Hydrothermal Activity and Seismicity at Teahitia Seamount: Reactivation of the Society Islands Hotspot?

Along mid-ocean ridges, submarine venting has been found at all spreading rates and in every ocean basin. By contrast, intraplate hydrothermal activity has only been reported from five locations, worldwide. Here we extend the time series at one of those sites, Teahitia Seamount, which was first shown to be hydrothermally active in 1983 but had not been revisited since 1999. Previously, submersible investigations had led to the discovery of low-temperature (≤30°C) venting associated with the summit of Teahitia Seamount at ≤1500 m. In December 2013 we returned to the same site at the culmination of the US GEOTRACES Eastern South Tropical Pacific (GP16) transect and found evidence for ongoing venting in the form of a non-buoyant hydrothermal plume at a depth of 1400 m. Multi-beam mapping revealed the same composite volcano morphology described previously for Teahitia including four prominent cones. The plume overlying the summit showed distinct in situ optical backscatter and redox anomalies, coupled with high concentrations of total dissolvable Fe (≤186 nmol/L) and Mn (≤33 nmol/L) that are all diagnostic of venting at the underlying seafloor. Continuous seismic records from 1986-present reveal a ∼15 year period of quiescence at Teahitia, following the seismic crisis that first stimulated its submersible-led investigation. Since 2007, however, the frequency of seismicity at Teahitia, coupled with the low magnitude of those events, are suggestive of magmatic reactivation. Separately, distinct seismicity at the adjacent Rocard seamount has also been attributed to submarine extrusive volcanism in 2011 and in 2013. Theoretical modeling of the hydrothermal plume signals detected suggest a minimum heat flux of 10 MW at the summit of Teahitia. Those model simulations can only be sourced from an area of low-temperature venting such as that originally reported from Teahitia if the temperature of the fluids exiting the seabed has increased significantly, from ≤30°C to ∼70°C. These model seafloor temperatures and our direct plume observations are both consistent with reports from Loihi Seamount, Hawaii, ∼10 year following an episode of seafloor volcanism. We hypothesize that the Society Islands hotspot may be undergoing a similar episode of both magmatic and hydrothermal reactivation

Characterizing swells in the southern Pacific from seismic and infrasonic noise analyses

International audienceA temporary network of 10 broad-band seismic stations has been installed in French Polynesia for the Polynesian Lithosphere and Upper Mantle Experiment (PLUME). All the seismic stations were installed either on volcanic islands or on atolls of the various archipelagos of French Polynesia in a manner which complements the geographic coverage provided by the regional permanent stations. The primary aim of PLUME is to image the upper mantle structures related to plate motion and hotspot activity. However, because of its proximity to all sites, the ocean is responsible for a high level of noise in the seismic data and we show that these data can also be used to analyse ocean wave activity. The power spectral density (PSD) analyses of the seismic data recorded in French Polynesia show clear peaks in the 0.05– 0.10 Hz band (periods between 10 and 20 s), which corresponds to swell frequencies. Clear peaks in this frequency band are also observed in infrasonic data recorded on Tahiti. Ground motion analysis shows that the swell-related seismic noise (SRSN) is linearly polarized in the horizontal plane and its amplitude decreases rapidly with the distance from the shore. The microseismic and the infrasonic 'noise' amplitudes show very similar variations from station to station and both are strongly correlated with the swell amplitudes predicted by the National Oceanic and Atmospheric Administration (NOAA), wind-forced, 'WaveWatch' models. The swell direction can be estimated from SRSN polarization analysis but this has to be done with care since, for some cases, the ground motions are strongly controlled by the islands' anisometric shapes and by swell refraction processes. We find cases, however, such as Tahiti or roughly circular Tuamotu atolls, where the azimuth of the swell is in good agreement with the seismic estimates. We, therefore, demonstrate that the SRSN and the infrasonic signal observed in French Polynesia can be used in such cases as a proxy for swell amplitude and azimuth. From the continuous analysis of the data recorded in 2003 at the permanent seismic station PPTL in Tahiti, transfer functions have been obtained. This could provide a way to quantify the swell activity during the last two decades and, therefore, assist in the investigation of climate changes

Unusual seismic activity in 2011 and 2013 at the submarine volcano Rocard, Society hot spot (French Polynesia)

International audienceWe analyze two seismic events that occurred on 27 May 2011 and 29 April 2013 at the Rocard submarine volcano which overlies the Society hot spot. The Polynesian Seismic Network recorded for the first time unusual associated short- and long-period signals, with perfectly monochromatic (0.0589 Hz) Rayleigh wave trains of long period and duration. None of the numerous observations of long-period (10-30 s) signals previously associated with volcanic activity in Japan, Italy, Mexico, Indonesia, Antarctica, and the Hawaiian Islands have the characteristics we observed at Rocard. We propose a tentative model for these unusual and rather enigmatic signals, in which the movement of lava excited the resonance of a shallow open conduit under a high hydrostatic pressure of ~400 bars

Seismic and hydroacoustic effects of the May 29, 2010 submarine South Sarigan volcanic explosion: Energy release and interpretation

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Collapse and reconstruction of Monowai submarine volcano, Kermadec arc, 1998�2004

Monowai submarine volcano is one of the three most historically active volcanoes of the Kermadec arc. Repeat multibeam surveys of Monowai Cone from September 1998 and September 2004 and T wave data recorded by the Réseau Sismique Polynésien network for the same period document the collapse and subsequent regrowth of the cone within this 6-a period. Grid differencing of the two bathymetric data sets, acquired 6 a apart, reveals that a landslide ?2230 m long occurred between the surveys, within which a postcollapse cone and talus ridge (?0.023 km3 in volume) subsequently formed. The volume of this collapse, minus postcollapse construction, is ?0.085 km3. We interpret an unusual, strong-amplitude T wave event on 24 May 2002 as recording �hot landsliding�, where the 100- to 160-m-thick collapse has �unroofed� the uppermost parts of the vent conduit, with the subsequent explosive interaction, and cooling, of hot magma and volcaniclastic rubble with ambient seawater. This interpretation is consistent with the lack of emergent events, sharp onset, and large amplitude of the 24 May 2002 T waves. The subsequent &gt;2500 T wave events, between November 2002 and September 2004, occurred in swarms with emerging and waning activity and with typical explosive volcanic acoustic signatures, which are interpreted as recording the regrowth of an ?90-m-high cone back to a near-1998 elevation, at an average rate of 47 m a?1. This study provides (1) a lower bound for frequency-magnitude relationships of landsliding for submarine arc volcanoes and (2) estimates of 0.013 km3 a?1 of submarine cone growth during eruptive cycles. <br/