Shallow Nonvolcanic Tremor Activity and Potential Repeating Earthquakes in the Chile Triple Junction: Seismic Evidence of the Subduction of the Active Nazca–Antarctic Spreading Center

Abstract

In southern Chile, at ∼46:2°S and ∼75:2°W, the active spreading center between the Nazca and Antarctic plates is colliding with the South American plate, forming the Chile triple junction (CTJ). For 1 yr, from March 2009 to February 2010, five ocean-bottom seismometers (OBSs) were deployed over the CTJ. We used a portion of the OBS data to study the seismic signatures of the subduction of the active Nazca–Antarctic spreading center. Using the envelope technique, we detected long episodes of shallow nonvolcanic tremor (NVT) activity. To improve the identified location of the NVTactivity, we cross-correlated the vertical and horizontal components of all located NVTs. In different months, we measured the local maximum of the lag-time correlation near 2 s, which is associated with the lag between the S and P waves (S − Ptime). Furthermore, we observed that in the days with intense tremor activity, the maxima corresponding to S − Ptime emerged in windows without observable NVTs. We suggest that days with intense tremor activity correspond to an almost continuous slow slip, which may accelerate and decelerates nearly randomly, with spatial and temporal heterogeneity. In addition, we detected some potential repeating earthquakes with an S − Ptime near 2 s, as well as NVTs. The detected NVTactivity and potential repeating earthquakes suggest the existence of a shallow region close to the CTJ that is able to generate brittle (earthquakes) and brittle–ductile (potential repeating earthquakes and NVTs) ruptures. Supplemental Content: Catalogs of nonvolcanic tremors (NVTs), local earthquakes, and repeating earthquakes detected using the ocean-bottom seismometer (OBS) data, and figure showing the hypocenters of localized earthquakes. INTRODUCTION The subduction of an active spreading center beneath a continental plate is a peculiar phenomenon in subduction environments. Currently, in southern Chile (at ∼46:2°S and 75.2° W), the Chile ridge (Nazca–Antarctic spreading center) is subducting beneath the continental South America plate, thereby forming the Chile triple junction (CTJ; Cande and Leslie, 1986; Bourgois et al., 1996; Fig. 1). The collision of the CTJ with the continent has generated local tectonic erosion, rapid uplift, normal faulting, and sedimentary mass wasting in the last 14 Ma (Maksymowicz et al., 2012; Folguera et al., 2018). The presence of a slab window south of the CTJ and beneath the South American plate is another unusual signature of the actual subduction of the Chile ridge (Breitsprecher and Thorkelson, 2009; Russo et al., 2010). Furthermore, some authors have proposed that the CTJ behaved as a barrier to the propagation of the 1960 ValdiviaMw 9.5 megathrust earthquake (Barrientos and Ward, 1990; Moreno et al., 2009). Seismically, the region around the CTJ has been characterized by a low rate of tectonic seismicity mostly related to shallow events (Mw ∼ 0:5–3:4) occurring in the overriding South American plate (Agurto-Detzel et al., 2014). In addition, signatures of slow earthquakes such as deep nonvolcanic tremors (NVTs; Ide, 2012; Gallego et al., 2013; Idehara et al., 2014) and low-frequency earthquakes (LFEs; Gallego et al., 2013) have been detected east of the CTJ (Fig. 1) at depths ranging between 30 and 50 km (Ide 2012; Idehara et al., 2014). Tectonic tremor activity around the CTJ occurs on the plate boundary interface and is controlled by tidal stress (Ide 2012; Gallego et al., 2013). Slow earthquakes occur in areas surrounding the seismogenic zone, where large tsunamigenic and destructive doi: 10.1785/0220180394 Seismological Research Letters Volume XX, Number XX – 2019 1 SRL Early Edition Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/doi/10.1785/0220180394/4801632/srl-2018394.1.pdf by Univ de Chile user on 24 July 2019 megathrust earthquakes occur (Obara and Kato, 2016). Shallow tremor, LFEs, and very low-frequency earthquakes (VLFEs) occur in the shallow part of the plate boundary (Obana and Kodaira, 2009; Sugioka et al., 2012; Nakano et al., 2018) where tsunamigenic earthquakes occur (Lay et al., 2012). On the other hand, different slow earthquakes signatures such as VLFEs, LFEs, and slow-slip event have been detected in the deepest zone of the seismogenic contact (Obara and Kato, 2016, and reference therein). Huge earthquakes such as the 2011 Tohoku-OkiMw 9.0 and the 2014 IquiqueMw 8.1 were preceded by the occurrence of slow earthquakes (Kato et al., 2012; Ito et al., 2013; Kato and Nakagawa, 2014; Ruiz et al., 2014). Studying the occurrence of slow earthquakes is crucial to better understand the generation of megathrust earthquakes. In this sense, in the present study, we attempt to answer the following questions: 1. What kind of seismic signature can be observed when an active spreading center begins to subduct? 2. Can the subduction of an active spreading center generate a slow earthquake? To answer these questions, we analyzed six months of data recorded by five ocean-bottom seismometers (OBSs) deployed above the current CTJ position (Fig. 1). This article is organized as follows. Descriptions of the data and instruments used are given in the Data section. In the NVTs in the CTJ section, we show the occurrence of shallow NVTs in the region. Later, in the S − P Time in NVTs section, we apply the S − P technique to the NVTs to constrain the tremor depth. In the Local Seismicity and Potential Repeating Earthquakes section, we show some examples of local earthquakes and repeaters. In the Discussion and Conclusions section, we discuss our results and analyze the relationship between the different observed seismic signatures. Finally, we propose that close to the CTJ, a region that is prone to break in a brittle–ductile manner generates potential repeating earthquakes and shallow NVT activity.