SOUTH ICELAND

 EARTHQUAKES 2000:

Damage and

Strong-Motion Recordings

INTRODUCTION

A damaging South Iceland Earthquake sequence started on 17 June 2000, 15:41, with a magnitude 6.6 earthquake with epicentre in Holt. It was followed by a great seismic activity in the entire South Iceland Seismic Zone, the Hengill Area and the Reykjanes Peninsula. The second earthquake exceeding magnitude 6 in the sequence occurred on 21 June, 00:52. The size was 6.5 and the epicentre was approximately 17 km west of the epicentre of the first event.

Totally the Icelandic Strong-Motion Network recorded 83 events in June 2000 and in the following months. This resulted in 774-ground response time series. The two biggest events, on 17 June and 21 June, were recorded on every station within a radius of 250 km. In these two events 147 ground response time series were recorded. The highest recorded peak ground accelerations in these two biggest events were, respectively, 64% and 84% g, which are among the highest values recorded in the world during the last decade. The surface traces of the causative faults of the earthquakes on 17 and 21 June were approximately 20 and 22 km, respectively. The total damage area containing significantly damaged buildings was approximately 800 km².

The buildings in the near-source area are mostly low-rise, single-family dwellings and farm buildings. Some of these buildings suffered severe damage, while others resisted the earthquake without visible damage. However, none of the residential buildings collapsed.  Approximately 5% of the dwellings, in the near-source area, where judged to be damaged beyond repair and most of them have now be demolished.  Fortunately, no significant injuries were reported.

DAMAGE

The earthquake-induced damage was widespread. However, the major damage was mostly confined to the epicentral region of the two biggest earthquakes. The damage areas lie from north to south around the main causative faults. Thus, the impact area of the first big earthquake can be said to be especially in Holta and Landssveit, but it also stretches somewhat into Rangárvellir.  The size of the damage area has been estimated to 440 km². The impact area of the second big earthquake, on the other hand, is especially in Skeid and on the east side of Flói and Grímsnes, but it also stretches east to Holt.  The size of it was about 360 km².

Most of the damage in the first earthquake occurred in a village named Hella, with regard to the number of both individuals and companies suffering damage. In the latter earthquake, there was also great damage, based on the number of buildings in the epicentral area, but the damage was mostly confined to individual farms, groups of summer cottages and utility and communication systems. The following account is based on [1].

Buildings

The buildings most damaged in the earthquakes were older structures particularly. Special mention should be made of houses built on poor foundations, houses constructed of building blocks made of lava aggregate and houses with floating base slabs resting on fill of poor quality, as well as houses with masonry partitions. Also, many unreinforced concrete houses were badly damaged, especially outbuildings in rural areas. However, it can be generally said that well-constructed wood houses and reinforced concrete houses withstood the earthquakes well, sustaining little or no damage. One can also find houses of masonry construction that withstood the earthquakes or sustained little damage despite great excitation. It often appeared somewhat a matter of chance which houses suffer damage.

Equipment and house hold articles

Great damage occurred to equipment and objects inside buildings, for both individuals and companies. Damage individuals’ household effects was in many places considerable, and it is worth noting that very heavy objects moved (slided or toppled) out of place, indicating that vertical acceleration was considerable, which is in accordance with measurements of the Earthquake Engineering Research Centre.

Agriculture and industry

The earthquakes originated in a flourishing agricultural district, and many producers of agricultural products sustained considerable damage. In this regard, damage to lighting installations in greenhouses should be mentioned. The damages may be attributed to the improper finishing of hanging fixtures for the lamps. Damage, both direct and indirect, occurring because of this was considerable in several greenhouses. Comparable damage also occurred in recent earthquakes originating in Hellisheidi in 1997 and 1998. Raisers of furbearing animals suffered considerable damage when cages turned on their side, and animals escaped. Chicken farmers also suffered damage resulting from similar causes. In addition feeding and water systems were disconnected, machines were damaged, and eggs broke. In this context, it should however be mentioned that it was to a great extent lucky that the earthquakes occurred in the summer when most of farmers’ livestock was in the fields and not inside buildings. Many outbuildings in the earthquakes’ regions of origin were greatly damaged, and it is obvious what the consequences would have been for animals housed inside them.

The most noteworthy damage to industrial companies occurred at Samverk Glassworks in Hella. Great damage occurred there to both the inventory and finished panes of glass.

Communication structures

Some damage occurred to communication structures. Most of the damage occurred to roads in the epicentral regions where cracks ran through pavements, and roads subsided, for example, at bridges. Emergency repairs were carried out immediately after the earthquakes, being completely both swiftly and expediously.

Thjórsá River Bridge was subjected to great strain in both the earthquakes, although especially the latter one, when peak ground acceleration went up to 85% g and differential displacements of supports were excessive. However, it withstood the excitation without considerable damage, since measures had been taken to strengthen the bridge. The most efficient protective part of the strengthening program was to be replacing the original bearings in the bridge with new lead-rubber bearings.

Water supply and changes of ground water level

Damage to and disruption of water supply systems were common, especially in the first earthquake. In the following earthquakes, utility systems suffered smaller disruption.

In the first earthquake, on 17 June, great damage occurred to the part of the geothermal heating system in Rangárvallasýsla, where the distribution system was made of asbestos pipes. This involved an older part of the conduit between Hella and Raudalækur, on the one hand, and Hella and Djúpidalur, on the other. Fortunately, a new steel pipe was under construction between Hella and Raudalækur. It was not damaged, and its connection was expedited. Hot water was back on in Hella a little over 24 hours after the earthquake. Hot water began to flow into Hvolsvöllur in the wee hours of 19 June. On the other hand, street pipes in Hella were considerably damaged in several places, and about half of the houses in Hella were without hot water for approximately 24 hours after the main line had been fixed. Lyngás between Hella and Raudalækur was also without hot water for approximately two days. In addition, the earthquake put the hot water utility for Vindás and nearby farms out of order. In the earthquake of 21 June, substantial damage was done to the piping systems of Sólheimar in Grímsnes for hot water, cold water and drainage. Also, the main water line between Selfoss and Eyrarbakki broke, but water was restored in the morning.

In both the earthquakes, considerable changes occurred in the water flow in boreholes in the epicentral areas, with the water level either rising or falling. If we look at the fault plane solutions of the two biggest earthquakes we see that the water level changes followed the same patterns as seen on the “beach ball” diagrams. In other words the water level rose in the pressure zones and sank in the tension zones. The changes of water level in boreholes indicated with fair accuracy the epicentres of the two biggest events.

There were examples of pumps in boreholes being flooded and the pumps’ electrical systems being damaged. It is to be expected that it could take some time for the boreholes to recover, but the general belief is that earthquakes, from a long-term perspective, have a positive effect on geothermal areas. In this regard, it can be mentioned that the great geyser Geysir in Haukadalur Valley has been revived to some extent and has erupted rather regularly since the second earthquake occurred. On 13 July, a full eruption of the geyser occurred without help for the first time since 1919.

Electrical power supply

In the earthquake on 17 June, the electricity went out in Hvolsvöllur, the Eyjafjöll Mountains district, Vík and the Westman Islands. About 15 minutes passed before electricity was restored in Hvolsvöllur, and it took about two hours to restore electricity to the Westman Islands and Vík. The reason was that the main switch in the substation had to be circumvented as its insulators broke in the earthquake. They are three meters high, made of ceramic material and are perched on three-metre-high pillars. When they broke, a flaming splash of oil ignited dry grass next to a security fence surrounding the substation at Hvolsvöllur. No substantial fire resulted, and it burned itself out. One transformer went out at the Búrfell Power Station and with it one of the six generators. It seems that a sensor broke the current because of shaking, but there was no damage to transformers or machinery. At the same time, electricity went out at one of the Icelandic Aluminium Company Ltd.’s pot rooms and one furnace of the Iceland Alloys Ltd. ferrosilicon plant. Electricity was restored approximately 10 minutes later.

Power lines sagged in the surrounding area of Selfoss and a total of six National Power Company high-voltage masts were damaged in the earthquakes when guy anchors and foundations subsided by up to 20-30 cm. They nevertheless continued to perform their function. Repairs began 21 June and were finished 22 June.

Telephone

No disruption occurred in the general telephone system, but part of the GSM -system of Iceland Telecom- went out for a short time when the earthquake rumbled through, and a disturbance occurred in part of an exchange in the GSM system in the Breidholt neighbourhood in Reykjavik. This caused some GSM transmitters to drop out, but not others. This disturbance lasted only a few minutes, and at 16:15 all the GSM transmitters in Iceland were back on line. Because of great load in particular places, on the other hand, transmitters could not handle all the traffic, especially, in some places in South Iceland and in Reykjavik. Where electricity was lost, telephone exchanges operated on reserve power until the electricity came back on. There were no reports of fibreoptic cables or telephone lines breaking.

Rock fall and landslides

Rock fall and landslides were very prominent in these earthquakes, and this seems to coincide with previous experience. It is therefore right to draw attention to the risk to inhabited areas stemming from such occurrences and houses that are built too near steep hills where there is a risk of falling rocks and landslides.

Fear – role of mass media

Some fear and dread seems to have gripped people when the earthquakes occurred. Many left their houses and awaited further developments. The length of time passing after the first earthquake occurred before the mass media issued their first news of the earthquake seems to have increased people’s uncertainty.  It is desirable that the Iceland Civil Defence, along with civil defence committees in parishes, issues press release to the State Broadcasting System’s related to such events as quickly as possible. Not more than a quarter of an hour should pass between an event and the presentation of the first announcements about it. The content of such notices does not have to be very substantial to achieve their goal, which is to calm people. It is often enough for people to know that someone is addressing the matter, and that more detailed information will be presented as soon as it is available. The performance of the mass media and civil defence was better in the second earthquake, which is normal.

People’s fear also surfaced in places faraway from the epicentral areas. Examples that may be mentioned include people in the Westman Islands, where there was a great deal of rock falls, and instances in high-rise building in Reykjavik where considerable magnification of vibration occurred. It can be assumed that the acceleration of the top stories was around or more than 20% of g in several places in the first earthquake. In this regard, it can be pointed out that in this earthquake elevators went out of order in several buildings.

Mass media have a tendency to emphasise the reporting news on ground dislocation and damage most, but there is often little discussion of all the structures that are not damaged or other positive aspects. This can result in the news’ giving an unrealistic picture of the overall effects of earthquakes, which, in turn, can have various negative consequences. In this regard, foreign mass media, among others, can be mentioned. For a while, it was feared that exaggerated news coverage by foreign mass media would have a bad effect on Icelandic tourism, with corresponding financial damage. Undoubtedly, some trips to Iceland were cancelled because of the earthquakes. On the other hand, the fact was that foreign tourism in Iceland increased in June, compared with the previous year.

Regarding tourism, positive aspects may also be pointed out. Iceland’s turbulent nature no doubt plays a part in making the country a more exciting and different destination. Also, it can be mentioned that the great Geysir, which has for many decades been one of the main tourist attractions, became active again following the earthquakes, as was previously mentioned.

Accidents involving people

No serious accidents occurred in the earthquakes, but many things indicate that luck had a great deal to do with this, for example, the fact that the first earthquake occurred on Iceland’s National Day, 17 June, when many people are gathered outdoors or indoors in well-built meeting houses. There were, however, several minor injuries.

Summary

Clearly, the earthquakes caused great damage to structures, communications and utility systems as well as household effects. Ground dislocation and subsidence were also considerable in areas in the vicinity of earthquake origins. Furthermore, the inhabited areas in the western part of Holt and around the Thjórsá River were greatly impacted by the earthquakes. The consequence of this was incremental damage to the buildings that were damaged in the first earthquake. Aftershocks also resulted in damage to structures increasing with time.

When the effect of the earthquakes is assessed, and the destruction following in their wake is examined, it can be said that South Iceland came through amazingly well and better than one would have expected. There are many reasons for this. Here, it should be mentioned that the first earthquake occurred when many people were gathered outdoors or inside in well-built meetinghouses. Also, in the impact area of the latter earthquake, there was no large inhabited area core. It is also of substantial importance that most buildings in the impact area of the earthquakes are low-rise and generally simple in construction and robust. It should also be kept in mind that the great damage stemming from fires in the wake of an earthquake in many places abroad is almost unknown in Iceland. Last but not least, it is also unequivocal that precautionary measures that companies, municipalities and individuals have worked at over the previous decades have produced the desired results of reducing the damage from what it otherwise would have been [2].

MEASUREMENTS

The earthquakes were measured by different agencies, both domestically and abroad. Herein only recordings of the Icelandic Strong-Motion Network will be dealt with. The network is operated by University of Iceland, the Earthquake Engineering Recearch Centre, under agreements with the National Power Company, the Public Road Administration, the City Engineer of Reykjavik and the municipalities in South and North Iceland. The network comprises 36 (free field) ground response stations besides arrays in buildings and structures.

The biggest earthquakes were recorded on every station within a radius of about 150 km from the epicentres. The greatest epicentral distance of a recording was, however, about 270 km. In this context it should be kept in mind that the measurement threshold is generally maintained at 0.4 to 0.6% g. Tables 1 and 2 provide an overview of the peak ground acceleration recorded at the most important sites in the earthquakes on 17 and 21 June, 2000, respectively. From the tables, one can see that the acceleration in the near source area was substantial and even greater than one would have expected in earthquakes of this magnitude.

The recordings obtained in these earthquakes are now accessible within the framework of the ISESD project (Internet-Site for European Strong-Motion Data), supported by the European Commission, Research-Directorate General, Environment and Climate Programme [3]. The web site will give access to both uncorrected and corrected three-component strong-motion acceleration records and their corresponding (linear) response spectra, along with information on their seismological, instrumental and site-specific parameters.

Table 1. Overview of peak ground acceleration in the earthquake on 17 June 2000, at 15:40:42, recorded by the Icelandic Strong-Motion Network [1]

Table 2. Overview of peak ground acceleration in the earthquake on 21 June 2000, at 00:51:48, recorded by the Icelandic Strong-Motion Network [1]