Two‐stage rupture of the 1999 Mw 6.0 Southern Lake Baikal earthquake

• Published in: Geophysical journal international 2008-01, Vol.172 (1), p.302-310
• Main Authors: Peacock, Sheila, Douglas, Alan
• Format: Article
• Language: English
• Subjects: Asia; Earthquake source observations
• ISSN: 0956-540X; 1365-246X
• DOI: 10.1111/j.1365-246X.2007.03629.x

SUMMARY The 1999 February 25, 1858 hr, Southern Lake Baikal earthquake is evidently a two‐stage rupture when compared with its prominent foreshock (mb 4.4) at 0537 hr on the same day and a prominent nearby earthquake (mb 5.1) on 2000 May 31. The foreshock and 2000 May 31 earthquake have similar waveforms recorded at short‐period array stations at Δ= 21–79°. The main shock P wave train (including the surface reflections pP and sP) appears most similar to that from the 2000 May 31 earthquake after a shift of ∼8.5 s from the P wave first arrival. A model of two‐stage rupture with a smaller stage followed 8.5 s later by a larger rupture fits many aspects of the seismograms of the main shock, whereas the 2000 May 31 seismograms can be fitted with a single source. The main shock first‐stage rupture has moment ∼17 per cent of that of the second‐stage rupture. Their hypocentres are at depth ∼15 ± 2 km, from modelling pP and sP phases using a crustal model of a low‐wave speed sedimentary basin at the epicentral region. The hypocentres of the two stages are the same to within the resolution of the modelling. The 2000 May 31 hypocentre is at depth ∼13 km in the same model. Model seismograms at stations to the northwest and southeast have different time intervals between initial P and the surface reflections: the longer P–pP interval for the northwest stations can be explained by a thicker sedimentary basin at pP bounce points northwest of the epicentre. The two‐stage rupture of the main shock could be responsible for the wide range of depths (7–21 km) and mb (4.9–6.1) published in the International Seismological Centre catalogue.