Assessing the Role of Urban Forests as Natural Seismic Metamaterials in USCS Classified Soils and Islamabad's Subsoils

Abstract

The devastating 2005 earthquake in Pakistan underscored the urgent need for seismic-resilient infrastructure, prompting a reassessment of seismic-resistant measures. Simultaneously, the escalating risks associated with climate change have propelled Pakistan to prioritize the incorporation of nature-based solutions, evident in the updated National Climate Change Policy (NCCP-2021). This study delves into the dual role of Urban Forests as natural seismic metamaterials, addressing both seismic hazards and climate change-related risks. Existing literature highlights the efficacy of periodic barriers, acting as vertical resonators, in mitigating low-frequency Rayleigh waves. Urban forests, organized in a periodic manner, have demonstrated their potential as natural seismic metamaterials, offering a nature-based solution for reducing seismic hazards. Building on previous studies focusing on the applicability of trees as vertical resonators, this research advances by evaluating the effectiveness of urban forests as seismic shields in diverse soil types classified under the Unified Soil Classification System (USCS). Moreover, the study extends to the examination of urban forests as seismic isolators in soils of 11 different sectors of Islamabad. The findings reveal insights into the frequency band gaps and vibration attenuation characteristics of different soils and Islamabad-specific soils. On this basis, USCS soils are classified into three classes; that is, (i) soils having elastic modulus up to 10 MPa which show narrow band gaps lower than 15 Hz, (ii) Soils having elastic modulus from 10-25 MPa which exhibit wide band gaps of almost 5 Hz within frequency range of 15-25 Hz and (iii) Soils having elastic modulus from 25-65 MPa showing wide band gaps of more than 5 Hz within 25-40 Hz range. Notably, Islamabad-specific soils exhibit band gaps within a consistent range of 20-30 Hz with wide band gaps between 20-25 Hz, indicating the prevalence of a uniform soil type in the region. This study provides a foundational framework for region-specific investigations, incorporating hazardous Rayleigh wave frequencies. By adopting this approach, feasible parametric optimization allows tailored interventions in seismic-prone regions. Additionally, the outcomes provide a framework for the integration of urban forests into town planning schemes and building design codes, offering dual benefits as seismic shields and contributors to climate change risk mitigation.