Banca de DEFESA: GILBERTO DA SILVA LEITE NETO

Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.
STUDENT : GILBERTO DA SILVA LEITE NETO
DATE: 12/07/2024
TIME: 08:00
LOCAL: Videoconferência / Canal Youtube PPGG - Youtube.com/@ppggufrn
TITLE:

Deep earthquakes in the Amazon region (Acre): focal mechanisms and source parameters

KEY WORDS:

Deep earthquakes, South America, Transformational faulting, Focal mechanisms, Source parameters

PAGES: 284
BIG AREA: Ciências Exatas e da Terra
AREA: Geociências
SUBÁREA: Geofísica
SPECIALTY: Geofísica Aplicada
SUMMARY:

Deep earthquakes occur at depths greater than 70 km, in the cold core of the subducting
lithosphere, where the extreme pressure and temperature conditions should prevent brittle failure. Thus,
alternative physical processes are necessary in order to explain the occurrence of these events. At depths
exceeding 300 km, earthquakes are attributed to transformational faulting, a mechanism that relies on
the presence of a metastable olivine wedge (MOW) in cold slabs. However, these events also occur in
warm slabs, such as the Nazca plate, which makes explaining these phenomena even more challenging.
This study aims to investigate the generating mechanism in South America by determining focal
mechanisms and source parameters for a sequence of 42 deep-focus events in the Amazonian region
between Peru and Brazil (2014-2022). To achieve this goal, data from several broadband stations
distributed across the continent were utilized, ensuring unprecedented seismic coverage of these events.
Initially, focal mechanisms and centroid depths were determined for a total of 28 events (Mw 4.2-7.5)
using the Cut-and-Paste method. The mechanisms reveal a predominance of normal faults reflecting a
down-dip compression (DDC) regime. Nodal planes, oriented approximately 45o from the axis of
maximum compression, indicate the formation of new faults, consistent with transformational faulting.
Centroid depths demonstrate a seismic zone confined between 557 and 659 km. These observations 

suggest that seismicity may result from transformational faulting within a MOW preserved by a colder
slab segment attached at depth to a warmer Nazca plate. Seismic moments, corner frequencies, and
radiated energies were determined for 13 events (Mw 4.8-7.4) from P- and S-wave spectra. From these
parameters, stress drops (5-90 MPa) and radiation efficiencies (0.1-1.4) were calculated, revealing
significant variations. The variability in radiation efficiencies suggests the coexistence of brittle and
dissipative ruptures, possibly influenced by the degree of melting in the fault zone and favored by the
dehydration of hydrous phases near the source. Thus, this work suggests that deep-focus earthquakes in
the region result from the combination of transformational faulting and thermal runaway, the latter
involving more or less melting depending on the presence of water near the source.

COMMITTEE MEMBERS:
Interno - 1451214 - ADERSON FARIAS DO NASCIMENTO
Interno - 350640 - FRANCISCO HILARIO REGO BEZERRA
Interno - 1863578 - JORDI JULIA CASAS
Externo à Instituição - Juraci Mario de Carvalho - UnB
Externo à Instituição - MARCELO SOUSA DE ASSUMPÇÃO - USP