of the response of local authorities, particularly in San Antonio, Valparaiso

and Vina del Mar. Both areas suffered many breaks in water and sewage lines and over 90% of the population in these towns were without a water supply immediately after the event. An apparently efficient system of distribution by water bowsers was rapidly instituted and we were also impressed by the speed with which permanent repairs were being carried out. Arrangements for sheltering the homeless either in public buildings such as schools or in tented camps also appeared to be working well. As an example of the degree of control, we were particularly impressed by the

Fig. 9. Bridge over River Maipo, south of San Antonio.

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English orderliness of the queue we saw of perhaps 100 people waiting for their food rations.

It must be emphasized that these are only impressions gained from a rather hectic week in country and so their validity may be questioned. Yet the overwhelming impression gained was that morale in the general population was high, even in the worst affected areas. There was no sign of apathy at this stage; on the contrary, everywhere we went, we saw people getting on with the jobs of clearance and reconstruction of their properties. The Chileans are clearly a warm and friendly people, but the fact that they were prepared to communicate with foreigners wandering through devastated back streets taking photo- graphs - and that they even offered invitations to inspect their ruined homes - speaks a lot for the Chilean resilience to earthquakes, as well as their more human virtues.

THE LONG TERM CONSEQUENCES

As has already been pointed out, the effects of the earthquake may begin to bite more as the winter draws in and initial reserves of energy and cash are exhausted. It will be interesting to observe how the authorities set about long term reconstruction - how socially equitable it is perceived as being, whether the relative success of timber houses in resisting the earthquake will cause a change in the type of construction, whether the failures in low cost reinforced housing will lead to changes in codes of practice and increases in the amount that is spent on seismic resistance. It is certain that Chile has not suffered her last major earthquake and there are reasons for supposing that the next one could well be an even larger event. The lessons from this recent earthquake, both for immediate disaster relief and for longer term measures, are waiting to be learnt, and, if they are not lost, will enable Chile to withstand her next one in better shape.

Acknowledgements - Thanks are due to Professor Ambraseys, of Imperial College and Richard Hughes of RDI, for helpful discussions and to Dr. Robin Adams, of the International Seismological Centre, for the information in Table 1.

REFERENCES

Booth E., Quake team ponders Chile anomolies, New Civil Engineer, London (3rd April 1985).

Coburn A. et al., Damage assessment and ground motion in the Italian earthquake of 23rd November 1980, 7th European Conference on Earthquake Engineering, Athens (1982).

Conner I., The San Antonio Chile Earthquake of 3rd March 1985, Bull. N.Z. SOC. Earthquake Engineering 18(2), (June 1985). Dowrick D., Preliminary field investigation of the Chilean earthquake of 3rd March 1985, Bull N.Z. SOC. Earthquake Engineering 18(2), (June 1985).

Earthquake in Chile, 3rd March 1985, EERI Newsletter 1961, (May 1985).

EERI (Earthquake Engineering Research Institute), Berkeley, CA. Full report on the Chile Earthquake of 1985 (to be published). EQE, The Chilean Earthquake of March, 1985, EQE Incorporated, San Francisco (March 1985).

Gere J. and Shah H., Terra non firma: Understanding and preparing for earthquakes, W. Freeman, New York (1 984). Taylor C. and Booth E., The Chile earthquake of 3rd March 1985. Earthquake Engineering Field Investigation Team, Thomas Telford (to be published).

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