Memory effects govern scale-free dynamics beyond universality classes

TL;DR

This paper reveals that memory effects influence scale-free avalanche dynamics beyond traditional universality classes, explaining larger exponents observed in natural phenomena through simulations of the OFC earthquake model.

Contribution

It introduces a unified framework showing how memory effects cause a transition from critical to quasi-critical dynamics with larger exponents, beyond classical universality classes.

Findings

Memory effects induce a transition to larger exponents.

Dissipation and memory effects drive quasi-critical behavior.

A power-law distribution of critical distances explains the transition.

Abstract

Scale-invariant avalanches -- with events of all sizes following power-law distributions -- are considered critical. Above the upper critical dimension of four, the mean-field solution with a robust $3/2$ size exponent describes the dynamics. In two and three dimensions, spatial constraints yield smaller yet robust exponent values governed by universality classes. However, both earthquake data and experiments often show exponent values larger than $3/2$, challenging those theoretical arguments based on critical behavior. Through extensive simulations in the classical OFC earthquake model, here we show a clear transition from the theoretical expected behavior of a robust exponent value, to a regime of quasi-critical dynamics with larger than $3/2$ exponents that depend on dissipation. While the first critical regime exhibits an inherently memoryless behavior, both the transition and the second regime are driven by memory effects provoked by the growth of avalanches over the traces left by previous events, due to dissipative mechanisms. The system hovers at a distance $d_{cp}$ from the critical point, and accounting for a power-law distribution of $d_{cp}$, validated by susceptibility measurements, captures the transition. This framework provides a unified description of both critical and quasi-critical behavior, and thus of the full spectrum of scale-free dynamics observed in nature.