Aldo Zollo

Earthquake magnitude estimation from peak amplitudes of very early seismic signals on strong motion records

[1] We show that the low-pass filtered, peak amplitudes of initial P- and S-wave seismic signals recorded in the vicinity of an occurring earthquake source correlates with the earthquake magnitude and may be used for real-time estimation of the event size in seismic early warning applications. The earthquake size can be therefore estimated using only a couple of seconds of signal from the P- or S-wave onsets, i.e. while the rupture itself is still propagating and rupture dimension is far from complete. We argue that dynamic stress release and/or slip duration on the fault in the very early stage of seismic fracture, scales both with the observed peak amplitude and with the elastic energy available for fracture propagation. The probability that a fracture grows to a larger size should scale with the energy initially available. Citation: Zollo, A., M. Lancieri, and S. Nielsen (2006), Earthquake magnitude estimation from peak amplitudes of very early seismic signals on strong motion records, Geophys. Res. Lett., 33, L23312, doi:10.1029/ 2006GL027795.

Seismic Evidence for a Low-Velocity Zone in the Upper Crust Beneath Mount Vesuvius

A two-dimensional active seismic experiment was performed on Mount Vesuvius: Explosive charges were set off at three sites, and the seismic signal along a dense line of 82 seismometers was recorded. A high-velocity basement, formed by Mesozoic carbonates, was identified 2 to 3 kilometers beneath the volcano. A slower (P-wave velocity VP ∼ 3.4 to 3.8 kilometers per second) and shallower high-velocity zone underlies the central part of the volcano. Large-amplitude late arrivals with a dominant horizontal wave motion and low-frequency content were identified as a P to S phase converted at a depth of about 10 kilometers at the top of a low-velocity zone (VP < 3 kilometers per second), which might represent a melting zone.

Geophysical and geochemical modelling of the 1982–1984 unrest phenomena at Campi Flegrei caldera (southern Italy)

Abstract Based on both geophysical and geochemical data collected during the 1970–1972 and 1982–1984 volcano-seismic crises at Campi Flegrei caldera, the possible interpretative models of the events are reviewed and discussed. It is shown that models involving exclusively an overpressure in the magma chamber cannot reconcile all the data sets and that convective heat transfer to confined shallow aquifers may be an important process contributing to unrest phenomena at Campi Flegrei.

Earthquake early warning system in southern Italy: Methodologies and performance evaluation

We investigate the effect of extended faulting processes and heterogeneous wave propagation on the early warning system capability to predict the peak ground velocity (PGV) from moderate to large earthquakes occurring in the southern Apennines (Italy). Simulated time histories at the early warning network have been used to retrieve early estimates of source parameters and to predict the PGV, following an evolutionary, probabilistic approach. The system performance is measured through the Effective Lead-Time (ELT), i.e., the time interval between the arrival of the first S-wave and the time at which the probability to observe the true PGV value within one standard deviation becomes stationary, and the Probability of Prediction Error (PPE), which provides a measure of PGV prediction error. The regional maps of ELT and PPE show a significant variability around the fault up to large distances, thus indicating that the systems capability to accurately predict the observed peak ground motion strongly depends on distance and azimuth from the fault.

Real-time evolutionary earthquake location for seismic early warning

An effective early-warning system must provide probabilistic estimates of the location and size of a potentially destructive earthquake within a few seconds after the event is first detected. In this work we present an evolutionary, real-time location technique based on an equal differential time (EDT) formulation and a probabilistic approach for describing the hypocenter estimation. The algorithm, at each timestep, relies on the information from triggered arrivals and not-yet-triggered stations. With just one recorded arrival, the hypocentral location is constrained by the Voronoi cell around the first triggering station constructed using the travel times to the not-yet-triggered stations. With two or more triggered arrivals, the location is constrained by the intersection of the volume defined by the Voronoi cells for the remaining, not-yet-triggered stations and the EDT surfaces between all pairs of triggered arrivals. As time passes and more triggers be- come available, the evolutionary location converges to a standard EDT location. Synthetic tests performed using the geometry of the Irpinia seismic network, southern Italy (ISNet), and the simulation of an evolutionary location for the 2000 Mw 6:6 Western Tottori, Japan, earthquake indicate that when a dense seismic net- work is available, reliable location estimates suitable for early-warning applications can be achieved after 1-3 sec from the first event detection. A further simulation with an Mw 6:7 southern Greece earthquake shows that at a regional scale, the real-time location can provide useful constraints on the earthquake position several seconds before a non-real-time algorithm. Finally, we show that the robustness of the algorithm in the presence of outliers can be effectively used to associate phase arrivals coming from events occurring close in time, and we present a preliminary algorithm for event detection.

A seismological study of the 1835 seismic gap in south-central Chile

We study the possible seismic gap in the Concepcion–Constitucion region of south-central Chile and the nature of the M = 7.8 earthquake of January 1939. From 1 March to 31 May 1996 a seismic network of 26 short period digital instruments was deployed in this area. We located 379 hypocenters with rms travel time residuals of less than 0.50 s using an approximate velocity distribution. Using the VELEST program, we improved the velocity model and located 240 high precision hypocenters with residuals less than 0.2 s. The large majority of earthquakes occurred along the Wadati–Benioff zone along the upper part of the downgoing slab under central Chile. A few shallow events were recorded near the chain of active volcanos on the Andes; these events are similar to those of Las Melozas near Santiago. A few events took place at the boundary between the coastal ranges and the central valley. Well constrained fault plane solutions could be computed for 32 of the 240 well located events. Most of the earthquakes located on the Wadati–Benioff zone had “slab-pull” fault mechanism due to tensional stresses sub-parallel to the downgoing slab. This “slab-pull” mechanism is the same as that of eight earthquakes of magnitude around 6 that are listed in the CMT catalog of Harvard University for the period 1980–1998. This is also the mechanism inferred for the large 1939 Chilean earthquake. A very small number of events in the Benioff zone had “slab-push” mechanisms, that is events whose pressureaxis is aligned with the slab. These events are found in double layered Wadati–Benioff zones, such as in northern Chile or Japan. Our spatial resolution is not good enough to detect the presence of a double layer, but we suspect there may be one.

An image of Mt. Vesuvius obtained by 2D seismic tomography

Abstract A high-resolution seismic tomography of Mt.Vesuvius was started in May 1994, with the aim of reconstructing the detailed shallow crustal structure underneath the volcano and define its feeding system. The first phase of the experiment was to perform a 2D profile, using three underground explosions as active sources. Data from controlled sources and microearthquakes were jointly used to determine the shallow structure of the volcano. A high-velocity body (Vp=3.5–4 km/s) was identified at about 2 km beneath the Somma-Caldera. It is likely to represent a sub-volcanic structure, formed by a dense network of solidified dikes. A prominent converted P-to-S phase at about 10 km of depth indicates the occurrence of a sharp transition to a very low-velocity zone. This may represent the top of an extended magmatic reservoir.

Seismic Sources and Attenuation Properties at the Campi Flegrei Volcanic Area

AbstractMicroearthquake digital data collected at Campi Flegrei during the recent (1982–1985) ground uplift episode have been analyzed in order to infer source and medium seismic properties. The main results obtained from these analyses are:1.Hypocenter distribution and the size of the seismic zone do not change with time and do not depend on the ground uplift rate. Events occurred clustered in time with no simple causal relations between the cluster occurrences and their energy.2.Anelastic attenuation does not depend strongly on frequency, showing a constant pattern at high frequencies. The observed values of low and high frequency attenuation, due to the short source receiver distances, do not seriously affect the spectral content of signals radiated by the sources.3.A constant Brune stress drop pattern (∼4–5 bars) as a function of seismic moment is observed. This indicates that the manner of fracturing is almost independent on magnitude of earthquakes (hypothesis of self-similarity (Aki, 1967)). Seismic processes in a prefractured medium can explain the observed small stress drop values.4.Focal mechanisms from moment tensor estimates show that radiation patterns are mostly well interpreted in terms of double couple source models.5.The scaling of peak ground motion parameters (Amax andVmax vs seismic moment) can be explained by an ω2 source model (constant stress drop) multiplied by an exponential function with a small decay parameter, which takes into account the measured attenuation. These results support the hypothesis of earthquakes generated by simple shear fractures along prefractured structures as a response to changes in the stress field due to the ground deformation.

Accurate fault mechanism determinations for a 1984 earthquake swarm at Campi Flegrei caldera (Italy) during an unrest episode: Implications for volcanological research

We have analyzed 42 microearthquakes (1<ML <3) that occurred at Campi Flegrei volcanic area during an intense swarm-like activity (more than 500 events in half a day) in the last period of strong ground uplift (1982–1984). Focal mechanisms of these earthquakes have been computed by a probability method, which uses P wave polarity and S wave polarization data. Composite focal mechanisms for different groups of earthquakes have also been computed by the same technique. Results show a well-defined fault plane, in agreement with the elongation of the epicentral pattern. The orientation and dip of this fault plane, together with other observations about hypocenter locations and focal mechanisms of the whole seismicity accompanying the ground uplift episode, suggest the presence of an elliptical fracture system at the caldera center. This fracture system, dipping toward the center of the caldera, should represent the limit of the most recent caldera collapse, which is also indicated by a negative Bouguer gravity anomaly. Moreover, an important differential feature among various groups of earthquakes has been shown, namely, a rotation up to about 30° of the strike of the compressive stress axis, between the easternmost and the westernmost events. The resulting pattern indicates that the axis is always oriented towards the town of Pozzuoli, which roughly represents the very center of the caldera, and also the center of symmetry of the ground deformation. These observations strongly suggest that earthquakes are generated by the variation of the stress field associated with the deformation, along prefractured zones, because of their greater weakeness. This study provides, then, the first direct evidence that both seismicity and ground deformations have a common origin.

Source parameters and three‐dimensional attenuation structure from the inversion of microearthquake pulse width data: Qp imaging and inferences on the thermal state of the Campi Flegrei caldera (southern Italy)

The three-dimensional P wave attenuation structure of the Campi Flegrei caldera and the estimate of source parameters for 87 local microearthquakes is obtained by the nonlinear inversion of pulse width and rise time measurements by using the method described by Zollo and de Lorenzo (this issue). Source radii represent the better resolved parameters with values ranging from 70 m to 230 m; the dip and strike angles defining fault orientations are usually affected by larger uncertainties and are well constrained only for 11 events. The dip fault is usually confined in the range 30°–60° (with an average uncertainty of 12°); the fault strikes mainly range between −60° and 60° and seem to define preferential directions oriented radially from the symmetry axis of the ground deformation. Stress drop estimates indicate rather low values (0.01–1 MPa) which suggest low strength properties of the incoherent and brittle materials filling the caldera (primarily yellow tuffs). The three-dimensional Qp images obtained from the inversion of P pulse durations show two significant low-Qp anomalies between 0 and 1 km of depth, in the north-eastern sector and at 2–3 km of depth in the central eastern sector of the caldera. The high degree of spatial correlation of the low-Qp zone and low-Vs (as inferred by Aster and Meyer (1988)) at 0–1 km in depth and other geophysical and geochemical observations suggest that this anomaly can be related to the presence of densely fractured, porous, and fluid-filled rocks in the NE sector of the caldera. The deeper low-Qp anomaly is interpreted as being related to a dominant thermal effect. We used the surface and deep borehole temperature measurements available in the area to obtain a local calibration curve to convert Qp in temperature at Campi Flegrei. The retrieved T(Qp) map shows a high thermal deep disturbance (450°–500°C) at depths between 2 and 3 km in the eastern sector of the caldera, where the most recent eruptive activity is concentrated. The present-day temperature field retrieved by Qp images has been interpreted by using a three-dimensional thermal conduction model assuming an extended heat source (initial temperature of 800°C) located underneath the attenuation anomalous region. The results indicate that the Qp-inferred temperature field can be related to the heat conduction effect of one or more molten bodies whose top should be at about 4-km depth, consistent with recent seismic estimates of the magma chamber top at Campi Flegrei (Ferrucci et al., 1992). This study suggests that the present thermal state and rock rheology of the inner caldera could be controlled by the cooling of molten bodies that originally intruded at depths of 1.4–1.6 km, during one or more recent (time of <10 kyr) eruptive events.