TSUNAMI
Tsunami merupakan perkataan dari bahasa Jepun yang membawa erti "ombak pelabuhan". Dalam bahasa Inggeris, gelombang tsunami dikenali sebagai ombak pasang surut (Tidal wave).Gelombang tsunami sebenarnya merujuk kepada ombak besar yang membawa kemusnahan kepada bangunan tepi pantai.Bagi penduduk Malaysia, kita beruntung disebabkan kita berada jauh daripada kawasan gunung berapi. Malaysia tidak terdedah kepada ancaman bencana alam gunung berapi dan bencana alam yang seiring dengannya seperti gelombang tsunami, bagaimanapun kita perlu tahu mengenai bencana alam memahami bumi dan alam sekitar kita. Di Indonesia, Pulau Jawa, pantai yang sering dilanda tsunami adalah pantai selatan Jawa Barat, Pantai Selatan Cilacap dan Pantai Selatan Jawa Timur. Sejak tahun 1990, Indonesia mencatat 15 kali bencana alam dilanda tsunami yang berlaku di sepanjang zone-mendak serta zone aktif seismik.
Good thread...keep it up....
PL :bgrin:
[ Last edited by PeNinGLaLaT on 8-9-2004 at 02:21 PM ]
Punca tsunami
Ombak laut kebiasaannya di sebabkan oleh pergerakan angin yang menghasilkan ombak yang menghempas ke pantai. tetapi gelombang tsunami merupakan gelombang laut yang teramat kuat yang terhasil oleh letupan gunung berapi, tanah runtuh bawah laut, gempa bumi, dan hentaman oleh asteroid atau komet pada permukaan laut.Gelombang tsunami mampu bergerak sejauh seribu batu merentasi lautan dan masih mampu menghancurkan bandar berhampiran laut, memusnahkan dan mengakibatkan kehilangan nyawa yang tidak terkira jumlahnya. Gelombang tsunami yang menyebabkan kehilangan jiwa paling ramai direkodkan berlakuselepas letupan gunung berapi Krakatoa pada tahun 1883. Dianggarkan seramai 36,000 people mati disebabkan letupan tersebut yang menghasilkan ombak setinggi 12 tingkat bangunan. Kebanyakannya disebabkan oleh ombak tsunami yang melanda perkampungan persisiran pantai, sejauh sehingga 120 kilometer dari gunung berapi Krakatoa di Selat Sunda Indonesia.
Selain itu jiwa yang terkorban akibat tsunami di Indonesia adalah seperti berikut,19 Ogos 1977 tsunami di daerah Sumba mengorbankan seramai 189 nyawa. 12 Disember 1992 di daerah Flores, gelombang tsunami mengorbankan 2,100 nyawa dan pada 3 Jun 1994 bencana tsunami melandaa daerah Banyuwangi mengorbankan seramai 208 orang.
Hentaman komet atau asteroid juga mampu menyebabkan tsunami gergasi. Walaupun tiada sesiapa pernah melihatnya, simulasi komputer menunjukkan bahawa gelombang tsunami gergasi yang mampu menghapuskan pencakar langit Manhattan boleh dan sudah tentunya pernah berlaku pada masa lalu.
Pakar sains di Los Alamos National Laboratory di New Mexico menjangkakan bahawa sekiranya asteroid sebesar tiga batu lebar menghentam pertengahan lautan Atlantik, gelombang tsunami akan menengelamkan Pantai Barat atas sehingga Pergunungan Appalachian dan menengelamkan persisiran Perancis dan Portugis. http://www.geocities.com/semua_hal/GambarTsunami.gif
Bagaimana tsunami terhasil
Punca utama gelombang tsunami adalah pergerakan muka bumi dasar laut yang berpunca daripada gempa bumi dasar laut. Gempa bumi tsunami berlaku pada zon terbenam "subduction zones" di mana kepingan kerak bumi atau "lithosphere" yang dikenali sebagai plak tetonik bertembung sesama sendiri menyebabkan satu daripadanya terbenam di bawah yang satu lagi. Terdapat zon terbenam "subduction zones" di pinggir negara Chile, Nicaragua, Mexico, dan Indonesia yang mencetuskan tsunamis pembunuh dalam tempoh 100 tahun lalu. Di lautan Pasifik, terdapat rekod 17 tsunami dari tahun 1992 sehingga 1996 yang bertanggungjawab mengakibatkan kehilangan 1,700 nyawa. Negara Indonesia terdedah kepada bencana alam disebabkan kedudukannya yang merupakan pertembungan 3 plat tetonik, iaitu kepingan Euro-Asia, kepingan Indo-Australia dan kepingan Pasifik. Selain ketiga kepingan yang besar itu, Indonesia juga menjadi pertemuan plak tetonik dengan skala kecil seperti kepingan Filipina.Apabila kepingan kerak bumi terbenam kedasar, pergerakannya bergerak tersekat-sekat - melekat sekejab sebelum tergelincir. Apabila ia terlekat pada tebing benua, tekanan terhasil. Apabila kawasan yang terlekat terbebas, sebahagian daripada dasar laut mungkin melantun naik seperti papan anjal, ketika tekanan terbebas; sementara bahagian lain mungkin tenggelam. Seketika selepas gempa bumi, air yang tertolak membentuk gelombang dan tsunami pun terhasil.
Zon terbenam "subduction zones" terdapat di seluruh dunia, tetapi lokasi yang paling dikenali adalah di sepanjang lingkaran Cincin Api Pasifik. "Pacific Ring of Fire". Lingkaran Cincin Api Pasifik merupakan sempadan pertembungan antara dua plak tetonik. Lingkaran Cincin Api Pasifik terletak sepanjang pantai barat Tengah dan Selatan Amerika, Jepun, dan Filipina, merentasi New Zealand, ke lautan Atlantik. Gunung berapi juga wujud di tengah laut sepanjang rabung laut. Rabung Tengah Atlantik merupakan contoh gunung berapi dasar laut. Apabila gunung berapi dasar laut ini meletus, ia mampu menghasilkan gelombang tsunami. Dalam lautan lepas, gelombang tsunami mencecah kelajuan 500 batu sejam, selaju jet penumpang. Tetapi di lautan lepas, gelombang tsunami itu tidak dapat dibezakan dengan ombak biasa. Di lautan dalam, ombak tersebar dan merunduk, dengan jarak beratus batu antara puncak ombak yang hanya kelihatan beberapa kaki tinggi. Tetapi sebenarnya, puncak gelombang tsunami hanyalah hujung jisim air menggunung yang bergerak. Berbeza dengan ombakyang disebabkan oleh angin yang berkesan pada lapisan atas air, gelombang tsunami terbenam sehingga beribu kaki di dalam laut.
Oleh kerana pergerakan ombak begitu kuat, dan sifat pedam air, gelombang tsunami mampu bergerak beribu batu dan kehilangan hanya sedikit tenaga. Sebagai contoh, gelombang tsunami yang disebabkan oleh gempa bumi di persisiran Chili pada tahun 1960 menghasilkan gelombang tsunami yang mempunyai cukup tenaga untuk membunuh 150 di Jepus setelah bergerak selama 22 jam sejauh 10,000 batu. Gelombang tsunami tersebut kemudiannya berulang-alik menyeberangi lautan selama beberapa hari.
Apabila gelombang tsunami sampai kepantai, ia menjadi perlahan disebabkan dasar laut menjadi cetek, dan kehilangan laju ini diiringi oleh peningkatan dalam ketinggian ombak. Ombak tersebut terhimpit antara satu sama lain seperti akordian dan meningkat naik. Bergantung kepada jenis punca gelombang tsunami itu, gelombang tsunami boleh menyebabkan air laut surut menyebabkan ikan tertinggal di dasar lautan. Ini menarik orang ramai untuk berkumpul melihat keadaan ini sebelum gelombang tsunami menghentam menyebabkan kematian mereka.
Gelombang tsunami juga mampu melanda dengan tiba-tiba tanpa sebarang amaran. Gelombang tsunami yang melanda tidak bergulong dikemuncak seperti ombak biasa. Mereka yang terselamat menggambarkan gelombang tsunami sebagai dinding air yang gelap. Ombak gergasi tsunami yang terdiri daripada jisim air yang menggunung ini akan menghentam ke pantai dan menenggeamkan kawasan persisiran, menumbangkan pokok seperti ranting kering, merobohkan tembok batu, dan rumah api, dan memusnahkan bangunan seperti bangunan permainan.
Bentuk permukaan dasar laut seperti berbentuk parit, dan persisiran pantai memainkan peranan penting dalam menentukan jenis glombang tsunami yang terhasil, kadang kala dengan keputusan yang mengejut dan merbahayakan Pada tahun 1993, satu glombang tsunami menghentam bandar, di Jepun, ombak purata setinggi 15 hingga 20 meter (50 - 65 kaki). Tetapi pada suatu tempat, ombah tertumpu oleh bentuk 'V' memampatkan ombak dalam ruang yang semakin sempit dan akhirnya menghasilkan gelombang tsunami setinggi 32 meter (90 kaki) dari aras laut, setinggi bangunan 8 tingkat.
Antara tempat di mana berlakunya tsunami yg hebat...
http://www.geophys.washington.edu/tsunami/general/historic/images/historicworld.gif1929 Grand Banks, Canada
1946 Aleutian Islands, Alaska
1952 Kamchatka Peninsula, Russia
1957 Aleutian Islands, Alaska
1960 Chile
1964 Prince Williams Sound, Alaska
1975 Hawaii
1946 Aleutian Tsunami
On April 1, 1946, at 12:29 GMT, an earthquake occurred in the Aleutian Islands of Alaska. A Pacific-wide tsunami was triggered by the earthquake, which had a surface-wave magnitude of 7.8, an epicenter of 52.8 Scotch Cap Lighthouse on Unimak Island, Alaska as it looked before the earthquake and tsunami. The structure was built in 1940. It was 40 feet above the sea and was five stories high. Photograph Credit: U.S. Coast Guard. Source: National Geophysical Data Center.http://www.geophys.washington.edu/tsunami/general/historic/images/64800101.jpg All that remained of Scotch Cap Lighthouse on Unimak Island, Alaska after the tsunami. Photograph Credit: U.S. Coast Guard. Source: National Geophysical Data Center.
http://www.geophys.washington.edu/tsunami/general/historic/images/64800102.jpg
1952 Kamchatka Peninsula Tsunami
On November 4, 1952, at 16:52 GMT, an earthquake occurred off the coast of the Kamchatka Peninsula, Russia. A Pacific-wide tsunami was triggered by the earthquake, which had a surface-wave magnitude of 8.2, an epicenter of 52.8 The tsunami had caused severe damage to Kamchatka Peninsula and then proceeded throughout the pacific. Midway Island was innundated with 1 m of water, flooding streets and buildings. On the Hawaiian Islands the waves destroyed boats, knocked down telephone lines, destroyed piers, scoured beaches, and flooded lawns. In Honolulu Harbor a cement barge was thrown into a freighter. In Hilo Bay a small bridge connecting Cocoanut Island to the shore was destroyed by a wave when it lifted off its foundation and then smashed down.Aerial view of Kaika Bay near Haleiwa on the north shore of Oahu shows the fourth wave climbing up beach toward the beach houses and shows the extent of inundation from previous waves. Photograph Credit: George Curtis. Source: National Geophysical Data Center.
http://www.geophys.washington.edu/tsunami/general/historic/images/64800108.jpg
[ Last edited by jofizo on 6-9-2004 at 01:14 PM ]
1957 Aleutian Tsunami
On March 9, 1957, at 14:22 GMT, an earthquake occurred south of the Andreanof Islands, in the Aleutian Islands of Alaska. A Pacific-wide tsunami was triggered by the earthquake, which had a surface-wave magnitude of 8.3, an epicenter of 51.5 The Island of Kauai, Hawaii, was hit twice as hard by this tsunami than by the Aleutian Islands tsunami in 1946. Houses were washed out and destroyed at Wainiha and Kalihiwai. At Haena, the waves reached heights of 16 m. In addition to that bridges were destroyed and sections of highways were flooded. At Hilo, Hawaii, the run-up was reached 3.9 m and damaged buildings. In Hilo Bay, Cocoanut Island was covered by 1 m of water and the bridge connecting it to shore was destroyed.Second photo in a series of three sequential photos show the arrival of a major wave at Laie Point on the Island of Oahu, Hawaii. Photograph Credit: Henry Helbush. Source: National Geophysical Data Center.
http://www.geophys.washington.edu/tsunami/general/historic/images/64800110.jpg
[ Last edited by jofizo on 6-9-2004 at 01:19 PM ] Although the northwest side of the Hawaiian Islands received high levels of water, the rest of the islands only received elevated water levels on average of 2 to 3 m. Both the 1946 and 1957 tsunamis occurred in the same general location (the Aleutian Islands). Even though the 1957 earthquake released more energy than the earthquake of 1946. The tsunami generated by this 1957 event caused less damage than the tsunami of 1946. This uncertainty of the potential destructive power of a tsunami forces Pacific Tsunami Warning System to issue warnings even when a tsunami may have little or no effect.
Third photo in a series of three sequential photos show the arrival of a major wave at Laie Point on the Island of Oahu, Hawaii. Photograph Credit: Henry Helbush. Source: National Geophysical Data Center.
http://www.geophys.washington.edu/tsunami/general/historic/images/64800111.jpg
[ Last edited by jofizo on 6-9-2004 at 01:23 PM ]
1960 Chilean Tsunami
On May 22, 1960, at 19:11 GMT, an earthquake occurred off the coast of South Central Chile. A Pacific-wide tsunami was triggered by the earthquake, which had a surface-wave magnitude of 8.6, an epicenter of 39.5 Ianya tidak berakhir disitu, tsunami chile ini terus bergerak menghala ke-arah hawaii..After the tsunami had passed the Hawaiian Islands damage costs were estimated at $24 million and 61 people had died. Hilo, on the main island of Hawaii, was the hardest hit city in the islands. The tsunami arrived at Hilo 14.8 hrs after the it originated off the coast of South Central Chile. The - at Hilo was measured at 10.7 m.
Aftermath of the Chilean tsunami in the Waiakea area of Hilo, Hawaii, 10,000 km from the generation area. Parking meters were bent by the force of the debris-filled waves. Note the scattered debris and the gutted foundation. In the area of maximum destruction, only buildings of reinforced concrete or structural steel, and a few others sheltered by these buildings, remained standing--and even these were generally gutted. Frame buildings either were crushed or floated to the limits of flooding. Photograph Credit: U.S. Navy. Source: National Geophysical Data Center.
http://www.geophys.washington.edu/tsunami/general/historic/images/64800113.jpg Downtown Hilo, Hawaii, was left devastated by the tsunami. Photo Credit: The Honolulu Advertiser.
http://www.geophys.washington.edu/tsunami/general/historic/images/waterfront60.jpg
Along the Peru-Chile coast the estimated lost of life from the tsunami ranged from 330 to 2000 people and the - was measured as high as 25m. A city along the western coast of the United States which received notable run-up was Crescent City, California, where the run-up reached 1.7 m and the first wave arrived 15.5 hrs after the tsunami was triggered.
Parking meters along the Hilo bayfront were bent from the tremendous force of the waves. Photo Credit: U.S. Army Corps of Engineers.
http://www.geophys.washington.edu/tsunami/general/historic/images/meters60.jpg
1964 Prince William Sound Tsunami
On March 28, 1964, at 03:28 GMT, an earthquake occurred in Prince William Sound of Alaska triggering a Pacific-wide tsunami. The earthquake had a surface-wave magnitude of 8.4, an epicenter of 61.1 wah devastating :ocam dilanda hurricane je The greatest amount of damage suffered by any location was Alaska. In Alaska the death toll was 106 and there was $84 million in damage. Among Alaskan areas the run-up measurements varied from 24.2 m at Blackstone Bay, 27.4 m at Chenega, 9.1 m at Valdez, and 6.1 m at Kodiak. Outside Alaska it took 5.4 hrs for the first wave to arrive at Hilo, Hawaii, where the run-up was measured at 3.0 m. Another city outside Alaska that received measurable run-up was Crescent City, CA, where a 4.3 m run-up was recorded 4.1 hrs after the tsunami was triggered.
Both the railroad yard and seaport facilities at Seward, Alaska, received heavy damage.
http://www.geophys.washington.edu/tsunami/general/historic/images/classic64.jpg
Even though the regional tsunami was very destructive the local tsunamis also caused significant damage. The local tsunamis were generated by landslides, which were triggered by the earthquake. At the Valdez Inlet a large landslide was triggered by the earthquake generated a tsunami that had a run-up measured at 67.0 m in the inlet. In areas where local tsunamis were generated by landslides nearby cities were given no warning of the oncoming waves. The inability to properly warn the Alaska region prompted the creation of the Alaska Tsunami Warning Center. The warning center can quickly warn towns of any threat of local tsunamis.