USING SAFETY INSPECTION DATA TO ESTIMATE SHAKING INTENSITY FOR THE 1994 NORTHRIDGE EARTHQUAKE

We map the shaking intensity suffered in Los Angeles County during the 17 January 1994, Northridge earthquake using municipal safety inspect ion data. The intensity is estimated from the number of buildings give n red, yellow, or green tags, aggregated by census tract. Census tract s contain from 200 to 4000 residential buildings and have an average a rea of 6 km(2) but are as small as 2 and 1 km(2) in the most densely p opulated areas of the San Fernando Valley and downtown Los Angeles, re spectively. In comparison, the zip code areas on which standard MMI in tensity estimates are based are six times larger, on average, than the census tracts. We group the buildings by age (before and after 1940 a nd 1976), by number of housing units (one, two to four, and five or mo re), and by construction type, and we normalize the tags by the total number of similar buildings in each census tract. We analyze the seven most abundant building categories. The fragilities (the fraction of b uildings in each category tagged within each intensity level) for thes e seven building categories are adjusted so that the intensity estimat es agree. We calibrate the shaking intensity to correspond with the mo dified Mercalli intensities (MMI) estimated and compiled by Dewey ef n l. (1995); the shapes of the resulting isoseismals are similar, althou gh we underestimate the extent of the MMI = 6 and 7 areas. The fragili ty varies significantly between different building categories (by fact ors of 10 to 20) and building ages (by factors of 2 to 6). The post-19 40 wood-frame multi-family (greater than or equal to 5 units) dwelling s make up the most fragile building category, and the post-1940 wood-f rame single-family dwellings make up the most resistant building categ ory.