Ketika penduduk Jakarta banyak yang terlambat pulang ke rumah sampai tengah malam/dini hari karena dihadang banjir di mana-mana, pada saat yang sama penduduk pulau-pulau sebelah selatan Mentawai diserbu tsunami yang menyerbu kampung-kampung mereka. Gempa Mentawai (Pagai selatan) 25 Oktober 2010 pukul 21.42.22 WIB semalam, meskipun sempat menimbulkan perdebatan yang menyebabkan dicabutnya peringatan tsunami, jelas tsunamigenic earthquake. Keempat syaratnya dipenuhi: 1. gempa berepisentrum di laut, 2. pematahannya vertikal (thrust, reverse, normal faults), 3. kedalamannya dangkal (<30 km), dan 4. magnitudenya termasuk besar (>7,0 M). Episentrum gempa berlokasi di 3.484 S dan 100.114 E (sebelah selatan Pulau Pagai, Mentawai; 280 km selatan Padang), pematahan gempa berdasarkan solusi momen tensor adalah sesar anjak (thrust) dengan jurus UBL-STenggara (326 deg NE, dip 12 deg), kedalaman 20,6 km dan magnitude 7,7 M. Kompas.com mencatat tinggi run up tsunami 2,0 meter dan sebuah TV sore ini melaporkan 23 orang tewas dan lebih dari 100 orang hilang tersapu gelombang. Melihat arah jurus pematahan batuan yang UBL-STenggara dan slip sesar naik dengan jelas menunjukkan bahwa gempa kemarin masih satu kelompok dengan gempa-gempa besar di wilayah ini sejak gempa Aceh Desember 2004. Kedalaman gempa dan posisi episentrum di selatan Pagai menunjukkan bahwa pematahan hampir terjadi di batas lempeng (interface) antara lempeng samudera Hindia dan kerak akresi sebelah barat Sumatra. Konvergensi kedua lempeng di sini berjalan dengan kecepatan 57-69 mm/tahun. Dalam konvergensi ini sering terjadi pematahan batuan secara sesar naik sebagai bagian deformasi di prisma akresi. Zone ini kaya gempa dan telah menimbulkan rupture zone yang luas dari Aceh ke Bengkulu. Lokasi episentrum gempa yang sekarang terjadi di dekat lokasi episentrum gempa bermagnitude 8,5 dan 7,9 Mw pada September 2007, terjadi di rupture zone kedua gempa tiga tahun yang lalu itu. Gempa Aceh 9,1 M pada Desember 2004, 800 km ke arah utara dari gempa semalam, gempa Nias 8,6 M pada 2005, 700 km ke arah utara, dan gempa Padang tahun 2009 yang lalu (7,5 M) 300 km ke arah utara dari episentrum gempa semalam berada di rupture zone yang sama. Juga gempa semalam berlokasi di rupture zone gempa 8,7-8,9 Mw tahun 1797 dan gempa 8,9-9,1 Mw tahun 1833. Hati-hati, untuk semua penduduk di pulau-pulau sebelah barat Sumatera dan pantai barat Sumatera, mereka bersebelahan atau berlokasi di dalam rupture zone gempa-gempa besar sejak 220 tahun yang lalu sepanjang sekitar 1200 km, dan banyak gempa di sini berhubungan dengan mekanisme penyesaran naik (Sunda-megathrust) yang kerap menyebabkan tsunami kalau episetrumnya di laut... Ada peringatan tsunami atau tidak, bila di pantai terasa gempa, segeralah lari menuju tempat tinggi dan tunggu satu-dua jam di sana sebelum situasi benar-benar aman... salam, Awang
--- Pada Sel, 26/10/10, Rovicky Dwi Putrohari <rovi...@gmail.com> menulis: Dari: Rovicky Dwi Putrohari <rovi...@gmail.com> Judul: [Forum-HAGI] Indonesia tsunami kills 23, leaves scores missing Kepada: "IAGI" <iagi-net@iagi.or.id>, "Forum HAGI" <fo...@hagi.or.id>, geologi...@googlegroups.com, "MBencana" <benc...@googlegroups.com>, "MJBP" <jabarped...@googlegroups.com> Tanggal: Selasa, 26 Oktober, 2010, 5:06 PM Indonesia tsunami kills 23, leaves scores missing By NINIEK KARMINI, Associated Press Niniek Karmini, Associated Press 21 mins ago AKARTA, Indonesia – A powerful earthquake triggered a tsunami that pounded villages on remote islands off western Indonesia, killing at least 23 people and leaving more than 160 others missing, witnesses and officials said Tuesday. The death toll from the 7.7-magnitude quake, which struck 13 miles (20 kilometers) beneath the ocean floor late Monday, was expected to climb with reports about damage and injuries just starting to trickle in the next day. Mujiharto, who heads the Health Ministry's crisis center, said a 10-foot (three-meter) -high wave washed away hundreds of houses on Pagai and Silabu, part of the remote and sparsely populated Mentawai island chain. "We have 200 body bags on the way, just in case," he said. Indonesia, the world's largest archipelago, is prone to earthquakes and volcanic activity due to its location on the so-called Pacific Ring of Fire. The fault that ruptured Monday, running the length of the west coast of Sumatra island, also caused the 9.1-magnitude quake that unleashed a monster tsunami around the Indian Ocean in 2004, killing 230,000 people in a dozen countries. Getting to the Mentawais, a popular surfing spot 175 miles (280 kilometers) from the Sumatra coast takes 12 hours and the islands are reachable only by boat. A group of Australians said they were hanging out on the back deck of their chartered surfing vessel, anchored in a nearby bay, when the temblor hit. It generated a wave that caused them to smash into a neighboring boat, and before they knew it, a fire was ripping through their cabin. "We threw whatever we could that floated — surfboards, fenders — then we jumped into the water," Rick Hallet told Australia's Nine Network. "Fortunately, most of us had something to hold on to ... and we just washed in the wetlands, and scrambled up the highest trees that we could possibly find and sat up there for an hour and a half." By daytime Tuesday, the toll from the quake and tsunami was rising. Ade Edward, a disaster management agency official, said 23 bodies were found in coastal villages — mostly on the hardest hit island of Pagai — and another 167 people were missing. Water in some places reached roof tops, and in Muntei Baru, a village on Silabu, 80 percent of the houses were damaged. Some 3,000 people were seeking shelter Tuesday in emergency camps, Edward said, and the crews from several ships were still unaccounted for in the Indian Ocean. The quake also jolted towns along Sumatra's western coast — including Padang, which last year was hit by a deadly 7.6-magninuted tremor that killed more than 700. Mosques blared tsunami warnings over their loudspeakers. "Everyone was running out of their houses," said Sofyan Alawi, adding that the roads leading to surrounding hills were quickly jammed with thousands of cars and motorcycles. "We kept looking back to see if a wave was coming," said 28-year-old resident Ade Syahputra. ___ Associated Press writer Irwan Firdaus contributed to this report. http://news.yahoo.com/s/ap/20101026/ap_on_re_as/as_indonesia_earthquake/print -----Berikut adalah Lampiran dalam Pesan----- ______________________________________________ The Indonesian Assosiation Of Geophysicists mailing list. fo...@hagi.or.id | www.hagi.or.id ---*** for administrative query please send your email to itweb.supp...@hagi.or.id
