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

Seismic Amplification of Typical New York City Soil Profiles

Amplification studies for New York City (NYC) soil sites are summarized herein. Ten (10) typical soil profiles from Brooklyn, Queens, and Manhattan, are analyzed using one-dimensional SHAKE methods. Dynamic soil properties are derived using state-of-practice correlations with standard penetration resistance and compared to available in-situ geophysical measurements. Three different rock motions are utilized, each modified from real records to match 500- and 2500-year probabilistic spectra. Results are presented in terms of dimensionless ratios of response spectra (RRS) and surface response spectra. The effect of the impedance contrast between soil and rock on soil amplification is examined. It is shown that although seismic hazard in the area is only moderate, significant soil effects can be generated and lead to large soil amplifications. By comparing the derived spectra with the design spectra defined by the 1998 NYC Department of Transportation guidelines, it is shown that the latter may be unconservative at short periods. Comparison of the results with the design spectra of the 1995 NYC Seismic Code shows that the Code provides conservative design parameters, but unconservative amplification values

Foundation Optimization and Design for Replacement of the Woodrow Wilson Bridge

The new Woodrow Wilson Bridge (WWB) will replace the existing bridge over the Potomac River to connect Alexandria, Virginia to Prince Georges County, Maryland. The new WWB will extend approximately 1.1 miles across the river, with a 367-ft long bascule span in the main river channel where the water depth is about 36 ft. The subsurface soil profile consists of up to 50 ft of a soft organic silty clay layer that is very vulnerable to scour, underlain by a deep deposit of hard sandy clay. This paper will present results from a Pile Demonstration Program (PDP) that was conducted as part of the bridge replacement project, discuss the various aspects of the seismic design and analysis, and describe how those data were applied to optimize foundation design. The PDP included dynamic monitoring, static load tests and Statnamic load tests at several locations, to evaluate: (i) the pile driveability and associated parameters necessary for dynamic analysis; and (ii) the ultimate skin friction and end bearing values for design. The PDP provided a basis for eliminating static load tests during construction and construction quality control, and for evaluating potential settlement of the existing bridge. Although the seismicity of the region is low, considering the importance of this bridge and the consequences of potential damage during an earthquake, seismic issues were addressed thoroughly in the design of the new structure, including: (i) development of design spectra based on site-specific seismic hazard and ground motion analyses; (ii) implications of the complex soil profile and potential scour on the dynamic response of the foundations; (iii) Soil-Structure Interaction (SSI) analyses for the various foundation alternatives; and (iv) evaluation of the significance of the kinematic SSI effect on the piles. The presented case study proves how results of a pile demonstration program and extensive seismic studies can enable significant optimization of the foundation design and cost savings, and provide significant quality control during construction

Liquefaction Susceptibility: Proposed New York City Building Code Revision

A simplified procedure is presented for evaluating liquefaction susceptibility of cohesionless saturated soils based on available technology. In 2001, a Committee of engineers working in the New York City (NYC) area was formed under the direction of the first Author, to review the liquefaction aspects of the 1995 New York City Building Code. The purpose was to gain consensus on a possible revision and augmentation of the exisiting regulations as part of the ongoing Code review by the Structural Engineers Association of New York (SEAoNY). This article summarizes the recommendations of the Committee, as compiled in 2002. The following topics are reviewed: (a) history of the current code; (b) seismicity and design motions in NYC; (c) updated screening criteria for liquefaction susceptibility. With reference to the topic in (c), recommendations are developed for Code language pertaining to: (1) method of analysis; (2) site classification schemes; (3) design considerations for bearing capacity and displacements of foundations in liquefied soil; (4) maximum depth of liquefaction; (5) field methods to evaluate soil resistance; (6) parameters to be considered in analyses; (7) treatment of sloped strata. Analytical results for typical NYC profiles subjected to 500-year rock motions are presented. Based on the these results, the Committee proposed a revised liquefaction screening diagram

THE 7.8 Mw EARTHQUAKE AND TSUNAMI OF 16th April 2016 IN ECUADOR: Seismic Evaluation, Geological Field Survey and Economic Implications

We evaluated the recent earthquake and tsunami responsible for considerable damage and 663 deaths due to a 7.8Mw movement on the 16th of April 2016. The seismic event filled tens of thousands in refugee camps and affected some two million persons directly. The potential of high losses and damage with a total of 29,672 properties, including family houses, is also given by the fact that the infrastructure of the fishing, tourism and other industries and the movement to live along the beaches, have been highly developed within the last decades along the Ecuadorian coasts. The geological survey and determination of field data were performed three days after the main seismic event, allowing to obtain 290 data coseismic effects on the ground that allowed to evaluate the maximum macroseismic intensities as well as the predominant geomorphological features.The results of these sampling stations allowed to reconstruct a geological map with isoseismals fields of intensities. With all the compiled and recorded coseismic data in the field of higher macroseismic intensities, we proceeded to produce a map of intensities applying the definitions and degrees of the ESI 2007 scale. We also evaluated the distribution and intensities of the aftershocks demonstrating the spatial- temporal affinities. The occurred tsunami, although less destructive than previous in the same region has been documented with all details available. The economical assessment included in this study concludes that this earthquake impacted a large part of a variety of coastal cities destroying between 70 and 80% of some close-by villages and cities with a distance of 140-150 km of the epicenter, which suffered damages of their buildings within 40 to 55%, in which lines of electricity transmission, infrastructure of water supply, hospitals, schools, private and public buildings, main roads and highways have been severely affected or even completely destroyed. The costs of the damages of the mentioned infrastructure are summing up an approximate loss of some 3.3 billion USD, being equivalent to 3.31 % of the Ecuadorian GDP. In addition to losses in infrastructure and properties, over 28 thousand jobs were lost and about 300 million US$ in trade and businesses