by Kuofong Ma, Mark D. Zoback, Stephen Hoyt Hickman

The 1999 Chi-Chi, Taiwan earthquake (Mw7.7) produced spectacular surface faulting with vertical displacement of up to 8m on the Chelungpu fault. The rupture behavior of this earthquake was well resolved from the strong motion records (~600 stations) and GPS (130 stations). It shows a large asperity at shallow depth near the surface with large slip of 10 to 15 m, and slip velocity of up to 3 to 4 m/sec in in the north, but with lower content of high-frequency accerleration, which produced relatively low levels of damage. The intriguing observations and results yield the important question in seismology. What physical properties or dynamic processes control the distribution of areas and behavior of large slips. The low angle thrust Chelunpu fault (dip30E) with large amount of fault slip at or near the surface provides a unique opportunity to study first-hand the physical mechanisms involved in faulting during large earthquakes through drilling. The Taiwan Chelungpu Drilling Program (TCDP) is extremely appealing both from a scientific and operational standpoint for the following reasons. 1. Clear Scientific Target: A rare and unique opportunity to understand the mechanism of generating a large earthquake. 2. Achievable Drilling Target: A shallow thrust fault (dip 30E) with large slip near the surface. 3. Well Defined Fault Plane: Careful observations from geological investigations and geophysical studies have clearly delineated the drilling target. 4. International Interests: Scientists from Taiwan, Japan, US and France have shown great interest in this drilling project. This might be the best chance ever to drill through a recently ruptured fault, which generated large slip of up to 10 m near the surface. In recognition of these favorable factors, we had submitted a proposal for funding of a 2-km-deep hole with continuous coring at the bottom 1 km (The target region with large slip will be reached at the depth of about 1.5 km) to the National Science Council (NSC) in Taiwan, and funding for this hole has been assured to the amount of about US$2.3 million. There are few scientists in Taiwan who are familiar with scientific drilling and coring projects. We write the proposal to envision the help of the ICDP in technique and training course support. We do believe that the Taiwan Chelungpu Drilling Project is quite unique in that it will lead to fundamental new insights into the rupture process for large earthquakes.

• The drilling plan proposed here is framed around a set of specific hypotheses, derived from major outstanding questions in fault mechanics and earthquake physics and through the criterion of being realistic to address by drilling. It will provide data bearing on each hypothesis and, whatever the results, will undoubtedly contribute a large step forward in fault zone studies. Hypothesis #1: Fault zone was lubricated while generating large slip and slip velocity. Hypothesis #2: Plate boundary fault is weak; i.e. slip under conditions of low resolved shear stress. Hypothesis #3: Physical properties, chemistry and state of the fault zone after a large earthquake with anomalous large slip. The hypotheses mentioned above are keys to understand the physical mechanism of earthquake rupture. The low angle thrust with cearly defined fault zone and well resolved spatial slip distribution of the source region provide the great opportunity of solving the world puzzle on fault zone propperties and earthquake faulting mechanism.

Active Fault Zones, Chelungpu Fault, Collision Zones, Convergent Margins, Earthquake Physics, Fault Lubrication, Fault Mechanics, Fault Properties, ICDP-2003/06, Taiwan, TCDP

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