Preliminary Results from a Prototype Ocean-Bottom Pressure Sensor Deployed in the Mentawai Channel, Central Sumatra, Ind
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Pure and Applied Geophysics
Preliminary Results from a Prototype Ocean-Bottom Pressure Sensor Deployed in the Mentawai Channel, Central Sumatra, Indonesia LEE FREITAG1 and EMILE A. OKAL2 Abstract—We analyze data retrieved from an ocean floor pressure sensor continuously operated for 48 days in the Mentawai Strait during the Spring of 2016, as part of Project Hazard SEES. Initial processing through systematic spectrogram analysis has identified ten distant earthquakes recorded through the variation of pressure accompanying the passage of seismic waves on the bottom of the ocean. The analysis of the corresponding wavetrains allows the recovery of the standard magnitude Ms of seven of the events (two more being intermediate depth, and the tenth antipodal) with a residual not exceeding 0.3 logarithmic units. We also show that the classical energy-to-moment ratio computation can be successfully adapted by defining a response function of the pressure sensor to teleseismic P waves. In addition, six local earthquakes, occurring at distances of 58–670 km from the sensor, but with moment magnitudes less than 5.7, were also recorded. We show that an estimate of the seismic energy radiated by these events can be obtained from a simple integration of the square of the pressure signal. Thus our results indicate that meaningful quantitative estimates of the source characteristics of both teleseismic and regional events can be obtained through robust methods based on single-station pressure recordings on the ocean floor. Keywords: Tsunamis, ocean bottom sensors, seismic waves.
1. Background and Motivation The purpose of this paper is to report on the prototype operation of a precision ocean bottom pressure sensor, in order to estimate the magnitudes of both nearby and distant teleseismic events in the context of tsunami warning in the Mentawai Strait, bounded to the Southwest by the island of Siberut, and to the Northeast by the large island of Sumatra
1 Woods Hole Oceanographic Institution, MS 18, Woods Hole, MA 02543, USA. 2 Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL 60208, USA. E-mail: [email protected]
(Fig. 1). The sensor was installed in the Spring of 2016 for slightly less than 2 months as an initial test in this active seismic area, within the Mentawai Basin, a body of water approximately 1800 m deep separating the main island of Sumatra from a string of forearc islands The relevant section of the Sumatra subduction zone is believed to have ruptured in the mega earthquakes of 1797 (northern half) and 1833 (southern part), and to have been locked ever since (Zachariasen et al. 1999; Natawidjaja et al. 2006), contrary to the next segment to the South, where at least part of the tectonic convergence accumulated since 1833 was released during the 2007 Bengkulu earthquake (Borrero et al. 2009). This region is therefore interpreted as a seismic gap, where conditions are ripe for a megathrust event to take place in the next years or decades, which would generate a tsunami with
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