Modelling the seismic potential of the Indo-Burman megathrust

The Indo-Burman arc is the boundary between the India and Burma plates, north of the Sumatra–Andaman subduction zone. The existence of active subduction in the Indo-Burman arc is a debatable issue because the Indian plate converges very obliquely beneath the Burma plate. Recent GPS measurements in Bangladesh, Myanmar, and northeast India indicate 13–17 mm/y of plate convergence along a shallow dipping megathrust while most of the strike-slip motion occurs on several steep faults, consistent with patterns of strain partitioning at subduction zones. A short period of instrumentally recorded seismicity and sparse historical records are insufficient to assess the possibility of great earthquakes at the Indo-Burman megathrust. Using the advantage of the Block-and-Fault Dynamics model allowing simultaneous simulation of slow tectonic motions and earthquakes, we test the hypothesis whether the India-Burma detachment is locked and able to produce great earthquakes, or it slips aseismically? We have shown that the model of locked detachment is preferred because it more adequately reproduces observed tectonic velocities. The integral characteristics of synthetic seismicity, the earthquake size distribution, and the rate of seismic activity are consistent with those derived from observations. Our results suggest that the megathrust is locked and can generate great M8+ earthquakes. The estimated average return period of great events exceeds one thousand years. Earthquakes of this size pose a great threat to NE India, Bangladesh and Myanmar, the most densely populated areas of the world.

Viscous fault creep controls the stress-dependence of modelled earthquake statistics

The ability to estimate the likelihood of particular earthquake magnitudes occurring in a given region is critical for seismic hazard assessment. Earthquake size and recurrence statistics have been empirically linked to stress state, however there is ongoing debate as to which fault-zone processes are responsible for this link. We numerically model combined viscous creep and frictional sliding of a fault-zone, where applied shear stress controls the interplay between these mechanisms. This model reproduces the stress-dependent earthquake magnitude distribution observed in nature. At low stress, many fault segments creep and impede ruptures, limiting earthquake sizes. At high stress, more segments are close to frictional failure and large earthquakes are more frequent. Contrasts in earthquake statistics between regions, with depth and through time, may be explained by stress variation, which could be used in the future to further constrain probabilistic models of regional seismicity.

How cementation and fluid flow influence slip behavior at the subduction interface

Much of the complexity of subduction-zone earthquake size and temporal patterns owes to linkages among fluid flow, stress, and fault healing. To investigate these linkages, we introduce a novel numerical model that tracks cementation and fluid flow within the framework of an earthquake simulator. In the model, there are interseismic increases in cohesion across the plate boundary and decreases in porosity and permeability caused by cementation along the interface. Seismogenic slip is sensitive to the effective stress and therefore fluid pressure; in turn, slip events increase porosity by fracturing. The model therefore accounts for positive and negative feedbacks that modify slip behavior through the seismic cycle. The model produces temporal clustering of earthquakes in the seismic record of the Aleutian margin, which has well-documented along-strike variations in locking characteristics. Model results illustrate how physical, geochemical, and hydraulic linkages can affect natural slip behavior. Specifically, coseismic drops in fluid pressure steal energy from large ruptures, suppress slip, moderate the magnitudes of large earthquakes, and lead to aftershocks.

Synthetic analysis of the efficacy of the S-net system in tsunami forecasting

The Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S-net) is presently the world’s largest network of ocean bottom pressure sensors for real-time tsunami monitoring. This paper analyzes the efficacy of such a vast system in tsunami forecasting through exhaustive synthetic experiments. We consider 1500 hypothetical tsunami scenarios from megathrust earthquakes with magnitudes ranging from Mw 7.7–9.1. We employ a stochastic slip model to emulate heterogeneous slip patterns on specified 240 subfaults over the plate interface of the Japan Trench subduction zone and its vicinity. Subsequently, the associated tsunamis in terms of maximum coastal tsunami heights are evaluated along the 50-m isobath by means of a Green’s function summation. To produce tsunami forecasts, we utilize a tsunami inversion from virtually observed waveforms at the S-net stations. Remarkably, forecasts accuracy of approximately 99% can be achieved using tsunami data within an interval of 3 to 5 min after the earthquake (2-min length), owing to the exceedingly dense observation points. Additionally, we apply an optimization technique to determine the optimal combination of stations with respect to earthquake magnitudes. The results show that the minimum requisite number of stations to maintain the accuracy attained by the existing network configuration decreases from 130 to 90 when the earthquake size increases from Mw 7.7 to 9.1.

On the heterogeneity of the earthquake rupture

Summary The scaling of earthquake parameters with seismic moment and its interpretation in terms of self-similarity is still debated in the literature. We address this question by examining a worldwide compilation of corner frequency-based and elastic rebound theory (ERT)-based fault slip, area and stress drop values for earthquakes ranging in magnitude from -0.7 to 7.8. We find that corner frequency estimates of slip (and stress drop) scale differently than those inferred from the ERT approach, where the latter deviates from the generally accepted constant stress drop behavior of so-called self-similar scaling models. We also find that average slips from finite-source models are consistent with corner frequency scaling, whereas peak slip values are more consistent with the ERT scaling. The different scaling of corner frequency- and ERT-based estimates of slip and stress drop with earthquake size is interpreted in terms of heterogeneity of the rupture process. ERT-based estimates of stress drop decrease with seismic moment suggesting a self-affine behavior. Despite the inferred heterogeneity at all scales, we do not observe a clear effect on the Brune stress drop scaling with earthquake size.

GR_EST: An OCTAVE/MATLAB Toolbox to Estimate Gutenberg–Richter Law Parameters and Their Uncertainties

The estimation of the earthquake size distribution parameters is one of the most important parts in any seismic hazard study. GR_EST toolbox is a source code written for OCTAVE/MATLAB (Eaton et al., 2019; MATLAB, 2019) that allows estimating these parameters in a proper way, including the estimation of the associated uncertainties. The toolbox contains functions to make the parameter estimation both for instrumental and historical seismic catalogs, also considering time-varying completeness for magnitudes. Different functional forms for the magnitude–frequency distribution and different strategies for the estimation of its parameters and relative uncertainty are included. To guide the seismologists into the use of this toolbox, a set of complete examples is provided, to be used as “how to” use cases.

Effects of seismogenic width and low-velocity zones on estimating slip-weakening distance from near-fault ground deformation

SUMMARY Fault weakening process controls earthquake rupture propagation and is of great significance to impact the final earthquake size and seismic hazard. Critical slip-weakening distance (${D_c}$) is one of the key parameters, which however is of difficult endeavours to be determined on natural faults, mainly due to its strong trade-off with the fault strength drop. An estimation method of ${D_c}$ proposed by Fukuyama et al. provides a simple and direct reference of ${D_c}$ on real faults from the near-fault ground displacement at the peak of ground velocity (${D_c}^{\prime\prime}$). However, multiple factors may affect the observed near-fault ground velocity and thus need to be considered when estimating ${D_c}.$ In this work we conduct 3-D finite element numerical simulations to examine the effects of finite seismogenic width and near-fault low velocity zones (LVZs) on the results of ${D_c}^{\prime\prime}$. In uniform models with constant prescribed ${D_c}$, the derived ${D_c}^{\prime\prime}$ values increase with seismogenic width. Furthermore, the scaling between ${D_c}^{\prime\prime}$ and final slip in models with a constant ${D_c}$ indicates that the scale-dependent feature of ${D_c}^{\prime\prime}$ might not be related to variation in friction properties. With a near-fault LVZ, ${D_c}^{\prime\prime}$ values show significant magnification. The width of the LVZ plays a more important role in enlarging ${D_c}$ estimation compared to the depth of the LVZ. Complex wavefields and multiple wiggles introduced by the LVZ could lead to delay pick and then cause large deviation. The value of ${D_c}$ on the fault may be overestimated through ${D_c}^{\prime\prime}$ from limited stations only.

Early rupture signals predict the final earthquake size

SUMMARY When a seismic rupture starts, the process may evolve into multiple ways, generating different size earthquakes. Contrasting models have been proposed to describe the evolution of the rupture process while limited observations at the scale of real earthquake data are available, so that a unifying theory is still missing. Here we show that small and large earthquake ruptures are different before the arrest and they do not exhibit a common, size-independent, universal behaviour. For earthquakes with magnitude 4 < M < 9 occurred in Japan, we measure the initial rate of the P-wave peak amplitude and show that this quantity is correlated to the final event magnitude and not affected by distance attenuation, thus being a proxy for the initiation time of the rupture process. While opening new views on the rupture preparation process, our findings can have significant implications on the effective development of fast and reliable methods for source characterization and ground shaking prediction.