Jeff Freymueller

Abstract The Indian Ocean earthquake of 26 December 2004 led to significant
ground deformation in the Andaman and Nicobar region, accounting for !800 km
of the rupture. Part of this article deals with coseismic changes along these islands,
observable from coastal morphology, biological indicators, and Global Positioning
System (GPS) data. Our studies indicate that the islands south of 10" N latitude
coseismically subsided by 1–1.5 m, both on their eastern and western margins,
whereas those to the north showed a mixed response. The western margin of the
Middle Andaman emerged by #1 m, and the eastern margin submerged by the same
amount. In the North Andaman, both western and eastern margins emerged by #1 m.
We also assess the pattern of long-term deformation (uplift/subsidence) and attempt
to reconstruct earthquake/tsunami history, with the available data. Geological evidence
for past submergence includes dead mangrove vegetation dating to 740 !
100 yr B.P., near Port Blair and peat layers at 2–4 m and 10–15 m depths observed
in core samples from nearby locations. Preliminary paleoseismological/tsunami evidence
from the Andaman and Nicobar region and from the east coast of India, suggest
at least one predecessor for the 2004 earthquake 900–1000 years ago. The history of
earthquakes, although incomplete at this stage, seems to imply that the 2004-type
earthquakes are infrequent and follow variable intervals.

During the past 5-10 Ma, the Yakutat terrane has been colliding with and subducting beneath the Alaskan part of the North American Plate. Modeling of GPS data and analysis of deep-crustal structure bolster a hypothesized connection between the terrane collision and seismogenesis. GPS data collected near Prince William Sound indicate that forearc subsidence following the 1964 Great Alaska Earthquake (Mw=9.2)

Although the horizontal velocity field for the Tibetan Plateau and its surrounding area is now very familiar, few estimates of vertical motions have been made. Measuring precise vertical motions without a dense continuous GPS network is not easy, but the measurement history of many sites in Tibet now extends for 15 years or more, making such measurements possible. We present the most complete vertical velocity field for Tibet yet presented, based on data collected over the last 2 decades, mostly by Chinese sources. We use a new data analysis strategy that improves both precision and accuracy relative to out previous work, and consider different estimates of the geocenter motion and other limitations of the geodetic reference frames. Preliminary results show that uplift rates in southern Tibet and northern Nepal can exceed 5 mm/yr, consistent with results from leveling data collected decades ago. Uplift rates in the Yarlung valley area of southern Tibet are smaller, generally 2-3 mm/...