High-resolution <i>P</i>-wave tomography of the 1999 Izmit and Düzce earthquake rupture zones

SUMMARY High-resolution seismic tomographic images of active fault zones are essential for linking physical processes and observations during large ruptures. The 1999 Izmit and Düzce earthquakes offer a valuable opportunity to study local fault geology, fault structure and rupture characteristics. By analysing seismic data from the aftershocks of these earthquakes and long-term seismic observations in the region, I computed P-wave velocity variations along ∼160 km long ruptured segments of the North Anatolian Fault (NAF) at km-scale resolution. The large velocity perturbations (±10 per cent) at depths of less than 5 km are associated with known geological structures alongside the fault zone. Lower velocities are observed in the Çınarcık, Izmit, Adapazarı and Düzce basins, while higher velocities are associated with the metamorphic and ultramafic rocks of the Armutlu–Almacık zone. A sharp velocity contrast (±10 per cent) is observed along the 1999 Izmit rupture zone at depths of less than 5 km, but this contrast diminishes at greater depths. The Izmit rupture zone does not hold a large-scale bimaterial interface, but instead exhibits heterogeneties on shorter length-scales. The low-velocity fault zones, ∼3 km thick, detected only in a limited section of the rupture zone. The Almacık block, with its high-velocity core (6.0–6.6 km s−1) extending to a depth of about 13 km, plays a crucial role in shaping the multiple branches of the NAF and in the partitioning of strain. The rupture arrests of both 1999 Izmit and Düzce earthquakes occurred in the transition of Düzce and Karadere faults, where a transtensional structure develops at depth of 8–12 km within the zone along the northern boundary of the Almacık block. There appears to be a link between the coseismic slip during the Izmit earthquake and P-wave velocity gradient along the fault zone. Nonetheless, this apparent correlation requires further evaluation through dynamic rupture simulations.