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Seismic Swarm Dynamics in the Atacames Region, Ecuador

Description: 

The April 2016 Mw 7.8 Pedernales megathrust earthquake offshore central Ecuador triggered a seismic swarm 8 months later, approximately 70 km northwest of the epicenter, in the Atacames region. The northern part of the megathrust ruptured in 1906 (M 8.8) and 1958 (M 7.6). The Atacames swarm includes 278 events (Ml <1 to 5.8; depths ~2 to 18 km) between December 2 and 28, 2016. Four events ≥Ml 5 contributed to ground shaking that damaged buildings in the region. The swarm was recorded by a dense network of temporary stations deployed as a rapid response effort to the Pedernales mainshock. Using PhaseNet and GaMMA, we built a catalog of 8 P and 6 S phases and located the 278 events using NonLinLoc and HypoDD. The shallow seismicity aligns along a south-dipping structure above a high-velocity basement uplift, imaged in a 3D tomographic inversion, suggesting activation of a previously unrecognized crustal fault. The swarm evolved through three distinct bursts of seismicity separated by quiescent phases. The first burst (December 2–3) included 9 events in a localized central area. The second burst (December 10–14) included 45 events and expanded westward and eastward. The third and most significant burst (December 19–28) contributed over 75% of the total events, migrating northeast and deeper. The spatiotemporal evolution of epicenters suggests dynamic processes, including stress redistribution, localized fault slip, and potential fluid migration. Waveform cross-correlation identified 26 event families and three repeaters, signifying repetitive rupture processes likely driven by localized stress reloading or pore pressure changes. While these observations are consistent with fluid-related mechanisms, ongoing investigations aim to refine our understanding of their specific role in driving seismicity. The Atacames swarm highlights the need to include moderate-magnitude earthquakes along crustal faults, in addition to the rupture of the megathrust, in hazard assessments in general, and as part of earthquake sequences following large ruptures like those in the last two centuries.

Session: Advanced Geophysical Observations, Analytical Methods, and New Insights for Earthquake Swarms [Poster]

Type: Poster

Date: 4/15/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Aracely

Student Presenter: Yes

Invited Presentation: 

Poster Number: 112

Authors