Mathematics – Logic
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.s13a0217n&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #S13A-0217
Mathematics
Logic
1816 Estimation And Forecasting, 1829 Groundwater Hydrology, 7223 Earthquake Interaction, Forecasting, And Prediction (1217, 1242)
Scientific paper
A near future large earthquake is estimated to occur off Miyagi prefecture, northeast Japan within 20 years at a probability of about 80 %. In order to predict this earthquake, we have observed groundwater temperature in a borehole at Sendai city 100 km west of the asperity. This borehole penetrates the fault zone of NE-trending active reverse fault, Nagamachi-Rifu fault zone, at 820m depth. Our concept of the ground water observation is that fault zones are natural amplifier of crustal strain, and hence at 820m depth we set a very precise quartz temperature sensor with the resolution of 0.0002 deg. C. We confirmed our observation system to work normally by both the pumping up tests and the systematic temperature changes at different depths. Since the observation started on June 20 in 2004, we found mysterious intermittent temperature fluctuations of two types; one is of a period of 5-10 days and an amplitude of ca. 0.1 deg. C, and the other is of a period of 11-21 days and an amplitude of ca. 0.2 deg. C. Based on the examination using the product of Grashof number and Prantl number, natural convection of water can be occurred in the borehole. However, since these temperature fluctuations are observed only at the depth around 820 m, thus it is likely that they represent the hydrological natures proper to the Nagamachi-Rifu fault zone. It is noteworthy that the small temperature changes correlatable with earth tide are superposed on the long term and large amplitude fluctuations. The amplitude on the days of the full moon and new moon is ca. 0.001 deg. C. The bottoms of these temperature fluctuations always delay about 6 hours relative to peaks of earth tide. This is interpreted as that water in the borehole is sucked into the fault zone on which tensional normal stress acts on the days of the full moon and new moon. The amplitude of the crustal strain by earth tide was measured at ca. 2∗10^-8 strain near our observation site. High frequency temperature noise of ca. 0.0005 deg. C is superposed on the cyclic fluctuation due to the earth tide. Using the earth tide as a reference, the resolution of our observation system is estimated to be higher than 10^-8 strain (0.5kPa). How small earthquakes off Miyagi Pref. can we detect by our observation system? Using a computer simulation code MICAP-G released by Okada (1992) and Naito & Yoshikawa (1999), we calculated the change in crustal strain at our observation site for assumed earthquakes off Miyagi Pref. with various sizes. These simulation results estimated that our system can detect earthquakes larger than about M6. Actually, we detected successfully the preseismic and coseismic temperature signals for the earthquake off Miyagi Pref. on Dec. 2, 2005 (M6.6) which is largest one since our observation started. The temperature began to decrease about 2.5 hours before the main shock, it was minimum (0.003 deg) one hour before the main, and abruptly increased by 0.002 deg. C 10 minutes after the main shock.
Nasuhara Y.
Otsuki Kaori
Yamauchi Takuya
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