Poster of the Tonga - Fiji Earthquakes of 19 August 2002 - Magnitude 7.6 and 7.7
On 19 August 2002, two extraordinary and large earthquakes occurred in an isolated portion of the Pacific Ocean between the Fiji Islands and North Island, New Zealand. Neither event was damaging or caused casualties and only the second shock was felt in the distant cities of Auckland, New Zealand and Suva, Fiji. The pair of shocks was interesting in that they occurred within 7 minutes and 315 km of each other, had large magnitudes (Mw = 7.60 and 7.70, respectively), and were at great depth (578 km and 694 km, respectively). Very large earthquakes are commonplace in this area of the Pacific (35 other shocks greater than magnitude 7.5 since 1900) and deep-focus earthquakes (depth greater than 300 km) are plentiful. What is unusual about this pair of shocks is the near coincidence in both time and space at the extreme lower depth limit of global earthquakes.
The earthquakes occurred in the Tonga - Fiji seismic zone, one of the most active on Earth. Earthquakes occur here because of the interaction of two tectonic plates (the Australian Plate and the Pacific Plate), both composed of relatively cold and rigid lithosphere (thin oceanic crust and uppermost mantle), moving towards one another at a rate of between 50 and 90 mm/yr. At the zone of contact, the cold Pacific Plate slides (subducts) under the overlying Australian Plate lithosphere and sinks into the hotter upper mantle. Where the rigid plates are in contact, a deep trench or trough forms (the Tonga Trench) and (often) large and numerous shallow-focus earthquakes occur, typically of the thrust-type mechanism expected in this zone of strong horizontal compression and brittle deformation. As the subducting plate continues to plunge to greater depths, earthquakes indicate strain relief within the relatively cold slab. The inclined pattern of earthquake foci from shallow (typically 0 km to 70 km) to intermediate (70 km - 300 km) and deep depths (greater than 300 km) is called the Wadati - Benioff zone. Volcanoes are frequently found above the 50 km depth of the Wadati - Benioff zone.
If the Australian Plate is assumed fixed, the westerly-moving Pacific Plate converges on the Tonga Trench at a rate of about 75 mm/yr and plunges into the upper mantle at an angle of about 45 deg. The first 19 August main shock is located in a cloud of deep-focus activity near the bottom of the Pacific Plate at its latitude. The second main shock is located at the greatest depth of the Wadati - Benioff zone at its latitude. Furthermore, the computed depth of 694 km is based upon a an excellent and large set of observations of "depth phases", characteristic seismic waves reflected off the underside of Earth's surface. Thus, the depth of the second main shock is nearly at a record value for the entire catalog of historical and recent seismic activity.
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