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Are Ground Motions Different for Aftershocks or Earthquakes Doublets?

Description: 

As seismic hazard analysis moves away from the Poisson assumption, there is a need for ground-motion models for dependent events. We revisit the question of whether ground motion levels are systematically different between mainshocks, aftershocks, and earthquake doublets. Differing ground motion could imply inherently different source processes or interactions and is important for hazard and risk analyses. This question lingers, in part, due to how an aftershock is defined: in the NGA-West2 project, on-fault aftershocks, close in space and time to a corresponding mainshock, were singled out. Previous studies have shown that high-frequency ground motion (e.g., Abrahamson et al., 2014) and stress drop (e.g., Baltay et al., 2019) for these aftershocks is about 35% lower than mainshocks. This could be attributed to on-fault aftershocks re-rupturing weakened patches on the mainshock fault plane, and thus less efficiently generating high-frequency radiation. On the other hand, Abrahamson et al. (2014) found that ground motion for longer periods is larger for aftershocks, with the physical explanation outstanding.

We reevaluate these observations using recent stress-drop estimates and ground-motion observations over a range of periods, for the 2019 Ridgecrest sequence, the 2023 Turkiye sequence, and the NGA-West3 database, keeping the same aftershock definition as in NGA-W2. The M6.4 and M7.1 Ridgecrest, and M7.8 and M7.5 Turkiye doublets show no relative difference in ground motion between each of the two main events at any period, implying that interactions between these doublet events do not affect their generated ground shaking, and thus we should continue to treat them as independent events in ground-motion modeling. While smaller high-frequency ground motion for aftershocks is good news, it is important to consider these systematic disparities when developing non-ergodic ground-motion models from data that includes many aftershocks. If on-fault aftershocks truly have different source physics, giving rise to depleted or enhanced ground motion, this sheds light into stress interactions and fault asperities.

Session: From Earthquake Recordings to Empirical Ground-Motion Modelling - III

Type: Oral

Date: 5/2/2024

Presentation Time: 02:45 PM (local time)

Presenting Author: Annemarie

Student Presenter: No

Invited Presentation: 

Authors