Antoni M. Correig

Attenuation and source parameters at Deception Island (South Shetland Islands, Antarctica)

Deception Island is the most active volcano of the South Shetland Islands-Antarctic Peninsula group, experiencing eruptions in 1967, 1969 and 1970. Local attenuation derived from coda analysis and source parameters derived from Brunes model, for well located seismic events, have been studied in order to complement the available geophysical information. Results show abnormally lowQ0 values and an abnormally high frequency dependence, as well as large dispersion. These factors are strongly dependent on the path travelled by the seismic wave. Retrieved values of the source parameters (stress drop, seismic moment and source radius), are again abnormally low compared to world-wide average values, for example, those obtained for the Oroville, California aftershock series between June and September, 1976. These results are consistent with some aspects of the geology of Deception Island, such as the very high degree of fracturing and faulting, and the existence of a strong hydrothermal alteration affecting most of the subaerial volcanic rocks. Moreover, the pattern defined for the lateral variations ofQ0 shows minimum values in the inner bay of the island, close to the most recent eruption vents. A large reduction in spectral amplitudes over a particular frequency range occurs in several observations, corresponding to the path crossing the zone of highest attenuation. This observation suggests the existence of a hot magmatic intrusion produced during the most recent eruption, and coincides with the superficial low density mass distribution obtained from the gravimetric model and the long wave magnetic field component obtained from magnetic surveys. The width of this intrusion is estimated to be about 200 m, in agreement with the previous results obtained analyzing residuals of the location of seismic events.

Aftershock series of event February 18, 1996: An interpretation in terms of self‐organized criticality

An aftershock interevent time series, initiated on February 18, 1996, in the eastern Pyrenees was analyzed. The threshold detection magnitude was set at 1.9, and the series was assumed to be complete for an interval of 77 days. The original time series does not fit Omoris law, probably because of sudden changes in the rate of occurrence, interpreted as an increase in the production rate. When the recorded interevent time series is classified in terms of leading aftershocks (those that satisfy a relaxation process) and cascades (those occurred at a nearly constant rate), the new time series of the leading aftershocks fits Omoris law quite well, with p = 0.94. Interpreted in terms of Dietrichs model, the series of leading aftershocks correctly predicts a return time for the main shock of the order of 50 years. To interpret the series of cascades, a minimalist, self-organized critical model was used. Although it is very simple, the model correctly reproduces the two-level structure in the observed time series, that is, the sequence of leading aftershocks and a cascade sequence emerging from each aftershock. This model may be given physical justification in terms of the Cotchard and Madariaga [1996] nucleation model.

A model for complex aftershock sequences

The decay rate of aftershocks is commonly very well described by the modified Omori law, n(t) ∝ t−p, where n(t) is the number of aftershocks per unit time, t is the time after the main shock, and p is a constant in the range 0.9 < p < 1.5 and usually close to 1. However, there are also more complex aftershock sequences for which the Omori law can be considered only as a first approximation. One of these complex aftershock sequences took place in the eastern Pyrenees on February 18, 1996, and was described in detail by Correig et al. [1997]. In this paper, we propose a new model inspired by dynamic fiber bundle models to interpret this type of complex aftershock sequences with sudden increases in the rate of aftershock production not directly related to the magnitude of the aftershocks (as in the epidemic-type aftershock sequences). The model is a simple, discrete, stochastic fracture model where the elements (asperities or barriers) break because of static fatigue, transfer stress according to a local load-sharing rule and then are regenerated. We find a very good agreement between the model and the Eastern Pyrenees aftershock sequence, and we propose that the key mechanism for explaining aftershocks, apart from a time-dependent rock strength, is the presence of dynamic stress fluctuations which constantly reset the initial conditions for the next aftershock in the sequence.

Attenuative body wave dispersion at La Cerdanya, eastern Pyrenees

Abstract Coda- Q for P- and S-waves has been measured from digitally recorded events occurring in the La Cerdanya region of the eastern Pyrenees. Interpreted in terms of a power law, Q ( f ) = Q 0 f η , Q-coda for P-waves is characterized by Q 0 = 14 and η = 1.07, and S-waves by Q 0 = 14 and η = 1.13. Using a generalization of a model due to Dainty (1981), we obtain a Q model for S-waves in which intrinsic- Q is 23, the frequency dependence (ζ) of intrinsic- Q is 1.17, and the turbidity factor is 0.051. Interpreted in terms of a continuous relaxation model, where Q m is minimum Q , and τ 1 and τ 2 are high- and low-frequency cutoffs, respectively, the values of the parameters are Q m = 5 and τ 1 = 0.37 when τ 2 is assumed to be 10 000. Body wave dispersion, as computed from the differences in arrival times of the wave filtered at 3, 6, 12 and 24 Hz relative to that at 6 Hz has been measured and found to range from 0.067 at 3 Hz to −0.075 at 24 Hz. This dispersion constrains τ 2 to be 43.

Attenuation of Rayleigh waves across the volcanic area of the Massif Central, France☆

Abstract Rayleigh wave attenuation is investigated for periods ranging from 20 to 90 s, along a 450 km-long profile following the Oligocene tensile zone of the French Massif Central. A model is deduced by inversion, assuming that the S-wave intrinsic quality factor Q β is frequency-independent, and yields a mean value Q β = 43 ± 10 for the first 100 km in the upper mantle. This value, far lower than the mean value obtained in Eurasia, is close to those obtained in other recent tensile areas, e.g., the western United States or mid-oceanic ridges. A velocity-depth model for S-waves, deduced in a previous study from surface-wave propagation, has been corrected for the attenuation effect. We find a discrepancy between the corrected S-model and P-wave residuals in the same area, implying that Q β must be frequency-dependent. This can be a clue for partial melting in the upper mantle beneath this region.

Regional variation of Rayleigh wave attenuation coefficients in the Eastern Pacific

Seismograms recorded for five earthquakes on the east Pacific rise have been analyzed to obtain the attenuation coefficients of the fundamental Rayleigh mode for the eastern Pacific in the 15–110 second period range. The attenuation coefficients have been obtained using two new methods, a reference-station method, and an iterative method by which the seismic moment and regionalized attenuation coefficient values are obtained simultaneously after considering the effect of the source directivity and time-function. The reference-station method was applied to the entire eastern Pacific, excluding paths along the east Pacific rise. When using the iterative method we divided the eastern Pacific into three sub-regions, designated as the north-eastern Pacific, the Nazca plate and the east Pacific rise. Although much scatter is present, the data suggest that attenuation coefficients for the Nazca plate are higher than those for the northeastern Pacific, and both are substantially higher than average values obtained for the entire Pacific Ocean. Two paths that lie along or near the east Pacific rise are characterized by especially high attenuation coefficients. These values suggest that a low-Q zone exists beneath that narrow feature.

Spatio-temporal seismicity patterns using mutual information application to southern Iberian peninsula (Spain) earthquakes

Abstract A quantitative method to characterize the spatio-temporal evolution of the seismic activity of a region for a given time interval is proposed in this paper. The epicentral region is divided in equal area parallelograms that we call “cells”. A probability P i is assign to every cell depending on its seismic activity level. The evolution of the mutual information, based on these probabilities, allows us to fit the optimal value for P i ’s. The analysis results show that is possible to monitor the change of seismic activity quantitatively. We show that a propagation model based on cross template model (CT model) give us a criteria whether or not an area (whose size β is determined by the model mathematically) is going to be considered seismically activated in a period of time τ (whose value is also the result of the application of the method). Our method is applied to the data of Southern Iberian Peninsula seismicity including more than 10,000 earthquakes: data available from the Andalusian Seismic Network is used from 1985 to 1995.

Body-Wave Dispersion: Measurement and Interpretation

Generalizing previous studies on short-period data, it is shown that body-wave dispersion can be measured from broad-band records of earthquakes of moderate magnitude. The method is based on the direct measurement of the arrival time of the frequency components of a seismic wave, and the arrival time is defined by its expectation value. The frequency components of the signal are obtained through a narrow band-pass filtering process. Previous to any interpretation, a correction of the arrival time for instrument response and group delay of the filter is needed. In the first step, body-wave dispersion is related to an absorption band to account for intrinsic attenuation, and thereafter we generalize this interpretation by considering a cascade of filters to account for medium parameters (attenuation and a layered crust) and source parameters (source time function and finiteness of fault). An inversion scheme to obtain the filter parameters can be devised by following, in a formal way, the same procedure as for the case of surface wave dispersion.

On the measurement of body wave dispersion

A method is devised to measure body wave dispersion in terms of the arrival time of a narrow band-pass filtered signal. The arrival time is defined as the expectation of the arrival time of the wave front minus half the duration of the first pulse. Body wave dispersion is related to an absorption band, and its transfer function is found. By interpreting the absorption band as a filter and identifying the dispersion as the (frequency dependent) group delay of the filter the measured dispersion is interpreted as due to a cascade of filters composed of the following elements: intrinsic attenuation, source time function, and finiteness of the fault.

Lateral variations of attenuation coefficients, group and phase velocities of rayleigh waves in Europe

Abstract Rayleigh wave group and phase velocities and attenuation coefficients are investigated in the 15–60-s period range for earthquakes occurring in the European area and registered at European stations. Observed group and phase velocities show clear differences in the period range 20–40 s, whereas attenuation coefficients are differentiated from 45 s to larger periods. From the shape of dispersion and attenuation curves, two broad zones have been isolated and designated as Western Europe and Southeastern Europe. Although some scatter is present, specially for attenuation, the data suggest that in general the shear velocity and Q β values are higher for Southeastern Europe than for Western Europe. For Southeastern Europe there is a channel of low velocity and low Q β , apparently correlated, located at the lower crust: in both regions there is a channel of low velocity and low Q β values located at the uppermost mantle. In each case there also exists a channel of high Q β values located at the lower crust but at slightly different depths.