Tide-modulated gas emissions and tremors off SW Taiwan Shu-Kun Hsu a,n , Shiou-Ya Wang a , Yen-Che Liao a , Tsanyao F. Yang b , Sen Jan c , Jing-Yi Lin a , Song-Chuen Chen a,d a Department of Earth Sciences, National Central University, Taiwan b Department of Geosciences, National Taiwan University, Taiwan c Institute of Oceanography, National Taiwan University, Taiwan d Central Geological Survey, Ministry of Economic Affairs, Taiwan article info Article history: Received 6 October 2012 Received in revised form 10 March 2013 Accepted 12 March 2013 Editor: P. Shearer Available online 16 April 2013 Keywords: tide gas emission tremor mud volcano Taiwan abstract The continental margin off SW Taiwan is in an incipient stage of orogeny and contains numerous active mud diapirs and mud volcanoes. Gas emissions out of the seabed off SW Taiwan are revealed by acoustic images from 38 kHz echo sounders or sub-bottom profilers. However, the mechanism for gas emissions is still poorly understood. In this study, we show that the gas emissions out of the seabed and the associated tremors off SW Taiwan are strongly correlated with the ocean tides, especially with the diurnal and semidiurnal constituents. Particularly, for each tidal day the strong gas emissions (expul- sions) and long-duration tremors occur mainly during both the rising periods to the higher high tides and the falling periods to the lower low tides. Both the gas emissions and tremor activities are generally quiescent near the tidal datum. On average, the daily intensity of gas emissions and the magnitude of tremors are positively proportional to the daily tidal range; a larger slope site implies a greater concentration of gas in the shallow sediments. Most of the individual tremors and long-duration tremors observed off SW Taiwan occur as results of the strong gas expulsions out of the gas-bearing seabed. The high-frequency components of the induced tremors quickly decay and are followed by horizontal low-frequency (~7 Hz) harmonic oscillations of the seabed. Long-duration tremors may enhance seafloor instability and increase the potential for submarine landslides. & 2013 Elsevier B.V. All rights reserved. 1. Introduction In addition to the methane emissions from fossil fuel related anthropogenic sources, geologic emissions of methane also have great impacts on the geosphere, the biosphere, the hydrosphere and the atmosphere (Etiope and Klusman, 2002; Judd, 2003). Among all, methane escape from the seafloor is a widespread phenomenon (Judd, 2003). The out-going free methane out of seabed is mainly derived from the dissociation of gas hydrate, formed at a high pressure and low temperature environment (Sloan, 1998). The bottom of the gas hydrate stability zone below the seafloor is often marked by a BSR (Bottom Simulating Reflector) in reflection seismic profiles. Gas hydrate may dissociate when the seafloor becomes shallow. The result can induce an over-pressure pore-fluid in marine sediments. The fluid, mainly methane and carbon dioxide, may migrate upwards through gas seepage and produce mud volcanoes or pockmarks (Kopf, 2002; Judd and Hovland, 2007; Dimitrov, 2003). The area off SW Taiwan is situated in the northern end of the Manila subduction zone. As shown in Fig. 1, the BSR is widely distributed off SW Taiwan (Chi et al., 2006; Liu et al., 2006) but there is almost no BSR at water depths less than ~600 m. It indicates that the gas hydrate in the area off SW Taiwan becomes unstable and disassociates at a water depth around 650 m. Free methane may be effectively produced in the area off SW Taiwan due to two main facts. First, the area is gradually uplifted and shoals due to a NW–SE plate convergence between the Philippine Sea Plate and the Eurasian Plate and an incipient stage of the Taiwan orogeny (e.g. Sibuet and Hsu, 2004; Lo and Hsu, 2005). Second, the global warming increases seafloor temperatures. Thus, gas-related mud diapirs and mud volcanoes are distributed from the offshore to the onshore area of SW Taiwan (Yang et al., 2004; You et al., 2004; Chiu et al., 2006)(Fig. 1). Moreover, high methane fluxes were detected in both cored sediments (Chuang et al., 2006, 2010; Lim et al., 2011) and water column samples (Yang et al., 2006). Among all the mud volcanoes, the submarine mud volcano MV1 is ~100 m high above a seafloor of ~450 m deep (Fig. 1). The MV1 is active as illustrated by its steep cone-shaped morphology and the flowing gas bubbles out of its summit into the seawater, which are detected as a gas plume by a 38 kHz echo sounder. Active mud volcanoes cannot only emit methane but also generate micro-earthquakes or tremors (Judd and Hovland, 2007). To understand the mechanism of gas emission from the seabed and the tremors off SW Taiwan, we deployed 8 short-period OBSs Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/epsl Earth and Planetary Science Letters 0012-821X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2013.03.013 n Corresponding author. Tel.: þ886 3 4268316; fax: þ886 3 4222044. E-mail address: hsu@ncu.edu.tw (S.-K. Hsu). Earth and Planetary Science Letters 369–370 (2013) 98–107