Monday, March 5, 2012

Percepatan-Pelambatan Rotasi Bumi

No comments
Rotasi Bumi Dipercepat oleh Gempa & Diperlambat oleh Pasang Surut Bulan.

Beberapa hari setelah gempa Chile 27 Februari 2010 peneliti JPL NASA mengabarkan bahwa gempa Chile berdampak pada percepatan rotasi bumi dan pergeseran poros ”gambar bumi”. Bagi sebagian orang, berita tersebut mengejutkan. Tetapi sebenarnya, percepatan dan pelambatan rotasi bumi terus terjadi. Bumi kita secara konstan diperlambat olah bulan dengan efek yang lebih kuat. Apa yang sesungguhnya terjadi?

===========================================================

http://www.jpl.nasa.gov/news/news.cfm?release=2010-071
Chilean Quake May Have Shortened Earth Days

March 01, 2010

The Feb. 27 magnitude 8.8 earthquake in Chile may have shortened the length of each Earth day.

JPL research scientist Richard Gross computed how Earth's rotation should have changed as a result of the Feb. 27 quake. Using a complex model, he and fellow scientists came up with a preliminary calculation that the quake should have shortened the length of an Earth day by about 1.26 microseconds (a microsecond is one millionth of a second).

Perhaps more impressive is how much the quake shifted Earth's axis. Gross calculates the quake should have moved Earth's figure axis (the axis about which Earth's mass is balanced) by 2.7 milliarcseconds (about 8 centimeters, or 3 inches). Earth's figure axis is not the same as its north-south axis; they are offset by about 10 meters (about 33 feet).

By comparison, Gross said the same model estimated the 2004 magnitude 9.1 Sumatran earthquake should have shortened the length of day by 6.8 microseconds and shifted Earth's axis by 2.32 milliarcseconds (about 7 centimeters, or 2.76 inches).

Gross said that even though the Chilean earthquake is much smaller than the Sumatran quake, it is predicted to have changed the position of the figure axis by a bit more for two reasons. First, unlike the 2004 Sumatran earthquake, which was located near the equator, the 2010 Chilean earthquake was located in Earth's mid-latitudes, which makes it more effective in shifting Earth's figure axis. Second, the fault responsible for the 2010 Chiliean earthquake dips into Earth at a slightly steeper angle than does the fault responsible for the 2004 Sumatran earthquake. This makes the Chile fault more effective in moving Earth's mass vertically and hence more effective in shifting Earth's figure axis.

Gross said the Chile predictions will likely change as data on the quake are further refined.
 
===========================================================
 
Pergeseran lempeng yang sebenarnya menyebabkan perubahan rotasi itu, bukan gempanya. Gempa sekadar indikator pelepasan energi akibat pergeseran lempeng bumi. Karena ada pergeseran lempeng bumi, maka kesetimbangan "bola" bumi berubah. Sesungguhnya "bola" bumi bukanlah bola sempurna, tetapi bola pepat di arah kedua kutubnya (oblate spherical) dengan distribusi massa yang tidak merata. Dalam geodesi, bentuk bumi yang tak mulus itu menjadi kajian menarik. Poros "gambar bumi" geodetik itu dirumuskan sebagai poros yang menjadikan massa bumi relatif setimbang di sekitar poros. Jadi poros "gambar bumi" (Earth's figure axis) tidak sama dengan poros astronomis (poros utara selatan) yang menggambarkan poros rotasi bumi.

Apa akibat pergeseran lempeng? Kesetimbangan "gambar bumi" sedikit berubah karena titik massa kulit bumi bergesar. Akibatnya, poros "gambar bumi" bergeser. Untuk kasus gempa Chile 2010 pergeserannya sekitar 8 cm (sudutnya bergeser 2,7 mili detik busur =0,00000075 derajat, terlalu kecil untuk dilihat). Dan untuk gempa Aceh 2004 pergeserannya 7 cm (sudutnya bergeser 2,32 mili detik busur = 0,00000064 derajat).

Akibat pergesaran kesetimbangan massa bumi, rotasi bumi dipercepat 1,26 mikro detik = 0,000000126 detik, manusia tidak mungkin merasakannya). Sebenarnya soal percepatan-perlambatan rotasi bumi, bukan hanya disebabkan oleh pergeseran lempeng. Efek pasang surut bulan juga menyebabkan bumi diperlambat 0,00002 detik per tahun.

No comments :

Post a Comment