Banca de QUALIFICAÇÃO: GILBERTO DA SILVA LEITE NETO

Uma banca de QUALIFICAÇÃO de DOUTORADO foi cadastrada pelo programa.
STUDENT : GILBERTO DA SILVA LEITE NETO
DATE: 30/06/2022
TIME: 15:00
LOCAL: auditório do CCET
TITLE:

Focal mechanism determination of deep earthquakes in the Amazonian region of Acre

KEY WORDS:

Deep-focus earthquakes; Rede Sismográfica Brasileira; Seismic moment tensor; Cut & Paste.

PAGES: 20
BIG AREA: Ciências Exatas e da Terra
AREA: Geociências
SUMMARY:

Deep earthquakes are defined as earthquakes occurring at depths greater than 70 km. Such earthquakes can be further divided into intermediate-focus (70 < d < 300 km) and deep-focus (d > 300 km) events. Deep seismicity is generally associated to subduction zones, where earthquakes are taking place inside the cold subducting oceanic lithosphere. Despite both deep and shallow earthquakes being well explained through double-couple sources, suggesting shear rupture along planar surfaces, ordinary brittle failure should be impossible at the pressure and temperature conditions where deep earthquakes take place. Thus, the mechanism responsible for deep events remains enigmatic. Particularly, the occurrence of deep-focus events (d > 500 km) is restricted to only a handful of regions worldwide. In South America, deep-focus seismicity is occurring in four distinct regions, two of them exhibiting regular activity with a linearly-trending distribution parallel to the trench. One of these main segments is along the Peru-Brazil border in the Amazonian region. Since 2014, several broadband stations from the Brazilian Seismic Network (RSBR) were installed in the region, which considerably improved the azimuthal coverage of these deep-focus events. From 2014 to 2022, the USGS catalog listed up to 40 deep-focus events in the Peru-Brazil area, with 25 occurring in 2015, with magnitudes and depths ranging from 4.0 to 7.6 Mw and 515 to 655 km, respectively. Aiming at investigating the physical mechanism responsible for these deep-focus events, we are using a combination of regional and teleseismic recordings from the RSBR and other networks in the continent to determine focal mechanisms for earthquakes (M > 4) between 2014 and 2022 along the Peru-Brazil border. The mechanisms are being determined through a Cut and Paste (CAP) approach, which compensates for inaccuracies in the velocity model through independent relative time shifts between observations and predictions for P, SV and SH wave trains sampling both the upper and lower hemispheres of the focal sphere. Prior to this investigation, we implemented and tested the CAP method using RSBR data. Initially, we tested its applicability to shallow intraplate events by performing an inversion of the Mw 4.4 2017 Maranhão mainshock, and the Mw 4.1 2020 Amargosa event. Finally, we tested its applicability for deep events at the Peru-Brazil border using the first event of the well-investigated 2015 doublet (Mw 7.5). Our results were fully compatible with those obtained using other methodologies, showing that the CAP can be successfully used to routinely determine focal mechanisms of shallow and deep earthquakes in Brazil recorded by the RSBR even when using imprecise velocity models. After that, we applied the CAP to the deep sequence of 2014-2022. Preliminary inversions using only RSBR data were performed for the 2015 Mw 7.5 doublet as well as for other 15 smaller earthquakes (4.3 < M < 7.1). All of these inversions resulted in normal mechanisms with T axes oriented roughly E-W. This apparent uniformity of the focal mechanisms for the South-American deep-focus earthquakes, with near-vertical P axes and near-horizontal (east-west-oriented) T axes, strongly suggests vertical compressionalong the subducting plate is the main source of stress driving deep-focus seismicity. Down-dip compression is expected from either buoyancy forces, equilibrium phase transformations or a metastable olivine wedge. The next steps are to include data from other networks in the inversions and to determine other source parameters (stress drop, radiated seismic energy, seismic efficiency) to investigate mechanism behind deep-focus earthquakes.

COMMITTEE MEMBERS:
Externo à Instituição - GERMAN ANDRES PRIETO GOMEZ
Interno - 1451214 - ADERSON FARIAS DO NASCIMENTO
Interno - 350640 - FRANCISCO HILARIO REGO BEZERRA
Presidente - 1863578 - JORDI JULIA CASAS