THE DISTRIBUTION OF SEISMIC INTENSITIES AND SURFACE RUPTURE IN THE LANCANG-GENGMA (YUNNAN PROVINCE,CHIN A ) EARTHQUAKES OF NOVEMBER 6, 1988

On November 6, 1988, two earthquakes with magnitude > 7 occurred on the Lancang- Gengma fault zone in south- west China.The extensive destruction and loss of lives resulted mainly from widespread collapse of unreinforced masonry and mud brick structures ; the maximum preliminary intensity of the Lancang earthquake was IX on the Chinese scale, which is similar to the Modified Mercalli scale,and the highest preliminary intensity of the Gengma earthquake was probably X.The surface manifestation of tectonic activity of the Lancang earthquake was the occurrence of the earthquake- related extensional ground cracks and small fault scarps in the epicentral region. The cracks with small fault scarps occurred mainly in four relatively continuous north- northwest- trending linear zones that ranged from a few hundred meters to 6km in length. The area within which the cracks and small scarps occurred is 35 km long by 3 km wide. The maximum net throw and the dextral horizontal offset were l-5m and l-4m respectively.Clear evidence of new surface faulting caused by the Gengma earthquake includes a series of relatively continuous north- northwest- trending linear ground crack zones and a 5 km long section of fault scarps. The total length of the surface rupture zones of the Gengma earthquake is about 24km, with 3.5m maximum net throw and 3m maximum right- lateral slip.Both earthquakes were associated with surface faulting showing a combination of normal and right lateral motion. The distribution of seismic intensities and surface rupture characteristics of these two earthquakes are discussed in this paper.