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Chandrayaan-1 finds signatures of recent shallow moonquakes

K. S. Jayaraman

doi:10.1038/nindia.2016.34 Published online 5 March 2016

India's Chandrayaan-1 mission to the Moon has helped discover the most likely locations of "shallow" moonquakes that resemble those on Earth, scientists report1.

Chandrayaan-1 image showing the distribution of shallow thrust faults (lobate scarps – yellow line) and the boulder falls (boulders containing trails) on basin wall of Schrödinger. © Senthil Kumar et al.

"Like earthquakes, other planets in the Solar System also experience quakes," P. Senthil Kumar, senior planetary scientist at the National Geophysical Research Institute (NGRI) in Hyderabad and lead author of the paper told Nature India. "After Earth, we only have direct measurements of quakes on the Moon, using the seismometers installed by Apollo astronauts during 1969–1977." They recorded thousands of deep moonquakes, hundreds of surface quakes due to meteorite impacts and very few shallow moonquakes every year.

Since the discovery of these "shallow" quakes, many researchers attempted to locate their sources without much success. Kumar and co-workers from institutions under the Indian Space Research Organization (ISRO) used images obtained by Chandrayaan-1 high-resolution terrain mapping camera and the Narrow Angle Camera of NASA's Lunar Reconnaissance Orbiter that was launched in 2009. "We report new occurrences of very young thrust faults – similar to those that cause earthquakes in the Himalayan range – at four different locations in the 320-km diameter Schrödinger basin near the lunar south pole that was created by an asteroid impact about 3.8 billion years ago," Kumar said.

These faults, just a few tens to hundreds of meters deep, are known as 'lobate scarps' – because of their 'topographic bump' – and are believed to have been formed when the lunar crust was pushed together as the Moon contracted. The researchers dated these faults using a well-proven age determination method (crater counting) and found them to be surprisingly young, the youngest being only 10 million years old. "The young age of the fault scarps indicates that the Moon has contracted very recently," Kumar said. On Earth, this type of faults are usually associated with repeated seismic activities. "Whether the lobate scarps on the Moon are seismically active presently or were only in the past is unknown," Kumar said. By mapping the distribution and determining the size of the newly detected and previously known 'lobate scarps', the tectonic and thermal history of the Moon can be reconstructed over the past billion years, he said.

P. Senthil Kumar The more interesting finding for the scientists was the occurrence of several hundreds of 'boulder falls' very near those young faults, where the boulders rolled, tumbled and jumped along the slopes, leaving many trails or tracks on their way. "These boulder tracks that are also very young, are potential indicators of recent ground shaking and seismic activity on the Moon," Kumar said.

The new observations of youthful 'lobate scarps' and abundant boulder falls on nearby slopes in the Schrödinger basin have important implications for seismotectonics and mass wasting processes on the Moon. The study also suggests the need to re-examine the Apollo seismic observations to confirm if these events were indeed due to meteorite impacts, as interpreted by NASA, or due to shallow moonquakes in the Schrödinger basin.

The moonquakes may also provide new insights about mechanism of earthquakes, Kumar said adding that ISRO's Moon rover in the Chandrayaan-II mission slated for 2017 should, therefore, carry a seismometer to study these activities in the Schrödinger basin.