Seismic Microzonation of Densely Populated Area of Kathmandu Valley of Nepal using Microtremor Observations

Abstract

The Kathmandu Valley with an estimated current urban population of 5 million has been shaken many times in the past by damaging earthquakes. The valley was formerly a lake and although it is clear that much of the historical damage in the valley arose from seismic amplification associated with thick lake sediments, which in places exceed 500 m, few studies have been undertaken to understand the variability of seismic site response throughout the valley. In this study, we map the spatial variability of the dominant period of amplification using a horizontal-to-vertical spectral ratio technique derived from recordings of ambient noise. The resulting map is a first step towards seismic microzonation. We find that the area near the center of the valley, corresponding to the thickest sedimentary deposits, is associated with dominant periods that approach 2 s. Long-period shaking during future earthquakes is expected to be especially severe in this part of the valley since it corresponds to the urban center where numerous tall structures and long-span bridges with relatively long natural periods are to be found.

Keywords:

• Seismic Microzonation
• Nepal
• Microtremors
• H/V Spectral Ratio
• Dominant Period

Acknowledgments

The authors are grateful to Mr. Ramhari Dahal (The Department of Education, Government of Nepal) and Mr. Prakash Poudyal (Kathmandu University) for their help during the extensive microtremor survey in the Kathmandu valley. The constructive comments made by Dr. Ranjan K. Dahal (Senior Lecturer, Trichandra Campus, Tribhuwan University; currently at Ehime University as a JSPS Postdoctoral Research Fellow) and Dr. Jishnu K. Subedi (Lecturer, Institute of Engineering, Tribhuwan University Nepal) while preparing the manuscript are sincerely acknowledged, and help provided by Ms. Manita Timilsina (Ph.D. candidate, Graduate School of Science and Engineering, Ehime University) in preparing a few maps is greatly appreciated. The authors would also like to express their special appreciation to Dr. Shinichiro Mori (Associate Professor, Graduate School of Science and Engineering, Ehime University) for enabling the first author to analyze the microtremor survey data. The help received from Ms. Ruth Vergin (Associate Professor, Ehime University) for an improved English of the first manuscript is also sincerely appreciated. Two anonymous reviewers have greatly helped us bring this article to this level, and the Editor, Prof. Dr. Roger Bilham, who also has a long experience of working in the Himalayan seismicity, has put his own great efforts to instruct us to largely improve the quality of this article. We express our sincere gratitude to the Editor and the anonymous reviewers.

Finally, the financial support received from the Government of Japan as a Grant-in-Aid for Overseas Scientific Research and Investigation (Team Leader: Ryuichi Yatabe, Ehime University, AY2009∼AY2011, Integrated approach to studying rain- and earthquake-induced disasters in the Himalayan Watersheds and development of a strategic disaster education system) is sincerely acknowledged.