Meg Segou - Academia.edu (original) (raw)

Papers by Meg Segou

Bulletin of the Seismological Society of America, 2020

Coseismic stress changes have been the primary physical principle used to explain aftershocks and... more Coseismic stress changes have been the primary physical principle used to explain aftershocks and triggered earthquakes. However, this method does not adequately forecast earthquake rates and diverse rupture populations when subjected to formal testing. We show that earthquake forecasts can be impaired by assumptions made in physics-based models such as the existence of hypothetical optimal faults and regional scale invariability of the stress field. We compare calculations made under these assumptions along with different realizations of a new conceptual triggering model that features a complete assay of all possible ruptures. In this concept, there always exists a set of theoretical planes that has positive failure stress conditions under a combination of background and coseismic static stress change. In the Earth, all of these theoretical planes may not exist, and if they do, they may not be ready to fail. Thus, the actual aftershock plane may not correspond to the plane with the...

Bulletin of the Geological Society of Greece, 2018

The Manassi section in Levkas Island belongs to the Pre-Apulian (Paxos) zone, the most external d... more The Manassi section in Levkas Island belongs to the Pre-Apulian (Paxos) zone, the most external domain of the Hellenic realm. Its Early Tortonian sediments contain a rich foraminiferal fauna dominated, in numbers of individuals, by planktic species. Its benthic foraminiferal assemblage is characterized by a high number of taxa, with low numbers of individuals. Their study provides a basis for interpreting the paleobathymetry of the basin.The Manassi section represents deposition in upper to lower bathyal depths, during a period of intense tectonic activity. Downslope transport of fauna by turbidity currents partly overprints the signal of paleobathymetrically-diagnostic foraminifera distribution. The recognition of allochthonous taxa is used, together with %P, to identify turbidite beds intercalated with in-situ marly sediments. The micropaleontological and paleobathymetrical analyses of the studied sediments indicate that these correspond to distal atypical flysch deposited in the ...

Journal of Geophysical Research: Solid Earth, 2013

ABSTRACT [1] We perform a retrospective forecast test using Northern California seismicity for th... more ABSTRACT [1] We perform a retrospective forecast test using Northern California seismicity for the period between 1980 and 2009. We compare 7 realizations of the short-term clustering epidemic-type aftershock sequence (ETAS) model, and 21 models combining Coulomb stress change calculations and Rate/State theory (CRS) to forecast seismicity rates in 10 day time intervals. We employ a common learning phase (1974–1980) for CRS models to ensure consistency, and we evaluate the forecasts with log likelihood statistics to detect any spatial inconsistencies and compare the total numbers of forecasts versus observed events. We find that: (1) ETAS models are better forecasters of the spatial evolution in seismicity in the near-source region, (2) CRS models can compete with ETAS models away from the mainshock rupture, and for short periods after mainshocks, (3) CRS models with optimally oriented receiver fault planes perform better in the first few days after mainshocks, whereas mapped fault planes should be implemented for longer-term forecasting, and (4) CRS models based on shear stress change calculations have comparable performance with Coulomb stress change models, with the benefit of lesser parameters involved in stress calculations. We conclude that physics-based and statistical forecast models are complimentary to each other and that future forecasts should be based on statistical models for near-source regions, and physical models for longer periods and distances. However, the realization of the CRS models involves a number of critical parameters (reference seismicity rates, regional stress field, and loading rates), which should be retrospectively tested to improve the predictive power of physics-based models.

Geophysical Research Letters, 2014

We calculate stress changes resulting from the M = 6.0 West Napa earthquake on north San Francisc... more We calculate stress changes resulting from the M = 6.0 West Napa earthquake on north San Francisco Bay area faults. The earthquake ruptured within a series of long faults that pose significant hazard to the Bay area, and we are thus concerned with potential increases in the probability of a large earthquake through stress transfer. We conduct this exercise as a prospective test because the skill of stress-based aftershock forecasting methodology is inconclusive. We apply three methods: (1) generalized mapping of regional Coulomb stress change, (2) stress changes resolved on Uniform California Earthquake Rupture Forecast faults, and (3) a mapped rate/state aftershock forecast. All calculations were completed within 24 h after the main shock and were made without benefit of known aftershocks, which will be used to evaluative the prospective forecast. All methods suggest that we should expect heightened seismicity on parts of the southern Rodgers Creek, northern Hayward, and Green Valley faults.

Seismological Research Letters, 2014

Seismological Research Letters, 2011

Journal of Seismology, 2012

The Seismic Hazard Harmonization in Europe (SHARE) project, which began in June 2009, aims at est... more The Seismic Hazard Harmonization in Europe (SHARE) project, which began in June 2009, aims at establishing new standards for probabilistic seismic hazard assessment in the Euro-Mediterranean region. In this context, a logic tree for ground-motion prediction in Europe has been constructed. Ground-motion prediction equations (GMPEs) and weights have been determined so that the logic tree captures epistemic uncertainty in ground-motion prediction for six different tectonic regimes in Europe. Here we present the strategy that we adopted to build such a logic tree. This strategy has the particularity of combining two complementary and independent approaches: expert judgment and data testing. A set

Earthquake Spectra, 2013

In recent years stochastic optimization has become popular in modeling highly complex natural sys... more In recent years stochastic optimization has become popular in modeling highly complex natural systems as opposed to traditional approaches, such as regression analysis. This paper tests the efficiency of regression analysis over mathematical optimization by evaluating the performance of stochastic (genetic algorithms, simulated annealing) and deterministic algorithms (nonlinear least squares, pattern search) in solving minimization problems. Qualitative and quantitative comparison of these solvers reveals that the implementation of genetic algorithms with initial population development, using Latin Hypercube sampling, outperformed other algorithms. Using the aforementioned solver, ground motion prediction equations are derived for the estimation of horizontal peak ground acceleration, velocity, and 5% damped spectral acceleration ordinates at discrete period estimators between 0.05 s and 2 s. Ground motion for stiff and soft soil sites is amplified by a constant factor with respect ...

Computers & Geosciences, 2010

Tectonophysics, 2014

The aftershock zone of each large (M≥7) earthquake extends throughout the shallows of planet Eart... more The aftershock zone of each large (M≥7) earthquake extends throughout the shallows of planet Earth. Most aftershocks cluster near the mainshock rupture, but earthquakes send out shivers in the form of seismic waves, and these temporary distortions are large enough to trigger other earthquakes at global range. The aftershocks that happen at great distance from their mainshock are often superposed onto already seismically active regions, making them difficult to detect and understand. From a hazard perspective we are concerned that this dynamic process might encourage other high magnitude earthquakes, and wonder if a global alarm state is warranted after every large mainshock. From an earthquake process perspective we are curious about the physics of earthquake triggering across the magnitude spectrum. In this review we build upon past studies that examined the combined global response to mainshocks. Such compilations demonstrate significant rate increases during, and immediately after (~45 minutes) M>7.0 mainshocks in all tectonic settings and ranges. However, it is difficult to find strong evidence for M>5 rate increases during the passage of surface waves in combined global catalogs. On the other hand, recently published studies of individual large mainshocks associate M>5 triggering at global range that is delayed by hours to days after surface wave arrivals. The longer the delay between mainshock and global aftershock, the more difficult it is to establish causation. To address these questions, we review the response to 260 M≥7.0 shallow (Z≤50 km) mainshocks in 21 global regions with local seismograph networks. In this way we can examine the detailed temporal and spatial response, or lack thereof, during passing seismic waves, and over the 24 hour period after their passing. We see an array of responses that can involve immediate and widespread seismicity outbreaks, delayed and localized earthquake clusters, to no response at all. About 50% of the catalogs we studied showed possible (localized delayed) remote triggering, and ~20% showed probable (instantaneous broadly distributed) remote triggering. However, in any given region, at most only about 2-3% of global mainshocks caused significant local earthquake rate increases. These rate increases are mostly composed of small magnitude events, and we do not find significant evidence of dynamically triggered M>5 earthquakes. If we assume that the few observed M>5 events are triggered, we find that they are not directly associated with surface wave passage, with first incidences being 9-10 hours later. We note that mainshock magnitude, relative proximity, amplitude spectra, peak ground motion, and mainshock focal mechanisms are not reliable determining factors as to whether a mainshock will cause remote triggering. By elimination, azimuth, and polarization of surface waves with respect to receiver faults may be more important factors.

Coseismic stress changes have been the primary physical principle used to explain after-shocks an... more Coseismic stress changes have been the primary physical principle used to explain after-shocks and triggered earthquakes. However, this method does not adequately forecast earthquake rates and diverse rupture populations when subjected to formal testing. We show that earthquake forecasts can be impaired by assumptions made in physics-based models such as the existence of hypothetical optimal faults and regional scale invariability of the stress field. We compare calculations made under these assumptions along with different realizations of a new conceptual triggering model that features a complete assay of all possible ruptures. In this concept, there always exists a set of theoretical planes that has positive failure stress conditions under a combination of background and coseismic static stress change. In the Earth, all of these theoretical planes may not exist, and if they do, they may not be ready to fail. Thus, the actual aftershock plane may not correspond to the plane with th...

Geophysical Research Letters (Accepted Articles) : We calculate stress changes resulting from the... more Geophysical Research Letters (Accepted Articles) : We calculate stress changes resulting from the M = 6.0 West Napa earthquake on north San Francisco Bay area faults. The earthquake ruptured within a series of long faults that pose significant hazard to the Bay area, and we are thus concerned with potential increases in the probability of a large earthquake through stress transfer. We conduct this exercise as a prospective test because the skill of stress-based aftershock forecasting methodology is inconclusive. We apply three methods: (1) generalized mapping of regional Coulomb stress change, (2) stress changes resolved on Uniform California Earthquake Rupture Forecast (UCERF) faults, and (3) a mapped rate/state aftershock forecast. All calculations were completed within 24 hours after the mainshock, and were made without benefit of known aftershocks, which will be used to evaluative the prospective forecast. All methods suggest that we should expect heightened seismicity on parts of the southern Rodgers Creek, northern Hayward, and Green Valley faults.

Main shocks are calculated to cast stress shadows across broad areas where aftershocks occur. Thu... more Main shocks are calculated to cast stress shadows across broad areas where aftershocks occur. Thus, a key problem with stress-based operational forecasts is that they can badly underestimate aftershock occurrence in the shadows. We examine the performance of two physics-based earthquake forecast models (Coulomb rate/state (CRS)) based on Coulomb stress changes and a rate-and-state friction law for their predictive power on the 1989 Mw = 6.9 Loma Prieta aftershock sequence. The CRS-1 model considers the stress perturbations associated with the main shock rupture only, whereas CRS-2 uses an updated stress field with stresses imparted by M≥ 3.5 aftershocks. Including secondary triggering effects slightly improves predictability, but physics-based models still underestimate aftershock rates in locations of initial negative stress changes. Furthermore, CRS-2 does not explain aftershock occurrence where secondary stress changes enhance the initial stress shadow. Predicting earthquake occurrence in calculated stress shadow zones remains a challenge for stress-based forecasts, and additional triggering mechanisms must be invoked.

![Research paper thumbnail of Stress transfer by the 2008 M=6.4 Achaia earthquake` to the western Corinth Gulf normal faults and its relation with the 2010 Efpalio sequence (Central Greece),](https://attachments.academia-assets.com/34860037/thumbnails/1.jpg)

We investigate the interaction between transform faults and normal faults in western Greece, base... more We investigate the interaction between transform faults and normal faults in western Greece, based on seismological analysis and static stress transfer calculations associated with the June 8th, 2008 Mw=6.4 Achaia earthquake. We present a relocated earthquake catalog for the period between June 2008-January 2010, when two normal faulting events on January 18th (Mw=5.3) and 22nd (Mw=5.2) 2010 occurred at Efpalio (western Corinth Gulf), and were located approximately 70 km NE from the buried right-lateral fault, identified as the causative structure of the Achaia earthquake. The first Efpalio event ruptured a mapped normal fault that trends ENEWSW, dipping 55°- 60° to the south. We estimate ~2-fold seismicity rate-changes in the western Corinth gulf region for the interseismic period (June 2008-January 2010) and we find that inside this interval, the monthly event rate remained increased at a 2σ significance level. We calculate a Coulomb stress increase (0.1-0.6 bar) in the Efpalio region using optimally-oriented for failure faults, and a ~0.11 bar Coulomb stress increase at the hypocenters of the January 2010 events when incorporating geologydefined receiver planes. We conclude that the positive static stress changes following the Achaia event promoted the observed spatiotemporal clustering in the Corinth gulf for this specific period. We identify fault unclamping due to normal stress reduction as the physical mechanism in this case. The high seismic hazard character of the target region (0.24 g) in the National Building Code underlines the importance behind timedependent earthquake probabilities and stress-mediated fault interaction studies.

Strong motion records are the original input data for earthquake engineering studies and earthqua... more Strong motion records are the original input data for earthquake engineering studies and earthquake resistant building codes. Records originating from both, analogue and modern digital instruments should be subjected to processing in order to derive credible engineering parameters, such as spectral ordinates. A number of processing procedures have been proposed by many researchers through the past few decades; the goal, however, is always the same, identification and removal of noise and performing necessary adjustments related to the operating instrument or the record's quality. A careful inspection of previous processing schemes and the design of several development tests related to major processing steps proved to be helpful in determining the necessary features that the presented software should include. These have been incorporated in a Matlab application in order to produce a reliable and user-friendly interface giving the researcher several options on major processing steps. It is noteworthy that this computer program can be used either as an interactive process tool when individual record processing corresponds to the researcher's needs or, for processing a vast number of records without user supervision and even through a scheduled task.

The Seismic HAzard haRmonization in Europe (SHARE) project, which began in June 2009, aims at est... more The Seismic HAzard haRmonization in Europe (SHARE) project, which began in June 2009, aims at establishing new standards for probabilistic seismic hazard assessment (PSHA) in the Euro-Mediterranean region. In this context, a logic tree for ground-motion prediction in Europe has been constructed. Groundmotion prediction equations (GMPEs) and weights have been determined so that the logic tree captures epistemic uncertainty in ground-motion prediction for six different tectonic regimes in Europe. Here we present the strategy that we adopted to build such a logic tree. This strategy has the particularity of combining two complementary and independent approaches: expert judgment and data testing. A set of six experts was asked to weight pre-selected GMPEs while the ability of these GMPEs to predict available data was evaluated with the method of . Results of both approaches were taken into account to commonly select the smallest set of GMPEs to capture the uncertainty in ground-motion prediction in Europe. For stable continental regions, two models, both from Eastern North America, have been selected for shields and three GMPEs from active shallow crustal regions have been added for continental crust. For subduction zones, four models, all non-European have been chosen. Finally, for active shallow crustal regions, we selected four models, each of them from a different host region but only two of them were kept for long periods. In most cases, a common agreement has been also reached for the weights. In case of divergence, a sensitivity analysis of the weights on the seismic hazard has been conducted, showing that once the GMPEs have been selected, the associated set of weights has a smaller influence on the hazard.