Assessment of the Port Vila Earthquake Vanuatu 2nd January 2002

The largest earthquake recorded to date in the vicinity of Port Vila occurred on 2 nd January 2002 at 17:22 universal time. The shock measured Ms 7.3 and was initially located 50 km west of Efate at a depth of 21 km. A tsunami generated by the event struck Port Vila Harbour about 15 minutes later. SOPAC and IRD teams intervened in the week following the earthquake to assist the Department of Geology, Mines & Water Resources of Vanuatu (DGMWR) to study and assess the effects of the earthquake. The combined team ensured the smooth running of the seismic network of Port Vila to record the seismic crisis, investigated the earthquake intensity distribution on the island, evaluated the damage caused by the earthquake and investigated the changes of relative sea level during the earthquake and tsunami. As a result of the more accurate determinations from the 16 stations of the IRD networks in Vanuatu and New Caledonia, the Port Vila earthquake was later relocated to 45 km west of Port Vila at a depth of 18 km. The earthquake was caused by a slip on a fault of about 20 x 40 km resulting in an elastic vertical displacement of several centimetres of subsid- ence of the western coast of Efate. The intensity distribution according to the Modified Mercalli scale varied across the island, ranging from a maximum of MM 10 in the west to MM 6 in the east. However, the averaged value of intensity by district in the rural areas of Efate generally ranged from MM 7.5 in the west to MM 6 in the east. The highest intensity from the earthquake, MM 10, was felt in Port Vila, but averaged values by district in the city area ranged from MM 8.5 to 6.5. A comparison of the distribution of felt intensities in Port Vila and an earthquake microzoning scheme recently devel- oped for the city indicated that the microzonation scheme was generally successful in discriminating the areas more severely affected. In general, areas zoned as high risk experienced an increase in felt intensity to MM 8 from a back- ground level in the city area of MM 7. Anomalous results were generally related to topographical effects which were not resolved well in the earlier work. The main shock was followed by thousands of aftershocks in the first fortnight with a few significant ones having magnitudes ranging from Ms 4.7 to Ms 6.4. The locations and magnitudes of the aftershocks reported by USGS NEIC placed epicentres on a northwest-southeast line to the west of Efate, but the more precise IRD network grouped the aftershocks on a slightly more northerly-oriented line. The tsunami generated with the earthquake was recorded by the National Tidal Facility gauge Port Vila Harbour with a peak-trough height of 0.8 m. However, eyewitnesses reported a maximum effect up to about 3 m in other parts of the harbour. The wave was large enough to have caused serious flooding of the central business district if it had not coincided with one of the lowest tides of the year. Modelling of the tsunami characteristics suggested that faulting occurred on the steeper west-dipping nodal plane.   Despite its magnitude, the earthquake was sufficiently far enough from Efate to cause only moderate damage. Direct earthquake effects caused structural damage to the extent where several public buildings were condemned. Founda- tion failure damaged several structures on reclaimed areas of the city waterfront, and severely damaged several bridges around Port Vila. Slope failures caused large landslides that blocked access to the country's main wharf, cut road access to the west of Port Vila and caused damage to a coastal resort.