Self-Organized Criticality on Self-Similar Lattice: Exponential Time Distribution between Extremes ?

Abstract

In 1987, Bak, Tang, and Wiesenfeld introduced a mechanism (hereafter, the BTW mechanism) that underlies self-organized critical systems. Extreme events generated by the BTW mechanism are believed to exhibit an unpredictable occurrence. In spite of this general opinion, the largest events in the original BTW model are e ciently predictable by algorithms that exploit information that is hidden in applications. Intending to relate the predictability of self-organized critical systems with the level of its asymmetry, we examine the inter-event distribution of extreme avalanches generated by the BTW mechanism on symmetrical and asymmetrical self-similar lattices. Initially, we claim that the main part of the size-frequency relationship is power-law independent of the asymmetry, but the asymmetry reduces the range of scale-free avalanches in the domain of small avalanches. Further, we turn to extremes and claim that they are located on the downward bend of the distribution of the avalanches over their sizes. Finally, we compare the probability distribution of waiting time between two successive extremes with the exponential distribution. The latter gives the reference point of the complete unpredictability naturally measured in terms of the sum of two rates related to type I and II statistical errors: the rate of the unpredicted avalanches and the alarm time rate. We posit that the deviations of the observed probability distribution from the exponential one do not a ect the unpredictability of extremes drawn from the waiting time between them.

Keywords

• sandpile
• power-law

Introduction

Extreme events constitute an essential phenomenon of complex systems. Their economic and social consequences are di cult to overestimate. Nevertheless, yet several decades ago, scholars primary focused on regular behavior of complex systems. Only recently the scenarios of extremes become better understood, but the prediction of extremes still remains a challenge for researchers, especially if the system exhibits so called self-organized criticality associated with a power-law size-frequency distribution of \normal" events [ 12 ]. Bak et al. [ 2 ] introduced a simple mechanism (hereafter, the BTW mechanism) that generates self-organized criticality. Two multi-scale processes: slow loading and quick stress release, which balance the stress on average, characterize the BTW mechanism. The stress release is modeled as an avalanche that redistributes the stress over the underlying system and carries the energy out at the boundary. The system attains a critical state observed through the power-law distribution of the avalanches over their size [ 9 ]. Typically, the existence of the power-law sizefrequency distributions signals that the prediction of extremes are hardly possible [ 1 ]. Under the BTW mechanism, a slight increase of loading a ects the system alike its local redistribution. However, the BTW mechanism generates an out-of-equilibrium system whose observed variables oscillate around average values. Such oscillations, in general, could open a door for an e ective prediction of extremes that topple the system from the super- to sub-critical state, if the oscillation exhibits a certain quasi-periodicity.

Bak et al. [ 2 ] gave the rst simple explanation of numerous critical phenomena reported by that time and hugely extended by nowadays [ 36 ]. The (truncated) power-laws describe the probability distributions of earthquakes [ 13 ], landslides [ 34 ], solar ares [ 7 ], sizes of large cities [ 10,15 ], city res [ 33 ], and nancial crashes [ 17 ]. Surprisingly, the power-law exponent observed in the BTW model is hardly tuned with the transformation of the BTW mechanism [ 3 ]. The random version of the model proposed by [ 16 ] results in a similar but distinct exponent [ 6,4 ]. However other numerous modi cations leave the models inside the two classes of universality determined by these two exponents [ 29 ].

The BTW mechanism itself primary exhibits a concept of multiscale processes that exhibits self-organized criticality, but does not describe details of observed systems. The impossibility to adjust the exponent of the power-law and leave the (very) limited set of the universality classes reduces the range of the direct applicability of the BTW model and its adjacent generalizations. The exponent is tuned, if the BTW mechanism is realized on fractals [ 8,5 ], site-percolation lattice [ 20 ], and self-similar lattice [ 31 ].

Typically, a downward bend follows the scale-free range of the size-frequency relationship at the right part of the graph. In the BTW model, the transition is explained by the nite-size e ect. The occurrence of the large events can exhibit time-clustering [ 28 ]. In this case, the largest events follow a foreshock activity or trigger aftershocks, or demonstrate the both phenomema [ 32 ]. Such timeclustering underlies the algorithms that predict the largest events. These events are characterized by speci c waiting time probability distribution [ 22 ]. The BTW model is dissipative on the boundary, and large avalanches drive energy out of the system. Therefore, a large loading seems to be required for a consecutive extreme avalanche to occur. The existence of the quiescent episodes in the dynamics generates a certain quasi-periodicity of the largest avalanches. These arguments are con rmed for the BTW model [ 30 ] and the laboratory experiment imitating the model [ 25 ]. As far as the waiting time probability distribution deviates from the exponential one, the waiting time itself serves as a precursor of the main event in the models and their applications [ 26,11,27,35 ]. However, the extension of the set of the largest events to smaller ones destroys the e ciency of this precursor, tending the waiting time distribution to be exponential [ 23,11 ]. The avalanches that are located on the power-law part of the size-frequency relation graph are considered as unpredictable [ 24,1 ]. [ 19 ] constructed the framework based on the fraction of the unpredicted events and the alarm time rate in order to quantify the prediction algorithms. The fall of the prediction e ciency with the size of the forecasted events remains a general feature of the system [ 14 ]

The goal of this paper is to extend the applicability of the BTW-like models to real-life processes by testing the existence of new universality classes generated by asymmetry of the underlying system and inspecting the predictability of extremes within the universality classes. Even if the universality classes are insensitive to the asymmetry of the system, the asymmetry itself may a ect the scenario of extremes. We examine the inter-event distribution of extreme avalanches generated by the BTW mechanism on the self-similar lattice. Initially, we address the question of how the asymmetry of the lattice a ects the size-frequency relationship of the avalanches, nding the scale-free part of the relationship and establishing that its exponent is insensitive to the asymmetry. Further, we turn to extremes and claim that they are located on the downward bend of the distribution of the avalanches over their sizes. Finally, we compare the probability distribution of waiting time between two successive extremes with the exponential distribution. The latter gives the reference point of the complete unpredictability naturally measured in terms of type I and II errors. We posit that the deviations of the observed probability distribution from the exponential one do not a ect the unpredictability of extremes drawn from the waiting time between them.

The rest of the paper is organized in the following way. We de ne the model in Section 2, discussing the size-frequency relationship in details. The waiting time probability distribution is studied in Section 3. The last section concludes. 2 2.1

Model

Self-Similar Lattice:

In order to de ne a model we introduce the following notation. Consider d d lattice. Let a pattern be an arbitrary partition of the lattice cells into two (marked and unmarked) sets. We note that it is enough to specify the marked set to determine the partition. The case d = 3 is considered. We deal with the two patterns that consist of four cells illustrated by Figure 1. The rst pattern is symmetrical. It consists of four corner cells, colored in grey in Figure 1a. The second pattern consists of three cells located along a side, whereas the forth cell adjoins the corner one, Figure 1b. This pattern can be treated as an example of the largest asymmetry. As the pattern repeats the move of the chess knight, we further refer to it as a knight We de ne a self-similar lattice that consists of cells of di erent sizes. The smallest cells determine the unit of measurements. The (linear) length of the lattice is L = 3n, where n is the depth of self-similarity realized in computations. In theoretical constructions, n tends to +1. The recursive algorithm, constructing the self-similar lattice, at each step takes all cells that does not belong to the pattern, divide them into 9 equal squares, and specify the pattern among these new, smaller squares. The patterns are pre-de ned; they are identical at each step.

More precisely, let a pattern be xed. We denote C0;L a lattice with L cells on the side. At every step r = 0; 1; : : : ; n 2 of the recursion, the algorithm repeats the following sub-steps: 1. Split each cell c 2 Cr;L into 9 equal squares with 3n r 1 cells on the side.

Four out of nine cells form the pattern Pr+1(c). 2. C^r+1;L denotes the set of all cells appeared at sub-step 1 that form the patterns Pr+1(c), c 2 Cr;L. 3. Cr+1;L denotes the set of all cells appeared at sub-step 1 that does not belong to the patterns Pr+1(c), c 2 Cr;L.

Note that step 1 is not applied to the cells from C^r+1;L. The nal step r = n di ers from the previous ones: 1 1. Split Cn 1;L into 9 equal squares with 1 cell on the side. 2. Cells corresponding to the pattern will be denoted as C^n;L. The set [r=1C^r;L is further referred to as self-similar lattice. Figure 2 illustrates n the self-similar lattices obtained with n = 3 and d = 3 The cells, i. e., c 2 C^r;L, where r = 1; : : : ; n, are assumed to be enumerated in some way c1, c2, : : :. We use the following notation.

{ hi is a number associated with the cell ci. Following tradition [ 2 ], the quantity hi is called a grain of sand, whereas the model is referred to as sandpile. { N (i) is the set of the cells that are adjacent to the cell ci by a common edge; these cells are called neighbors. jN (i)j denotes the number of the neighbors illustrated in Figure 2. { If the cell ci is located inside the lattice C0;L (i. e., neither edge belong to the lattice boundary) then we put Hi = jN (i)j. Otherwise, Hi is the sum of jN (i)j and the number of the ci's edges located on the lattice boundary. The cell ci is called stable if hi < Hi. At the beginning all cells are stable. For the sake of simplicity we put hi = 0 for all i. Our computer simulations give evidence that what follows is insensitive to the initial conditions.

The model dynamics consists of two stages: accumulation and avalanche. Accumulation. A grain falls on a randomly chosen cell: 1. A cell c of the self-similar lattice is chosen at random with the probability being proportional to the cell's area. If c 2 C^r;L, r = 1; : : : ; n, then the probability is p = 32(n r)=32n = 3 2r. 2. Let i be the number of the just chosen cell c, i. e., c = ci. Then a grain of sand \falls" on ci: hi ! hi + 1. 3. We perform a stability check. If hi < Hi, we repeat accumulation process.

Otherwise, the avalanche starts.

Avalanche. The unstable cell ci topples transferring grains to the neighbors equally: hi ! hi

Hi hj ! hj + 1; 8j 2 N (i) (1) (2) The transfer (1) and (2) conserves the total amount of grains in the lattice when an inner cell topples. If at least one neighbor becomes unstable, the transfers continue. In other words, while there are unstable cells ci, the rules (1) and (2) are applied. Each avalanche is nite because the transfers at the boundary are dissipative. We call the number of sand grains that are displaced during the avalanche the size of the avalanche. 2.3

Power-Law Size-Frequency Relationship

First, we investigate the probability distribution of the avalanches over their sizes. Let N (s) be the number of avalanches that have the size s. Then we de ne We substitute Q(s) for N (s) to stabilize the graph in the domain of small sizes. Figure 3 displays the graphs of Q(s) for the symmetrical (a) and knight (b) patterns. The power-law part of the graphs turns to a downward bend at the right. The power-law part and the structure of the downward bend can be studied in more details when the exponential binning is applied. Let f (s) be the fraction of the avalanches in the catalogue that have the size s;

F (s) =

X 2[s= s;s s) f ( ); (3) where s is a parameter. We stress that if f (s) follows a power-law function, then F (s) also does, but the exponent of the power-law is increased by 1. Indeed, the summation in equation (3) serves as the integral that transforms 1=s into 1=s 1. For instance, if = 1, the integral over the exponential bin is Z s s s= s

s s s 1 = ln s s= s = 2 ln s = 2s0 ln s:

Waiting Time Distribution

In this section, we investigate the waiting time probability distribution and relate it to the prediction of rare events. Rare events are de ned parametrically. We will call an avalanche the rare event if its size is bigger than some S . The inter-event probability distribution rather accurately follows the exponential function while S belongs to the power-law range of sizes (we do not support this statement by a graph). If S is located on the right part of the downward bend, this probability distribution deviates from the exponential function; see Figure 5, where the complement cumulative distribution function, 1 cdf is displayed. Under symmetrical pattern, the deviation is in the domain of a large waiting time: a large time gap between successive rare events occurs more frequency then the exponential random probability distribution predicts. In the case of the knight pattern, the observed waiting time distribution is convex in the linear-log scale, but the convexity is small. We introduce a simple algorithm predicting the next rare event based on the record of the previous one. If a rare event occurs at the time moment t0, then an alarm is raised for the interval (t0; t0 + T ], where the parameter T > 0 will be adjusted later. The alarm means that the algorithm predicts the occurrence of the consecutive rare event during time window (t0; t0 + T ]. The unit of time is associated with the grain falling. It falls one grain per the time unit. Each alarm continues either up to the occurrence of a new rare event (and then a new alarm is raised) or T units of time.

The rare events that occurs when the alarm is raised are predicted. The other rare events are unpredicted. Let be the fraction of the unpredicted rare events. The rst rare event is unpredicted by the construction of the algorithm. Therefore, it is ignored when is computed. Further, let be the alarm rate (the sum of the intervals with the raised alarm divided by length of the whole time interval) and " = + . The quantities and are related to type I and II statistical errors [ 18 ]. Their sum " describes the e ciency of the prediction algorithm. The equation " 1 characterizes unpredictability. If the underlying probability distribution of the waiting time is exponential then our algorithm results in " = 1 (this is the result of an elementary mathematics skipped here). The both errors as functions of the parameter T are displayed in Figure 6 whereas their sum is shown in 7. In our computer simulations, S is chosen as 32000 (symmetrical pattern) and 20000 (knight pattern), that leads to the amount of rare events equalled to 193 and 139 respectively. The quantity " oscillates around 1. This suggests that the waiting time probability distribution deviates from the power one insigni cantly. 4

Conclusion

We constructed the BTW mechanism on the self-similar lattice. The existence of the asymmetry in the geometry of the self-similar lattice does not a ect the selforganized criticality of the underlying system. The size-frequency relationship follows the power-law and the power-law exponents are identical. Their value was earlier predicted by [ 31 ]. The power-law part of the size-frequency relation graphs is transformed to a downward bend located at the right. There are traces of another power-law in this bend whose exponent is (very) roughly estimated as 3. The best approximation to the downward bend deserves a separate study.

We found that the waiting time distribution between rare events deviates from the exponential one, if the low border of these events is su ciently large. However we did not detect a power-law inter-event distribution as [ 22 ] identi ed for the classical BTW sandpile. In the sandpile on the self-similar lattice studied here, the prediction of the rare avalanches drawn from their waiting time distribution fails completely. We construct the algorithm that expects the occurrence of a new extreme within T time units after the previous extreme. This algorithm is ine ective for any value of the parameter T .

This result does not give evidence against the prediction of extremes in the sandpile model on the self-similar lattice in general. We believe that the predictability of large avalanches found by [ 28 ] is translated onto the case of the BTW-mechanism on the self-similar lattice. At least, it is worth trying to construct a prediction algorithm that switches on the alarm when the surplus of sand grains on the lattice is observed. This algorithm could be e ective since large avalanches are expected upon the system becomes supercritical.

The BTW-like models considered in the paper can be used when studying earthquake formation processes, as the exponent in the power law is tuned through an appropriate choice of the self-similarity determinant. Narkunskaya & Shnirman [ 21 ] scrutinized long-term dynamics of the distribution of the earthquakes over their magnitude and constructed a precursor of strong earthquakes drawn from an upward bend of this distribution. The existence of a similar precursor in the model would further justify its applicability to the earthquake formation process. Then the prediction algorithms adjusted on our arti cial system can be further tested when predicting strong earthquakes.

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