The Distribution of Red and Yellow Tags in the City of Napa

Abstract

The M 6.0 South Napa earthquake that occurred on 24 August 2014, strongly shook the City of Napa, damaging residential and commercial buildings from Brown’s Valley through the historic downtown. The damage to wood‐frame houses occurred largely from broken and cracked chimneys, although a number of houses were shifted on their foundations, or suffered racking or failure of cripple walls. In the downtown area, many masonry buildings, both unreinforced and retrofitted, were damaged, including the 1870 Napa courthouse. When earthquakes damage municipalities, it is important for planners and residents to understand the factors that condition this damage, because these factors can determine where damage will recur in future earthquakes. For areas subjected to strong ground motion, that is, peak accelerations exceeding 25% g and peak velocities exceeding 40 cm/s, municipal‐tagging data can provide a spatial detail of building damage that greatly exceeds what can be obtained from ground‐motion recordings or intensity surveys. In this paper, we use the red‐ and yellow‐tagging data assembled by the City of Napa to identify zones of extensive damage. We compare these zones with the pre‐1950 development of Napa, the recent alluvial geology of the Napa Valley (Witter et al. , 2006), and the depth of the underlying sedimentary basin, as inferred from the Bouguer gravity anomaly (Jachens et al. , 2006; Aagaard et al. , 2010). The extent of the central damage zone is strongly correlated with the pre‐1950 buildings and with the underlying sedimentary basin, but poorly correlated with the alluvial geology. The City of Napa, with the assistance of structural engineers who volunteered from many areas across California, tagged and retagged damaged buildings throughout the city (Earthquake Engineering Research Institute, 2014). The tagging data provide a complete municipal reporting of prohibited (red tags) or restricted (yellow tags) access to earthquake‐damaged structures. Some buildings received multiple …