Ortensia Amoroso

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.

S‐Wave Identification by Polarization Filtering and Waveform Coherence Analyses

High-resolution imaging with microseismic events requires the use of large and consistent data sets of seismic phase arrival times. In particular the S phase is important to derive physical parameters of the subsurface. Typically this phase is identified on one of the horizontal seismogram components by a change of signal amplitude and frequency as compared to the previous P phase. However, reliable S-phase identification can be difficult for local events because of a signal overlap with the P coda, the presence of converted phases, and possible S-wave splitting due to anisotropy. In this study we propose a new data processing technique aiming at un- iquely identifying the S-phase arrival using all available records from a seismic net- work. The technique combines polarization analysis of single three-component recordings of an event with analysis of lateral waveform coherence across the net- work. This makes it possible to construct seismic sections in which the first arrival is the S phase. This graphical representation can support an operator in both the analysis of single events and in semiautomatic analyses of large datasets. In addition, an automated stacking velocity analysis provides S-wave velocities from these sec- tions. We demonstrate the applicability of this technique using synthetic seismograms, and we evaluate the efficacy on a dataset of three-component velocimeter records from local earthquakes of the Campania-Lucania Apennines (southern Italy) recorded by the Irpinia Seismic Network (ISNet).

3D ultra-high resolution seismic imaging of shallow Solfatara crater in Campi Flegrei (Italy): New insights on deep hydrothermal fluid circulation processes

Seismic tomography can be used to image the spatial variation of rock properties within complex geological media such as volcanoes. Solfatara is a volcano located within the Campi Flegrei, a still active caldera, so it is of major importance to characterize its level of activity and potential danger. In this light, a 3D tomographic high-resolution P-wave velocity image of the shallow central part of Solfatara crater is obtained using first arrival times and a multiscale approach. The retrieved images, integrated with the resistivity section and temperature and the CO2 flux measurements, define the following characteristics: 1. A depth-dependent P-wave velocity layer down to 14 m, with Vp < 700 m/s typical of poorly-consolidated tephra and affected by CO2 degassing; 2. An intermediate layer, deepening towards the mineralized liquid-saturated area (Fangaia), interpreted as permeable deposits saturated with condensed water; 3. A deep, confined high velocity anomaly associated with a CO2 reservoir. These features are expression of an area located between the Fangaia, water saturated and replenished from deep aquifers, and the main fumaroles, superficial relief of the deep rising CO2 flux. Therefore, the changes in the outgassing rate greatly affect the shallow hydrothermal system, which can be used as a “mirror” of fluid migration processes occurring at depth.

A three‐dimensional Qp imaging of the shallowest subsurface of Campi Flegrei offshore caldera, southern Italy

To improve the knowledge of the shallowest subsurface of Campi Flegrei caldera, a 3-D P wave attenuation tomography of the area was performed. We analyzed about 18,000 active seismic traces, which provided a dataset of 11,873 Δt* measurements, e.g. the differential traveltimes to quality factor ratios. These were inverted through an adapted tomographic inversion procedure. The 3-D tomographic images reveal an average QP about 70, interpreted as water-saturated volcanic and marine sediments. An arc-like, low-QP structure at 0.5-1 km depths was interpreted as a densely fractured, fluid saturated rock volume, well matching the buried rim of Campi Flegrei caldera. The spatial distribution of high and low-QP bodies in the inner caldera is correlated with low-VP values and may reflect either the differences in the percentage of fluid saturation of sediments or the presence of vapor state fluids beneath fumaroles manifestations.

Accurate estimation of seismic source parameters of induced seismicity by a combined approach of generalized inversion and genetic algorithm: Application to The Geysers geothermal area, California

The accurate determination of stress drop, seismic efficiency and how source parameters scale with earthquake size is an important issue for seismic hazard assessment of induced seismicity. We propose an improved non-parametric, data-driven strategy suitable for monitoring induced seismicity, which combines the generalized inversion technique together with genetic algorithms. In the first step of the analysis the generalized inversion technique allows for an effective correction of waveforms for attenuation and site contributions. Then, the retrieved source spectra are inverted by a non-linear sensitivity-driven inversion scheme that allows accurate estimation of source parameters. We therefore investigate the earthquake source characteristics of 633 induced earthquakes (Mw 2-3.8) recorded at The Geysers geothermal field (California) by a dense seismic network (i.e., 32 stations, more than 17.000 velocity records). We find a non-self-similar behavior, empirical source spectra that require an ωγ source model with γ > 2 to be well fit and small radiation efficiency ηSW. All these findings suggest different dynamic rupture processes for smaller and larger earthquakes, and that the proportion of high frequency energy radiation and the amount of energy required to overcome the friction or for the creation of new fractures surface changes with earthquake size. Furthermore, we observe also two distinct families of events with peculiar source parameters that in one case suggests the reactivation of deep structures linked to the regional tectonics, while in the other supports the idea of an important role of steeply dipping faults in the fluid pressure diffusion.