Global ubiquity of dynamic earthquake triggering

Earthquakes can be triggered by local changes in the stress field (static triggering) due to nearby earthquakes or by stresses caused by the passage of surface (Rayleigh and Love) waves from a remote, large earthquake (dynamic triggering). However, the mechanism, frequency, controlling factors and the global extent of dynamic triggering are yet to be fully understood. Because Rayleigh waves involve compressional and dilatational particle motion (volumetric changes) as well as shearing, whereas Love waves only involve shearing, triggering by either wave type implies fundamentally different physical mechanisms. Here, we analyse broadband seismograms from over 500 globally distributed stations and use an automated approach to systematically identify small triggered earthquakes—the lowamplitude signals of such earthquakes would normally be masked by high-amplitude surface waves. Our analysis reveals that out of 15 earthquakes studied ofmagnitude (M) greater than 7.0 that occurred after 1990, 12 are associated with significant increases in the detection of smaller earthquakes during the passage of both the Love and Rayleigh waves. We conclude that dynamic triggering is a ubiquitous phenomenon that is independent of the tectonic environment of themain earthquake or the triggered event. Triggering of earthquakes has been classified into two primary areas of study: static and dynamic. Static triggering occurs within a few fault lengths of a mainshock rupture, and results from slip-induced changes in the local stress field. Anomalous earthquake rate increases, associated with mainshock timing at distances beyond the influence of static stress increases, must be triggered by another process. These events, called dynamically (or remotely) triggered events, usually correlate with the passage of large-amplitude and long-duration transient signals: seismic surface waves. Surface waves, usually the largest-amplitude arrivals on a seismogram, produce increased strain as they travel along the surface of the Earth. Surface-wave amplitudes can be highly impacted by the nature of an earthquake rupture, such as its depth, focal mechanism and rupture style (for example, directivity effects). Surface-wave propagation also contains dispersion, where velocities are a function of frequency. These waveforms are characterized by long durations, long periods (dominant periods ∼20 s but range from several to hundreds of seconds) and emergent wave-train signals, as opposed to P and S waves, which are generally impulsive arrivals. Furthermore, small, local earthquakes recorded on a single station can be hidden within these frequency-dependent, large-amplitude signals. To investigate the role of surface waves in dynamic triggering, we used broadband seismograms from open network data (for example, Global Seismic Network, USArray and so on) available at the Incorporated Research Institutions for Seismology (IRIS) Data Management Center (DMC). Using this data set, we systematically determined the presence of small signals embedded within the surfacewave signals and documented the global extent of dynamically triggered earthquakes. We identified triggered events using filtered broadband data optimized for the detection of small, local events near a seismic station. In particular, local events hidden in large-amplitude surface waves were detected by applying a high-pass filter to include frequencies greater than 5Hz. Figure 1 shows displacement seismograms for the 26 December 2004 Mw = 9.2 Sumatra–Andaman islands earthquake recorded at station OTAV (∆ = 173) in Ecuador, which is located near the antipode off the coast of South America. The figure shows unfiltered Love and Rayleigh waves, plus the high-pass-filtered (corner at 5Hz) vertical component of ground motion. Note the triggered event on the high-pass-filtered seismogram that occurs during the peak amplitude of the Rayleigh wave. This event is too small to be recorded in the global US Geological Survey Preliminary Determination of Epicenters catalogue. The Sumatra–Andaman islands earthquake triggered events at Mount Wrangell, Alaska, thousands of kilometres away from the epicentre, it triggered this event, and as we will show, it triggered events in many regions throughout the world. Global catalogues generally detect and associate P waves from multiple stations to locate an event; thus, many small events with P waves recorded on fewer than 3–5 stations may not be located. This event criterion determines the detection threshold of a network, which is a direct result of the station coverage. For example, the US Geological Survey Preliminary Determination of Epicenters catalogue typically records earthquakes of M ≥ 4.5 throughout the world, but this lower threshold varies depending on the global network station coverage. Regional catalogues, such as in California, Nevada, and Utah typically record much smaller (M ∼ 1) events. Our approach is to detect small events near broadband stations using all data available from open sources, independent of global or regional catalogues. To systematically identify small events typical of dynamic triggering, we developed an efficient and effective automated approach and systematically analysed data from 15 large to