Strike-slip seismicity at the Andaman-Sumatra Subduction Zone: Role of the fracture zones and age of the subducting lithosphere

A better comprehension of the shallow strike-slip seismicity along the Sunda Trench is achieved by superimposing the Wharton basin fracture zones, magnetic isochrones, and focal mechanisms to the geometry of the subducting plate. The shallow strike-slip seismicity is generally associated with the north-south trending fracture zones of the basins, which segment the subducting lithosphere into eight main compartments named A-H. Strike-slip earthquakes with magnitudes higher than 4.5 Mw in the time period of 2004-2019 are examined with respect to the thickness and density of the subducting lithosphere, estimated from the lithospheric age. Bulk density ranges from similar to 3258 to similar to 3272 and lithospheric thickness from similar to 74 to similar to 112 km between the younger fossil spreading axis and the older Compartment H, respectively. 'Subductability', a parameter calculated from lithospheric thickness and bulk density, is coined to represent the easiness of subduction and varies for each spreading compartment depending on the lithospheric age. We propose the correlation between the shallow strike-slip seismicity and the contrasts of lithospheric thickness, density and subductability. Considering the available seismic records from the past four decades, a prominent cluster of strike-slip earthquakes is observed along Fracture Zone B-C, which displays a strong subductability contrast, whereas other fracture zones displaying only minor seismicity correspond to low subductability contrasts. Despite a high subductability contrast, slab tearing on the subducting Investigator Fracture Zone between Compartments G and H prevents the generation of strike-slip earthquakes. This result emphasizes the importance of physical and mechanical properties of the subducting lithosphere in localizing the seismicity at subduction zones.

Automated summary

By integrating fracture zones, magnetic isochrones, and focal mechanisms, this study provides a better understanding of the shallow strike-slip seismicity along the Sunda Trench. The research reveals a correlation between the shallow strike-slip seismicity and the contrasts of lithospheric thickness, density, and subductability. The importance of physical and mechanical properties of the subducting lithosphere in localizing seismicity at subduction zones is emphasized.