Statistical monitoring of aftershock sequences: a case study of the 2015 Mw7.8 Gorkha, Nepal, earthquake

Yosihiko Ogata,Hiroshi Tsuruoka

doi:10.1186/s40623-016-0410-8

Abstract

Early forecasting of aftershocks has become realistic and practical because of real-time detection of hypocenters. This study illustrates a statistical procedure for monitoring aftershock sequences to detect anomalies to increase the probability gain of a significantly large aftershock or even an earthquake larger than the main shock. In particular, a significant lowering (relative quiescence) in aftershock activity below the level predicted by the Omori–Utsu formula or the epidemic-type aftershock sequence model is sometimes followed by a large earthquake in a neighboring region. As an example, we detected significant lowering relative to the modeled rate after approximately 1.7 days after the main shock in the aftershock sequence of the Mw7.8 Gorkha, Nepal, earthquake of April 25, 2015. The relative quiescence lasted until the May 12, 2015, M7.3 Kodari earthquake that occurred at the eastern end of the primary aftershock zone. Space–time plots including the transformed time can indicate the local places where aftershock activity lowers (the seismicity shadow). Thus, the relative quiescence can be hypothesized to be related to stress shadowing caused by probable slow slips. In addition, the aftershock productivity of the M7.3 Kodari earthquake is approximately twice as large as that of the M7.8 main shock.

Highlights

On April 25, 2015, a strong earthquake of Mw7.8 occurred along the Himalayan front close to Kathmandu, Nepal
Operational quasi-real-time statistical monitoring of anomalies of aftershock sequences should be implemented together with probability forecasting based on real-time earthquake datasets using either the Omori– Utsu model or the Epidemic-type aftershock sequence (ETAS) model
We illustrate this type of statistical monitoring for the early aftershock sequence of the Gorkha earthquake of M7.8 using the Preliminary Determination of Epicenters (PDE) datasets and the Advance National Seismic Network (ANSS) catalog

Summary

Introduction

On April 25, 2015, a strong earthquake of Mw7.8 occurred along the Himalayan front close to Kathmandu, Nepal. Seventeen days after the main shock, the largest aftershock of May 12, the M7.3 Kodari earthquake, occurred in the eastern extension of the primary aftershock zone. Relevant to this issue, Utsu (1979) and Aki (1981) emphasized the role of seismic anomalies in predicting the enhancement of the likelihood (probability gain) of having a substantially larger earthquake than the secular probability of the same size of earthquake. After the appearance of an anomaly, we need to evaluate the probability that it will be a precursor to a large earthquake; i.e., we need to forecast whether the probability in a space–time zone will increase to an extent, relative to that of the reference probability. It is desirable to search for anomalous phenomena that enhance the probability gains

Methods

Discussion

Conclusion