The Role of Faults as Barriers in Confined Seismic Sequences: 2021 Seismicity in the Granada Basin (Betic Cordillera)

Abstract

Fault barriers are key structures for studying seismic hazard in regions of intense brittle deformation. The interaction between fault sets affects their seismogenic behavior, if some of them act as barriers. The Granada Basin, in the Betic Cordillera, is a region affected by shallow brittle deformation, as it was the scenario for the recent Granada 2021 seismic sequence. This seismicity presented a swarm behavior at the beginning of the sequence, followed by mainshock‐aftershock features. Geological and gravity data presented here reveal that the basement is affected by two sets of NW‐SE and NE‐SW normal faults and intensely deformed by vertical NW‐SE joints. Improved relocation of the Granada 2021 seismicity reveals a confined chimney‐shape seismicity caused by the activity of a 2 km long NW‐SE normal fault segment. The confinement of the sequence is associated with the NE‐SW fault set acting as a barrier that restricts the rupture area, limiting the maximum magnitude, and favoring the recurrence of events with smaller magnitude. The chimney‐shape of the seismic sequence suggests that the deformation is propagated vertically to the surface, facilitated by preexisting fractures. The shallow extensional deformation during the uplift of the central Betic Cordillera drove the activity of the local structures obliquely to the regional extensional trends, as evidenced by the seismic sequence. This multidisciplinary study improves the knowledge on the origin of the Granada Basin and underlies the important role of preexisting fractures on fault segmentation and seismic propagation, decreasing the seismic potential of this area.

Insights of Active Extension Within a Collisional Orogen From GNSS (Central Betic Cordillera, S Spain)

“…The Granada Fault System is responsible for the 1431 Otura‐Alhendín earthquake as well as the 1806 Pinos SW of Armilla and 1956 Albolote Earthquakes. This fault system also produced the 2021 Granada Seismic Swarm (Lozano et al., 2022; Madarieta‐Txurruka et al., 2021, 2022). The Baza Fault is the seismogenic source of the 1531 Baza earthquake ( I = VIII–IX, M ms = 6.0) (Alfaro et al., 2008).…”

“…Other geodetic studies quantified the local extension rates of some of the main active faults within the Central Betic Cordillera (Alfaro et al., 2021; Gil et al., 2002, 2017). This active deformation is responsible for significant seismicity, such as the 1884 Andalusia earthquake (I = VIII‐IX, M ms = 6.5) (Reicherter et al., 2003), the 1531 Baza earthquake ( I = VIII–IX; Mw = 6.0) (Alfaro et al., 2008), and the 2021 Granada seismic swarm (Madarieta‐Txurruka et al., 2021, 2022).…”

Active Shortening Simultaneous to Normal Faulting Based on GNSS, Geophysical, and Geological Data: The Seismogenic Ventas de Zafarraya Fault (Betic Cordillera, Southern Spain)

“…Seismicity data can be found in the IGN online database (National Geographic Institute, 2023). The relocated seismicity data is available in Table S1 and are archived into Zenodo repository (Madarieta‐Txurruka, González‐Castillo, et al., 2023). Earthquake focal mechanisms are compiled in Table S2, sourced from both the literature and the IGN online database (SMT, 2023).…”

“…The Betic Cordillera is currently affected by a NNW–SSE to NW–SE 4–6 mm/yr Eurasia‐Nubia convergence (DeMets et al., 2010; McClusky et al., 2003; Nocquet & Calais, 2003). At the same time, a nearly 2 mm/yr ENE–WSW extension perpendicular to regional compression occurs in the central Betic Cordillera (Galindo‐Zaldívar et al., 2015; Martín‐Rojas et al., 2023; Palano et al., 2015; Serpelloni et al., 2007), forming active high‐ and low‐angle normal faults (Galindo‐Zaldívar et al., 1999; Lozano et al., 2022; Madarieta‐Txurruka et al., 2021, 2022; Sanz de Galdeano et al., 2003). Today, the Granada Basin is the main extensional active basin of the central Betic Cordillera (Morales et al., 1990; Rodríguez‐Fernández & Sanz de Galdeano, 2006).…”

“…It is affected by low-to-moderate recurrent seismicity in its eastern part (Galindo-Zaldívar et al, 1999) related to NW-SE striking normal faulting that indicates W-E to NE-SW extension (Madarieta-Txurruka et al, 2021). This sector is also affected by perpendicular NE-SW striking normal faults (Madarieta-Txurruka et al, 2022) and microfaults also evidencing radial extension in the region (Galindo-Zaldívar et al, 1999). In addition, GNSS data available for the region (Galindo-Zaldívar et al, 2015;Gil et al, 2017) point to an extension that rotates and decreases in rate from E-W in the north to NE-SW in the south (Madarieta-Txurruka et al, 2022).…”