W. Plastino

Hydrogeochemical anomalies in Kamchatka (Russia)

Abstract Data concerning discharge, melted iones and gases from underground waters obtained in Kamchatka during the last ten years are presented. Some hydrogeochemical parameters from a volcanic site reveal clear coseismic and postseismic anomalies on the occasion of five earthquakes with magnitude M > 6.5 that occurred at distances closer than 250 km from the measurement site. Hydrogeochemical parameters from a tectonic site show anomalies on the occasion of three of the above earthquakes. Such earthquakes are characterized by the highest values of the deformation parameter ϵ and the corresponding anomalies are preseismic ones, starting from 1 to 4 months prior to the earthquake. Also, some hydrogeochemical parameters from another tectonic site show clear preseismic anomalies on the occasion of just one of these earthquakes. This last one is located inland while the epicentres of the other earthquakes are well offshore. Models able to explain the phenomenologies we described are proposed.

Field strength variations of LF radio waves prior to earthquakes in central Italy

Abstract The electric field strength of the LF radio broadcasting RMC (Principality of Monaco) which operates at 216 kHz has been recorded since January 1991 by two receivers in central Italy. During the monitoring period we observed two evident attenuations of the field strength in one receiver, with durations of 6–10 days. The geomagnetic and ionospheric observations carried out in the same time interval do not seem able to explain the attenuation of the radio signal. An analysis of the seismic activity occurring in the area between transmitter and receiver has revealed that some days after the attenuations the energy released by earthquakes reaches a maximum. The observed attenuation might therefore be precursors of earthquakes. We also checked meteorological conditions and found that advections of warm air occurred during both the two anomalous periods. It seems possible that these conditions can help the action of preseismic effects in generating irregularities in the vertical gradient of the tropospheric radio refractivity able to produce defocusing of LF radiobroadcast propagation.

Helium content in thermal waters in the Caucasus from 1985 to 1991 and correlations with the seismic activity

Abstract A wide data set of groundwater helium content collected in the Caucasus from 1985 to 1991 has been analyzed. The strongest earthquakes that occurred in this area during the helium measurement period were the: Paravani (May 13th, 1986, M = 5.6), Spitak (December 7th, 1988, M = 6.9) and Georgian (April 29th, 1991, M = 6.9) earthquakes. The analysis of the helium content data revealed statistically significant increases in this parameter on the occasion of the Paravani and Spitak earthquakes and no increase on the occasion of the Georgian earthquake. These results corroborate those obtained by the analysis of scanty helium content data presented in previous papers. With the exception of the anomalous increases, the helium content data were subjected to randomness tests and spectral analysis and cross correlation with meteorological parameters and areal seismicity were also attempted. The main result obtained is that the destructive Spitak earthquake produced some statistical changes in the groundwater helium content fluctuations some years before and after its occurrence.

Anomalies in different parameters related to the M=3.9 Gran Sasso earthquake (1992)

Abstract The Gran Sasso chain is located in the Central Apennines and contains one of the largest aquifers of Central Italy. From 1986 up until now the most remarkable event was a seismic sequence that occurred on August 25, 1992 characterized by a M=3.9 earthquake, followed by several aftershocks with M≤2.0. Ten days before the main shock, the electromagnetic and seismoacoustic equipment set up in a natural cave of the Gran Sasso chain began to record anomalous acoustic signals and an anomalous decrease of the natural electric and magnetic emissions. Flow rates of three springs located at the northern border zone of the chain revealed in 1992 a clear anomaly in the characteristic cyclic trend. Tilt trend recorded at a measurement site located in the underground laboratories inside the chain also showed a marked anomaly in its characteristic annual trend. All these anomalies seem to be precursory phenomena of the earthquake described. We present a possible explanation for the observed phenomenology, in terms of the earthquake being induced by an anomalous hydraulic head variation in the Gran Sasso aquifer. This variation is thought to be connected with the rapid melting of the 2–3m mantle of snow on the Gran Sasso chain of mountains, provoked by an abrupt temperature increase.

Electromagnetic and seismoacoustic signals revealed in karst caves (Central Italy)

SummarySince 1988–89 equipment for detecting electric, magnetic and seismoacoustic signals has been running inside the Amare cave. The Amare cave is placed on the southern slope of the Gran Sasso chain, that is one of the largest karst areas of the Italian Apennines. In 1992, a similar equipment was installed inside the Cervo cave. This cave is located in another karst area of the Central Apennines, at about 50 km southwestwards of the Amare cave. In both these measurements sites, the signals are recorded every ten minutes in a digital form; the equipment is able to record signals, the frequency of which ranges from some hundred Hz to some hundred kHz. The data collected up to now seem to identify two different states that we call «quiet» and «perturbed» state. In the quiet state only electric and magnetic signals with the highest frequencies appear. These signals are connected with radio broadcastings and with the general lightnings activity of the Earth. A perturbed state is characterized by the sudden appearance of seismoacoustic signals coupled with electric and magnetic ones. This phenomenology is connected with local processes. Rainfall, atmospheric-pressure variations and some thermal effects are responsible for these local processes. A possible model is proposed to justify the observed phenomenology: micromovements of the limestone blocks that constitute the roof of the caves are invoked for the production of seismoacoustic signals. The electrification generated by these movements is invoked for the production of electric and magnetic signals.

A 2-D model for tilt and strain fields associated to earthquakes in crustal block structures

Abstract A 2-D model for slow crustal movements, including tilt and strain anomalies before earthquakes as well as shape and time lag of each preseismic anomaly with respect to the time of origin of the corresponding earthquake, is proposed. The model represents a numerical study to evaluate the behavior of a series of crustal rigid blocks separated by narrow fault zones filled with viscoelastic material. Rheology of the fault material is described by constitutive equations of standard linear solid. Vertical displacement and rotation of the blocks result from the vertical uplift of one (central) block due to a vertical force. Displacements and tilts of blocks are computed from a system of linear differential equations which are solved numerically. The main characteristic feature of the model is that the viscoelastic parameters of the fault zone, where fracture occurs (at the boundary of the rising block), may change in time, simulating the earthquake preparation. Numerical modelling shows that tilt behavior at successive blocks reveals anomalies which are similar in shape to the observed ones. The anomalies are of opposite signs in adjacent blocks, and show a time lag with distance from the fracturing zone.