Depremin Trambolin Etkisi
Japonya’da yapılan bir araştırma, depremlerin yeri yalnızca iki yöne doğru sallamadığını, yeryüzeyini ”trambolin üzerindeymiş gibi” yukarı ve aşağı doğru ittiğini de ortaya koydu.
Araştırmacılar, 14 Haziran 2008′de Japonya’nın kuzeyinde meydana gelen ve 12 kişinin ölümüne neden olan 6.9 büyüklüğündeki depremle ilgili kayıtlı verileri inceleyerek, dikey sarsıntılarla ilgili de kanıtlar buldu. Uzmanlar, depremin dikey hareketini, ”trambolin üzerinde zıplayan bir atlete” benzettiler.
Science dergisinde yayınlanan araştırmada*, ”atlete etki eden iki güç olduğu, birinin aşağı doğru olan yerçekimi, diğerinin ise atletin çarptığı trambolinin itme gücüyle oluşan dikey güç” olduğu kaydedildi.
Biliminsanları, trambolinin itici gücünün de yerçekiminden güçlü olduğunu söylediler. Yeni bulguların, deprem bölgelerinde hem dikey hem de yatay güce dayanıklı binalar inşa edilmesine yardımcı olması umuluyor.
Trampoline Model of Vertical Earthquake Ground Motion
Seismic sensors at the surface of a borehole near the epicenter of a magnitude-6.9 earthquake this year in Japan revealed unpredicted asymmetry in the vertical wave amplitudes at the soil surface: The largest upward acceleration was more than twice that of the largest downward acceleration. The data also showed that the soil surface layer was tossed upward at nearly four times the gravitational acceleration— more than twice the peak horizontal acceleration. These findings run contrary to current structural engineering models, which presume that seismic waves from earthquakes shake the ground horizontally more than vertically. Shin Aoi and colleagues at Japan’s National Research Institute for Earth Science and Disaster Prevention propose what they call a trampoline model to explain the observed nonlinear bouncing behavior. In their model, the soil undergoes compression in the upward direction and behaves as a rigid mass with no intrinsic limit on acceleration, much like an acrobat rebounding from a trampoline (figures 1 and 3). In the downward direction, though, dilatational strains break up the soil and the loose particles fall freely at or below gravitational acceleration (figures 2 and 4). The observed seismographic data were simulated by combining the theoretical waveform from the trampoline model with selected borehole data that resembled elastic deformation of a deformable mass. The researchers say that other events need to be analyzed to learn how material conditions affect vertical ground response during large earthquakes. (S. Aoi et al., Trampoline Effect in Extreme Ground Motion, 2008. 31 October, Science, V. 322, pp. 727-730.)*
*Trampoline Effect in Extreme Ground Motion
Shin Aoi, Takashi Kunugi, Hiroyuki Fujiwara
In earthquake hazard assessment studies, the focus is usually on horizontal ground motion. However, records from the 14 June 2008 Iwate-Miyagi earthquake in Japan, a crustal event with a moment magnitude of 6.9, revealed an unprecedented vertical surface acceleration of nearly four times gravity, more than twice its horizontal counterpart. The vertical acceleration was distinctly asymmetric; the waveform envelope was about 1.6 times as large in the upward direction as in the downward direction, which is not explained by existing models of the soil response. We present a simple model of a mass bouncing on a trampoline to account for this asymmetry and the large vertical amplitude. The finding of a hitherto-unknown mode of strong ground motion may prompt major progress in near-source shaking assessments.
Bu haber, AA, Physics Today ve Science Magazine sitelerinden derlenmiştir.
