Local Geology of New York City and Its Effect on Seismic Ground Motions

A thorough understanding of the local geologic and tectonic environment, the seismological history as well as very detailed site specific geotechnical and geophysical data are essential to the proper prediction of local site effects and seismic design in New York City (NYC). The site response in the NYC metropolitan area is affected by the widely varying geologic conditions encountered in the five boroughs. Along the spine of Manhattan Island rock extends well above sea level at the northern reaches, and falls to depths in excess of 250 m at the barrier islands at the southern extremities of NYC. Large areas in the City have been filled to cover soft sediments and marshes to accommodate the need for building space, such as the present area of Chinatown that is built on fills that have replaced a large lake known as Collect Pond; the World Fairs site in Long Island Sound Embayment in Flushing, Queens, and the ground on which JFK Airport is constructed by placing hydraulic sand fill in the south shore of Brooklyn. The highly variable geologic conditions, along with the lack of strong ground motion recordings create uncertainty in predicting site response. This paper will present an overall review of the geological and seismological characteristics of the NYC metropolitan area and will examine how current, applicable codes deal with predicting soil amplification and evaluating liquefaction hazard. Issues of concern not covered in codes, such as the effect of high impedance contrast between hard bedrock and soft soil and the response of soft-high-plasticity organic clays and silts will be examined using typical NYC soil profiles and state-of-practice design motions and hazard levels