The precursory phase of large earthquakes.

The existence of an observable precursory phase of slip on the fault before large earthquakes has been debated for decades. Although observations preceding several large earthquakes have been proposed as possible indicators of precursory slip, these observations do not directly precede earthquakes, are not seen before most events, and are also commonly observed without being followed by earthquakes. We conducted a global search for short-term precursory slip in GPS data. We summed the displacements measured by 3026 high-rate GPS time series-projected onto the directions expected from precursory slip at the hypocenter-during 48 hours before 90 (moment magnitude ≥7) earthquakes. Our approach reveals a ≈2-hour-long exponential acceleration of slip before the ruptures, suggesting that large earthquakes start with a precursory phase of slip, which improvements in measurement precision and density could more effectively detect and possibly monitor.

Instantaneous tracking of earthquake growth with elastogravity signals.

Rapid and reliable estimation of large earthquake magnitude (above 8) is key to mitigating the risks associated with strong shaking and tsunamis1. Standard early warning systems based on seismic waves fail to rapidly estimate the size of such large earthquakes2-5. Geodesy-based approaches provide better estimations, but are also subject to large uncertainties and latency associated with the slowness of seismic waves. Recently discovered speed-of-light prompt elastogravity signals (PEGS) have raised hopes that these limitations may be overcome6,7, but have not been tested for operational early warning. Here we show that PEGS can be used in real time to track earthquake growth instantaneously after the event reaches a certain magnitude. We develop a deep learning model that leverages the information carried by PEGS recorded by regional broadband seismometers in Japan before the arrival of seismic waves. After training on a database of synthetic waveforms augmented with empirical noise, we show that the algorithm can instantaneously track an earthquake source time function on real data. Our model unlocks 'true real-time' access to the rupture evolution of large earthquakes using a portion of seismograms that is routinely treated as noise, and can be immediately transformative for tsunami early warning.

Slip bursts during coalescence of slow slip events in Cascadia.

Both laboratory experiments and dynamic simulations suggest that earthquakes can be preceded by a precursory phase of slow slip. Observing processes leading to an acceleration or spreading of slow slip along faults is therefore key to understand the dynamics potentially leading to seismic ruptures. Here, we use continuous GPS measurements of the ground displacement to image the daily slip along the fault beneath Vancouver Island during a slow slip event in 2013. We image the coalescence of three originally distinct slow slip fronts merging together. We show that during coalescence phases lasting for 2 to 5 days, the rate of energy (moment) release significantly increases. This observation supports the view proposed by theoretical and experimental studies that the coalescence of slow slip fronts is a possible mechanism for initiating earthquakes.

The Ionospheric view of the 2011 Tohoku-Oki earthquake seismic source: the first 60 seconds of the rupture.

Using the specific satellite line of sight geometry and station location with respect to the source, Thomas et al. [Scientific Reports, https://doi.org/10.1038/s41598-018-30476-9] developed a method to infer the detection altitude of co-seismic ionospheric perturbations observed in Global Positioning System (GPS) - Total Electron Content (TEC) measurements during the Mw 7.4 March 9, 2011 Sanriku-Oki earthquake, a foreshock of the Mw 9.0, March 11, 2011 Tohoku-Oki earthquake. Therefore, in addition to the spatio-temporal evolution, the altitude information of the seismically induced ionospheric signatures can also be derived now using GPS-TEC technique. However, this method considered a point source, in terms of a small rupture area (~90 km) during the Tohoku foreshock, for the generation of seismo-acoustic waves in 3D space and time. In this article, we explore further efficacy of GPS-TEC technique during co-seismic ionospheric sounding for an extended seismic source varying simultaneously in space and time akin to the rupture of Mw 9.0 Tohoku-Oki mainshock and the limitations to be aware of in such context. With the successful execution of the method by Thomas et al. during the Tohoku-Oki mainshock, we not only estimate the detection altitude of GPS-TEC derived co-seismic ionospheric signatures but also delineate, for the first time, distinct ground seismic sources responsible for the generation of these perturbations, which evolved during the initial 60 seconds of the rupture. Simulated tsunami water excitation over the fault region, to envisage the evolution of crustal deformation in space and time along the rupture, formed the base for our model analysis. Further, the simulated water displacement assists our proposed novel approach to delineate the ground seismic sources entirely based on the ensuing ionospheric perturbations which were otherwise not well reproduced by the ground rupture process within this stipulated time. Despite providing the novel information on the segmentation of the Tohoku-Oki seismic source based on the co-seismic ionospheric response to the initial 60 seconds of the event, our model could not reproduce precise rupture kinematics over this period. This shortcoming is also credited to the specific GPS satellite-station viewing geometries.

Mega-earthquakes rupture flat megathrusts.

The 2004 Sumatra-Andaman and 2011 Tohoku-Oki earthquakes highlighted gaps in our understanding of mega-earthquake rupture processes and the factors controlling their global distribution: A fast convergence rate and young buoyant lithosphere are not required to produce mega-earthquakes. We calculated the curvature along the major subduction zones of the world, showing that mega-earthquakes preferentially rupture flat (low-curvature) interfaces. A simplified analytic model demonstrates that heterogeneity in shear strength increases with curvature. Shear strength on flat megathrusts is more homogeneous, and hence more likely to be exceeded simultaneously over large areas, than on highly curved faults.