Final answer:
The Chikyu drillship's expedition after the 2011 Tohoku-Oki Earthquake focused on studying the geological effects of the earthquake, related to Earth's plate tectonics and the potential for predicting future seismic events.
Step-by-step explanation:
Shortly after the 2011 Tohoku-Oki Earthquake, the Chikyu drillship began an expedition to study the geological effects of the earthquake. This expedition was crucial to understanding how the earthquake occurred and to learn more about the Earth's plate tectonics in the region, which includes fault zones and subduction zones, as these areas are known to cause serious earthquakes and can trigger tsunamis when they occur near the sea. Gathering this data is also essential for predicting future seismic events and improving safety measures for nuclear facilities and other critical infrastructure.
Considering the research focus of the Chikyu concerning the earthquake's impact on the Earth's geology, option 2, "The geological effects of the earthquake," is the correct answer. Marine ecologists and other scientists might study the impact on marine life, and economists or sociologists would examine economic and social impacts respectively, but the expedition's main focus was on geology.