Quantitative mapping of precursory seismicity rate changes along the Indonesian island chain

In this study, the possibility of detecting a significant seismicity rate change prior to a hazardous earthquake was examined along the Indonesian Sunda margin (ISM), Indonesia, using the seismological Z value method. The completeness inter- and intraplate seismicity data with a Mw ≥ 4.0 recorded during 1980–2016 was the dataset analyzed in this study. Based on an iterative test with 13 major or great earthquake case studies, the most suitable free parameters for detecting the precursory seismic quiescence by Z value analysis along the ISM were N = 25 events and Tw = 2.5 years. Investigation of the Z value in both temporal variation and spatial distribution revealed at least seven areas along the ISM with prominent Z value anomalies that are still quiescent from any hazardous earthquake and so are potential earthquake sources. These are northern and southern Praya, northern and southern Bajawa, eastern Dili, southern Ambon and southeastern Palu. Three of these seven areas (northern Praya, northern Bajawa and southwestern Pala) conform fairly well to the previously proposed eight risk areas in the Indonesian island chain derived from analysis of the b value of the frequency–magnitude distribution model. Therefore, it is concluded that there is a high possibility in the near future of seismic and/or tsunami hazard impact in the Indonesian island chain due to ISM seismic activities.