4 August 2009

Drilling method helps understand earthquakes

Just about 58 km southeast of Japan and fighting through harsh atmospheric and ocean conditions, the deep-sea drilling vessel CHIKYU, for the first time in the history of scientific ocean drilling, successfully drilled down to 1,603.7 meters beneath the sea floor (at water depth of 2,054 meters).

The CHIKYU is drilling deep into the upper portion of the great Nankai Trough earthquake zone to gain measurements into geological formations and stress-strain characteristics. The Japan Agency for Marine-Earth Science and Technology (JAMSTEC) a partner in the Integrated Ocean Drilling Program (IODP) operates the CHIKYU.

Lead scientists Lisa McNeill and Tim Byrne examine core retrieved from beneath the sea floor, from an earthquake-generating zone called the Nankai Trough, off the coast of Japan.

Researchers used riser-drilling technology from about 700 meters below the sea floor to the bottom of the hole. Riser-drilling involves the circulation of drilling fluid that helps maintain pressure balance within the borehole. Researchers were able to cull cuttings from the circulated drilling fluid and analyze them to gain a better overall picture of downhole changes in lithology and age. They were also able to grab core samples between depths of 1,510 and 1,593.3 meters below the seafloor.

“This state-of-the-art technology enables scientists to access an unknown area,” said Co-Chief Scientist Lisa McNeill of University of Southampton, U.K. “It will provide a lot of important information about what has happened in the seismogenic zone in the past and its present condition.”

Following drilling operations that included “measurement -while-drilling” to obtain real-time geophysical characteristics, wireline logging instruments went into the borehole to measure formation temperature, resistivity, porosity, density, gamma ray, and borehole diameter. The riser-drilling technology enabled dynamic formation testing using the logging instrumentation for the first time during IODP scientific ocean drilling operations; this instrumentation measures stress, water pressure, and rock permeability.

“These two parameters, stress magnitude and pore pressure are both important to understanding earthquake processes,” said Co-Chief Scientist Timothy Byrne of University of Connecticut.

In addition, researchers conducted vertical seismic profiling to obtain accurate details of the geological structure of the plate boundary system. The activity involved an array of 16 seismographs vertically lowered into the borehole and eight ocean-bottom seismographs placed on the sea floor. An air-gun array on the JAMSTEC research vessel KAIREI generated elastic waves, which traveled through the formation so the borehole and sea floor instruments could record them.

“The seismic sensor array was installed in this hole below the thick sediment layer,” said Co-Chief Scientist Eiichiro Araki of JAMSTEC. “It acts like a telescope exploring the structure of faults in detail, which are responsible for causing large earthquakes such as the one that occurred here in 1944.”

For related information, go to www.isa.org/instruments.