ENTERTAINING A GREAT EARTHQUAKE IN WESTERN NEPAL: HISTORIC INACTIVITY AND GEODETIC TESTS FOR THE PRESENT STATE OF STRAIN

A 500-800 km long segment of the Himalaya bordered by the rupture zones of the great Bihar, 1934, and Kangra, 1905, earthquakes has not experienced a great earthquake for at least 200 years, and perhaps for as long as 750 years. The rate of occurrence of earthquakes is evidently too low to accomodate Indo/Tibetan slip which must therefore be accomodated by creep or occasional great earthquakes. Creep processes do not appear to be sufficiently fast, at least in central Nepal, where leveling data in the last two decades have been interpreted to account for at most 30% of the inferred ≈20 mm/yr convergence signal. The measurement of 19th century geodetic networks in northern India, which have hitherto been neglected, potentially provides an estimate the rate of accumulation of elastic strain in W. Nepal. In view of the disastrous consequences to the many tens of millions inhabitants of northern India and Nepal who would be affected by a great earthquake, an intense effort to explore further the historic record and the geographic limits of historic and future rupture is desirable. INTRODUCTION-SEISMIC HAZARD AND INVISIBLE FAULTS Each year India is believed to approach Tibet by 15-20 mm. This convergence rate has been inferred indirectly from geological and seismological evidence and from global plate-circuit closures (Molnar, 1990), and new data from GPS observations promises soon to provide a direct measure of its instantaneous rate. Data to date are consistent with a 20 mm/year convergence rate although the uncertainty of the measurements is currently of the same order of magnitude as the 1991-1992 signal (Jackson and Bilham, 1994a). If we assume that the convergence rate is 20 mm/a and that the last few great earthquakes of the Himalaya were associated with slip of the order of 5-10 m, the renewal time for their recurrence is 250-500 years . Current methods employed to estimate seismic hazards in Nepal depend on identifying active faults, assigning probable recurrence intervals for earthquakes of a given magnitude on these faults, and estimating at selected nearby sites the probable accelerations from all probable ruptures on these identified faults in a given time window. Necessary refinements include assessing the dispersion and attenuation of seismic waves with distance from each hypothetical earthquake, and assessing soil and geometrical conditions that can result in local amplification of these waves. The problem with this approach is that the great earthquakes that accommodate most of the Himalayan convergence are not well-represented by surface faulting. This would not be a serious omission were not the causal fault(s) of these great earthquakes, namely the sub-horizontal faults associated with the Himalayan-detachment of Seeber and Armbruster (1981), fewer than 6 km beneath the surface, and thus the closest fault to many villages in Nepal and northern India. The fault (or faults) is believed to separate the Indian plate from an accretionary prism of Himalayan sediments, but its geometry and location are uncertain because microearthquakes near it are rare, and its reflected image in seismic refraction surveys is weak. Because it is horizontal or gently dipping to the