Meyer, Lauriane

ULB thesis supervised by Lecocq, Thomas; Van Noten, Koen (Master)

Abstract: The Ijen volcano is one of the most dangerous volcanoes on the island of Java in Indonesia. Signs of volcanic activity are expressed as solfatare and bubbles in the volcanic lake. Such volcanoes must be constantly monitored in order to warn the population in case of an earthquake or volcanic crisis. Currently, seismic monitoring only occurs by real-time seismic measurements. This master thesis aims to develop and validate a new seismic method in continuous monitoring over a long time. Currently, H/V spectral ratio (HVSR) analysis of continuous seismic measurement has never been applied on a volcanic system. H/V is usually calculated from a single measurement on a short seismic record of e.g. twenty minutes. In order to develop a continuous HVSR monitoring system, first the automation of the H/V all seismic data needed to be developed. To account for this problem, a Python script was written to apply an HVSR algorithm on the available data. Second, the Python script was validated by comparing the automated results with the existing program GEOPSY in which data needs to be manually operated. The developed script only applies the basics of HVSR as an advanced and accurate calculation, such as applied in GEOPSY, is not necessary because only long term variations are discussed in this study. Afterwards, the developed continuous HVSR method was validated by comparing the H/V results obtained for two regions in Belgium (Court-St-Etienne and Uccle) with the H/V results applied on seismic data of the Kawah Ijen. This comparison is important as these two completely different types of environments give a wide spectrum of results. After all, a volcano is a young structure with active signs of seismic and volcanic activities whereas Belgium lies in an old and stable continental environment. The results of these two case studies show interesting results on the different responses of the developed method. The impact of human activity on ambient noise, a factor that is almost nonexistent on the volcano. Human activity in Belgium is so high that it disrupts the H/V results. In order to obtain the correct response of the environment studied, this study concludes that it is imperative to get rid of this human activity and the HVSR measurements need to be performed when human activity is low. Furthermore, the importance of continuous HVSR is shown in this work as the disturbances caused by the crisis of April 2015 were clearly detected. These results are conclusive as continuous HVSR has now for the first time been validated in a study on the activity of a volcano. The quality of the obtained results of this study will be further improved during the PhD of Raphael De Plaen (University of Luxembourg).

Keyword(s): HVSR ; Volcano-seismology ; Indonesia ; Kawah Ijen