Dibawah sana penelitian pengaruh bulan yg sudah dimulai sejak lama dipublikasikan di majalah Science 2002. Saya rasa memang tidak salah kalau kita harus hari-hati pada saat-saat bulan purnama ataupun bulan mati. Dan saya sudah percaya sejak lama, cuman baru ketemu artikelnya.
Pengetahuan ttg pengaruh bulan ini bukan hal yg baru tentunya. Mas Tujuh bisa meneruskannya pula tentunya. RDP ============================================ Science 19 July 2002: Vol. 297. no. 5580, pp. 348 - 349 Perspectives GEOPHYSICS: Tides, Earthquakes, and Volcanoes Junzo Kasahara* Large earthquakes and volcanic eruptions occur at subduction zones, major faults, volcanoes, and oceanic ridges. They are driven mainly by plate motion, but other factors can also trigger earthquakes and volcanism. For example, tides have long been implicated in their generation, although evidence has been sparse. A recent paper in Geology (1) makes the strongest case yet for tidal forcing of earthquakes and volcanism at mid-ocean ridges. Gravitational forces exerted by the Moon and the Sun cause ocean tides in the ocean and Earth tides in the solid Earth, with diurnal and semidiurnal periods. In and near the ocean, Earth tides and ocean tides are tightly coupled. The elastic strain resulting from Earth tides is extremely small, on the order of 10-8, which seems too small to trigger earthquakes and volcanism (2). Nevertheless, the idea that tides may influence these geophysical events has been discussed since 1930, when an interesting earthquake sequence was observed during an earthquake swarm east of Ito on the Izu Peninsula, central Japan. The Ito swarm was thought to be related to volcanism, although magma was not identified at the time. Nasu et al. (3) observed that for several days, the hourly numbers of earthquakes were higher during low tide than during high tide. They suggested that the swarm was triggered by the ocean tide, but did not offer a convincing triggering mechanism. Very few diurnal or semidiurnal earthquake activities of this kind have been observed during the Ito swarms, although month-long swarms have occurred frequently since 1930. But at least one other example of semidiurnal variation in earthquake swarm activity was detected near Ito in 1978 (4). Analysis of stress due to ocean loading effects suggested a strong influence of ocean tides. A statistical examination of the correlation between tidal force and earthquake activity has shown a slightly higher probability of earthquake occurrence for a normal fault source mechanism (5). The probability is highest for mid-ocean ridge earthquakes. But because of the dearth of examples of diurnal and/or semidiurnal earthquake sequences, tidal effects on earthquakes were not accepted until recently. The eruptions of Miyake-jima, ~180 km south of Tokyo, in 1983 and 2000 proved to be a turning point. In October 1983, earthquake activity started 1.5 hours before a huge eruption, and massive lava flows occurred at Miyake-jima. An ocean bottom seismometer (OBS) deployed nearby on the sea floor recorded numerous earthquakes (2). The eruption began at low tide. For the next 2 weeks, the hourly number of earthquakes showed maxima at either high tide or low tide. Earthquakes activity was strongly correlated with low tide or high tide for several days. Miyake-jima erupted again on 8 July 2000. During this event, the 1.6-km-wide summit region collapsed and subsided to a depth of 500 m. After the summit collapse, five tiltmeters recorded 46 steplike changes accompanying intensive earthquakes; diurnal and/or semidiurnal periodicities were observed in the data (6). Thirty-three of the 46 tilt-steps coincided with maximum or minimum shear strain, which is strongly influenced by ocean tides. Tidal effects on volcanism were also proposed for the Pavlof volcano in Alaska (7), several Hawaiian volcanoes (8), Mount St. Helens (9), and the Mayon volcano (10). The effects of tides on submarine volcanism were not observed until the summer of 1994, when the U.S. Navy Sound Surveillance System (SOSUS) array identified intense earthquake activity around Axial Volcano on the Juan de Fuca Ridge (11). The ridge is located about 400 to 800 km west to southwest off the western coast of North America. The data showed a clear correlation between tidal change and earthquake activities on two occasions (1, 12). Shortly after the beginning of the earthquake activity, an OBS array was deployed at Axial Volcano at 130ºW and 46ºN. On the basis of 402 earthquakes observed over a 2-month period, Tolstoy et al. (1) found a strong correlation between earthquake activity and ocean tides. The correlation between pressure change at the ocean floor caused by the ocean tide and the peaks of seismic activity is extremely good at low tide (see panel A in the figure). Spectral analysis of earthquake activity shows a semidiurnal peak (panel B). Harmonic tremors observed by the OBSs at Axial Volcano also showed semidiurnal spectral peaks, but did not show a clear correlation with low or high tide. Such tremors are often observed in volcanic regions and are characterized by sinusoidal ground oscillations. Schultz and Elderfield have suggested that harmonic tremor is caused by hydrothermal circulation in the oceanic crust, which in turn is induced by ocean tide (13). Thus, harmonic tremors may be caused indirectly by tidal waves. --------------------------------------------------------------------- ----- PIT IAGI ke 35 di Pekanbaru ----- Call For Papers until 26 May 2006 ----- Submit to: [EMAIL PROTECTED] --------------------------------------------------------------------- To unsubscribe, send email to: iagi-net-unsubscribe[at]iagi.or.id To subscribe, send email to: iagi-net-subscribe[at]iagi.or.id Visit IAGI Website: http://iagi.or.id Pembayaran iuran anggota ditujukan ke: Bank Mandiri Cab. 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