MACROSSISMICITY ASSOCIATED WITH THE ARRAIOLOS EARTHQUAKE OF JANUARY 15, 2018 WITH M = 4.9 AND POSSIBLE IMPLICATIONS IN THE GEOMETRY OF THE RUPTURE

Abstract

is well known a low, diffuse and persistent seismic activity in the region north of Évora, particularly concentrated between Arraiolos, Pavia and Vimieiro. On 15th January , 2018, at 11:51 am there was an earthquake of magnitude considerably higher than usual, M = 4.9 according to the Portuguese Institute of the Sea and Atmosphere (IPMA). This earthquake occurred at a depth of 11 km and its epicenter was located between Aldeia da Serra and São Gregório villages. The effect was felt in a very large area, even in Oporto city, according to what was reported in the media. According to the magnitude of the earthquake, we carried out a study of macrossismicity at regional level, in order to construct a map of seismic intensities. The fieldwork took place between 17th and 24th of the same month, using the questionnaire which IPMA has available at https://www.ipma.pt/pt/geofisica/informe/. The data collection in the field, covered the area lying approximately between latitudes 37.9o N and 39.5o N and between longitudes 7.3o W and 9o W. At the same time, we provided the same questionnaire on the Internet, and collected responses from many parts of the country between 18th and 26th January. The results obtained allowed to construct a map of intensities showing a clear elongation of the intensities in a N-S direction. This orientation can be explained by the differences in the behavior of seismic waves depending on the nature of the rocks or by a directivity effect associated with the rupture along a fault with this direction. On the one hand, an earthquake of magnitude 4.9 with a hypocenter at 11km will hardly have the capacity to produce a clear directivity effect, however on the other hand the analysis of the Portuguese Geological Chart seems to discard the first hypothesis. In fact, regional geological heterogeneities that explain this distribution of intensities are unknown. IPMA put available on its page a strike slip focal mechanism for this earthquake (E-W dextral and N-S sinistral). The conjugation of this focal mechanism with the distribution of intensities, seems to reinforce the hypothesis of rupture occurring along an N-S plane. In the Geological Chart of Portugal at 1/50000 scale is mapped a fault with orientation near N-S with some kilometers of extension and with geomorphological expression in the proximities of Aldeia da Serra. This fault could be a superficial evidence of the structure that generated the earthquake of January 15. Focal mechanisms similar to this are known, associated with previous earthquakes in this region. In the work of Araújo et al (2010), it was related, not to N-S ruptures, but to WNW-ESE structures present in the region, with geological and geomorphological evidence of recent tectonic activity. It should also be noted that in the period between 2000 and 2009 there was a linear distribution of microseismicity according the WNW-ESE direction, from this region to the town of Benavente. The information collected from the questionnaires suggests a rupture along a N-S fault. However, given that in this type of studies there is always a factor of subjectivity inherent to the sensitivity of people who were questioned, these data are not enough to affirm that the earthquake occurred associated with a fault with that direction. The combination of these data with the information published in previous works allows to consider the possibility that the "Arraiolos seismic cluster" corresponds to an intersection of active strike slip faults with WNW-ESE and N-S orientations. Assuming that there are alternating ruptures in both systems, this will cause a temporary block in the system that has been cut, leading to a local stress concentration. Though, a movement that in depth can be ductile, aseismic throughout most of the area of the faults, in the zone of intersection of the two systems the crust will have to respond necessarily as brittle to the tension, generating earthquakes and explaining the existence of this "seismic cluster".