Elsevier

Tectonophysics

Volume 790, 5 September 2020, 228547
Tectonophysics

Seismicity and seismotectonics of Madagascar revealed by the 2011–2013 deployment of the island-wide MACOMO broadband seismic array

https://doi.org/10.1016/j.tecto.2020.228547Get rights and content

Abstract

The seismicity and seismotectonics of Madagascar have been studied using an island-wide distribution of broadband seismic stations. The 28-station MAdagascar-COmoros-MOzambique (MACOMO) array was deployed for a 23-month period between 2011 and 2013. MACOMO data were supplemented by seven temporary stations from the Seismological Signatures in the Lithosphere/Asthenosphere system of Southern Madagascar (SELASOMA) project, ten temporary stations from the Réunion Hotspot and Upper Mantle - Réunions Unterer Mantel project (RHUM-RUM), and 11 permanent stations. A total of 695 earthquakes with magnitudes between ML1 and ML5.3 located within Madagascar were recorded, a new local magnitude scale was developed, and focal mechanisms were determined for 23 well-recorded events. Most of the seismicity is clustered within central Madagascar, forming NW-SE trends in the Ankaratra region and NE-SW trends in the Alaotra and Ankay regions that roughly align with extensional tectonic features. The northern and southern parts of Madagascar also show seismicity clusters that align parallel to existing tectonic features, primarily Precambrian shear zones. Focal mechanisms exhibit a wide orientation of nodal planes, show predominantly normal faulting throughout Madagascar, and provide no evidence for a sharp east-west striking plate boundary between the Lwandle and Somalian plates in the middle of the island. However, a diffuse plate boundary cannot be excluded. We suggest that topographically-generated extensional stresses is the cause of most seismicity, given that significantly fewer earthquakes are located in the lower elevation areas of the island compared to regions of higher elevations. The frequency-magnitude distribution has a b-value of ~1.2 and indicates that events with magnitude ML5 or greater should occur with an average repeat time of ~1.4 years. However, no earthquake exceeding ML6 has been recorded in the last century, suggesting that the frequency-magnitude distribution could be truncated.

Introduction

Madagascar, the world's fourth-largest island, is located approximately 1200 km east of the East African Rift (EAR) system, 300 km from mainland Africa and 900 km north of the Southwest Indian Ridge. Most of what is known about seismic activity in Madagascar comes from studies that used data from seismic stations concentrated in the center of the island (e.g., Bertil and Regnoult, 1998). In common with other continental intraplate settings, the earthquakes in Madagascar tend to be small in both number and magnitude and associated with ancient zones of tectonic activity, such as terrane sutures, failed rifts, and shear zones. The clusters of low-magnitude events are generally aligned with known faults and grabens (Fourno and Roussel, 1993; Bertil and Regnoult, 1998; Rindraharisaona et al., 2013). The level of seismic activity is generally described as ‘moderate’. However, the rate of seismicity is somewhat larger than expected for a typical continental intraplate region, with about 90 earthquakes greater than M3 occurring annually and about five in the range of M5 to M6 every decade. However, no earthquakes larger than M6 have been reported since record-keeping began in 1835.

Recent studies based on geodetic, seismic, topographic and bathymetric observations have postulated the existence of several microplates in the region between the East African Rift system and the Southwest Indian Ridge (Chu and Gordon, 1999; Calais et al., 2006), and, more specifically, that the boundary between the Lwandle and Somalian plates passes through the central part of Madagascar (Horner-Johnson et al., 2007; Stamps et al., 2008; Saria et al., 2013, Saria et al., 2014).

We present a study of Madagascan seismicity based on a distribution of broadband stations that provides significantly better coverage than previous studies, particularly in the northern and southern parts of the island. The 28-station 2011–2013 MAdagascar-COmoros-MOzambique (MACOMO) array (Wysession et al., 2012) is supplemented by 7 stations from the 2012–2014 Seismological Signatures in the Lithosphere/Asthenosphere system of Southern Madagascar (SELASOMA) network (Tilmann et al., 2012), 10 stations from the Réunion Hotspot and Upper Mantle – Réunions Unterer Mantel (RHUM – RUM) project (Sigloch and Barruol, 2013; Van Wyk de Vries and Merle, 1998) and 11 stations from various permanent networks (GSN, GEOSCOPE, GEOFON, Madagascar National Seismic Network) (Table S1; Fig. S1).

The MACOMO and SELASOMA data sets have already been used to investigate the seismic velocity structure of crust and upper mantle of Madagascar (Andriampenomanana et al., 2017; Rindraharisaona, 2017; Pratt et al., 2017) and seismic anisotropy (Ramirez et al., 2018). Here we exploit the fairly uniform distribution of seismic stations to investigate the seismicity and seismotectonics of the entire island, and to evaluate the hypothesis that a plate boundary passes through it.

Section snippets

Geologic setting

During the Jurassic period, present-day Madagascar (Fig. 1) was situated near the heart of the Gondwana supercontinent, close to the suture between Eastern and Western Gondwana (Segoufin and Patriat, 1980; Coffin and Rabinowitz, 1987; Geiger et al., 2004). The island separated from mainland Africa at ca. 165 Ma along the Davie Fracture Zone, and India at ca. 95 Ma. The transform fault that governed the northeastward movement of the Indian subcontinent bounds the eastern side of the island (Gnos

Data

Most of the data used to investigate the seismicity of Madagascar comes from the 26 temporary broadband seismic stations deployed at 28 sites during 2011–2013 as part of the MACOMO project (Wysession et al., 2012) (Fig. 2, Table S1). These stations, provided by the PASSCAL program of the Incorporated Research Institutions for Seismology (IRIS), were deployed in two phases: Phase A (10 stations) was installed in October 2011 and recorded for two years, except for two stations (VATO, MARO) that

Seismicity

The continuous seismic records were loaded into an Antelope database, which is a commercial software package for seismic data processing developed by BRTT (Boulder Real Time Technology), 2004BRTT. Events were identified using the Antelope dbdetect routine, which uses a short-term-average/long-term-average (STA/LTA) detector, and initial event association was performed using the routine dbgrassoc. P- and S-wave arrivals were then manually picked on the vertical and horizontal components,

Discussion

The increased number and wider distribution of seismic stations used in this study has enabled us to develop a new local magnitude scale and gain a better understanding of the seismicity and the seismotectonics of Madagascar. Previous magnitude scales were based on signal duration rather than amplitude, while the seismicity patterns reported in earlier studies may have been influenced by the concentration of seismic stations near the center of the island (Rakotondrainibe, 1977; Bertil and

Conclusions

An analysis of the seismicity and seismotectonics of Madagascar has been carried out using seismic data recorded in a 23-month period during 2011–2013 by both temporary and permanent broadband seismic stations spread fairly evenly across the island. Hypocenters were determined for 695 earthquakes using P- and S-waves arrival times, and double-difference relocations were obtained for 422 of these events. A new earthquake magnitude scale was determined, the recurrence period of felt earthquakes

Acknowledgements

We gratefully acknowledge the field support from IRIS-PASSCAL, which provided the seismic equipment for the MACOMO project (FDSN code: XV, doi:https://doi.org/10.7914/SN/XV_2011); SELASOMA project (FDSN code: ZE, doi:10.14470/MR7567431421), IRIS (FDSN code: II, doi:https://doi.org/10.7914/SN/II),CEA/DASE (FDSN code: RD, doi.org/10.15778/RESIF.RD) GEOFON (FDSN code: GE, doi:10.14470/TR560404), and GEOSCOPE (FDSN code: G, doi:10.18715/GEOSCOPE.G) for additional seismic data. Figures in this paper

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