A probabilistic method for the estimation of earthquake source parameters from spectral inversion : application to the 2016-2017 Central Italy seismic sequence

We develop a probabilistic framework based on the conjunction of states of information between data and model, to jointly retrieve earthquake source parameters and anelastic attenuation factor from inversion of displacement amplitude spectra. The evaluation of the joint probability density functions (PDFs) enables us to take into account between-parameter correlations in the final estimates of the parameters and related uncertainties. Following this approach, we first search for the maximum of the a-posteriori PDF through the basin hopping technique that couples a global exploration built on a Markov chain with a local deterministic maximization. Then we compute statistical indicators (mean, variance and correlation coefficients) on source parameters and anelastic attenuation through integration of the PDF in the vicinity of the maximum likelihood solution. Definition of quality criteria based on the signal to noise ratio and the similarity of the marginal PDFs with a Gaussian function enable us to define the frequency domain for the inversion and to get rid of unconstrained solutions. We perform synthetic tests to assess theoretical correlations as a function of the signal to noise ratio and to define the minimum bandwidth around the corner frequency for consistent parameter resolution. As an application, we finally estimate the source parameters for the 2016-2017 Central Italy seismic sequence. We found that the classical scaling between the seismic moment and the corner frequency holds, with an average stress drop of $\Delta\sigma$ = 2.1 +- 0.3 MPa. However, the main events in the sequence exhibit a stress drop larger than the average value. Finally, the small seismic efficiency indicates a stress overshoot, possibly due to dynamic effects or large frictional efficiency.Comment: 57 pages, 16 figure

Early magnitude and potential damage zone estimates for the great Mw 9 Tohoku-Oki earthquake

The Mw 9.0, 2011 Tohoku-Oki earthquake has reopened the discussion among the scientific community about the effectiveness of earthquake early warning for large events. A well-known problem with real-time procedures is the parameter saturation, which may lead to magnitude underestimation for large earthquakes. Here we measure the initial peak ground displacement and the predominant period by progressively expanding the time window and distance range, to provide consistent magnitude estimates (M = 8.4) and a rapid prediction of the potential damage area. This information would have been available 35 s after the first P-wave detection and could have been refined in the successive 20 s using data from more distant stations. We show the suitability of the existing regression relationships between early warning parameters and magnitude, provided that an appropriate P-wave time window is used for parameter estimation. We interpret the magnitude under-estimation as a combined effect of high-pass filtering and frequency dependence of the main radiating source during the rupture process

1-D P-velocity Models of Mt. Vesuvius Volcano from the Inversion of TomoVes96 First Arrival Time Data

—We applied a revised version of the 1-D τ–p inversion method to first P-arrival times from the active seismic experiment performed at Mt. Vesuvius (southern Italy) in 1996 (TomoVes96 Project). The main objective of this work is to obtain 1-D velocity models of Mt. Somma-Vesuvius volcano complex and surrounding area. Moreover we show that combining the 1-D information we provide a reliable 2-D initial model for perturbative tomographic inversions. Seismic and geological surveys suggest the presence of a refractor associated with the contrast between carbonate basement and volcanic/alluvial sediments; synthetic simulations, using a realistic topography and carbonate top morphology, allowed us to study the effect of topography on the retrieved velocity models and to check that the 1-D τ–p method can also approximately retrieve the refractor depth and velocity contrast. We analysed data from 14 on-land shots recorded at stations deployed along the in-profile direction. We grouped the obtained models in three subsets according to the geology of the sampling area: Models for carbonate outcrop area, models for the Campanian Plain surrounding the volcano edifice and models for Mt. Somma-Vesuvius volcano complex. The found 1-D P-velocity models show important vertical and lateral variations. Very low velocities (1.5–2.5 km/s) are observed in the upper 200–500 m thick shallow layer. At greater depths (3 km is the maximum investigated depth) P velocities increase to values in the range of 4–6 km/s which are related to the presence of the carbonatic basement. Finally we interpolated the 1-D models to demonstrate an example of misfit for a 2-D interpolated model whose residuals are confined in a narrow band around zero

A threshold-based earthquake early warning using dense accelerometer networks

Most earthquake early warning systems (EEWS) developed so far are conceived as either ‘regional’ (network-based) or ‘on-site’ (stand-alone) systems. The recent implementation of nationwide, high dynamic range, dense accelerometer arrays makes now available, potentially in real time, unsaturated waveforms of moderate-to-large magnitude earthquakes recorded at very short epicentral distances (<10–20 km). This would allow for a drastic increase of the early warning lead-time, for example, the time between the alert notification and the arrival time of potentially destructive waves at a given target site. By analysing strong motion data from modern accelerograph networks in Japan, Taiwan and Italy, we propose an integrated regional/on-site early warning method, which can be used in the very first seconds after a moderate-to-large earthquake to map the most probable damaged zones. The method is based on the real-time measurement of the period (τ_c) and peak displacement (Pd) parameters at stations located at increasing distances from the earthquake epicentre. The recorded values of early warning parameters are compared to threshold values, which are set for a minimum magnitude 6 and instrumental intensity VII, according to the empirical regression analyses of strong motion data. At each recording site the alert level is assigned based on a decisional table with four alert levels defined upon critical values of the parameters Pd and τ_c, which are set according to the error bounds estimated on the derived prediction equations. Given a real time, evolutionary estimation of earthquake location from first P arrivals, the method furnishes an estimation of the extent of potential damage zone as inferred from continuously updated averages of the period parameter and from mapping of the alert levels determined at the near-source accelerometer stations. The off-line application of the method to strong motion records of the M_w 6.3, 2009 Central Italy earthquake shows a very consistent match between the rapidly predicted (within a few seconds from the first recorded P wave) and observed damage zone, the latter being mapped from detailed macroseismic surveys a few days after the event. The proposed approach is suitable for Italy, where, during the last two decades, a dense network of wide dynamic-range accelerometer arrays has been deployed by the Department of Civil Protection (DPC), the Istituto Nazionale di Geofisica e Vulcanologia (INGV) and other regional research agencies

Application of AVO attributes for gas channels identification, West offshore Nile Delta, Egypt

Abstract The offshore part of the Nile Delta is considered one of the most prolific provinces for gas production and for future petroleum exploration. Amplitude Variation with Offset (AVO) analysis supported by composite logs considered as one of the best-advanced techniques enables the interpreter to understand the seismic data. It is used to generate a new view of the output results, especially for the identification of gas zones as gas channels by using pre-stack AVO attributes that based on the intercept product gradient (A*B) and the Scaled Poisson's Ratio Change. Free gas, regardless of the percentage show obvious AVO anomaly. Well log data, including gamma-ray, resistivity, and Vp sonic logs are used in the seismic data to well tie and in the generation of the synthetic seismogram. Seismic data conditioning processes with check-shot correction is performed to improve the resolution of the reflection events and signal to noise ratio. AVO attributes technique, as a direct hydrocarbon indicator, including AVO gradient analysis and AVO crossplot separate the gas levels in the sand-shale sequences. The identified gas-bearing zones consistent with well log data responses. As the main conclusion, we show the visual evidence for the gas-bearing zones by interpreting different AVO responses due to the changes in fluid and rock types

High Resolution Attenuation Images From Active Seismic Data: The Case Study of Solfatara Volcano (Southern Italy)

&lt;p&gt;The anelastic attenuation of rocks strongly depends on the contained fluid physical state and saturation. Furthermore, it is more sensitive than elastic parameters to changes in the physical state of materials. In a geologically complex&amp;#160; volcanic context, where fluids play a very important role, anelastic imaging of the subsoil is therefore a very powerful tool for a better understanding of its dynamics.&lt;/p&gt;&lt;p&gt;In this study we present a robust workflow aimed at retrieve accurate 1-D and 3-D anelastic models from the processing of active seismic data, in terms of lateral and depth variations of P-wave quality factors Q&lt;sub&gt;P&lt;/sub&gt;. This methodology has been applied to data collected during a high resolution active seismic experiment in a very small-scale volcanic volume, the Solfatara crater, within Campi Flegri caldera, Southern Italy. The presented methodology is developed in three distinct steps: 1) the active seismic data have been properly processed and analyzed for measuring the t* attenuation parameter for all possible source-receivers couples. First, the source contribution has been removed by cross-correlating the recorded signal with the sweep function of the Vibroseis, which was the adopted active seismic source. Then, the spectral decay method has been applied in order to compute the t* values. 2) A reference 1-D attenuation model has been retrieved by means of a grid search procedure aiming at finding the 1-D Qp structure that minimizes the residual between the average observed t* and the theoretical t* distributions. The obtained starting reference model allowed to build a preliminary map of t* residuals through which the retrieved t* dataset has been validated. 3) The 15,296 t* measurements have been inverted by means of a linearized, perturbative approach, in a 160 x 160 x 45 m&lt;sup&gt;3 &lt;/sup&gt;tomographic grid.&lt;/p&gt;&lt;p&gt;The retrieved 3-D attenuation model describes the first 30 m depths of Solfatara volcano as composed of very high attenuating materials, with Qp values ranging between 5 and 40. The very low Qp values, correlated with low Vp values retrieved by a previous tomographic work carried out in the area, indicate the low consolidation degree of very superficial volcanic materials of Solfatara volcano. Finally, in the NE part of the crater, lower attenuating bodies have been imaged: it is a further hint for characterizing this area of the volcano as the shallow release of the CO&lt;sub&gt;2 &lt;/sub&gt;plume through the main fumaroles of the crater.&lt;/p&gt

Identification of gas zones and chimneys using seismic attributes analysis at the Scarab field, offshore, Nile Delta, Egypt

Seismic attributes supported by composite logs are the best way that can enable the interpreter to understand seismic data very well and generate a new view of the output results. Detection of the reservoir zone can be enhanced by analyzing wells log data based on Gamma-ray, Resistivity, and Vp sonic logs respectively. Composite logs of Scarab-1, Scarab-De, Scarab-Da, Scarab-Dd, and Scarab-2 wells indicate the lateral and vertical variation of the gas reservoir in El Wastani Formation. However, there are several seismic attributes that can be used to support reservoirs identification. For enhancement the detection of the hydrocarbon reservoirs, it is important to carefully analyze the 2D seismic data, which in this study will be primarily prepared to enhance seismic attributes results for the identification of gas chimneys, gas zones as channels, enhance stratigraphic and structural interpretations. In this article, we have performed data conditioning, quality control and seismic well ties including the preliminary wavelet extractions to get accurate output. Then, we have extracted of several classes of physical, geometrical and complex attributes as a direct hydrocarbon indicator to identify the gas zones, channels and chimneys and to identify the faults and discontinuities. The main contribution of this work is to provide a more detailed seismic reflection image supported by several seismic attributes classes and well logs to show a visual and quantitative evidence to identify the gas channels and gas chimneys with improving the detection of the faults and discontinuities. Keywords: Seismic attributes, Offshore nile delta, Gas channels, Gas bearing sand zones, Gas chimney, Well loggin

Test of a Threshold‐Based Earthquake Early‐Warning Method Using Japanese Data

Most of existing earthquake early‐warning systems are regional or on‐site systems. A new concept is the integration of these approaches for the definition of alert levels and the estimation of the earthquake potential damage zone (PDZ). The key element of the method is the real‐time, simultaneous measurement of initial peak displacement (P_d) and period parameter (τ_c) in a 3‐s window after the first P‐wave arrival time at accelerometer stations located at increasing distances from the epicenter. As for the on‐site approach, the recorded values of P_d and τ_c are compared to threshold values, which are set for a minimum magnitude M 6 and instrumental intensity I_MM VII, according to empirical regression analysis of strong‐motion data from different seismic regions. At each recording site the alert level is assigned based on a decisional table with four entries defined by threshold values of the parameters P_d and τ_c. A regional network of stations provides the event location and transmits the information about the alert levels recorded at near‐source stations to more distant sites, before the arrival of the most destructive phase. We present the results of performance tests of this method using ten M>6 Japanese earthquakes that occurred in the period 2000–2009 and propose a very robust methodology for mapping the PDZ in the first seconds after a moderate‐to‐large earthquake. The studied cases displayed a very good matching between the rapidly predicted earthquake PDZ inferred from initial P‐peak displacement amplitudes and the instrumental intensity map, the latter being mapped after the event, using peak ground velocity and/or acceleration, or from field macroseismic surveys

New tools for scientific learning in the EduSeis project: the e-learning experiment

The Educational Seismological Project (EduSeis) is a scientific and educational project, the main aim of which is the development and implementation of new teaching methodologies in Earth Sciences, using seismology as a vehicle for scientific learning and awareness of earthquake risk. Within this framework, we have recently been experimenting with new learning and information approaches that are mainly aimed at a high school audience. In particular, we have designed, implemented and tested a model of an e-learning environment in a high school located in the surroundings of the Mt. Vesuvius volcano. The proposed e-learning model is built on the EduSeis concepts and educational materials (web-oriented), and is based on computer-supported collaborative learning. Ten teachers from different disciplines and fifty students at the ITIS «Majorana» technical high school (Naples) have been taking part in a cooperative e-learning experiment in which the students have been working in small groups (communities). The learning process is assisted and supervised by the teachers. The evaluation of the results from this cooperative e-learning experiment has provided useful insights into the content and didactic value of the EduSeis modules and activities. The use of network utilities and the «Learning Community» approach promoted the exchange of ideas and expertises between students and teachers and allowed a new approach to the seismology teaching through a multidisciplinary study