Variation of Principal Stress Direction with Depth in North Sumatra, Indonesia

Abstract Sumatra, Indonesia, is situated in a highly seismically active region. The Sumatra Fault Zone, which spans 1,900 kilometers, runs along the island’s spine and develops within or around active volcanic arcs. The area of interest is located at the boundary between the Indo-Australian and Eurasian tectonic plates, resulting in high pressure and increasing the risk of earthquakes. The Sumatra Fault Segment Renun is characterized by a strike-slip type fault with a west-northwest to southeast orientation in the northern part of the island. This study aims to identify variations in the principal stress direction with depth in the subduction zone, the Sumatra Fault Segment Renun, and transition zone between two structures mentioned. The study used earthquake data from 1977 to 2023, which contains 72 events with focal mechanisms dominated by strike-slip. The earthquakes’ depths were categorized into three groups: 10-40 km, 41-60 km, and 61-100 km. Iterative joint inversion method was applied to determine the rotation of stress that occurred by utilizing data on focal mechanisms to estimate stress within the Earth’s crust. The results included a principal stress diagram with P/T (Pressure/Tension) axis, confidence level of principal stress, Mohr circle, and shape ratio. The principal stress direction (σ 1 ) and shape ratio for the three depth categories are: north-south with a ratio of 0.4-0.55, southwest with a ratio of 0.7-0.9, and west-northwest to southeast with a ratio of 0-0.38. This suggests that the orientation of principal stress in Sumatra Fault Segment Renun is predominantly north-south at shallow depths of 10-40 km, which is positively correlated with the Sumatra Fault and indicates that this stress drives the movement of the Sumatra Fault Segment Renun with a west-northwest to southeast orientation.