Engineering-geological investigation and risk assessment for disaster management in the Purworejo Regency, Central Java, Indonesia

摘要

In the Purworejo regency, Central Java, there is a mountainous area, called Menoreh Hills. It is known as a disaster prone, tectonically representing volcanic arcs of the central part of Java Island of the tertiary era. The stratigraphy of the area varieties from the oldest to the youngest rocks, respectively consists of volcanic rocks of the Kaligesing and Dukuh formations, limestones of the Jonggrangan Formation, marl-calcarenite intermittent of Sentolo Formation, and alluvial deposits. The Kaligesing Formation consists of andesite pyroclastic breccia, tuffaceous breccia, and lava breccia, the Dukuh Formation consists of laharic breccia, Jonggrangan Formation comprises reef limestones, Sentolo Formation comprises layered calcarenite limestone, alternating with marl, while alluvial deposits include loose materials with various grain size. The geological structure found in the Menoreh Hills is in the form of joints and lateral slip faults with a general pattern of North West - Southeast and Northeast - Southwest. Especially the volcanic rocks in the Menoreh Hills have experienced extensive weathering, firming soil with a thickness reaches up to 5 meters. Because of the topography with a quite steep slope (>20° or >36%), the soil of quite thick (>2m), the presence of fracture structures that quite tight, triggered by high rainfall (.2000mm/year), Menoreh Hills can be classified as landslide prone areas. In connection with this, a risk assessment needs to be carried out in the context of developing disaster management in the Purworejo Regency area.