Seismic Activities And Tectonic Characteristics Of Longqi Hydrothermal Field At Southwest Indian Ridge

Human has found another important window to explore our planet's last frontier since the discovery of hydrothermal activity in the 1970 s.The scientists focused on the research of the deep structure and hydrothermal circulation beneath the hydrothermal field at mid-ocean ridge.Compared with the faster spreading ridge,the knowledge of hydrothermal process in the hydrothermal field at ultraslow spreading ridge is insufficient for lacking of investigation.In this paper,we performed OBS monitoring experiments near the Longqi hydrothermal field as a typcal representative at the ultraslow spreading ridge in order to study the tectonic structure beneath the hydrothermal field and model the dymatics process of the hydrothermal circulation.People have realized that passive seismic monitoring experiments using OBSs are the efficient method to explore magmatic and tectonic activities beneath the hydrothermal field.The crustal structure and the models of hydrothermal circulation at faster spreading ridge are obtained by using OBS data.Our OBS monitoring experiments near Longqi hydrothermal vent are the first observation of hydrothermal system at the ultraslow spreading environment in the word.It is of great significance to study the mechanism of hydrothermal circulation at the ultraslow spreading ridge.In this paper,we firstly processed the OBS data with the correction of clock drift and bottom position offset.Time correction of the OBS by the ambient noise cross-correlation was applied in case of no GPS re-synchronization.We discussed the influence of the parameters selection and demonstrated this approach was feasible under the appropriate parameters.A significant clock drift of ~2s was estimate between OBSs sets through linear regression.After that,we classified these events into the following three classes based on their time-frequency characteristics:(1)volcano-tectonic micro-earthquakes related to the tectonic and volcanic activities;(2)short duration events point to fluid driven in the pipes and cractks of the hydrothermal system(3)regional earthquakes for confirming the seismic sensor orientation;(4)possible marine mammals' sound.Finally,according to the spatial and temporal distribution characteristics of different signals,we have studied their corresponding tectonic and hydrothermal processes respectively.The main results are as follows:1.Short duration event is a kind of resonace of fluid-filled cracks induced by impulsive pressure transient.It occurred intermittently in time characteristics.Based on their spectral content and damping characteristics,a short duration event is a superposition of several monocharomatic harmonic waves in different pipes.Each monochromatic harmonic wave might come from the fluids resonating process in a certain case such as specific shape of a pipe or crack,velocity of flow and pore pressure.2.Under the condition of limited OBSs,we use a single station location method through polarization analysis of the P first phase to locate volcano-tectonic micro-earthquakes.The results show the epicenters distribute focusly at 3~15 km beneath the axial volcano ridge im the middle of the valley.The distribution of the earthquakes located by the single station location method is consistent with the results of multiple OBSs employed later.3.We located microearthquakes accurately recorded by the OBS network using the double-difference method.The epicenters outline the subsurface geometry of twin detachment faults system.One is near-vertical detachament fault related to the Longqi vent.Such steep fault with tightly focused earthquakes may be active at the initiate stage for formation of detachment faults.Another mature detachment fault associated with the oceanic core complexes on the south is flexural rotated to a low single in the shallow.This fault may be inactivefor it locked up by excessive rotating in the subsurface.Both detachment faults can reach the 13 km beneath axial volcano ridge suggest the heat source is located beneath the root zone of detachment system.4.The temperature vibration of diffuse flow near the hydrothermal vent shows a strong correlation with tide,suggesting that decreasing stress of rocks facilitate increase of fluid fluxes when ocean loading is a minimum.This results in a slight increase of temperature at the tidal lows.However,microearthquake and short duration events show no periodicity with tide.We proposed the hydrothermal activities are influenced by the tide,but it is not strong enough to induce earthquake.5.It is obvious increasing of temperature within diffuse flow field near the Longqi vent from background 3? up to 12? lasted for 8 hour after the onset of the microearthquake swarm which is located 5-8 km along the detachment fault;however,the temperature of high-temperature vent is still 372? with no obvious increase.It may be possible related with the permeability change of detachment fault after seismic swarm.The significant increase of temperature in the diffuse flow area was released by the increased flow.But at the same time,an faulting induced permeability increasing allows high temperature fluids mixing with cold sea water resulting in no increase of vent temperature6.Combined with seismic monitoring results and geological and geophysical data,we propose a hydrothermal circulation model of Longqi hydrothermal field.The twin detachment faults provide main paths of hydrothermal circulation.The heat flow can extract from heat source 13 km beneath axial volcano ridge and transport through high-transmissibility paths to release at vents.Additionally,non-transform discontinue zone on the west margin of Longqi and local faults along the Longqi increase the permeability of shallow crust indicating that the local faulting system also facilitate hydrothermal circulation.