Magnitude 7.9 Earthquake in Tonga Trench
Thursday, March 19, 2009 at 18:17:37 UTC
11:17:37 AM Pacific Daylight Time
Epicenter: Latitude 23.015°S, 174.782°W (indicated by arrow on map below).
Magnitude = 7.9
Depth: 10 kilometers.
A major earthquake occurred Thursday morning Portland time on the subduction zone at the Tonga
Trench about 1800 km (1140 miles) northeast of Auckland, New Zealand. As shown on the map on
the left below, this earthquake occurred on the convergent boundary at the Tonga Trench where the
Pacific Plate subducts below the eastern edge of the India – Australia Plate. The rate of
convergence at the location of Thursday’s earthquake is about 60 mm/yr (6 cm/yr). The map below
on the right shows earthquakes recorded from 1990 to present with the March 19 earthquake shown
as a yellow star. This recent earthquake fits the depth pattern expected as the Pacific Plate dips
towards the west beneath the India – Australia Plate. Earthquakes along the line A – A’ through the
Tonga Trench are illustrated in map view and in cross-section on the next page. The earthquake
"generated a small tsunami," but there is "no evidence of destructive waves," said Stewart
Weinstein, assistant director of the Pacific Tsunami Warning Center in Hawaii. Another interesting
observation is that the earthquake struck a few hours after a volcano erupted off Tongatapu, Tonga,
about 200 km away. Ken Hudnut, a geophysicist at the U.S. Geological Survey in Pasadena,
California, said: "The association with the volcanic activity seems to be an interesting added
dimension to this. It's not clear at this point that there is a direct association, but it seems suggestive
at this point."
Tonga
Trench
A
A’
India Australia
Plate
Pacific
Plate
The record of the Tonga earthquake on the University of Portland seismometer (9200 km =
5800 miles = 83 degrees from the earthquake):
• P waves – 12 minutes 22 seconds to travel from the earthquake to Portland.
• PP waves – 15 minutes 30 seconds. PP waves have a path through the mantle that bounce off
the Earth’s surface between the epicenter and station. PP phase is quite visible on this record.
• S waves – 22 minutes 39 seconds to travel from the earthquake to Portland.
• Surface waves - traveled from the earthquake to Portland around the perimeter of the Earth,
the first waves arriving 35 minutes after the earthquake occurred.