Abstract
Usually, earthquakes develop after a strong main event. In literature they are defined as aftershocks and play a crucial role in the seismic sequence development: as a result, they should not be neglected. In this paper we analyzed several aftershock sequences triggered after a major earthquake, with the aimed at identifying, classifying and predicting the most energetic aftershocks. We developed some simple graphic and numeric methods that allowed us to analyze the development of the most energetic aftershock sequences and estimate their magnitude value. In particular, using a hierarchisation process related to the aftershocks sequence, we identified primary aftershocks of various orders triggered by the mainshock and secondary aftershocks of various orders triggered by the previous shock. Besides, by a graphic method, it was possible to estimate their magnitude. Through the study of the delay time and distance between the most energetic aftershocks and the mainshock, we found that the aftershocks occur within twenty-four hours after the mainshock and their distance remains within a range of hundreds of kilometers. To define the aftershocks sequence decay rate, we developed a sequence strength indicator (ISF), which uses the magnitude value and the daily number of seismic events. Moreover, in order to obtain additional information on the developmental state of the aftershocks sequence and on the magnitude values that may occur in the future, we used the Fibonacci levels. The analyses conducted on different aftershocks sequences, resulting from strong earthquakes occurred in various areas of the world over the last forty years, confirm the validity of our approach that can be useful for a short-medium term evaluation of the aftershocks sequence as well as for a proper assessment of their magnitude value.
Highlights
Studies conducted on seismicity have shown that earthquakes grouping in space and time does not happen randomly, but follows some rules based on interactions between the earthquakes.The earthquake launching the most energetic seismic activity is known as mainshock, which is caused by the release of previously accumulated energy into the lithospheric volume [1], while the groups of earthquakes that follow are known as aftershocks.A shock is considered an aftershock if it happens within the length of the rupture surface that generated the main event, or within a subsidence area
Aftershocks typically give birth to a series of primary and secondary aftershocks of different orders whose magnitude is lower compared to the main event’s, which can be identified through a sequence hierarchisation process
The primary aftershocks result from a triggering process due to mainshock-induced stress variation, and the secondary aftershocks are triggered by stress changes occurred in primary aftershocks
Summary
Studies conducted on seismicity have shown that earthquakes grouping in space and time does not happen randomly, but follows some rules based on interactions between the earthquakes. A shock is considered an aftershock if it happens within the length of the rupture surface that generated the main event, or within a subsidence area (the so-called aftershock area). It should occur before the seismicity rate of the area goes back to the basic values recorded in the period preceding the mainshock. Aftershocks-affected areas sometimes show a considerable expansion within days or years after the mainshock [19] [20] Based on this knowledge, this study has the aim to analyze the seismic sequence and the mainshok-aftershocks relation to define the future development of the aftershock stage
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