[1] The outer-arc islands of western Sumatra rise during great megathrust earthquakes, due to large slip on the underlying megathrust. In contrast, the islands subsided up to a few centimeters during the recent tsunamigenic earthquake of October 2010, due to slip far updip, near the trench. Coral microatolls on one of the islands recorded a much larger subsidence, at least 35 cm, during an event in approximately A.D. 1314. We calculate a suite of slip models, slightly deeper and/or larger than the 2010 event, that are consistent with this large amount of subsidence. Sea level records from older coral microatolls suggest that these events occur at least once every millennium, but likely far less frequently than their great downdip neighbors. The revelation that shallow slip events are important contributors to the seismic cycle of the Mentawai segment further complicates our understanding of this subduction megathrust and our assessment of the region's exposure to seismic and tsunami hazards.

<p>Mainland Southeast Asia is located on the route of moisture transport of the Indian summer monsoon where hydroclimate records from speleothems have rarely been investigated. Here we present a new multi-proxy data set (δ<sup>18</sup>O, δ<sup>13</sup>C, trace elements and grayscale values) of stalagmite KPC1 from Khao Prae cave in western Thailand spanning from approximately 500 CE to 1900 CE. Our multi-proxy data reveal high variability between wet and dry periods during 500-850 CE and 1150-1350 CE, a stable condition between 850-1150 CE, and an overall trend towards dry conditions since 1350 CE. The δ<sup>13</sup>C, trace elements and grayscale values suggest centennial-scale fluctuations driven by local hydrological process at the cave site. In contrast, variations in stalagmite δ<sup>18</sup>O reflect integrated changes in rainfall amount from the Indian summer monsoon, supported by two-year monitoring rainfall data. In comparison with other Asian Monsoon records for the last millennia, the KPC1 record shows similarity with speleothem δ<sup>18</sup>O records from India, as well as lakes and tree-ring data from mainland Southeast Asia but diverges from records from equatorial regions and the western Pacific. We conclude that hydroclimate variability in the western side of Mainland Southeast Asia is mainly driven by changes in moisture transport from the Indian summer monsoon and modulated by expansion and contraction of the Intertropical convergent zone (ITCZ). However, Pacific Walker circulation (PWC) may have been the overriding control on precipitation on the eastern sides of Mainland Southeast Asia located closely to the western Pacific.</p>

Radiocarbon (14 C) is a useful tracer for surface ocean circulation and mixing, which reflects air-sea CO 2 exchange. We present radiocarbon marine reservoir ages (R) and corrections (ΔR) in Holocene inferred from 18 paired 14 C and 230 Th ages on fossil corals from Lanyu Island offshore eastern Taiwan. The results show large fluctuations in the ΔR value, with averages of-330 and-5 14 C yr for 6000-5100 yr BP and the past 150 years, respectively. The extremely young R in the mid-Holocene indicate a well-equilibrated North Equatorial Current (NEC), likely stemmed from enhanced air-sea interactions and strengthened Pacific Walker circulation. This suggests a larger E-W gradient across the Equatorial Pacific and hence a La Niña-like condition, consistent with both model simulations and other paleo-proxy records. Combining the ΔR records in the northern South China Sea, the results imply an increasing influence of the NEC water on the subtropical western Pacific since the mid-Holocene.