Abstract
In this paper, we examine both the sequence and organisation of major shallow earthquakes occurred in various areas of the world from 1904 to 2017. We aim to describe their major features and how they are connected with foreshocks and aftershocks immediately close in time and space. Examining magnitude value’s fluctuations over time, we see that they form a basic pattern, consisting of three maxima, one of which is central, and two or more events preceding and following it, whose magnitude, in some cases, may be comparable. The retrospective analysis of earthquakes’ patterns of high comparable magnitude has allowed their classification along with the development of some statistically significant relationships between epicentral distance and magnitude difference and between time interval and delay among maxima as well as the identification of activation signals predicting their occurrence. The pattern we identified in seismic sequences analysis, in relation to minor shocks-generated activation signals’ positions may be used to obtain useful information for the evolutionary study of seismic sequences and for predicting double and multiple earthquakes. The graphic analysis procedure applied to the pattern enables us to know the period of seismic sequence’s greatest hazard after a strong earthquake.
Highlights
The analysis of historical earthquakes’ seismic sequences and, especially, of seismic patterns consisting of magnitude values’ fluctuations, identifies three types of shocks: Foreshocks: premonitory shock; Mainshock: primary shock—often single but sometimes multiple; Aftershocks: The preparation process that leads to the nucleation of large earthquakes usually is longer compared to smaller earthquakes’: this is supported by the branched structure that in stronger earthquakes shows greater hierarchisation [15]
In order to classify Doublet Earthquakes (DEQ) occurred in various parts of the world, we analysed several seismic sequences using the catalogs of the National Institute of Geophysics and Volcanology (INGV) [19] and the U.S Geological Survey (USGS) [20]
The observations made on magnitude values fluctuations over time, underline that foreshocks-mainshock-aftershocks sequence (FMA) pattern allows identifying the energy release phase closure as well as the mainshock compared to that of other major earthquakes in the pattern
Summary
Moderate and strong earthquakes may be preceded and followed by events of comparable magnitude forming a pattern only slightly different from the typical. The term Doublet Earthquakes (DEQ) is related to two seismic events showing similar magnitude and localisation values, and occurring after a short delay [1] [2]. DEQ are especially frequent in seismic areas featuring large asperities [3] and may occur along the same fault or a different fault. They may happen a few seconds or minutes after the first shock, or even weeks or months later
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