Chapter 16 - Time-frequency analysis of ground motions from the 1999 Chamoli earthquake

Abstract

On March 29, 1999, the Garhwal-Kumaun region in the western Himalayas was struck by a damaging Mw 6.6 earthquake. The earthquake was recorded at 11 stations in the state of Uttarakhand, India. The earthquake caused immense damage to the built environment in the region which primarily comprises low-rise masonry and reinforced concrete structures. Under severe cyclic motions, induced by earthquakes, these structures undergo damage leading to a reduction in strength and stiffness, thereby causing an increase in their natural period during the duration of an earthquake. The change in the resonant frequency of the structure, during an earthquake, implies that they may be impacted by multiple frequencies present in an earthquake wave. In this paper, the frequency content of 22 horizontal ground motions from the 1999 Chamoli earthquakes are examined using fast Fourier transforms. The time sequencing of frequencies cannot be inferred from fast Fourier transforms and are examined using continuous wavelet transforms. It is observed that the frequency content in strong ground motions is within the range of both the original and altered natural frequencies of low-rise structures. Further, at some stations, these damaging frequencies are observed to arrive at multiple time instants or are sustained in time. These observations correlate well with the observed damage to structures in the region.