Abstract
It has been emphasized that the temporal occurrence of earthquakes in various spatial areas and over ranges of magnitude may be described by a unique distribution of inter-earthquake intervals under suitable rescaling, implying the presence of a universal mechanism governing seismicity. Nevertheless, it is possible that some features in the fine temporal patterns of event occurrences differ between spatial regions, reflecting different conditions that cause earthquakes, such as relative motion of tectonic plates sharing a boundary. By abstracting the non-Poissonian feature from non-stationary sequences using a metric of local variation of event intervals Lv, we find a wide range of non-Poissonian burstiness present in the temporal event occurrences in different spatial areas. Firstly, the degree of bursty features in the occurrence of earthquakes depends on spatial location; earthquakes tend to be bursty in areas where they are less frequent. Secondly, systematic regional differences remain even if the overall correlation between burstiness and the rate of event occurrence is eliminated. Thirdly, the degree of burstiness is particularly high on divergent tectonic boundaries compared to convergent and transform boundaries. In this way, temporal patterns of event occurrences bear witness to the circumstances underlying event generation.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. The large-scale motion of tectonic plates induces stress, causing an abrupt discharge of energy or an earthquake. It has recently been emphasized that the temporal occurrence of earthquakes may be described by a unique distribution of inter-event intervals under suitable rescaling, implying the presence of a universal mechanism governing seismicity. Nevertheless, the plate boundaries that generate earthquakes are inhomogeneous and are classified into different types, based on the relative motion of adjacent plates: divergent, convergent, and transform. It is possible that the temporal patterns of event occurrences differ between spatial regions, reflecting the internal conditions causing events.
Main results. By abstracting the non-Poissonian feature using a metric of local variation of event-intervals, Lv, we find a wide range of non-Poissonian burstiness present in different spatial areas. Firstly, the burstiness in the occurrence of earthquakes depends on spatial location; earthquakes tend to be bursty in areas where they are less frequent. Secondly, systematic regional differences remain, even if the overall correlation between burstiness and the rate of event occurrence is eliminated. Thirdly, the degree of burstiness is in particular higher on divergent tectonic boundaries compared to convergent and transform boundaries.
Wider implications. Temporal patterns of event occurrences bear witness to the circumstances underlying event generation, including mechanical and biological signals, and human activity. This may be revealed by measuring the local variation of event-intervals while rescaling the time coordinate with the instantaneous rate to diminish rate fluctuation.
Figure. Sequences of earthquakes depicting different temporal patterns of occurrences and their locations on the map. (a) Event sequences recorded from various areas may exhibit different values of Lv. Sequences of n = 50 inter-event intervals assuming Lv~1.8, 1.4 and 1.0 (±0.1) exhibit bursty, semi-bursty and quasi-Poissonian random temporal patterns, respectively. (b) Histograms of intervals rescaled with the instantaneous rate, which is estimated from the adjacent 11 intervals (five in front, the present one and five in the rear). Sequences from individual areas are classified into three types (Lv > 1.6, 1.6 > Lv > 1.2, 1.2 > Lv). The dashed line represents the exponential distribution, which should be realized for an ideal Poisson random process. (c) Color of the cells where each cell measured 250 000 km2 and had more than 50 earthquakes ( > 4.5 M) from 1 January 1973 to 31 December 2009 are indicated with colours of Lv values.