The Relevance of Foreshocks in Earthquake Triggering: A Statistical Study

An increase of seismic activity is often observed before large earthquakes. Events responsible for this increase are usually named foreshock and their occurrence probably represents the most reliable precursory pattern. Many foreshocks statistical features can be interpreted in terms of the standard mainshock-to-aftershock triggering process and are recovered in the Epidemic Type Aftershock Sequence ETAS model. Here we present a statistical study of instrumental seismic catalogs from four different geographic regions. We focus on some common features of foreshocks in the four catalogs which cannot be reproduced by the ETAS model. In particular we find in instrumental catalogs a significantly larger number of foreshocks than the one predicted by the ETAS model. We show that this foreshock excess cannot be attributed to catalog incompleteness. We therefore propose a generalized formulation of the ETAS model, the ETAFS model, which explicitly includes foreshock occurrence. Statistical features of aftershocks and foreshocks in the ETAFS model are in very good agreement with instrumental results.

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Testing of the foreshock hypothesis within an epidemic like description of seismicity

Geophysical Journal International ◽

10.1093/gji/ggaa611 ◽

2020 ◽

Author(s):

G Petrillo ◽

E Lippiello

Keyword(s):

Seismic Activity ◽

Good Description ◽

Aftershock Sequence ◽

Short Term ◽

Etas Model ◽

Temporal Clustering ◽

Epidemic Type Aftershock Sequence ◽

Preparatory Phase ◽

Spatio Temporal ◽

The One

Summary The Epidemic Type Aftershock Sequence (ETAS) model provides a good description of the post-seismic spatio-temporal clustering of seismicity and is also able to capture some features of the increase of seismic activity caused by foreshocks. Recent results, however, have shown that the number of foreshocks observed in instrumental catalogs is significantly much larger than the one predicted by the ETAS model. Here we show that it is possible to keep an epidemic description of post-seismic activity and, at the same time, to incorporate pre-seismic temporal clustering, related to foreshocks. Taking also into-account the short-term incompleteness of instrumental catalogs, we present a model which achieves very good description of the southern California seismicity both on the aftershock and on the foreshock side. Our results indicate that the existence of a preparatory phase anticipating mainshocks represents the most plausible explanation for the occurrence of foreshocks.

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The prognostic value of foreshocks - a statistical reevaluation

10.5194/egusphere-egu21-8691 ◽

2021 ◽

Author(s):

Ester Manganiello ◽

Marcus Herrmann ◽

Warner Marzocchi

Keyword(s):

Physical Process ◽

Large Earthquake ◽

Aftershock Sequence ◽

Earthquake Forecasting ◽

Earthquake Triggering ◽

Epidemic Type Aftershock Sequence ◽

Sequence Selection ◽

Short Time ◽

Scientific Questions ◽

Large Earthquakes

The ability to forecast large earthquakes on short time scales is strongly limited by our understanding of the earthquake nucleation process. Foreshocks represent promising seismic signals that may improve earthquake forecasting as they precede many large earthquakes. However, foreshocks can currently only be identified as such after a large earthquake occurred. This inability is because it remains unclear whether foreshocks represent a different physical process than general seismicity (i.e., mainshocks and aftershocks). Several studies compared foreshock occurrence in real and synthetic catalogs, as simulated with a well-established earthquake triggering/forecasting model called Epidemic-Type Aftershock Sequence (ETAS) that does not discriminate between foreshocks, mainshocks, and aftershocks. Some of these studies show that the spatial distribution of foreshocks encodes information about the subsequent mainshock magnitude and that foreshock activity is significantly higher than predicted by the ETAS model. These findings attribute a unique underlying physical process to foreshocks, making them potentially useful for forecasting large earthquakes. We reinvestigate these scientific questions using high-quality earthquake catalogs and study carefully the influence of subjective parameter choices and catalog artifacts on the results. For instance, we use data from different regions, account for the short-term catalog incompleteness and its spatial variability, and explore different criteria for sequence selection and foreshock definition.

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Testing physics and statics based hybrid ETAS models

10.5194/egusphere-egu21-5157 ◽

2021 ◽

Author(s):

Shubham Sharma ◽

Shyam Nandan ◽

Sebastian Hainzl

Keyword(s):

Synthetic Data ◽

Large Earthquake ◽

Aftershock Sequence ◽

Additional Stress ◽

Etas Model ◽

Epidemic Type Aftershock Sequence ◽

Stress Changes ◽

Alternative Approaches ◽

Large Earthquakes ◽

Model Approach

Currently, the Epidemic Type Aftershock Sequence (ETAS) model is state-of-the-art for forecasting aftershocks. However, the under-performance of ETAS in forecasting the spatial distribution of aftershocks following a large earthquake make us adopt alternative approaches for the modelling of the spatial ETAS-kernel. Here we develop a hybrid physics and statics based forecasting model. The model uses stress changes, calculated from inverted slip models of large earthquakes, as the basis of the spatial kernel in the ETAS model in order to get more reliable estimates of spatiotemporal distribution of aftershocks. We evaluate six alternative approaches of stress-based ETAS-kernels and rank their performance against the base ETAS model. In all cases, an expectation maximization (EM) algorithm is used to estimate the ETAS parameters. The model approach has been tested on synthetic data to check if the known parameters can be inverted successfully. We apply the proposed method to forecast aftershocks of mainshocks available in SRCMOD database, which includes 192 mainshocks with magnitudes in the range between 4.1 and 9.2 occurred from 1906 to 2020. The probabilistic earthquake forecasts generated by the hybrid model have been tested using established CSEP test metrics and procedures. We show that the additional stress information, provided to estimate the spatial probability distribution, leads to more reliable spatiotemporal ETAS-forecasts of aftershocks as compared to the base ETAS model.

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Short-term earthquake forecasting experiment before and during the L’Aquila (central Italy) seismic sequence of April 2009

Annals of Geophysics ◽

10.4401/ag-6583 ◽

2015 ◽

Vol 57 (6) ◽

Author(s):

Maura Murru ◽

Jiancang Zhuang ◽

Rodolfo Console ◽

Giuseppe Falcone

Keyword(s):

Central Italy ◽

Aftershock Sequence ◽

Earthquake Forecasting ◽

Model Parameters ◽

Short Term ◽

Productivity Function ◽

Epidemic Type Aftershock Sequence ◽

Forecasting Performance ◽

The One ◽

Lower Magnitude

<div class="page" title="Page 1"><div class="layoutArea"><div class="column">In this paper, we compare the forecasting performance of several statistical models, which are used to describe the occurrence process of earthquakes in forecasting the short-term earthquake probabilities during the L’Aquila earthquake sequence in central Italy in 2009. These models include the Proximity to Past Earthquakes (PPE) model and two versions of the Epidemic Type Aftershock Sequence (ETAS) model. We used the information gains corresponding to the Poisson and binomial scores to evaluate the performance of these models. It is shown that both ETAS models work better than the PPE model. However, in comparing the two types of ETAS models, the one with the same fixed exponent coefficient (alpha) = 2.3 for both the productivity function and the scaling factor in the spatial response function (ETAS I), performs better in forecasting the active aftershock sequence than the model with different exponent coefficients (ETAS II), when the Poisson score is adopted. ETAS II performs better when a lower magnitude threshold of 2.0 and the binomial score are used. The reason is found to be that the catalog does not have an event of similar magnitude to the L’Aquila mainshock (Mw 6.3) in the training period (April 16, 2005 to March 15, 2009), and the (alpha)-value is underestimated, thus the forecast seismicity is underestimated when the productivity function is extrapolated to high magnitudes. We also investigate the effect of the inclusion of small events in forecasting larger events. These results suggest that the training catalog used for estimating the model parameters should include earthquakes of magnitudes similar to the mainshock when forecasting seismicity during an aftershock sequence.</div></div></div>

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Operational Earthquake Forecasting during the 2019 Ridgecrest, California, Earthquake Sequence with the UCERF3-ETAS Model

Seismological Research Letters ◽

10.1785/0220190294 ◽

2020 ◽

Vol 91 (3) ◽

pp. 1567-1578 ◽

Cited By ~ 3

Author(s):

Kevin R. Milner ◽

Edward H. Field ◽

William H. Savran ◽

Morgan T. Page ◽

Thomas H. Jordan

Keyword(s):

High Performance ◽

Aftershock Sequence ◽

Lessons Learned ◽

Ease Of Use ◽

Earthquake Forecasting ◽

Earthquake Rupture ◽

Etas Model ◽

Epidemic Type Aftershock Sequence ◽

Aftershock Sequences ◽

High Performance Computing Cluster

Abstract The first Uniform California Earthquake Rupture Forecast, Version 3–epidemic-type aftershock sequence (UCERF3-ETAS) aftershock simulations were running on a high-performance computing cluster within 33 min of the 4 July 2019 M 6.4 Searles Valley earthquake. UCERF3-ETAS, an extension of the third Uniform California Earthquake Rupture Forecast (UCERF3), is the first comprehensive, fault-based, epidemic-type aftershock sequence (ETAS) model. It produces ensembles of synthetic aftershock sequences both on and off explicitly modeled UCERF3 faults to answer a key question repeatedly asked during the Ridgecrest sequence: What are the chances that the earthquake that just occurred will turn out to be the foreshock of an even bigger event? As the sequence unfolded—including one such larger event, the 5 July 2019 M 7.1 Ridgecrest earthquake almost 34 hr later—we updated the model with observed aftershocks, finite-rupture estimates, sequence-specific parameters, and alternative UCERF3-ETAS variants. Although configuring and running UCERF3-ETAS at the time of the earthquake was not fully automated, considerable effort had been focused in 2018 on improving model documentation and ease of use with a public GitHub repository, command line tools, and flexible configuration files. These efforts allowed us to quickly respond and efficiently configure new simulations as the sequence evolved. Here, we discuss lessons learned during the Ridgecrest sequence, including sensitivities of fault triggering probabilities to poorly constrained finite-rupture estimates and model assumptions, as well as implications for UCERF3-ETAS operationalization.

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The overlap of aftershock coda waves and forecasting the first hour aftershocks

10.5194/egusphere-egu2020-2444 ◽

2020 ◽

Author(s):

Eugenio Lippiello ◽

Giuseppe Petrillo ◽

Cataldo Godano ◽

Lucilla de Arcangelis ◽

Anna Tramelli ◽

...

Keyword(s):

Aftershock Sequence ◽

Coda Waves ◽

Statistical Features ◽

Short Term ◽

Geophysical Research ◽

Ground Shaking ◽

Epidemic Type Aftershock Sequence ◽

Novel Approach ◽

Single Station ◽

Strong Ground

We show that short term post-seismic incompleteness can be interpreted in terms of the overlap of aftershock coda waves. We use this information to develop a novel procedure which gives accurate occurrence probabilities of post-seismic strong ground shaking within 30 minutes after the mainshock. This novel approach uses, as only information, the ground velocity recorded at a single station without requiring that signals are transferred and elaborated by operational units. We will also discuss how this information can be implemented in the Epidemic-Type Aftershock Sequence model in order to reproduce statistical features in time and magnitude of recorded aftershocks.Main references de Arcangelis L., Godano C. & Lippiello E. (2018) The Overlap of Aftershock Coda Waves and Short-Term Postseismic Forecasting. Journal of Geophysical Research: Solid Earth, 123: 5661-5674,doi:10.1029/2018JB015518Lippiello E., Petrillo G. , Godano G. , Tramelli A., Papadimitriou E. &, Karakostas V. (2019) Forecasting of the first hour aftershocks by means of the perceived magnitude. Nature Communications , 10, 2953, doi:10.1038/s41467-019-10763-3

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Background Seismicity before and after the 1976 Ms 7.8 Tangshan Earthquake: Is Its Aftershock Sequence Still Continuing?

Seismological Research Letters ◽

10.1785/0220200179 ◽

2020 ◽

Author(s):

Yue Liu ◽

Jiancang Zhuang ◽

Changsheng Jiang

Keyword(s):

Aftershock Sequence ◽

Tangshan Earthquake ◽

Aftershock Activity ◽

Background Rate ◽

Epidemic Type Aftershock Sequence ◽

Aftershock Sequences ◽

Background Seismicity ◽

Before And After ◽

Large Earthquakes ◽

Current Stage

Abstract The aftershock zone of the 1976 Ms 7.8 Tangshan, China, earthquake remains seismically active, experiencing moderate events such as the 5 December 2019 Ms 4.5 Fengnan event. It is still debated whether aftershock sequences following large earthquakes in low-seismicity continental regions can persist for several centuries. To understand the current stage of the Tangshan aftershock sequence, we analyze the sequence record and separate background seismicity from the triggering effect using a finite-source epidemic-type aftershock sequence model. Our results show that the background rate notably decreases after the mainshock. The estimated probability that the most recent 5 December 2019 Ms 4.5 Fengnan District, Tangshan, earthquake is a background event is 50.6%. This indicates that the contemporary seismicity in the Tangshan aftershock zone can be characterized as a transition from aftershock activity to background seismicity. Although the aftershock sequence is still active in the Tangshan region, it is overridden by background seismicity.

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