Time filter

Source Type

Papeete, French Polynesia

Isse T.,University of Tokyo | Sugioka H.,Kobe University | Ito A.,Japan Agency for Marine - Earth Science and Technology | Shiobara H.,University of Tokyo | And 2 more authors.
Earth, Planets and Space | Year: 2016

We determined the three-dimensional shear wave velocity structure beneath the South Pacific superswell down to a depth of 200 km by analyzing fundamental Rayleigh wave records from permanent and temporary land-based and seafloor seismometers in the Pacific Ocean. Data from the Tomographic Investigation by seafloor ARray Experiment for the Society hotspot (TIARES) project yield excellent spatial resolution of velocity anomalies in the central part of the superswell, near the Society hotspot. Localized slow anomalies are found near hotspots in the upper mantle, but the vertical profiles of the anomalies vary from location to location: Slow anomalies near the Samoa, Macdonald, Pitcairn, and Society hotspots extend to at least 200 km depth, while a slow anomaly near the Marquesas hotspot extends only to ~150 km depth. Owing to the recently deployed seafloor array, horizontal resolutions of slow anomalies near the Society hotspot are substantially improved: The slow anomalies are about 300 km in lateral extent and have velocity anomalies as low as -6 %. The lithosphere thickness is estimated to be ~70 km in the vicinity of all hotspots, which may indicate thermal erosion by mantle plumes. © 2016 Isse et al. Source

Suetsugu D.,Japan Agency for Marine - Earth Science and Technology | Shiobara H.,University of Tokyo | Sugioka H.,Japan Agency for Marine - Earth Science and Technology | Ito A.,Japan Agency for Marine - Earth Science and Technology | And 9 more authors.
Earth, Planets and Space | Year: 2012

We conducted geophysical observations on the French Polynesian seafloor in the Pacific Ocean from 2009 to 2010 to determine the mantle structure beneath the Society hotspot, which is a region of underlying volcanic activity responsible for forming the Society Islands. The network for Tomographic Investigation by seafloor ARray Experiment for the Society hotspot (TIARES, named after the most common flower in Tahiti) is composed of multi-sensor stations that include broadband ocean-bottom seismometers, ocean-bottom electro-magnetometers, and differential pressure gauges. The network is designed to obtain seismic and electrical conductivity structures of the mantle beneath the Society hotspot. In addition to providing data to study the mantle structure, the TIARES network recorded unprecedented data of pressure and electromagnetic (EM) signals by tsunamis associated with large earthquakes in the Pacific Ocean, including the 2010 Chilean earthquake (Mw 8.8). Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS). Source

Clement J.,Laboratoire Of Geophysique | Reymond D.,Laboratoire Of Geophysique
Pure and Applied Geophysics | Year: 2015

This paper presents the tsunami warning tools, which are used for the estimation of the seismic source parameters. These tools are grouped under a method called Preliminary Determination of Focal Mechanism_2 (PDFM2), that has been developed at the French Polynesia Warning Center, in the framework of the (Formula presented.) system, as a plug-in concept. The first tool determines the seismic moment and the focal geometry (strike, dip, and slip), and the second tool identifies the ”tsunami earthquakes” (earthquakes that cause much bigger tsunamis than their magnitude would imply). In a tsunami warning operation, initial assessment of the tsunami potential is based on location and magnitude. The usual quick magnitude methods which use P waves, work fine for smaller earthquakes. For major earthquakes these methods drastically underestimate the magnitude and its tsunami potential because the radiated energy shifts to the longer period waves. Since French Polynesia is located far away from the subduction zones of the Pacific rim, the tsunami threat is not imminent, and this luxury of time allows to use the long period surface wave data to determine the true size of a major earthquake. The source inversion method presented in this paper uses a combination of surface waves amplitude spectra and P wave first motions. The advantage of using long period surface data is that there is a much more accurate determination of earthquake size, and the advantage of using P wave first motion is to have a better constrain of the focal geometry than using the surface waves alone. The PDFM2 method routinely gives stable results at T0 + 45 minutes, with T0 being the origin time of an earthquake. Our results are then compared to the Global Centroid Moment Tensor catalog for validating both the seismic moment and the source geometry. The second tool discussed in this paper is the slowness parameter θ and is the energy-to-moment ratio. It has been used to identify tsunami earthquakes, which are characterized by having unusual slow rupture velocity and release seismic energy that has been shifted to longer periods and, therefore, have low θ values. The slow rupture velocity would indicate weaker material and bigger uplift and, thus, bigger tsunami potential. The use of the slowness parameter θ is an efficient tool for monitoring the near real-time identification of tsunami earthquakes. © 2014, Springer Basel. Source

Jamelot A.,Laboratoire Of Geophysique | Reymond D.,Laboratoire Of Geophysique
Pure and Applied Geophysics | Year: 2015

Tsunami warning is classically based on two fundamental tools: the first one concerns the source parameters estimations, and the second one is the tsunami amplitude forecast. We presented in the first companion paper how the seismic source parameters are evaluated, and this second article describes the operational aspect and accuracy of the estimation of tsunami height using tsunami numerical modelling on a dedicated supercomputer (2.5 T-flops). The French Polynesian tsunami warning centre developed two new tsunami forecast tools for a tsunami warning context, based on our tsunami propagation numerical model named Taitoko. The first tool, named MERIT, that is very rapid, provides a preliminary forecast distribution of the tsunami amplitude for 30 sites located in French Polynesia in less than 5 min. In this case, the coastal tsunami height distribution is calculated from the numerical simulation of the tsunami amplitude in deep ocean using an empirical transfer function inspired by the Green Law. This method, which does not take into account resonance effects of bays and harbour, is suitable for rapid and first estimation of the tsunami danger. The second method, named COASTER, which uses 21 nested grids of increasing resolutions, gives more information about the coastal tsunami effects about the flow velocities, the arrival time of the maximal amplitude, and the maximal run-up height for five representative sites in 45 min. The historical tsunamis recorded over the last 22 years in French Polynesia have been simulated with these new tools to evaluate the accuracy of these methods. The results of the 23 historical tsunami simulations have been compared to the tide-gauge records of three sites in French Polynesia. The results, which are quite encouraging, shows standard errors of generally less than a 2 factor : the maximal standard error is 0.38 m for the Tahauku Bay of Hiva-Oa (Marquesas islands). © 2014, Springer Basel. Source

Discover hidden collaborations