Abstract
Ocean Drilling Program (ODP) Leg 195 Hole 1200C was drilled and instrumented with a CORK subsurface borehole observatory in March of 2001. This CORK is located in the Mariana forearc region, 85 km away from the Mariana trench at 266 meters below seafloor and is installed atop South Chamorro Seamount, a blueschist serpentine mud volcano. The observatory is instrumented with pressure transducers, temperature thermistors and an osmotic fluid sampler. Data were downloaded from Hole 1200C in March 2003. Pressure data used in this study are measured hourly with sensors at the seafloor and within the formation. Two pressure transients were seen in the data recorded on October 12, 2001 and March 26, 2002 that correspond to pressure responses to tectonic strain events. These pressure transients can be used to estimate regional hydraulic properties of the Mariana forearc which is difficult and expensive to access. Over the two year period that the CORK in Hole 1200C was recording, 39 body wave magnitude five or greater earthquakes occurred within a 500 km radius from the CORK location. The two largest earthquakes were moment magnitude 7.0 and correspond to both pressure transients. They will be referred to as EQ1 and EQ2. CORK pressure data are processed and analyzed. The corrected data show that there is an initial pressure increase of 7.5 kilopascals (kPa) that occurs with EQ1 and 4 kPa that occurs with EQ2. The pressure increase with EQ1 displays an estimated volumetric strain of 1.95 x 10-6 while the pressure increase with EQ2 has an estimated volumetric strain of 1.0 x 10-6.These pressure increases occur at the same time as the two large earthquake events and are followed by a slow decay pressure. This decay is governed by the hydraulic properties of the formation and the long time period alludes to the hydraulic diffusivity of the formation. Literature suggests that this instantaneous response of pressures at the time of EQ1 and EQ2 as well as this slow decay of pressures are indicative of large scale contraction in which a large section of the plate contracts as slip occurs. It is proposed that two large contractions occurred within the Pacific plate or at the slab boundary, causing strain to build up at Hole 1200C at the time of both EQ1 and EQ2.