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Maruia Falls

New Zealand

At Maruia Falls, water cascades over grey sandstone eroding gravel deposits downstream. The boundary between sandstone and gravel may be an older earthquake fault plain. In the area the Alpine Fault coming from the south connects to the Malborough Fault System.

At Maruia Falls, water cascades over grey sandstone eroding gravel deposits downstream. The boundary between sandstone and gravel may be an older earthquake fault plain. In the area the Alpine Fault coming from the south connects to the Malborough Fault System.

Geological Period

Holocene

Main geological interest

History of geosciences

Location

Near Murchison, South Island, New Zealand
41°51’36”S, 172°15’08”E

At Maruia Falls, water cascades over grey sandstone eroding gravel deposits downstream. The boundary between sandstone and gravel may be an older earthquake fault plain. In the area the Alpine Fault coming from the south connects to the Malborough Fault System.

Murchinson earthquake (1929) causing Maruia Falls led to the Discovery of the Inner Earth Core.

In 1936, the Danish seismologist Inge Lehmann (1888–1993), suggested from the analysis of seismic P-wave data from the 1929 Murchison earthquake, which caused Maruia Falls and which were recorded at seismic stations in Greenland, that the Earth has an inner core – an important breakthrough in the understanding of the nature of the Earth’s interior.

Inge Lehmann, discoverer of the inner Earth's core, in 1932. (Image courtesy The Royal Library, National Libary of Denmark and University of Copenhagen University Library, under a Creative Commons License).

Inge Lehmann, discoverer of the inner Earth’s core, in 1932. (Image courtesy The Royal Library, National Libary of Denmark and University of Copenhagen University Library, under a Creative Commons License).

  • Geological description

On 17 June 1929, the MS 7.8 Murchison earthquake triggered numerous landslides in the north-western South Island, New Zealand. One of them diverted the course of the Maruia River westward, where it encountered unconsolidated gravel deposits, which it eroded over the years forming a waterfall. At present, its height is about 10 meters. Maruia Falls’ significance, however, goes far beyond local geology:
Seismic P waves of large earthquakes travel through the whole Earth. By 1929, it was already known that the Earth had a fluid core, as P waves reached the far side of the globe significantly later than would be expected in a homogeneous globe. Also, the core focused seismic waves acting as a huge lens, creating a shadow zone further out, in which no seismic waves were recorded. In the 1930s, better seismic detectors showed that this shadow zone was not devoid of seismic activity. P-Waves were weak only in terms of surface parallel movements. The vertical movement, however, was quite strong. Explaining seismic activity within the “shadow zone”, using data from the Murchison earthquake, Inge Lehmann postulated the existence of inner Earth’s core, which has a higher velocity of seismic waves compared to the outer core.

  • Scientific research and tradition

Inge Lehmann studied mathematics and science in Copenhagen and Newnham College Cambridge. She worked as a computer at Den Danske Gradmaaling from 1928 onwards being responsible for establishing seismic stations in Denmark and Greenland. She interpreted the institute’s seismograms, but also undertook original scientific research. In her 1936 paper P’, she postulated the existence of an inner core for the Earth.

Visualisation of Lehmann’s simplified earth model: The outer core focuses seismic waves creating a shadow zone, whereas the inner core acts as a diverging lens, shedding seismic energy into the shadow zone (Kölbl-Ebert, 2001).

Visualisation of Lehmann’s simplified earth model: The outer core focuses seismic waves creating a shadow zone, whereas the inner core acts as a diverging lens, shedding seismic energy into the shadow zone (Kölbl-Ebert, 2001).

  • Reference

Bolt, B.A. (1987) ‘50 years of studies on the inner core’, Eos, Transactions American Geophysical Union, 68(6), pp. 73–81. Available at: https://doi.org/10.1029/EO068i006p00073-01.

Bolt, B.A. and Hjortenberg, E. (1994) ‘Memorial essay: Inge Lehmann (1888-1993)’, Bulletin of the Seismological Society of America, 84(1), pp. 229–233. Available at: https://doi.org/10.1785/BSSA0840010229.

Brush, S.G. (1980) ‘Discovery of the Earth’s core’, American Journal of Physics, 48(9), pp. 705–724. Available at: https://doi.org/10.1119/1.12026.

Hancox, G.T. et al. (2016) Landslides caused by the MS 7.8 Murchison earthquake of 17 June 1929 in northwest South Island, New Zealand. Lower Hutt, N.Z. GNS Science report 2015/42. GNS Science, p. 131 pp. + 4 maps.

Kölbl-Ebert, M. (2001) ‘Inge Lehmann’s paper: “ P’”(1936)’, Episodes Journal of International Geoscience, 24(4), pp. 262–267. Available at: https://doi.org/10.18814/epiiugs/2001/v24i4/007.

Lehmann, I. (1936) ‘P’’, Bureau Central Séismologique International Strasbourg : Publications du Bureau Central Scientifiques, 14, pp. 87–115.

  • Author(s)

Martina Kölbl-Ebert.
International Commission on the History of Geological Sciences (INHIGEO) & Department of Earth and Environmental Sciences University of Munich, Germany.

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