Time-dependent and spatiotemporal statistical analysis of intraplate anomalous seismicity: Sarria-Triacastela-Becerreá (NW Iberian Peninsula, Spain)

Abstract

Summary The Sarria-Triacastela-Becerreá seismicity is located in an intraplate region considered seismically stable, but in 1995 started an unprecedented earthquake activity in the area. Since then an anomalous long-term continuous seismicity remains until now in the same location. Despite the long-term seismicity and the large magnitudes for the region standards (5.1 Mw), the origin and mechanisms of this seismicity remains unclear. The isolation of background from the mainshock-aftershock contribution by means of the application of the Epidemic Type Aftershock Sequences (ETAS) model, the spatial depiction, and the resemblance to near seismic regions allow us to identify several seismicity behaviours. From 1995 to 1998/99, the seismicity in this location mainly consists of intensive and deeper earthquake sequences linked to the 1995 and 1997 mainshocks. Our results suggest that the mainshocks triggered aftershocks and additionally initiated or facilitated aseismic processes. A likely scenario is that the mainshocks broke a sealed source at depth allowing a subsequent intrusion of high-pressurized fluids from depth. The resemblance of this period with Zamora seismic characteristics proposes that Triacastela seismicity was also initiated by tectonic activity. From 1998/99 to 2018, a change in seismicity is observed, the background contribution took control and swarm-type activity is predominant. While the earthquake rate decreases, the relative background contribution goes up. Actually, after 2013 the clusters almost disappear and background contribution achieves 55 per cent of the total activity. The spatial migration to the southeast and the upward trend to shallower depth support fluid migration as possible driving mechanism responsible for the transient seismicity in this period. The swarm-type activity in Triacastela in later periods and the resemblance of b-values with Ponte Caldelas and Ventaniella seismicity suggest that the seismicity in Triacastela is related to fluid migration and the reactivation of fractured areas. We propose that the mechanism of this anomalous and long-term seismicity in Triacastela is the mix of different mechanisms, starting with the tectonic seismicity, generated during the 1995 and 1997 seismic sequences, which initiated a fluid upward migration through fractured crustal fault patches, observed after 1998–99, and responsible for the seismicity during the following 20 years.

Identification and Temporal Characteristics of Earthquake Clusters in Selected Areas in Greece

“…Marsan et al [19] introduced an ETAS model with a time-dependent background component for the detection of aseismic transients. The modified ETAS model is used efficiently to reveal both main shock-aftershocks and earthquake swarms [20,21]. Finally, the Nearest-Neighbor metric proposed by Baiesi and Paczuski [22] adopts a nonparametric definition of a cluster considering the space-time-magnitude proximity among earthquakes.…”

“…ETAS parameters used for the synthetic earthquake catalog with m c = 2.5. The target area is Corinth Gulf, Greece, withΣx[t in , T] = [21.3 • E − 23.2 • E] × [37.9 • N − 38.6 • N] ×[2,20]. The number of clustered events is N = 4253 and the number of mother events is N bg = 1595.…”