Asymmetry in earthquake interevent time intervals

TL;DR

This paper investigates the asymmetry in earthquake interevent time series, revealing significant asymmetry patterns linked to aftershock sequences and spontaneous events, and compares models to better reproduce these features.

Contribution

It introduces a new asymmetry metric for earthquake interevent times and demonstrates its significance and modeling improvements over standard approaches.

Findings

Earthquake interevent times are significantly asymmetric.

Asymmetry exhibits a crossover at large lag indices.

Modified ETAS model better reproduces observed asymmetry.

Abstract

Here we focus on a basic statistical measure of earthquake catalogs that has not been studied before, the asymmetry of interevent time series (e.g., reflecting the tendency to have more aftershocks than spontaneous earthquakes). We define the asymmetry metric as the ratio between the number of positive interevent time increments minus negative increments and the total (positive plus negative) number of increments. Such asymmetry commonly exists in time series data for non-linear geophysical systems like river flow which decays slowly and increases rapidly. We find that earthquake interevent time series are significantly asymmetric, where the asymmetry function exhibits a significant crossover to weak asymmetry at large lag-index. We suggest that the Omori law can be associated with the large asymmetry at short time intervals below the crossover whereas overlapping aftershock sequences and the spontaneous events can be associated with a fast decay of asymmetry above the crossover. We show that the asymmetry is better reproduced by a recently modified ETAS model with two triggering processes in comparison to the standard ETAS model which only has one.