Pak Rovicky, Ketebalan kerak benua Sumatra 30 km (Malod dan Kemal, 1996). Bila kedalaman sumber gempa (fokus) sampai 50.8 km berarti pusat gempa ada di slab (kerak oseanik) yang menunjam di bawah kerak benua dan akresinya. Hanya, propagasi gaya pasti lebih mudah via sesar yang ada di sekitarnya, maka episentrum gempa ini muncul di jalur Sesar Mentawai (master atau splay fault-nya).
Masih ada kemungkinan lain juga, yaitu bahwa master fault Sesar Mentawai sebagai sesar mendatar regional bisa dalam, sedalam Sesar Sumatra, yaitu hampir 100 km sampai batas bawah lithospheric mantle (Malod dan Kemal, 1996). Saya pikir bila Sesar Mentawai setengah saja kedalaman Sesar Sumatra, maka ia dalamnya sudah 50 km, atau persis dengan kedalaman pusat gempa ini. Pada kedalaman 50 km saya pikir reologi kerak oseanik masih cukup brittle untuk terpatahkan dan menimbulkan gempa. Keberadaan Sesar Mentawai ini bisa menjadi semacam earthquake force thief zone, bila tak ada Sesar Mentawai, gaya gempa dari kedalaman 50 km tak akan menuju Sumatra, tetapi justru dibuang ke sebelah barat Siberut menuju Laut Hindia. Kita belajar tentang hal ini saat di utara Jawa beberapa tahun yang lalu ada gempa besar dengan kedalaman pusat gempa >300 km; yang rusak bukan pantai utara Jawa yang dekat dengan episentrum, tetapi justru wilayah pantai selatan Jawa sebab propagasi gaya gempa merambat sepanjang Benioff zone dari tempat dalam di utara Jawa menuju ke tempat dangkal di selatan Jawa. salam, Awang --- On Sun, 8/16/09, Rovicky Dwi Putrohari <rovi...@gmail.com> wrote: > From: Rovicky Dwi Putrohari <rovi...@gmail.com> > Subject: Re: [iagi-net-l] Gempa Siberut 16 Agustus 2009 (7.0 Mw) > To: iagi-net@iagi.or.id > Cc: "Geo Unpad" <geo_un...@yahoogroups.com>, "Forum HAGI" <fo...@hagi.or.id>, > "Eksplorasi BPMIGAS" <eksplorasi_bpmi...@yahoogroups.com> > Date: Sunday, August 16, 2009, 3:57 PM > Pak Awang > Kalau gempanya pada kedalaman 50 Km padahal ketebalan kerak > hanya 30-40 Km > apakah ini sesar geser Mentawai ? > Berapa ketebalan kerak benua di Sumatra ? > > RDP > > On Sun, Aug 16, 2009 at 4:26 PM, Awang Satyana <awangsaty...@yahoo.com>wrote: > > > Sebuah gempa cukup besar (7.0 Mw, 6.9 SR) baru saja > terjadi (pukul 14.38 > > WIB) di wilayah perairan Sumatra Barat dan menggoncang > wilayah di > > sekitarnya, terutama Provinsi Sumatra Barat. Kedalaman > gempa yang dilaporkan > > BMG (data sementara) adalah 38 km, meskipun menurut > data sementara USGS > > kedalaman fokus gempa 50,8 km (seperti data detail di > bawah). > > > > Secara tektonik, episentrum gempa tepat berlokasi di > sesar dextral Mentawai > > Fault, di sebelah timur Pulau Siberut, pada perairan > forearc basin. Momen > > tensor solution sampai tulisan ini dibuat belum > tercantum di data USGS; > > kemungkinan besar merupakan pemetahan mendatar > sebagaimana sesar > > regionalnya. > > > > Kerusakan yang terjadi dan korban belum diketahui. > > > > Earthquake Details (USGS) > > Magnitude 7.0 > > Date-Time Sunday, August 16, 2009 at 07:38:26 UTC > > Sunday, August 16, 2009 at 02:38:26 PM at epicenter > > Time of Earthquake in other Time Zones > > > > Location 1.409°S, 99.494°E > > Depth 50.8 km (31.6 miles) > > Region KEPULAUAN MENTAWAI REGION, INDONESIA > > Distances 110 km (65 miles) WSW of Padang, Sumatra, > Indonesia > > 305 km (190 miles) SW of Pekanbaru, Sumatra, > Indonesia > > 560 km (345 miles) SSW of KUALA LUMPUR, Malaysia > > 960 km (600 miles) WNW of JAKARTA, Java, Indonesia > > > > Location Uncertainty horizontal +/- 7.5 km (4.7 > miles); depth +/- 13.9 km > > (8.6 miles) Parameters NST= 54, Nph= 54, Dmin=683.7 > km, Rmss=1.03 sec, Gp= > > 40°, M-type=teleseismic moment magnitude (Mw), > Version=7 > > Source USGS NEIC (WDCS-D) > > > > Kemungkinan tsunami kecil menurut NOAA, tetapi itu > berdasarkan historic > > data. Diharapkan pejabat setempat mewaspadai, terutama > di radius 100 km dari > > episentrum gempa ke arah Pulau Sumatra. > > > > TSUNAMI BULLETIN NUMBER 001 > > PACIFIC TSUNAMI WARNING CENTER/NOAA/NWS > > ISSUED AT 0749Z 16 AUG 2009 > > > > THIS BULLETIN IS FOR ALL AREAS OF THE INDIAN OCEAN. > > > > ... TSUNAMI INFORMATION BULLETIN ... > > > > THIS MESSAGE IS FOR INFORMATION ONLY. > > > > THIS BULLETIN IS ISSUED AS ADVICE TO GOVERNMENT > AGENCIES. ONLY > > NATIONAL AND LOCAL GOVERNMENT AGENCIES HAVE THE > AUTHORITY TO MAKE > > DECISIONS REGARDING THE OFFICIAL STATE OF ALERT IN > THEIR AREA AND > > ANY ACTIONS TO BE TAKEN IN RESPONSE. > > > > AN EARTHQUAKE HAS OCCURRED WITH THESE PRELIMINARY > PARAMETERS > > > > ORIGIN TIME - 0738Z 16 AUG 2009 > > COORDINATES - 1.6 > SOUTH 99.4 EAST > > LOCATION - SOUTHERN > SUMATRA INDONESIA > > MAGNITUDE - 6.9 > > > > EVALUATION > > > > A DESTRUCTIVE WIDESPREAD TSUNAMI THREAT DOES NOT > EXIST BASED ON > > HISTORICAL EARTHQUAKE AND TSUNAMI DATA. > > > > HOWEVER - THERE IS A VERY SMALL POSSIBILITY OF A > LOCAL TSUNAMI > > THAT COULD AFFECT COASTS LOCATED USUALLY NO MORE > THAN A HUNDRED > > KILOMETERS FROM THE EARTHQUAKE EPICENTER. > AUTHORITIES IN THE > > REGION NEAR THE EPICENTER SHOULD BE MADE AWARE > OF THIS > > POSSIBILITY. > > > > salam, > > Awang > > > > > > > > > > > > > > > -------------------------------------------------------------------------------- > > PP-IAGI 2008-2011: > > ketua umum: LAMBOK HUTASOIT, lam...@gc.itb.ac.id > > sekjen: MOHAMMAD SYAIFUL, mohammadsyai...@gmail.com > > * 2 sekretariat (Jkt & Bdg), 5 departemen, banyak > biro... > > > > > -------------------------------------------------------------------------------- > > ayo meriahkan PIT ke-38 IAGI!!! > > yg akan dilaksanakan di Hotel Gumaya, Semarang > > 13-14 Oktober 2009 > > > > > ----------------------------------------------------------------------------- > > To unsubscribe, send email to: > iagi-net-unsubscribe[at]iagi.or.id > > To subscribe, send email to: > iagi-net-subscribe[at]iagi.or.id > > Visit IAGI Website: http://iagi.or.id > > Pembayaran iuran anggota ditujukan ke: > > Bank Mandiri Cab. 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