Opak fault: a comparative review

Abstract

The term of Opak fault as commonly used today refer to the subsurface rupture beneath Opak river, which is covered by young Merapi volcano sediment, located in Bantul Region southeast of Yogyakarta within the entire of southern mountain. The early concerning to the existing of Opak fault appear to be awakened since Dr. S.W. Visser reported Yogyakarta earthquake of 1867 which epicenter located near opak river. Since the Yogyakarta earthquake on 27 May 2006, Opak fault become famous and researcher believe that the earthquake caused by this fault. Further more, the conclusions regarding fault position remain equivocal are due to difference in each idea which resulting from their researches. It is understood because the main fault body exposure is limited and not available data at all to collecting due to covering by young Merapi sediment. However, as ontologically it is still remain question mark and has not being met the essence of opak fault existence. This paper proposes a preliminary result of the comparative review taking from the conclusions of the last several researchers regarding its opak fault existence. In order to determine either aspect of structure elements which have been conformed or still remain the uncertainties. Previous studies have reported some results regarding opak fault as summarized: 1) Opak fault system striking from the Southwest to the Northeast; 2) The lineament of Poncosari – Taman Tirto was identified as a fault which has striking parallel to opak fault; 3) Between Poncosari Tamantirto lineament and opak fault trending create a zone with corridor at approximately 10 km; 4) Different ideas of opak fault dip direction resulting from researchers, either opak fault has dip direction toward the Northwest or the Southeast. At least two group of ideas arising based upon the list explained above which have been identified.(1) Although they have coincident in the trending SW-NE of the opak fault. Yet in kinematic aspects appear different opinion due to yielding from different methods. Either they concluded in trending left lateral strike slip fault or thrust fault with dip direction to the Southeast. (2)The lineament Poncosari Tamantirto is still need to find field evident while this fault was interpreted from InSar to reveal surface deformation.(3) Poncosari-Tamantirto fault is shown in gravity pattern starting from Kasihan toward Glagah Jatinom, Klaten. (4) Updating of new opinion regarding dip direction to resulted the Southeast of dip direction and directed to the hanging wall area in Wonosari Gunung kidul. In the next stage of research, it is required an investigation in the field within corridor of opak fault zone to determine the areas where need to clarify and to find the evident at the edge of hills .