Data completeness of the Kumamoto earthquake sequence in the JMA catalog and its influence on the estimation of the ETAS parameters

Jiancang Zhuang,Ting Wang,Yosihiko Ogata

doi:10.1186/s40623-017-0614-6

Abstract

This study investigates the missing data problem in the Japan Meteorological Agency catalog of the Kumamoto aftershock sequence, which occurred since April 15, 2016, in Japan. Based on the assumption that earthquake magnitudes are independent of their occurrence times, we replenish the short-term missing data of small earthquakes by using a bi-scale transformation and study their influence on the maximum likelihood estimate (MLE) of the epidemic-type aftershock sequences (ETAS) parameters by comparing the analysis results from the original and the replenished datasets. The results show that the MLEs of the ETAS parameters vary when this model is fitted to the recorded catalog with different cutoff magnitudes, while those MLEs remain stable for the replenished dataset. Further analysis shows that the seismicity becomes quiescent after the occurrence of the second major shock, which can be regarded as a precursory phenomenon of the occurrence of the subsequent M_J7.3 mainshock. This relative quiescence is demonstrated more clearly by the analysis of the replenished dataset.Graphical abstract(Left 6 panels) Illustration of applying the replenishing algorithm to the short missing of aftershocks in the Kumamoto aftershock sequence. (Right 6 panels) ETAS parameters estimated from the Kumamoto aftershock sequence with different magnitude thresholds. See text for details.

Highlights

On April 16, 2016, an earthquake sequence bursted in the Kumamoto region of the Kyushu Island, Japan, on the Hinagu and Futagawa faults, which lie at the southern end of the Median Tectonic Line, forking in two directions from the Beppu-Haneyama Fault Zone
The assumptions of this model are: (1) The background seismicity is a stationary Poisson process; (2) every event, no matter whether it is a background event or it is triggered by a previous event, triggers its own offspring independently; (3) the expected number of direct offspring is an increasing function of the magnitude of the mother event; and (4) the time lags between triggered events and the mother event follow the Omori–Utsu formula
To study the seismicity of the Kumamoto aftershock sequence, the epidemic-type aftershock sequences (ETAS) model is firstly fitted to the original dataset

Summary

Introduction

On April 16, 2016, an earthquake sequence bursted in the Kumamoto region of the Kyushu Island, Japan, on the Hinagu and Futagawa faults, which lie at the southern end of the Median Tectonic Line, forking in two directions from the Beppu-Haneyama Fault Zone. After Ogata (1988) proposed this model and extended into space–time version (Ogata 1998), it has become a popular model for standard short-term clustering of seismicity The assumptions of this model are: (1) The background seismicity is a stationary Poisson process; (2) every event, no matter whether it is a background event or it is triggered by a previous event, triggers its own offspring independently; (3) the expected number of direct offspring is an increasing function of the magnitude of the mother event; and (4) the time lags between triggered events and the mother event follow the Omori–Utsu formula. This model can be formulated by its conditional intensity function (t)

Objectives

Findings

Conclusion