[1] We describe and present a new model of global subduction zone geometries, called Slab1.0. An extension of previous efforts to constrain the two-dimensional non-planar geometry of subduction zones around the focus of large earthquakes, Slab1.0 describes the detailed, non-planar, three-dimensional geometry of approximately 85% of subduction zones worldwide. While the model focuses on the detailed form of each slab from their trenches through the seismogenic zone, where it combines data sets from active source and passive seismology, it also continues to the limits of their seismic extent in the upper-mid mantle, providing a uniform approach to the definition of the entire seismically active slab geometry. Examples are shown for two well-constrained global locations; models for many other regions are available and can be freely downloaded in several formats from our new Slab1.0 website, http://on.doi.gov/d9ARbS. We describe improvements in our two-dimensional geometry constraint inversion, including the use of ‘average’ active source seismic data profiles in the shallow trench regions where data are otherwise lacking, derived from the interpolation between other active source seismic data along-strike in the same subduction zone. We include several analyses of the uncertainty and robustness of our three-dimensional interpolation methods. In addition, we use the filtered, subduction-related earthquake data sets compiled to build Slab1.0 in a reassessment of previous analyses of the deep limit of the thrust interface seismogenic zone for all subduction zones included in our global model thus far, concluding that the width of these seismogenic zones is on average 30% larger than previous studies have suggested.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2011JB008524

Slab1.0: A three‐dimensional model of global subduction zone geometries

We introduce a new method to determine the magnitude of complete- ness Mc and its uncertainty. Our method models the entire magnitude range (EMR method) consisting of the self-similar complete part of the frequency-magnitude dis- tribution and the incomplete portion, thus providing a comprehensive seismicity model. We compare the EMR method with three existing techniques, finding that EMR shows a superior performance when applied to synthetic test cases or real data from regional and global earthquake catalogues. This method, however, is also the most computationally intensive. Accurate knowledge of Mc is essential for many seismicity-based studies, and particularly for mapping out seismicity parameters such as the b-value of the Gutenberg-Richter relationship. By explicitly computing the uncertainties in Mc using a bootstrap approach, we show that uncertainties inb-values are larger than traditionally assumed, especially when considering small sample sizes. As examples, we investigated temporal variations of Mc for the 1992 Landers aftershock sequence and found that it was underestimated on average by 0.2 with former techniques. Mapping Mc on a global scale, Mc reveals considerable spatial variations for the Harvard Centroid Moment Tensor (CMT) (5.3 Mc 6.0) and the International Seismological Centre (ISC) catalogue (4.3 Mc 5.0).

Assessing the Quality of Earthquake Catalogues: Estimating the Magnitude of Completeness and Its Uncertainty

Extended-duration seismic signals occur episodically on some major faults, often in conjunction with aseismic or 'slow-slip' earthquake events. The mechanism underlying this tremor and its relationship to the aseismic slip are as yet unresolved. David Shelley et al. demonstrate that tremor beneath Shikoku, Japan can be explained as a swarm of small, low-frequency earthquakes, each of which occurs as shear faulting on the subduction zone plate interface. This suggests that tremor and slow slip are different manifestations of a single process. Tremor beneath Shikoku, Japan can be explained as a swarm of small, low-frequency earthquakes, each of which occurs as shear faulting on the subduction zone plate interface. This suggests that tremor and slow slip are different manifestations of a single process. Non-volcanic tremor is a weak, extended duration seismic signal observed episodically on some major faults, often in conjunction with slow slip events1,2,3,4. Such tremor may hold the key to understanding fundamental processes at the deep roots of faults, and could signal times of accelerated slip and hence increased seismic hazard. The mechanism underlying the generation of tremor and its relationship to aseismic slip are, however, as yet unresolved. Here we demonstrate that tremor beneath Shikoku, Japan, can be explained as a swarm of small, low-frequency earthquakes, each of which occurs as shear faulting on the subduction-zone plate interface. This suggests that tremor and slow slip are different manifestations of a single process.

Non-volcanic tremor and low-frequency earthquake swarms

Non-volcanic seismic tremor was discovered in the Nankai trough subduction zone in southwest Japan and subsequently identified in the Cascadia subduction zone. In both locations, tremor is observed to coincide temporally with large, slow slip events on the plate interface downdip of the seismogenic zone. The relationship between tremor and aseismic slip remains uncertain, however, largely owing to difficulty in constraining the source depth of tremor. In southwest Japan, a high quality borehole seismic network allows identification of coherent S-wave (and sometimes P-wave) arrivals within the tremor, whose sources are classified as low-frequency earthquakes. As low-frequency earthquakes comprise at least a portion of tremor, understanding their mechanism is critical to understanding tremor as a whole. Here, we provide strong evidence that these earthquakes occur on the plate interface, coincident with the inferred zone of slow slip. The locations and characteristics of these events suggest that they are generated by shear slip during otherwise aseismic transients, rather than by fluid flow. High pore-fluid pressure in the immediate vicinity, as implied by our estimates of seismic P- and S-wave speeds, may act to promote this transient mode of failure. Low-frequency earthquakes could potentially contribute to seismic hazard forecasting by providing a new means to monitor slow slip at depth.

https://courses.washington.edu/ess502/ShellyBerozaEtal_Nature_2006.pdf

Low-frequency earthquakes in Shikoku, Japan, and their relationship to episodic tremor and slip

A simple, yet effective, analytical model is proposed for the representation of near-field strong ground motions. The model adequately describes the impulsive character of near-fault ground motions both qualitatively and quantitatively. In addition, it can be used to analytically reproduce empirical observations that are based on available near-source records. The input parameters of the model have an unambiguous physical meaning. The proposed analytical model has been calibrated using a large number of actual near-field ground-motion records. It successfully simulates the entire set of available near-fault displacement, velocity, and (in many cases) acceleration time histories, as well as the corresponding deformation, velocity, and acceleration response spectra. Furthermore, a very simplified methodology for generating realistic synthetic ground motions that are adequate for engineering analysis and design is outlined and applied. Finally, it should be noted that the analytical model (along with the scaling laws of its parameters) proposed in the present work has the potential to facilitate the study of the elastic and inelastic response of conventional, nonconventional (e.g., base-isolated), and special structures (e.g., suspension bridges, fluid-storage tanks) subjected to near-source seismic excitations as a function of the model input parameters and thus, ultimately, as a function of earthquake size.

A Mathematical Representation of Near-Fault Ground Motions

With the availability of the Global Positioning System and other technical advances, a growing number of unusual earthquake phenomena occurring at relatively long periods have been recognized. They include deep episodic tremor, low-frequency earthquakes, slow slip events and 'silent' earthquakes. Based on data chiefly from western Japan, Ide et al. report that these 'slow' seismic events follow a unified scaling relationship that clearly differentiates their behaviour from that of 'regular' earthquakes. A diverse array of unusual earthquake phenomena occurring at relatively long periods, 'slow' events, follow a unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes, indicating that they comprise a new earthquake category. Recently, a series of unusual earthquake phenomena have been discovered, including deep episodic tremor1, low-frequency earthquakes2, very-low-frequency earthquakes3, slow slip events4 and silent earthquakes5,6,7,8,9. Each of these has been demonstrated to arise from shear slip, just as do regular earthquakes, but with longer characteristic durations and radiating much less seismic energy. Here we show that these slow events follow a simple, unified scaling relationship that clearly differentiates their behaviour from that of regular earthquakes. We find that their seismic moment is proportional to the characteristic duration and their moment rate function is constant, with a spectral high-frequency decay of f-1. This scaling and spectral behaviour demonstrates that they can be thought of as different manifestations of the same phenomena and that they comprise a new earthquake category. The observed scale dependence of rupture velocity for these events can be explained by either a constant low-stress drop model or a diffusional constant-slip model. This new scaling law unifies a diverse class of slow seismic events and may lead to a better understanding of the plate subduction process and large earthquake generation.

A scaling law for slow earthquakes

Strong spatial variation of rupture characteristics in the moment magnitude (Mw) 9.0 Tohoku-Oki megathrust earthquake controlled both the strength of shaking and the size of the tsunami that followed. Finite-source imaging reveals that the rupture consisted of a small initial phase, deep rupture for up to 40 seconds, extensive shallow rupture at 60 to 70 seconds, and continuing deep rupture lasting more than 100 seconds. A combination of a shallow dipping fault and a compliant hanging wall may have enabled large shallow slip near the trench. Normal faulting aftershocks in the area of high slip suggest dynamic overshoot on the fault. Despite prodigious total slip, shallower parts of the rupture weakly radiated at high frequencies, whereas deeper parts of the rupture radiated strongly at high frequencies.

https://science.sciencemag.org/content/sci/332/6036/1426.full.pdf

Shallow Dynamic Overshoot and Energetic Deep Rupture in the 2011 Mw 9.0 Tohoku-Oki Earthquake

Seismic energy is distributed across a wide fre- quency band so that limited bandwidth recording can lead tosubstantialunderestimates oftheradiatedseismic energy or introduce an articial upper bound of radiated energy. We estimate an adjustment factor to account for the proba- ble missing energy and apply it to three previously studied data sets with limited recording bandwidth. We nd that thisadjustment,togetherwithaccountingforpossibly miss- ing events, eliminates much of the moment dependence of radiated energy found previously. We obtain a nearly con- stant ratio of radiated energy to seismic moment, 310 5 , or 1 MPa of apparent stress drop, over 17 orders of seismic moment. This suggests that deviation from similarity of the energy radiation for seismic events essentially the entire observablerangeofearthquakesizemaynotyetberesolved.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001GL013106

Satoshi Ide

Papers

Non-volcanic tremor and low-frequency earthquake swarms

Low-frequency earthquakes in Shikoku, Japan, and their relationship to episodic tremor and slip

A scaling law for slow earthquakes

Shallow Dynamic Overshoot and Energetic Deep Rupture in the 2011 Mw 9.0 Tohoku-Oki Earthquake

Does apparent stress vary with earthquake size