Shear velocity structural characterization around the Lonar crater using joint inversion of ambient noise HVSR and Rayleigh wave dispersion

Abstract

The Lonar crater in India is the only known hyper-velocity impact crater formed in basalt target on the Earth formed by an extra-terrestrial meteorite impact, with a distinct shock-induced metamorphosed signature. To understand the effects of impact deformation, fracturing, emplacement of the shock-metamorphosed ejecta material and sedimentation structure in the target basalt around the crater, we investigate the sub-surface shear velocity structure around the Lonar crater region. We use three components, 24-hour records of ambient noise (or microtremor) H/V spectral ratio (HVSR) and Rayleigh wave dispersion data through cross-correlation of ambient noise, obtained from 20 broadband seismic stations around the Lonar crater. The HVSR pattern across the Lonar crater does not present common frequency peaks, suggesting a laterally varying lithological arrangement around the crater. We observe that the HVSR curves at the stations show one or more peaks, below 1 Hz and above 2 Hz. To obtain the average sub-surface shear velocity models, we use the joint inversion of the phase dispersion curves of fundamental and higher mode Rayleigh wave with HVSR data sets, using neighborhood algorithm, in the frequency range 0.2–20 Hz and explored up to 750 m depth. As a reliability-check for the shear velocity models, we compute the theoretical HVSR by forward modeling. The theoretical and observed HVSR peaks match fairly well at each broadband station spread across the Lonar crater. The results show the existence of the fractured/weathered and sedimentary layers and also of the fundamental and higher modes of Rayleigh waves altering the shape of the HVSR and the Rayleigh wave dispersion curves. A pattern of the multiple layers in the sediment–package is visible in the shallow and deeper parts up to 600 m depths across the Lonar crater, but with spatial variations, which may be due to variations in the extent of the fractured zone. This spatial asymmetry of shear velocity profiles might also be the effect of an obliquity impact at the Lonar crater. The significance of such a joint analysis of different datasets of surface waves and HVSR is intended to reduce non-uniqueness involved in the non-linear inversions and present more acceptable sub-surface models to enhance the near-surface geophysical understanding of the impacted structure around the Lonar Crater.