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Depth Variation in Megathrust Rupture Leads to Mature Tsunami Gap in Metropolitan Chile

Description: 

“Seismic gaps” refer to segments along active geologic faults that have not ruptured in a time period comparable with the recurrence intervals of past earthquakes, and are therefore expected to rupture at any time. This concept usually describes spatial variations in earthquake recurrence in the strike dimension. However, modern seismological observations show that rupture can occur at a variety of depths, so that the lack of a seismic gap in the strike direction may not indicate the lack of a seismic gap in the dip direction. This recognition has important hazard implications. For example, within a given strike segment of a subduction megathrust, an earthquake may rupture only one depth section, with another section still accumulating energy for a future large earthquake. Here, we present unambiguous historical evidence for the presence of such an unruptured depth section in metropolitan Chile. In this area, four earthquakes greater than magnitude 8 (1730, 1822, 1906 and 1985) were documented in the written history, but only the first (1730) of this sequence was accompanied by a large tsunami. By combining newly found first-hand accounts of coastal uplift and tsunamis with coupled deformation-tsunami models, we show that the three post-1730 earthquakes failed to produce large tsunamis because their rupture zones were confined to relatively large depths beneath land. In contrast, the large tsunami in 1730 was the consequence of large slip of the shallow section of the megathrust beneath the sea. Because the shallow section has not ruptured in a time period comparable with the recurrence intervals of prehistoric tsunamis (2-6 centuries), it creates a “tsunami gap” today along Chile’s most populated coast that could be filled by a large tsunami at any time. Similar tsunami gaps are likely present in other subduction zones and should be addressed in hazard analysis and risk mitigation.

Session: Six Decades of Tsunami Science: From the Source of the 1964 Tsunami to Modern Community Preparedness [Poster Session]

Type: Poster

Date: 5/2/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Matías

Student Presenter: No

Invited Presentation: 

Authors