[Paper Review] Evidence of Early Supershear Transition in the Mw 7.8 Kahramanmaraş Earthquake From Near-Field Records

Ares J. Rosakis, Mohamed Abdelmeguid+1 more|arXiv (Cornell University)|Feb 14, 2023

earthquake and tectonic studies8 citations

TL;DR

The paper uses near-field records to provide field evidence of an early supershear transition on the initiating splay fault, estimating a rupture speed of ~1.55 Cs and a transition length of ~19.45 km.

ABSTRACT

The Mw 7.8 Kahramanmaraş Earthquake was larger and more destructive than what had been expected for the tectonic setting in Southeastern Turkey. By using near-field records we provide evidence for early supershear transition on the splay fault that hosted the nucleation and early propagation of the first rupture that eventually transitioned into the East Anatolian fault. The two stations located furthest from the epicenter show a larger fault parallel particle velocity component relative to the fault normal particle velocity component; a unique signature of supershear ruptures that has been identified in theoretical and experimental models of intersonic rupture growth. The third station located closest to the epicenter, while mostly preserving the classical sub-Rayleigh characteristics, it also features a small supershear pulse clearly propagating ahead of the original sub-Rayleigh rupture. This record provides, for the first time ever, field observational evidence for the mechanism of intersonic transition. By using the two furthest stations we estimate the instantaneous supershear rupture propagation speed to be ~ 1.55 Cs and the sub-Rayleigh to supershear transition length to be around 19.45 km, very close to the location of the station nearest to the epicenter. This early supershear transition might have facilitated the continued propagation and triggering of slip on the nearby East Anatolian Fault leading to amplification of the hazard. The complex dynamics of the Kahramanmaraş earthquake warrants further studies.

Motivation & Objective

• Motivate understanding of rupturing dynamics in large intraplate-like earthquakes in Southeastern Turkey.
• Investigate whether the Kahramanmaraş rupture exhibited supershear characteristics during initiation and early propagation.
• Identify signatures in near-field ground motion that indicate intersonic rupture growth and supershear transition.

Proposed method

• Analyze near-field ground-velocity records from multiple stations to identify fault-parallel versus fault-normal components.
• Characterize signatures of supershear ruptures (fault-parallel dominance) and sub-Rayleigh characteristics in proximal records.
• Estimate instantaneous supershear rupture propagation speed from far-field-like near-field observations at the two furthest stations.
• Infer the sub-Rayleigh to supershear transition length from relative timing/location of recorded pulses.

Experimental results

Research questions

• RQ1 Do near-field records from the Kahramanmaraş earthquake show signatures of supershear rupture during early propagation?
• RQ2 What is the estimated instantaneous supershear rupture speed from available near-field data?
• RQ3 Where is the estimated sub-Rayleigh to supershear transition length, and how does it relate to station locations?

Key findings

• Two stations farthest from the epicenter show a larger fault-parallel velocity component relative to fault-normal, indicating supershear characteristics.
• The station closest to the epicenter preserves mostly sub-Rayleigh features but also records a small supershear pulse ahead of the sub-Rayleigh rupture.
• The instantaneous supershear rupture speed is estimated at approximately 1.55 times the shear wave speed (Cs).
• The sub-Rayleigh to supershear transition length is estimated to be around 19.45 km, near the location of the closest station.
• The observed early supershear transition may have facilitated continued propagation and triggering on the nearby East Anatolian Fault.