Tsunami



Istilah tsunami berasal dari bahasa Jepang. Tsu berarti "pelabuhan", dan nami berarti "gelombang", sehingga tsunami dapat diartikan sebagai "gelombang pelabuhan".
Istilah ini pertama kali muncul di kalangan nelayan Jepang. Karena panjang gelombang tsunami sangat besar, pada saat berada di tengah laut, para nelayan tidak merasakan adanya gelombang ini. Namun setibanya kembali ke pelabuhan, mereka mendapati wilayah di sekitar pelabuhan tersebut rusak parah. Karena itulah mereka menyimpulkan bahwa gelombang tsunami hanya timbul di wilayah sekitar pelabuhan, dan tidak di tengah lautan yang dalam.
Tsunami adalah gelombang air yang sangat besar yang dibangkitkan oleh macam-macam gangguan di dasar samudra. Gangguan ini dapat berupa gempa bumi, pergeseran lempeng, atau gunung meletus. Tsunami tidak kelihatan saat masih berada jauh di tengah lautan, namun begitu mencapai wilayah dangkal, gelombangnya yang bergerak cepat ini akan semakin membesar.
Tsunami juga sering disangka sebagai gelombang air pasang. Ini karena saat mencapai daratan, gelombang ini memang lebih menyerupai air pasang yang tinggi daripada menyerupai ombak biasa yang mencapai pantai secara alami oleh tiupan angin. Namun sebenarnya gelombang tsunami sama sekali tidak berkaitan dengan peristiwa pasang surut air laut. Karena itu untuk menghindari pemahaman yang salah, para ahli oseanografi sering menggunakan istilah gelombang laut seismik (seismic sea wave) untuk menyebut tsunami, yang secara ilmiah lebih akurat.

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Earth Quake


An Earthquake is an event that causes the ground to shake. A sudden movement of the Earth’s crust causes an earthquake. This releases energy within the ground.
HOW EARTHQUAKE HAPPEN ????
When tectonic plates collide, they can cause the Earth’s crust to crack. These cracks, called faults, usually run along the edges of the plates. A few are in the middle of the plates. Most faults run deep into the crust.
The rocks on both sides of the fault fit tightly together. They bend as stress builds up because of continental drift. If the stress gets strong enough, the rocks suddenly snap back into shape. They release shock waves reach the surface of the earth. The land starts to shake. An earthquake has begun.
The focus of the earthquake is the point where the stress releases. The epicenter is the place on the Earth’s surface directly above the focus.
The Sun Andreas Fault is the best known fault in North America. It runs through California for 800 miles ( 1,300 km ). It was the cause of the Great San Fransisco Earthquake of  1906. The earthquake caused massive fires and terrible destruction. It was one of the worst natural disasters ever to hit the United States.
Many people think of faults lines occurring near the edges of continents. In fact,faults can occur anywhere. If you live in the Southern of Midwestern United States, you might live near the New Madrid Fault Line.
MEASURING EARTHQUAKES
            Seismologists, scientists who study earthquakes, have several ways to measure them. If you’ve heard a news report on an earthquake, you’ve probably heard of the Richter Scale. The Richter Scale describes the strength, or magnitude, of an earthquake. It is named after American scientist Charles Richter. A machine called a seismograph measure the earthquake’s strength.
            The Richter scale ranges from 1 to 10. Each number of the scale stands for a tenfold increase in the strength of an earthquake. An earthquake that registers 5 on the scale is 10 times worse than an earthquake with a magnitude of four. 
 Earthquake magnitude and The Effects
1-3       - Can be recorded on a seismograph, but rarely causes damage
- Usually not felt by humans
3-6       - Can be felt by humans
- Damage is usually minor
- Some buildings can be affected
6-9+     - Is felt by humans
- Can cause great damage
- An earthquake with a magnitude over six can cause damage for 100 miles
- Anything greater than eight can cause severe damage over an area of hundreds     of miles
  The Mercalli Scale measures how much the Earth shakes. It is named after Italian scientist Gieuseppe Mercalli. Earthquakes with Mercalli intensity of I (one) are not felt. Special instruments detect them. Those with an intensity of XII (twelve) cause total destruction of cities and change the Earth’s Surface.



Source : ensyclopedia; volume 4 earth science; page: 30-31

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