EARTHQUAKES IN FIJI

Earthquakes occur throughout the world but certain areas are more prone to them than others. Unfortunately Fiji is within the "Pacific Rim of Fire", the zone of earthquakes and volcanoes which surrounds the Pacific Ocean. However, there are no volcanoes in Fiji and the number of earthquakes is somewhat less than for the most active areas around the Pacific, such as Japan, Alaska, Chile and nearby Tonga, Vanuatu and Papua New Guinea. The risk from earthquakes is similar to that in New Zealand which is relatively high by world standards.

The first known historical earthquake report describes an earthquake at Kadavu in about 1850. It states: "a recent earthquake had been felt throughout the greater part of Feejee, but was so tremendous in Kantavu that the earth opened in several parts and destroyed a great number of people. In one part it shook down a large cave and buried thirty or forty women who had taken shelter for the night, having been on a fishing excursion". Details of other severe earthquakes such as those which were reported in 1869 (upper Rewa River), 1984 (Macata), 1902 (Yasawas), 1919 (Rabi), 1932 (Rabi and Koro), 1953 (Suva) and 1979 (Taveuni) are given in the Table and their locations are shown in the earthquake map overleaf.

What are Earthquakes?

Earthquakes are due to failure of rocks under stress. Sometimes rocks are able to adjust by folding but if the stress is applied rapidly enough they will fracture and form a geological fault, which is usually too deep in the earth's crust to be seen. The fracturing along the fault causes vibrations or seismic waves which travel outwards in all directions from the fault and if the earthquake is large enough are recorded on sensitive seismographs areound the world and can cause the ground to quake strongly, close to the fault. Naturally, shallow earthquakes (depths less than 40 km) cause most damage because the Earth's surface is close to the earthquake fault.

Magnitude and Intensity

The magnitude of an earthquake is calculated from the recordings by seismographs and is a measure of of its size, or the energy released by it. It is stated according to the Richter Scale, which is longarithmic; thus a magnitude of 6, for example represents a disturbance iwth a ground motion 10 times that of one with a magnitude of 5, and an energy release amounting to about 30 times that of a magnitude-5 earthquake. Generally, an earthquake with a magnitude of 6 or more will cause major damage to buildings near the epicentre.

In Fiji since 1918, when reliable recording began, a potentially dangerous earthquake of magnitude 6 or more has occurred on an average of once every three years.

When an earthquake is recorded by distant overseas seismograph stataions another magnitude, MS, similar to the "Richter" magnitude, is determined by many of the stations. MS is given in the Table of important earthquakes.

Intensity is expressed as the Modified Mercalli (MM) intensity, which is a subjective expression of how severely the earthquake was felt at a particular place. It is measured in terms of the effects the earthquake produces, such as the degree of ground virbation and damage to buildings. an intensity of MM& or more will cause substantial damage to many man-made structures and probably cause severe land-sliding.

The Zones of Earthquake Activity

Earthquakes occur in several zones which are outlined in the map inset. Offshore activity occurs to the northeast and east of Vanua Levu and Taveuni, north of the Yasawas and Vanua Levu, to the west of Viji Levu and around Kadavu. Southeastern Viti Levu shows activity and another belt of seismicity which is not clearly definable possibly extends from near soughwestern Vanua Levu through Koro then arcs around to the southwest to south of Kadavu. The magnitude of the largest eartrhquake known to have occurred in each zone is given in the inset.

[Remind WebAdmin to acquire map as referred to above]

The northeast one is the most active in Fiji and the largest earthquakes have taken place here. The magnitude 7.0 quakes in 1928 and 1949 (see map) are the largest known to have occurred in Fiji. The area of Rabi and northern Taveuni was severely shaken (intensity MM8) by the 1919, 1932 and 1979 earthquakes, causing severe landslides, mud volcanoes, changes to natural water supplies and severe damage to buildings. The church built of stone at Napuka was badly damaged on each occasion.

The northern zone is the second most active one and earthquakes in 1884 and 1902 caused damage in northern Vanua Levu and the Yasawa Group respectively. Larger quakes in this zone are frequently felt in northern Vanua Levu, the Yasawas, the Nadi-Lautoka-Ba area and occasionally in Suva.

The western zone. Although earthquakes in this Zone are felt occasionally in western Viti Levu the only one to cause concern was in 1921 when the M6.7 quake shook the above area with an intensity MM5-6. Notable damage has not resulted from earthquakes in this area.

The Kadavu zone. The earthquake hazard in Kadavu is severe, and similar to that for Taveuni, Rabi and eastern Vanua Levu. The zone is not clearly defined but the pattern of smaller earthquakes indicates that Kadavu is included in it. The 1950 M6.5 earthquake was felt clearly in Suva and Navua and caused landslides in Kadavu. Had the earthquake focus been beneath Kadavu instead of offshore the quake could have been disastrous.

Southeastern Viti Levu. Scattered minor seismicity has been detected since a seismograph network centred on Suva was installed in 1979 and, for instance, Navua, Suva and Ovalau are often shaken by minor termors in their vicinities. However, intensities of MM7 or more have been reported only for quakes in 1869 and 1953.

The M6.8 1953 "Suva" earthquake, which caused damage costing about $500,000 (1953 values) and eight deaths, is remembered by everyone who was in Suva and surrounding areas of high earthquake intensities at the time. A spectular tsunami following the earthquake in Suva Harbour and Nakasaleka Bay (Kadavu) caused five of the earthquake's eight fatalities.

Zone through Koro. The zoning is uncertain. The only damaging earthquake in the zone is the 1932 Koro earthquake with M6.5. A report indicated that a landslide had taken part of a village into the sea and titled the lighthouse and put it out of action.

Rotuma and the Lau Group are not in recognisable earthquake zones. Rare reports of minor teremors come from the Lau Group. Several earthquake reports have orginated from Rotuma and a weakly active zone could exist near the island.

Tsunamis

Tsunamis (seas waves) have been reported after two earthquakes in Fiji, namely one off the north coast of Vanua Levu in about 1881 and the 1953 'Suva' earthquake. The Vanua Levu tsunami was reortedly about 1.8m in height and did not cause concern. However, the Suva waves were from 1.8 to 4.5m above low-tide level and caused great concern along water frontages.

One notable tsunami resulting from a distant earthquake occurred at Savusavu in 1877, when several 2.0-m waves were noted approximately 18 hours after a major Chilean earthquake.

Further Reading

Reports of earthquakes felt in Fiji, 1850-1940 by i.B. Everingham, MRD Report 48. 1983.

The 1953 Suva earthquake and tsunami, by R.E. Houtz, Bulletin of the Seismological Society of America vol. 52, no. 1, pages 1-12, 1962.

Some strong earthquakes felt in Fiji

WHAT TO DO ABOUT EARTHQUAKES

BEFORE THE QUAKE

Build expertly to withstand earthquakes and hurricanes Remember - earthquakes cause landslides.

Heavy items should be fixed to walls, floors i.e. stoves, refrigerators, tall furniture.

Select a place to shelter - under a table, bed, bench, suitable doorway to avoid being hit by falling items, hot water, broken glass and heavy furniture.

DURING THE QUAKE

DON'T PANIC. The earht's shaking is frightening but does not hurt you.

IF YOU ARE INDOORS, STAY THERE and take cover under selected shelters - table, bed, etc.

IF OUTSIDE move quickly away from buildings, tall trees, coconut palms and electricity wires.

DON'T RUN NEAR OR THROUGH BUILDINGS. The greatest danger is from heavy objects falling off outside walls.

IF YOU ARE IN A CAR - stop the car and stay there.

AFTER THE SHAKING STOPS

Check for fire. Check electricity and water supplies and turn off power or water supply if necessary.

Look for fallen power lines. Check danger from structural damage especially stairways.

Stay off telephone unless there is an emergency.

Do not go sight-seeing, and stay away from damaged buildings and landslip areas. Aftershocks may occur.

Should you be close to the sea, head for high ground in case a tsunami (sea wave) is generated.