To detect the precursors of deep groundwater changes related to anticipated large earthquakes off Miyagi Prefecture, we selected two deep boreholes located on the hanging walls of active reverse faults, and established an observation network with a real-time, automatic data-acquisition system. After taking into account the hydrological conditions and the effects of barometric pressure, the earth’s tide, and rainfall, as well as the resolution limits of temperature measurements of groundwater for the two boreholes, we identified plausible earthquake-related groundwater changes during the observation period (4 April 2004 to 31 December 2007). One of the events was a pre-seismic 0.003°C fall in groundwater temperature that began 2.7 hours prior to the main shock (MJ 6.6) of the 2 December 2005 off-Miyagi Prefecture earthquake. The second event was a preseismic 0.0012°C rise in groundwater temperature that started 2 hours before a shallow earthquake (MJ 4.5) located beneath Sendai Bay on 12 April 2007. The third event was a coseismic temperature fall by as much as 0.02°C associated with a shallow earthquake (MJ 4) on 29 May 2007, located close to one of the observation sites. The resolution limit of our temperature observation system indicates that it has the potential to detect the precursors of future large earthquakes off Miyagi Prefecture if associated with preseismic slip as large as that for a MJ 6.5 earthquake.
Anjo City is divided topographically into the Hekikai terrace (which represents the main area of the city), the Yahagigawa lowland (eastern area), and the Aburagafuchi lowland (southwestern area). The city is underlain by Holocene sediments, the Hekikai and Koromo formations, the Tokai Group, and basement rocks, in descending order. The Jomon transgression, which deposited a marine clay bed in the lower half of the Holocene sediments, caused inundation up to the midstream parts of narrow valleys carved into the Hekikai terrace. The Hekikai Formation, which forms the Hekikai terrace, consists of a sand member (upper half of the formation) and marine clay member (lower half), and is 16–36m thick. The marine clay and sand members correspond with the bottomset and foreset beds of a delta, respectively. The age of the Hekikai Formation ranges from about 120 to 70ka. The transgression that deposited the marine clay member of the Hekikai Formation started from Kinuura Bay (southwestern part of the Nishimikawa Plain) and inundated the western half of the city area. The upper part of the Koromo Formation consists of alternating beds of sand and mud, and is well consolidated compared with the Hekikai Formation. The basal surface of the Koromo Formation is located at an elevation of about –100m a.s.l. in the southwestern part of the city, –45m in the central part, and –25m in the northeast. A generalized geological cross-section through the Nishimikawa Plain is proposed as a tentative model of the local geology.
The results of a three-dimensional seismic survey of the continental slope off Shimokita Peninsula, NE Japan, reveal the detailed features of typical slumps and related fluid-migration paths in Pliocene and Quaternary formations. Dewatering structures, which are strongly dependent on the development of imbricate structures within slump sediments, acted to drain excess fluid that accumulated upon the slip planes related to slumping. Damped and dimmed reflections in the dewatering structures and in the slumps indicate that the distribution of natural gas is strongly related to fluid circulation in and around the slump sediments. Slumping and dewatering structures are probably typical phenomena in such a continental slope of high methane flux.
The cooling rate of a geological material is generally calculated using two isotopic ages with different closure temperatures; however, the closure temperature is itself dependent on the cooling rate. Here we present a simple procedure for calculating a consistent set of closure temperatures and cooling rate using Dodson’s mathematical formula.