Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted material contained within this report. FRONT COVER Coulomb stress change at 7.5 km depth imparted by the 1992 M=7.3 Landers, California, earthquake to the surrounding crust, together with the next 7 years of M≥2 aftershocks. The calculation is from King and others (1994). The Coulomb image has been draped over relief in ArcGIS by Serkan Bozkurt (AMEC/Geomatrix; formerly USGS).

Large earthquakes trigger very small earthquakes globally during passage of the seismic waves and during the following several hours to days, but so far remote aftershocks of moment magnitude M ≥ 5.5 have not been identified, with the lone exception of an M = 6.9 quake remotely triggered by the surface waves from an M = 6.6 quake 4,800 kilometres away. The 2012 east Indian Ocean earthquake that had a moment magnitude of 8.6 is the largest strike-slip event ever recorded. Here we show that the rate of occurrence of remote M ≥ 5.5 earthquakes (>1,500 kilometres from the epicentre) increased nearly fivefold for six days after the 2012 event, and extended in magnitude to M ≤ 7. These global aftershocks were located along the four lobes of Love-wave radiation; all struck where the dynamic shear strain is calculated to exceed 10(-7) for at least 100 seconds during dynamic-wave passage. The other M ≥ 8.5 mainshocks during the past decade are thrusts; after these events, the global rate of occurrence of remote M ≥ 5.5 events increased by about one-third the rate following the 2012 shock and lasted for only two days, a weaker but possibly real increase. We suggest that the unprecedented delayed triggering power of the 2012 earthquake may have arisen because of its strike-slip source geometry or because the event struck at a time of an unusually low global earthquake rate, perhaps increasing the number of nucleation sites that were very close to failure.

The Çınarcık Basin is a transtensional basin located along the northern branch of the northern North Anatolian Fault (NAF) in the Sea of Marmara, the eastern half of which has been identified as a seismic gap. During the SEISMARMARA (2001) experiment, a dense grid of multichannel seismic reflection profiles was shot, covering the whole Çınarcık Basin and its margins. The new seismic images provide a nearly three‐dimensional view of the architecture of the basin (fault system at depth and sedimentary infill) and provide insight into its tectonic evolution. Along both northern and southern margins of the basin, seismic reflection data show deep‐penetrating faults, hence long‐lived features, which have accommodated a large amount of extension. There is no indication in the data for a single throughgoing strike‐slip fault, neither a cross‐basin fault nor a pure strike‐slip fault running along the northern margin. Faster opening is presently observed in the eastern part of the basin. The Çınarcık Basin seems to have developed as a transtensional basin across strike‐slip segments of the northern NAF for the last few million years.

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The origin and prevalence of triggered seismicity and remote aftershocks are under debate. As a result, they have been excluded from probabilistic seismic hazard assessment and aftershock hazard notices. The 2004 M ¼ 9.2 Sumatra earthquake altered seismicity in the Andaman backarc rift-transform system. Here we show that over a 300-km-long largely transform section of the backarc, M ≥ 4.5 earthquakes stopped for five years, and over a 750-kmlong backarc section, the rate of transform events dropped by twothirds, while the rate of rift events increased eightfold. We compute the propagating dynamic stress wavefield and find the peak dynamic Coulomb stress is similar on the rifts and transforms. Longperiod dynamic stress amplitudes, which are thought to promote dynamic failure, are higher on the transforms than on the rifts, opposite to the observations. In contrast to the dynamic stress, we calculate that the mainshock brought the transform segments approximately 0.2 bar (0.02 MPa) farther from static Coulomb failure and the rift segments approximately 0.2 bar closer to static failure, consistent with the seismic observations. This accord means that changes in seismicity rate are sufficiently predictable to be included in post-mainshock hazard evaluations.Coulomb stress transfer | great earthquakes | seismicity rate changes