Common dependence on stress for the two fundamental laws of statistical seismology

Narteau, Clément
Byrdina, Svetlana
Shebalin, Peter
Schorlemmer, Danijel

¹Institut de Physique du Globe de Paris (UMR 7154, CNRS, Univ. P7), 4 Place Jussieu, 75252 Paris Cedex 05, France
²Laboratoire de Géophysique Interne et Tectonophysique, UMR 5559, CNRS, Université de Savoie, IRD, 73376 Le Bourget-du-Lac Cedex, France
³International Institute of Earthquake Prediction Theory and Mathematical Geophysics, 84/32 Profsouznaya, Moscow 117997, Russia
⁴Department of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, California 90089, USA

^*Correspondence and requests for materials should be addressed to C.N. ([email protected]).

Received 16 January 2009; accepted 28 Spetember 2009

Nature 462(7273):p 642-645, December 3, 2009. | DOI: 10.1038/nature08553

Two of the long-standing relationships of statistical seismology are power laws: the Gutenberg–Richter relationdescribing the earthquake frequency–magnitude distribution, and the Omori–Utsu lawcharacterizing the temporal decay of aftershock rate following a main shock. Recently, the effect of stress on the slope (thebvalue) of the earthquake frequency–magnitude distribution was determinedby investigations of the faulting-style dependence of thebvalue. In a similar manner, we study here aftershock sequences according to the faulting style of their main shocks. We show that the time delay before the onset of the power-law aftershock decay rate (thecvalue) is on average shorter for thrust main shocks than for normal fault earthquakes, taking intermediate values for strike-slip events. These similar dependences on the faulting style indicate that both of the fundamental power laws are governed by the state of stress. Focal mechanisms are known for only 2 per cent of aftershocks. Therefore,candbvalues are independent estimates and can be used as new tools to infer the stress field, which remains difficult to measure directly.