Absence of Near-Trench Early Triggering during the 2012 Mw 7.2 Indian Ocean Strike-Slip Earthquake: Evidence from One-Day Aftershocks

Dynamic earthquake triggering is a widely accepted mechanism of earthquake interaction, which plays a vital role in seismic hazard estimation, although its efficacy at regional distances is under debate. The 2012 Mw 7.2 Indian Ocean event is one of the first reported events to produce dynamic stress triggering at regional distances using backprojection (BP) techniques. Alternatively, the coherent radiators in BP images can be interpreted as localized water reverberation phases. We present further evidence against near-trench triggering during this event. We collected 24 hr seismic recordings of two nearby stations located near the trench. We adopted a waveform denoising algorithm and detected 125 aftershocks using two regional seismic stations with a minimum magnitude of ML∼2.7 and completeness magnitude of ML∼3.6, whereas none of these aftershocks occurred near the trench. The absence of immediate (within one day) aftershocks near the trench suggest the absence of dynamic triggering during the offshore mainshock.

Towards understanding earthquake triggering due to enhanced geothermal systems

The effect of fluid pulse driven fractures (FPDF) propagating in poroelastic media on fault slip in the presence of natural fractures is a complicated interplay between fracture propagation, fracture-fracture interaction, fracture-fault interaction, friction model governing fault slip and wave propagation associated with pulsing injection. Furthermore, the problem is stochastic due to the uncertainty associated with the existing fracture-fault topology.

Laboratory Evidence of Transient Pressure Surge in a Fluid-Filled Fracture as a Potential Driver of Remote Dynamic Earthquake Triggering

Seismic waves carrying tiny perturbing stresses can trigger earthquakes in geothermal and volcanic regions. The underlying cause of this dynamic triggering is still not well understood. One leading hypothesis is that a sudden increase in the fluid-pore pressure in the fault zone is involved, but the exact physical mechanism is unclear. Here, we report experimental evidence in which a fluid-filled fracture was shown to be able to amplify the pressure of an incoming seismic wave. We built miniature pressure sensors and directly placed them inside a thin fluid-filled fracture to measure the fluid pressure during wave propagation. By varying the fracture aperture from 0.2 to 9.2 mm and sweeping the frequency from 12 to 70 Hz, we observed in the lab that the fluid pressure in the fracture could be amplified up to 25.2 times compared with the incident-wave amplitude. Because an increase of the fluid pressure in a fault can reduce the effective normal stress to allow the fault to slide, our observed transient pressure surge phenomenon may provide the mechanism for earthquake dynamic triggering.

On the Common Features of Reservoir Water-Level Variations and Their Influence on Earthquake Triggering: An Inherency of Physical Mechanism of Reservoir-Triggered Seismicity

ABSTRACT Impoundment of hydroelectric water reservoir influences the stability of nearby faults that may lead to reservoir-triggered seismicity (RTS). Various qualitative empirical relations, relating reservoir water-level variations with earthquake triggering and their frequency, have been deduced. With the goal to give a theoretical causation (in terms of time) to these empirical relations, a detailed theoretical analysis of the physical mechanism of RTS phenomenon, in terms of mechanical loading and changes in the pore-fluid boundary condition in the underlying rockmass, is undertaken. Three components, namely elastic stress, diffusion pore pressure, and stress-induced pore pressure, are simulated by considering a simple and schematic reservoir water-level time series using the Green’s function solution of poroelastic equations and frictional failure criterion. Various factors may influence the occurrence of RTS, but here definite role and nature of the poroelastic components in governing the empirical relations are simulated. The analysis suggests that (1) all the components contribute in RTS cases that are associated with higher reservoir water level, (2) diffusion pore pressure contributes mainly in RTS cases that are associated with longer duration of high reservoir water level, and (3) contribution of stress-induced pore pressure dominates in the RTS cases that are associated with rate of change of reservoir water level. Further, detailed simulations corroborate that the rapid type of earthquake triggering in RTS cases is mainly influenced by the immediate increase of stress and stress-induced pore pressure, whereas delayed type of triggering is mainly influenced by the diffusion pore pressure, and continuing type of triggering, inter alia, is influenced by the effect of dynamic changes in seasonal water cycle on the three components. The analyses lead us to conclude that the empirical relations are governed by the physical mechanism of RTS within the ambit of poroelastic theory.

A Nonparametric Hawkes Model for Forecasting California Seismicity

ABSTRACT A variety of nonparametric models have been proposed for estimating earthquake triggering. We investigate the ability of the model-independent stochastic declustering method developed by Marsan and Lengliné (2008) to estimate variable spatial triggering that can vary with direction, magnitude, and region. We develop an approach for local fault estimation and demonstrate forecasting methods that use the nonparametric estimates. Simulation studies are conducted to verify the effectiveness of the method, and the nonparametric estimates are applied to a California earthquake catalog. Model forecast performance is evaluated retrospectively by comparing our models with the long-term forecast of Helmstetter et al. (2007), using both deviance and Voronoi residuals. We show improved performance compared with Helmstetter et al. (2007) in various regions while using a full nonparametric estimation and forecasting approach.

Responding to Media Inquiries about Earthquake Triggering Interactions

In the aftermath of a significant earthquake, seismologists are frequently asked questions by the media and public regarding possible interactions with recent prior events, including events at great distances away, along with prospects of larger events yet to come, both locally and remotely. For regions with substantial earthquake catalogs that provide information on the regional Gutenberg–Richter magnitude–frequency relationship, Omori temporal aftershock statistical behavior, and aftershock productivity parameters, probabilistic responses can be provided for likelihood of nearby future events of larger magnitude, as well as expected behavior of the overall aftershock sequence. However, such procedures generally involve uncertain extrapolations of parameterized equations to infrequent large events and do not provide answers to inquiries about long-range interactions, either retrospectively for interaction with prior remote large events or prospectively for interaction with future remote large events. Dynamic triggering that may be involved in such long-range interactions occurs, often with significant temporal delay, but is not well understood, making it difficult to respond to related inquiries. One approach to addressing such inquiries is to provide retrospective or prospective occurrence histories for large earthquakes based on global catalogs; while not providing quantitative understanding of any physical interaction, experience-based guidance on the (typically very low) chances of causal interactions can inform public understanding of likelihood of specific scenarios they are commonly very interested in.

Machine-Learning-Based High-Resolution Earthquake Catalog Reveals How Complex Fault Structures Were Activated during the 2016–2017 Central Italy Sequence

The 2016–2017 central Italy seismic sequence occurred on an 80 km long normal-fault system. The sequence initiated with the Mw 6.0 Amatrice event on 24 August 2016, followed by the Mw 5.9 Visso event on 26 October and the Mw 6.5 Norcia event on 30 October. We analyze continuous data from a dense network of 139 seismic stations to build a high-precision catalog of ∼900,000 earthquakes spanning a 1 yr period, based on arrival times derived using a deep-neural-network-based picker. Our catalog contains an order of magnitude more events than the catalog routinely produced by the local earthquake monitoring agency. Aftershock activity reveals the geometry of complex fault structures activated during the earthquake sequence and provides additional insights into the potential factors controlling the development of the largest events. Activated fault structures in the northern and southern regions appear complementary to faults activated during the 1997 Colfiorito and 2009 L’Aquila sequences, suggesting that earthquake triggering primarily occurs on critically stressed faults. Delineated major fault zones are relatively thick compared to estimated earthquake location uncertainties, and a large number of kilometer-long faults and diffuse seismicity were activated during the sequence. These properties might be related to fault age, roughness, and the complexity of inherited structures. The rich details resolvable in this catalog will facilitate continued investigation of this energetic and well-recorded earthquake sequence.