Time-dependent Seismic Hazard Assessment Research Articles

Time-Dependent Seismic Hazard Assessment Based on the Annual Consultation: A Case from the China Seismic Experimental Site (CSES)

Time-Dependent Seismic Hazard Assessment Based on the Annual Consultation: A Case from the China Seismic Experimental Site (CSES)

Nontrivial clustering of microseismicity induced by hydraulic fracturing

AbstractFor induced microseismicity associated with hydraulic fracturing, the frequency‐magnitude distribution is typically characterized by a falloff with increasing magnitude that is significantly faster than for seismicity along active fault systems. This characteristic may arise from a break in scale invariance, possibly due to mechanical layering that typifies fine‐grained sedimentary rocks in many shale gas and tight oil reservoirs. The latter would imply the presence of spatiotemporal magnitude correlations. Using three microseismic catalogs for well stimulations in widely separated locations with varying hydraulic‐fracturing methods, we show that events with similar magnitudes indeed tend to cluster in space and time. In addition, we show that the interevent time distribution can be described by a universal functional form characterized by two power laws. One exponent can be related to the presence of interevent triggering as in aftershock sequences and is a consequence of the Omori‐Utsu law. The other one is a reflection of the intrinsic spatial variation in the microseismic response rates. Taken together, these features are indicative of nontrivial spatiotemporal clustering of induced microseismicity and, hence, of direct relevance for time‐dependent seismic hazard assessment.

Self-similar aftershock rates.

In many important systems exhibiting crackling noise-an intermittent avalanchelike relaxation response with power-law and, thus, self-similar distributed event sizes-the "laws" for the rate of activity after large events are not consistent with the overall self-similar behavior expected on theoretical grounds. This is particularly true for the case of seismicity, and a satisfying solution to this paradox has remained outstanding. Here, we propose a generalized description of the aftershock rates which is both self-similar and consistent with all other known self-similar features. Comparing our theoretical predictions with high-resolution earthquake data from Southern California we find excellent agreement, providing particularly clear evidence for a unified description of aftershocks and foreshocks. This may offer an improved framework for time-dependent seismic hazard assessment and earthquake forecasting.

Can Apparent Stress be Used to Time-Dependent Seismic Hazard Assessment or Earthquake Forecast? An Ongoing Approach in China

The approach in China since the last 1.5 decade for using apparent stress in time-dependent seismic hazard assessment or earthquake forecast is summarized. Retrospective case studies observe that apparent stress exhibits short-term increase, with time scale of several months, before moderate to strong earthquakes in a large area surrounding the ‘target earthquake’. Apparent stress is also used to estimate the tendency of aftershock activity. The concept relating apparent stress indirectly to stress level is used to understand the properties of some ‘precursory’ anomalies. Meanwhile, different opinions were reported. Problems in the calculation also existed for some cases. Moreover, retrospective studies have the limitation in their significance as compared to forward forecast test. Nevertheless, this approach, seemingly uniquely carried out in a large scale in mainland China, provides the earthquake catalogs for the predictive analysis of seismicity with an additional degree of freedom, deserving a systematic review and reflection.

Seismicity rate changes in association with the evolution of the stress field in northern Aegean Sea, Greece

SUMMARY Seismicity rate changes in northern Aegean area were studied by applying the Dieterich (1994) Rate/State formulation. The study area was divided into four smaller ones that exhibit seismotectonic homogeneity and accommodate strong events with similar to identical focal mechanisms. Seismicity rate evaluation concerns the ‘study period’ between 1981 December 19 and 2010 August 31, whereas the reference seismicity rate was estimated for the ‘learning period’ lasting from 1970 January 1 to 1981 December 19. The coseismic slip of the strongest events (Mw≥ 5.8) that occurred during the study period was considered to contribute to the stress field evolution along with the continuous tectonic loading. Stress changes were calculated just before and after each strong event and their influence was then examined in connection with the occurrence rate of the smaller magnitude events above the individually determined magnitude of completeness in each subarea and for the respective time intervals. After defining the probability density function of seismicity distribution, a Rate/State model was used to combine static Coulomb stress changes (ΔCFF) with seismicity rates and to compare the observed with the expected rates of earthquake production for each time period and subarea. Different parameter values combinations were tested to evaluate the model sensitivity. Qualitative and quantitative correlations were performed for each one of the selected study periods. The modelling approach resulted to satisfactory correlation between real and synthetic seismicity rates and is expected to constitute a useful mean for the time-dependent seismic hazard assessment.

The SISMA prototype system: integrating Geophysical Modeling and Earth Observation for time-dependent seismic hazard assessment

An innovative approach to seismic hazard assessment is illustrated that, based on the available knowledge of the physical properties of the Earth structure and of seismic sources, on geodetic observations, as well as on the geophysical forward modeling, allows for a time-dependent definition of the seismic input. According to the proposed approach, a fully formalized system integrating Earth Observation data and new advanced methods in seismological and geophysical data analysis is currently under development in the framework of the Pilot Project SISMA, funded by the Italian Space Agency. The synergic use of geodetic Earth Observation data (EO) and Geophysical Forward Modeling deformation maps at the national scale complements the space- and time-dependent information provided by real-time monitoring of seismic flow (performed by means of the earthquake prediction algorithms CN and M8S) and permits the identification and routine updating of alerted areas. At the local spatial scale (tens of km) of the seismogenic nodes identified by pattern-recognition analysis, both GNSS (Global Navigation Satellite System) and SAR (Synthetic Aperture Radar) techniques, coupled with expressly developed models for interseismic phase, allow us to retrieve the deformation style and stress evolution within the seismogenic areas. The displacement fields obtained from EO data provide the input for the geophysical modeling, which eventually permits to indicate whether a specific fault is in a “critical state.” The scenarios of expected ground motion (shakemaps) associated with the alerted areas are then defined by means of full waveforms modeling, based on the possibility to compute synthetic seismograms by the modal summation technique (neo-deterministic hazard assessment). In this way, a set of deterministic scenarios of ground motion, which refer to the time interval when a strong event is likely to occur within the alerted area, can be defined both at national and at local scale. The considered integrated approach opens new routes in understanding the dynamics of fault zones as well as in modeling the expected ground motion. The SISMA system, in fact, provides tools for establishing warning criteria based on deterministic and rigorous forward geophysical models and hence allows for a well-controlled real-time prospective testing and validation of the proposed methodology over the Italian territory. The proposed approach complements the traditional probabilistic approach for seismic hazard estimates, since it supplies routinely updated information useful in assigning priorities for timely mitigation actions and hence it is particularly relevant to Civil Defense purposes.

Probabilistic approach to earthquake prediction.

The evaluation of any earthquake forecast hypothesis requires the application of rigorous statistical methods. It implies a univocal definition of the model characterising the concerned anomaly or precursor, so as it can be objectively recognised in any circumstance and by any observer.A valid forecast hypothesis is expected to maximise successes and minimise false alarms. The probability gain associated to a precursor is also a popular way to estimate the quality of the predictions based on such precursor. Some scientists make use of a statistical approach based on the computation of the likelihood of an observed realisation of seismic events, and on the comparison of the likelihood obtained under different hypotheses. This method can be extended to algorithms that allow the computation of the density distribution of the conditional probability of earthquake occurrence in space, time and magnitude. Whatever method is chosen for building up a new hypothesis, the final assessment of its validity should be carried out by a test on a new and independent set of observations. The implementation of this test could, however, be problematic for seismicity characterised by long-term recurrence intervals. Even using the historical record, that may span time windows extremely variable between a few centuries to a few millennia, we have a low probability to catch more than one or two events on the same fault. Extending the record of earthquakes of the past back in time up to several millennia, paleoseismology represents a great opportunity to study how earthquakes recur through time and thus provide innovative contributions to time-dependent seismic hazard assessment. Sets of paleoseimologically dated earthquakes have been established for some faults in the Mediterranean area: the Irpinia fault in Southern Italy, the Fucino fault in Central Italy, the El Asnam fault in Algeria and the Skinos fault in Central Greece. By using the age of the paleoearthquakes with their associated uncertainty we have computed, through a Montecarlo procedure, the probability that the observed inter-event times come from a uniform random distribution (null hypothesis). This probability is estimated approximately equal to 8.4% for the Irpinia fault, 0.5% for the Fucino fault, 49% for the El Asnam fault and 42% for the Skinos fault. So, the null Poisson hypothesis can be rejected with a confidence level of 99.5% for the Fucino fault, but it can be rejected only with a confidence level between 90% and 95% for the Irpinia fault, while it cannot be rejected for the other two cases. As discussed in the last section of this paper, whatever the scientific value of any prediction hypothesis, it should be considered effective only after evaluation of the balance between the costs and benefits introduced by its practical implementation.

Comment on "Layered Seismogenic Source Model and Probabilistic Seismic-Hazard Analyses in Central Italy" by Bruno Pace, Laura Peruzza, Giusy Lavecchia, and Paolo Boncio

Pace et al. (2006) define a seismogenic model aimed to provide a (time-dependent) seismic-hazard assessment for Central Italy. One of the main novelties of the model is that it introduces seismogenic structures (faults) in seismic- hazard calculations, and it imposes a time behavior on these structures, leading to a time-dependent seismic-hazard assessment. In my opinion, the approach used in this article (1) does not explain the “observed” time distribution of historical earthquakes (one of the few empirical evidence that can be used to check the reliability of any seismic-hazard or earthquake-forecasting model), and (2) uses values of parameters of the statistical distribution that are clearly unreliable. These problems introduce significant biases that potentially make the hazard assessment made by the authors less reliable that the one obtained by more traditional approaches. In the following, I discuss in detail these points. I focus the attention only on earthquakes with M 5.5+ that are considered “destructive” for the Italian territory. Sometimes, for the sake of conciseness, I drop the specification of the magnitude of events, and then it should always be considered equal to or larger than 5.5. Traditionally, seismic hazard and earthquake forecasting in Italy are based on regular spatial grids or on the definition of seismogenic provinces (Slejko et al. , 1998; Albarello et al. , 2000; Meletti et al. , 2000; Scandone and Stucchi, 2000; Gruppo di Lavoro, 1999, 2004; Romeo et al. , 2000; Lucantoni et al. , 2001; Faenza et al. , 2003; Cinti et al. 2004). This choice has two main rationales; first, despite the common acceptance that almost all earthquakes occur on faults that …

Time-independent and Time-dependent Seismic Hazard Assessment for the State of California: Uniform California Earthquake Rupture Forecast Model 1.0

Ground-shaking hazard maps based on sound earth-science research are effective tools for mitigating damage from future earthquakes. Assuming that future earthquakes will occur on active faults or near previous events and that the ground shaking from these events will fall within the range of globally recorded ground motions leads to probabilistic hazard maps that predict ground-shaking potential across a region. Developing hazard and risk maps requires technical interactions between earth scientists and engineers to estimate the rate of potential earthquakes across a region, quantify likely ground-shaking levels at a site, and understand how buildings respond to strong ground shaking. For the past 25 years the U.S. Geological Survey (USGS) and California Geological Survey (CGS) have cooperated with government officials and professional organizations to incorporate hazard maps and other hazard products in public and corporate documents such as building codes, insurance rate structures, and earthquake risk mitigation plans (Algermissen and Perkins 1982; Frankel et al. 1996, 2002; Petersen et al. 1996). Because these hazard products are used in making public-policy decisions, it is essential that the official USGS-CGS hazard models reflect the “best available science.” This qualification is also required by the statute that regulates the California Earthquake Authority, which provides most earthquake insurance in the state. To adequately represent the “best available science,” hazard maps need to be updated regularly to keep pace with new scientific advancements. The methodologies, computer codes, and input data used in developing these products need to be openly available for review and analysis. Input parameters and codes for current hazard maps may be obtained at http://earthquake.usgs.gov/hazmaps and http://www.consrv.ca.gov/cgs. At these Web sites a user may access documentation that describes methodologies and parameters, a relational database and tables that contain explanations of how the fault parameters and uncertainties were chosen, interactive …

Perspectives for earthquake prediction in the Mediterranean and contribution of geological observations

Abstract Accelerating seismic strain caused by the generation of intermediate-magnitude preshocks in a broad (critical) region, accompanied by decelerating seismic strain caused by the generation of smaller preshocks in the seismogenic region are systematically observed before strong mainshocks. On the basis of this seismicity pattern a model has been developed that seems promising for intermediate-term earthquake prediction, called the ‘Decelerating in-Accelerating out Seismic Strain Model’. Recent seismological data for the Mediterranean region are used here for backward and forward testing of this model. The selection of the broader Mediterranean region as a test area was motivated not only by the interest of time-dependent seismic hazard assessment in a high-seismicity and highly populated region but also by the fact that the Mediterranean is a natural geophysical and geological laboratory where both complex multi-plate and continuum tectonics are found in a more or less convergent zone. Within this complex geotectonic setting several geological phenomena such as subduction, collision, orogen collapse and back-arc extension take place, leading to the generation of a broad spectrum of mainshocks, reaching M W = 8.0 or greater for subduction-related thrust events and a variety of corresponding seismicity levels and neotectonic activity ranging from very low (e.g. large parts of Iberian peninsula) to very high (broader Aegean area). The backward procedure shows that all six strong (M ≥ 6.8) mainshocks that have occurred in the Mediterranean since 1980 had been preceded by preshock sequences that followed this seismicity pattern and satisfy all model constraints. Application of the model for future mainshocks has led to the identification of nine regions (in the Pyrenees, Calabria, NE Adriatic, Albania, Northern Greece, SE Aegean, NW Anatolia, western Anatolia, NE Anatolia) where current intermediate-magnitude seismicity satisfies the constraints of the model and corresponds to strong (M ≥ 6.2) mainshocks. The magnitudes, epicentres and origin times of these probably ensuing mainshocks, as well as their corresponding uncertainties, are estimated, so that it is possible to evaluate the model potential during the next decade (2006–2015). Furthermore, it is shown that geological observations of surface fault traces can contribute to the accurate location of the foci of future strong mainshocks in the Mediterranean and to an estimation of their sizes. For this purpose, globally valid relations between fault parameters based on geological observations (surface fault length, L S , and fault slip, u S ) and measures of mainshock size (mainshock magnitude, subsurface fault length, L , and fault slip, u ) are proposed.

Accelerated Preshock Deformation of Broad Regions in the Aegean Area

—Twenty-four regions where accelerating deformation has been observed for a few decades before corresponding strong (M = 6.0–7.5) mainshocks are identified in the broader Aegean area. To a first approximation these preshock regions have elliptical shapes and the radius, R (in km), of a circle with an area equal to the corresponding ellipse is related to the moment magnitude, M, of the mainshock by the equation:¶log R = 0.42 M-0.68.¶The dimension of each preshock region is about seven to ten times larger than the rupture zone (fault length) of the corresponding mainshock. The time variation of the cumulative Benioff strain was satisfactorily fitted by a power-law relation, which is predicted by statistical physics if the mainshock to which accelerating strain rates leads is considered as a critical point. The duration, t (in years), of the accelerating Benioff strain release period is given by the relation:¶¶log t = 5.94-0.75 log s r ¶where s r is the mean Benioff strain rate release (per year for 104 km2) in the preshock region calculated by the complete available data (M≥5.2) for the entire instrumental period (1911–1998). The importance of identifying and investigating such regions for better understanding the dynamics of the active part of the lithosphere as well as for earthquake prediction and time-dependent seismic hazard assessment is discussed.