Seismic Activity in Fiji

Fiji is an island country located on the Pacific Ring of Fire, the world’s greatest active volcano and earthquake belt, where 81 percent of recorded earthquakes occur. In addition to earthquakes and volcanic eruptions, Fiji is highly exposed to other natural hazards including annual cyclones and floods as well as landslides, droughts, and tsunamis, all of which damage property and cause long-term cumulative economic harm.

Although Fiji is situated within a relatively quiet seismic area, the Pacific Ring of Fire that surrounds it coincides with tectonic plate boundaries, which are extremely active seismic zones capable of generating large earthquakes.

According to the United States Geological Survey (USGS), in 2020, 19 earthquakes with a magnitude of more than 4.0 occurred within 200 km of Fiji (among the strongest were four between 5.0 to 6.0). The following year, in 2021, again 19 earthquakes of 4.0 or more occurred (among them were three with a magnitude above 5.0, with two of those above 6.0). This year, the USGS has reported 12 earthquakes above magnitude 4.0, with none of them reaching a magnitude of 5.0.

Earthquakes in this region are caused when the boundaries of tectonic plates subduct or slip beneath each other. The two largest instrumentally recorded earthquakes along the Circum-Pacific belt were a magnitude 9.5 in Chile in 1960 and a magnitude 9.2 in Alaska in 1962¹.

The islands of Fiji were formed as volcanic island arcs caused by subduction processes, but luckily Fiji and Lau are no longer active island arcs².

Being an island, Fiji is greatly exposed to tsunamis as well. Fijian Assistant Minister Mr. Joeli Cawaki shared his worry in the following way: "We are at high risk and can be hit by any of the mentioned natural hazards and we live at a location that is exposed to the risk. It is like we are sitting on a time bomb and don't know when it will burst³."

Picture 1: World map and Fiji flag.

Fiji is located on the top of a complex tectonic setting along the boundary between the Australian Plate and the Pacific Plate, between two opposite-facing subduction zones. The Pacific Plate furthermore subducts beneath the Australian Plate along the Tonga Trench and forms the Tonga Ridge island arc system (the most linear, fastest converging, and most seismically active subduction boundary on Earth, which consequently has the highest density of submarine volcanoes) and the Lau Basin back-arc basin (a subduction zone that moves towards the subducting plate)⁴.

The strongest earthquake in the Fiji region occurred on August 19, 2018. It registered a magnitude of 8.2 on the Richter scale with a focal depth of 600 km in the South Pacific Ocean, 347 km east of Suva. Since it was so deep, it did not cause any damage, according to the authorities. Its epicenter was located close to the Fijian island of Lakeba and around 270 km from the small town of Levuka on Ovalau.

Picture 2: 2018 Fiji earthquake location.

Another mainshock struck the islands on September 6^th with a magnitude of 7.9 and at a depth of 670 km. Although neither caused extensive damage or injuries, both were widely felt. Small "tsunami" waves were also spotted along the coast, although because of its depth, a larger tsunami was not triggered. More than 250 aftershocks were felt up to 34 days after the main event³.

The USGS also reports that deep-focus earthquakes are common around the location of these two events. Over the past century, nine earthquakes with a magnitude above 7.0 have occurred within 250 km of there, all at depths greater than 300 km. You are welcome to read more details about it here.

The deadliest and most destructive earthquake that struck Fiji, particularly Suva, was the 1953 magnitude 6.8 “Suva” earthquake. Eight fatalities were reported, five of them caused by the tsunami which followed the mainshock, and more than 20 others were seriously injured. The earthquake and the resulting tsunami that damaged Suva were unusually well-observed and clearly illustrated a specific causative relation between the earthquake and the sea wave. Several hundred thousand people felt the shock and observed the approach of the tsunami⁵.

Picture 3: Fiji streets.

According to studies, Fiji has a 40% chance in the next 50 years of experiencing, at least once, moderate to strong levels of ground shaking⁶. In 2011, Fiji’s government reported that they expected to incur, on average, 79 million USD per year in losses due to earthquakes and tropical cyclones. In the next 50 years, Fiji has a 50% chance of experiencing a loss exceeding 750 million USD and casualties larger than 1,200 people, and a 10 % chance of experiencing a loss exceeding 1.5 billion USD and casualties larger than 2,100 people. However, up until today, Fiji’s earthquake average annual loss is 2.5 million USD6.

Currently, the five largest earthquakes in the Fiji region are:
- M5.7 Kadavu region 2019-10-20 (Minor damage/Landslides);
- M7.9 Viti Levu region 2018-09-06;
- M8.2 Lakeba island region 2018-08-19;
- M6.9 Nggilanggila Island 1979-11-16 (Moderate damage);
- M6.9 Suvasavu 1919-10-03 (Moderate damage).

Sources:
1) United States Geological Survey (USGS). Where do earthquakes occur? Accessed on 14-Jun-2022. Available at: https://www.usgs.gov/faqs/where-do-earthquakes-occur
2) Seach, John. Volcanoes of Fiji - John Seach. Accessed on 14-Jun-2022. Available at: http://volcanolive.com/fiji.html
3) UNDRR. 2009. Fiji Prone Disaster. Accessed on 14-Jun-2022. Available at: https://www.preventionweb.net/news/fiji-prone-disaster
4) Rahiman, T.I.H. Pettinga, J.R. Watts, P. 2007. The Source Mechanism and Numerical Modeling of the 1953 Suva Tsunami, Fiji. Marine Geology. 237 (1-2): 55-70. Doi:10.1016/j.margeo.2006.10.036
5) Houtz, R.E. 1962. The 1953 Suva earthquake and tsunami. Bulletin of the Seismological Society of America (1962) 52 (1): 1–12. Doi: https://doi.org/10.1785/BSSA0520010001
6) Global Facility for Disaster Reduction and Recovery (GFDRR). 2011. Pacific Catastrophe Risk Assessment and Financing Initiative. Accessed on 14-Jun-2022. Available at: https://www.gfdrr.org/sites/default/files/publication/PCRAFI%20AIR%20Brochure-%20Fiji.pdf