Active Fault Delineation Using Magnetotelluric Data in the Western Region of East Java

The subsurface geological condition of East Java is an area that is crossed by an active fault geological structure. The National Earthquake Center revised the Indonesian Earthquake Hazard and Source Map of 2017 with the discovery of new active faults surrounding the Kendeng Zone, East Java. This research was conducted using Magnetoteluric (MT) method which has high penetration therefore it was able to map the geometry of active deep fault structures beneath the surface. Measurements were made at 11 measuring points focusing on the Blumbang Fault (M6.6) in Lamongan District. 4 (four) lines were made from 11 existing measuring points. The lines were then modelled in 2D to describe the distribution of subsurface resistivity. The results showed an anomaly with low resistivity value which was indicated as Blumbang Fault in 4 section models. Section of line 1 and 2 indicated a fault in the NS-SE direction continuity to the northwest which became deeper to the south from 6 km to 9 km. Lines 3 predicted the fault with low resistivity value (<30 ωm) close to M07 point with a depth of 8 km. However, it did not clearly show its continuity. Line 4 assumed the suspected structure as Blumbang Fault with a resistivity value below 30 ωm that is shallower to the west with 6 km of depth.