Monitor Seismic Velocity Change In Volcanic Zones Applying Ambient Seismic Noise

The Mt.Ontake volcano erupted on 27 September 2014 in Japan and the Kilauea volcano erupted in early May 2018 in the US of Hawaii island.The Mt.Ontake volcano was a sudden steam eruption.There were no significant changes in subsurface fluid pressure and crustal deformation observed within a few days before the volcanic eruption.Only little tilt change was observed within a few minutes before the eruption.With the lava ascending and descending movements and seismic activities,the Kilauea volcano activities roughly started from 25 th April 2018,and small eruptions occurred in the crater.On 4 May 2018,south flank of Pu‘u‘?‘? occurred the Mw6.9 earthquake.The lava level of the Halema'uma'u crater fell tens of meters,and volcanic activities ended until August 4th.The Kilauea crater area recorded significant crustal deformation during the volcanic activities,and GPS observed vertical displacements about 85 m of the collapse of the crater area.Accompanying nearly daily collapse events,the tilt station observed continuous surface uplift and subsidence changes.The volcanic eruption involves the interaction between magmatic activity and tectonic stress.The accumulation and release of stress will cause changes in the pressure within the magma chamber.Monitoring the temporal change characteristics of the crustal medium before and after of volcanic eruption is of great significance for understanding the dynamic process of magma in the volcanic area.Based on the ambient seismic noise cross-correlation,this paper studied the relative seismic wave velocity changes before and after the above of two volcanic eruptions.During the study of the Mt.Ontake volcano,we collected 2014 continuous data of vertical components of the 6 seismic stations near the volcanic area.We used moving-window cross-spectral method to measure travel time shifts in frequency 0.4-0.8 Hz,and we obtained relative seismic velocity changes of mean of all pairs and inter-station pair respectively.By analyzing the relative seismic velocity changes of mean of all pairs of 30 days moving window showed that the seismic velocity significantly dropped about 0.1% around 10 days before volcanic eruption.After eruption,the seismic velocity gradually increased and recovered to the background value.During the study of the Kilauea volcano,we collected 2018 continuous data of vertical component of 10 seismic stations around the Kilauea crater.We measured relative travel time shifts in the frequency 0.5-1.0 Hz using same way,then we obtained relative seismic velocity changes of mean of all pairs.By analyzing the relative seismic velocity changes of mean of all pairs of 5 days moving window showed that 10 days before the crater started large eruption on 4 May 2018,the seismic velocity dropped about 0.5%.The eruption of the crater ended until August 4th,and seismic velocity gradually increased and recovered to the background value after August 4th.We discussed the physical mechanism that may cause the mean relative seismic velocity change before and after the two volcanic eruptions.Combined with the results of the focal mechanism before and after the eruption of the Mt.Ontake volcano,which showed that the change of seismic velocity in the summit area was related to the surrounding stress change.The seismic velocity change in the Kilauea crater may be related to the pressure change of the magma.