Continental strike-slip faults often display complex, along-strike geometries with branches and splays, which play an important role in earthquake rupture processes. We use a digital elevation model and a ground-penetrating radar survey to analyse the Awatere Fault in New Zealand and demonstrate that the number of branch faults and the width of the fault zone increases as the fault passes from bedrock to unconsolidated alluvial sediments. With analogue models of a strike-slip fault, we test whether this observation can be reproduced. Each model incorporates a sand body that represents a basin filled with less consolidated sediments above a basement of corn-starch. Fault branch-points repeatedly formed at the basin-basement boundary in the analogue model, indicating that the geometrical complexity of strike-slip faults can be strongly controlled by lateral changes in the properties of the host material. The change of friction coefficient at the material boundary promotes splays and branch fault nucleation and also fosters fault-bend formation. The thicker the sedimentary cover overlying the basement, the wider the zone of deformation. This implies that the lateral passage of faults from bedrock into unconsolidated material leads to a widening of the deformation zone, which is confirmed by the ground-penetrating radar survey across the Awatere Fault. The results of this study have wide application to active strike-slip faults that run into sedimentary basins, as is, e.g., the case for the Newport-Inglewood Fault in the Los Angeles Basin. • We reproduce GPR field observations by conducting a series of analogue models to test the effect of lateral changes in rock properties (e.g., lithological boundaries) on the geometry of developing strike-slip faults and compare the analogue models to strike-slip faults on the scale of a sedimentary basin. • We show that the intricate structural style of strike-slip faults could be simply the result of changes in the properties of the lithosphere.

We estimated the resistivity structure in the region of the 1997 Northwestern Kagoshima earthquake doublet (March 26, Mw6.1; May 13, Mw6.0) by dense magnetotelluric (MT) observations to investigate the influence of low-resistivity zones on the initiation and termination of large inland earthquake ruptures. In addition to MT data from previous studies, we acquired new broadband MT and telluric data at 42 sites. With a total of 86 sites in approximately 50 km × 50 km area , we constructed a high-resolution resistivity structure through 3-D inversion. A comparison between the slip distribution and resistivity structure showed that the rupture of the mainshock in March initiated at the western edge of a low-resistivity zone, propagated westward (with a maximum slip of 0.8 m), and was ultimately arrested by another low-resistivity zone. Based on geological structures and 3 He/ 4 He ratios in hot springs, we suggest that magma-derived fluids, located beneath a shallow, low-permeability smectite-rich zone and granodiorite, contributed to the initiation of the mainshock rupture. Another smectite-rich zone, along with underlying high pore pressure fluids, played a role in arresting the rupture. The rupture of the mainshock in May initiated at the edge of another low-resistivity zone and propagated in two directions (with a maximum slip of 0.4 m): one along the edge of the low-resistivity zone and the other toward the low-resistivity zone, before finally stopping within the low-resistivity zone. Our findings are consistent with those from the 2016 Kumamoto earthquake (M7.3), supporting the idea that resistivity structures can help assess the spatial potential of large inland earthquakes. • Dense MT observations were conducted on past inland earthquakes. • Rupture of the Mw6.1 mainshock initiated at the edge of a low-resistivity zone. • Rupture of the Mw6.1 mainshock terminated at the edge of another low-resistivity zone. • Magma-derived fluids contributed to the initiation of mainshock rupture. • A smectite-rich zone and underlying fluids played a role in arresting the rupture.