Expedition Menu
1. Introduction
2. 
The Question
3. Funding
4. Study Area
5. Background
6.
Seismogenic Zone
7. Chief Scientists
8. The Journey
9. Ship Tour
10. Leaving Port
11. Seismic Method
12. Seismic Source
13. Seismic Recording
14. 3-D Acquisition
15. Life at Sea
16. In the Lab
17. Time to Depart
18. Data Processing
19. The 3-D Volume
20. Interpretation
21. Your Turn
22. Comparison
23, Publication
24. IODP Drilling

NSF Logo
This material is based
upon work supported
by the
National Science Foundation
under
Grant No. 0633234


 
 


 

NanTroSEIZE in 3-D

Imaging an Active Plate Boundary Fault
5. Scientific Background

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Seafloor map of study region

Start YouTube video in lower left corner now

Let's next examine the general geological background for this study.

Seafloor Features in the region

The map in the upper left corner shows the seafloor features and land masses in the vicinity of our study. The islands of Japan, specifically Honshu, Kyushu and Shikoku are in the upper middle portion of the map. Japan is separated from the Asia mainland by the deep water area of the Sea of Japan. The Korean peninsula and the continental margin of the People's Republic of China, colored green and gray, are shown along the western edge of the map, and Taiwan is in the lower left corner. The triangular-shaped dark blue or purple region, south of Japan, marks the deep water region of the Philippine Sea. The Nankai Trough lies along the boundary between the northwestern margin of the Phillipine Sea and southeastern Japan. The orange, gray, and turquoise-colored area extending southward from central Honshu, and lying along the eastern edge of the Philippine Sea, marks a volcanic ridge that is capped by active volcanoes of the Izu and Bonin islands. A deep-sea trench lies along the eastern boundary of this ridge, shown by a narrow region in dark purple. Further to the east is the flat abyssal plain, or deep water area of the western Pacific Ocean.

The Plate Boundaries

Below the seafloor map is a map showing the tectonic plates and plate boundaries in the region. The blue lines with triangles or teeth represent convergent plate boundaries where two plates are moving together with one of the plates sinking below the other. The major plates, which form the rigid other shell of the earth in the region are: the Pacific plate on the east, the Phillipine Sea plate in the middle, and the Eurasian plate on the west. Note the arrow extending from the Phillipine Sea Plate twords the Eurasian plate (pointing towards the Korean peninsula); this arrow shows the direction of motion of the Phillipine Sea plate towards the Eurasian plate and the number "48" gives the convergence rate of the two plates, in other words, 48 millimeters per year, or slightly less than two inches per year of plate convergence across the Nankai Trough.

Convergent Plate Boundary and Subduction

Here is an animation showing plate convergence produced by the Educational Multimedia Visualization Center at UC Santa Barbara. Note how one plate, shown in green, with the red arrows, sinks beneath another plate, which is called subduction. This our study area, this represents the Phillipine Sea plate. The other plate with the landmass and volcanoes of Japan is thrust over the Phillipine Sea plate. The volcanoes of Japan are fed by molten rock, called magma, which is generated in the earth's mantle as a result of the subduction process. The boundary between the two plates, the sinking and overthrusting plates, is marked by many faults, which are cracks or fractures, along which pieces of the earth's crust in the region slip and slide past each other.

Subduction Earthquakes and Tsunamis

Here is another animation, this time by Miho Aoki of the University of Alaska Fairbanks. Here you see a close-up of one of these faults along the boundary between the convergening plates. Sometimes friction along individual faults prevents slip between the crustal slices, at least for a while, even though the motion between the plates continues. Energy, called strain, builds up in the rocks in the form of deformation, sometimes causing a slowly growing bulge in the overlying plate. When the stresses associated with plate convergence exceed the frictional forces on the fault surface, slip occurs, causing the earthquake, or rapid slip along the fault, with seismic energy propagating in all directions. The elastic response of the rocks adjacent to the fault, like that of a rubber band, causes rapid motion of the overlying seafloor over a few tens of seconds or even minutes, pushing away the overlying ocean. Consequently a wave is created that propagates in all directions away from the area of the earthquake.

Topography of Study Area Showing Japan, Philippine Sea

Plate Map

Map showing of plate boundaries in region

Subduction, Earthquakes and Tsunamis

©Copyright 2009
July 23, 2009

Send to Don Reed

Department of Geology
San Jose State University
NanTroSEIZE Drilling Objectives
Modified from IODP Nankai Trough Seismogenic Zone Experiment


With this general knowledge, let's return to the scientific question being addressed by this study.