Pak Dwi, Gempa Pulau Andaman pada 10 Agustus 2009 (magnitude 7.6 Mw, depth 33 km) merupakan gempa overriding plate pada segmen Burma plate, bukan pada slab Indian oceanic crust yang menunjam di bawahnya. Gempa itu terjadi pada ujung rupture zone gempa besar Aceh 26 Desember 2004. Sementara gempa Siberut kemarin merupakan slab earthquake di bagian atas oceanic crust yang menunjam di bawah kerak benua/akresi Sumatra. Mekanismenya berbeda. Bisa saja gempa yang berbeda hampir seminggu kejadiannya ini saling memicu (distant earthquake trigger) - tetapi ini perlu evaluasi lebih lanjut. Untuk ke Jawa umumnya kecil kemungkinannya karena mekanismenya sudah semakin berbeda. Saat gempa besar Aceh 26 Des 2004 pun, tak ada pemicunya ke Jawa kan; hanya sampai Nias saat itu yang beda waktunya beberapa bulan.
salam, Awang > > --- On Sun, 8/16/09, Awang Satyana <[email protected]> > wrote: > > > From: Awang Satyana <[email protected]> > > Subject: [iagi-net-l] Re: [Geo_unpad] Gempa Siberut 16 > Agustus 2009 (7.0 Mw) > > To: [email protected] > > Cc: "IAGI" <[email protected]>, > "Forum HAGI" <[email protected]>, > "Eksplorasi BPMIGAS" <[email protected]> > > Date: Sunday, August 16, 2009, 8:38 AM > > Pak Iyan, selamat malam. > > > > Sesar Mentawai ada, itu bisa dilihat buktinya dari > beberapa > > penampang seismik di dalam publikasi Malod dan Kemal > (1996) > > : The Sumatra margin : oblique subduction and lateral > > displacement of the accretionary prism -Geol.Soc. > Spec.Publ. > > no. 106, pp. 19-28. Di dalam penampang2 seismik itu > > keberadaan Sesar Mentawai ditunjukkan oleh > pergeseran2 > > horizon seismik yang menunjukkan pola flower > structuring. > > > > Bila kita unduh data historic momen tensor solution > dari > > database USGS pun jelas bahwa "beach ball" momen > tensor > > solution tersebut dominan menunjukkan penyesaran > sesar > > mendatar di sepanjang sisi barat Sumatra. Tidak perlu > > diragukan lagi Sesar Mentawai ada. > > > > Tidak seperti Sesar Sumatra yang menjadi tempat > munculnya > > gunungapi2 Kuarter di sepanjang Bukit Barisan, maka > Sesar > > Mentawai sepi dari gunungapi. Ini tak perlu menjadi > alasan > > keraguan akan keberadaan Sesar Mentawai. Seperti kita > tahu, > > gunung2api di Sumatra atau di pinggir lempeng aktif > muncul > > karena proses partial melting atau peleburan sebagian > slab > > (oceanic crust) dan air yang masuk ke bawah kerak > kontinen. > > Kedalaman peleburan ini umumnya terjadi di antara > 100-200 km > > Benioff zone (Dickinson, 1971). Nah Sesar Sumatra > tepat > > berada di zone partial melting slab, maka melted > materials > > naik ke atas via konduit Sesar Sumatra dan jadilah > gunungapi > > di permukaan. > > > > Sementara itu, Sesar Mentawai hanya berada di atas > batas > > prisma akresi dengan kerak kontinen/akresinya dengan > > kedalaman subducted slab maksimal hanya 30 km (Malod > dan > > Kemal, 1996). Maka wajar di sepanjang Sesar Mentawai > tak ada > > satu pun gunungapi sebab tak ada melted material dari > slab > > yang naik ke atas. > > > > Semua di alam punya kaidah dan hukum; kalau kaidah > dan > > hukum tak dipenuhi tak akan terjadilah sesuatu itu. > Kata > > Einstein, Tuhan tak bermain dadu dengan Alam Semesta. > > > > salam, > > Awang > > > > > > > > > > > > --- On Sun, 8/16/09, Iyan Haryanto <[email protected]> > > wrote: > > > > > > From: Iyan Haryanto <[email protected]> > > Subject: Re: [Geo_unpad] Gempa Siberut 16 Agustus 2009 > (7.0 > > Mw) > > To: [email protected] > > Date: Sunday, August 16, 2009, 7:26 PM > > > > > > > > > > > > > > > > > > > > > > > > Slamat malam Pak Awang, > > Saya ada sedikit pertanyaan walaupun tidak terkait > langsung > > dengan gempa tersebut. Mengenai Sesar Mentawai, apakah > benar > > ada sesar tersebut? mengingat di sebagian besar jalur > sesar > > tersebut tidak dijumpai gunungapi aktif, seperti > halnya > > sesar Sumatra yang memunculkan banyak gunungapi. > Jika > > memang betul ada, kenapa tidak seperti di Sumatra? > > terimakasih Pak Awang. > > Salam Iyan. > > > > --- On Sun, 8/16/09, Awang Satyana <awangsatyana@ > > yahoo.com> wrote: > > > > > > From: Awang Satyana <awangsatyana@ yahoo.com> > > Subject: [Geo_unpad] Gempa Siberut 16 Agustus 2009 > (7.0 > > Mw) > > To: "IAGI" <iagi-...@iagi. or.id>, "Geo Unpad" > > <geo_un...@yahoogrou ps.com>, "Forum HAGI" > <[email protected]. > > id>, "Eksplorasi BPMIGAS" <eksplorasi_bpmigas@ > > yahoogroups. com> > > Date: Sunday, August 16, 2009, 8:26 AM > > > > > > > > > > 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. 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