An experimental study of the dynamic features of shadow areas of caustics in response to loading/unloading fracture

IntroductionCHEN (1981), GENG, et al (1980), YAO (1981) and others have found out that different stress paths can lead to fracture. They have further studied the precursory features of wave velocity ratio, acoustic emission, and so on, in this situation. YIN (1987), YIN and YIN (1991), Yin, et al (1994, 1995, 2000) have put forward the concept of loading-unloading response ratio. When a mechanical test system is in a stable state, the path of its response to loading is the same as that…     

加载破裂和卸载破裂的焦散阴影区动态特征的实验研究

以实时全息干涉计量方法和焦散阴影光学法，用有机玻璃试样直观、形象地研究加载过程和卸载过程破裂时的应力（应变）场、塑性区或成核区（阴影区）不同地动态特征变化．结果发现，在低应力时，预制裂缝端点先出现应变核（条纹密集区），进而形成焦散阴影区．这些核和阴影区会随荷载增加变大，随卸载而减小．在一定的压力范围内重复加载卸载（循环加载），焦散阴影区则反复增大缩小；但在高应力时，尤其在将出现大破裂时，加载和卸载的焦散阴影区变化是不可逆的，其变化也完全不同．加载破裂时的焦散阴影区由慢到快迅速扩展；而卸载时的焦散阴影区则扩展较慢，并且在尖角形前方有一圆形阴影．      

Thermal-mechanical Coupling in Shear Deformation of Viscoelastic Material as a Model of Frictional Constitutive Relations

-- We propose a thermal-mechanical model of shear deformation of a viscoelastic material to describe the temperature-dependence of friction law. We consider shear deformation of one-dimensional layer composed of a Maxwell linear viscoelastic material under a constant velocity V and temperature T_w at the boundary. The strain rate due to viscous deformation depends both on temperature and shear stress. The temperature inside the layer changes owing to frictional heating and conductive cooling. Steady-state calculations show that the sign of d†^ss/dV, where †^ss is steady-state stress, changes from positive to negative as V increases, and that the threshold velocity above which the sign of d†^ss/dV is negative increases with increasing T_w. These results are in accordance with the conjecture that the downdip limit of seismogenic zones is marked by the transition in the sign of d†^ss/dV due to temperature rise with depth. We also find that the response of steady state to a step change in V is quite similar to the response of frictional slip with constitutive laws which employ state variables. These findings suggest that by further improving the present model a model of constitutive relations along faults or plate boundaries can be developed which contains temperature-dependence in a physically-sound manner.     

Earthquake Hazard Parameters in Crete Island and its Surrounding Area Inferred from Bayes Statistics: An Integration of Morphology of the Seismically Active Structures and Seismological Data

-- The study addresses the evaluation of earthquake hazard parameters such as maximum regional magnitude (M_max) and the slope of Gutenberg-Richter law # (where b=# log e) for the Hellenic Wadati-Benioff zone and the overriding lithospheric plate in the area of Crete and its surroundings. The seismicity of the area is divided in a cellular (1.0° 2 1.0°) manner allowing analysis of the localized earthquake hazard parameters and graphical representation of their spatial variation. Our approach incorporates the recently updated earthquake catalogue for Greece and the adjacent areas, the consideration of the morphology of the deep seismically active structures in the studied area and use of a probabilistic procedure for estimating the earthquake hazard parameters.¶One of the main inconsistencies in the earthquake hazard assessment is the estimation of the maximum magnitude and the related uncertaint y. The Bayesian approach, applied in the present, is a straightforward technique for evaluating the earthquake hazard parameters and is based on the following assumptions: Poissonian character of seismic events flow, a frequency-magnitude law of Gutenberg-Richter's type with cutoff maximal value for estimated parameter and a seismic catalogue, having a rather sizeable number of events (i.e., 50 events at least per cell). For five cells in which the number of events is less than 50, an effort is made to produce synthetic data. The re-assessed parameters obtained from the synthetic data show no significant difference and the real data (of the five cells) are finally taken into account although the estimated uncertainty is high.¶For four random cells we constructed hazard curves showing the probabilities that a certain magnitude M will be exceeded in one year and the return periods (in years) that are expected for a given magnitude. These are particularly useful for the mapping of earthquake hazard in regions of either low or high seismic activity, as is Crete and the adjacent area.¶The obtained results show that the W and E parts of both subducting and overriding plates differ in the spatial distribution of all the estimated earthquake hazard parameters. The M_max distribution indicates strong coupling between the western portions of the interacting plates (M_max > 6.3) to the south of 36°N. The smaller values of M_max (M_max < 6.3) estimated in the SE part of the studied area indicate weak coupling between the eastern portions of the subducting and overriding plates.¶Values of b > 1.0 are found to the south and east of Crete for the Wadati-Benioff zone, and over the central part of the island and the area to the northeast of it (cell 11) for the continental wedge, which suggests nonuniform stress field and/or heterogeneous material.     

Relocation of Swarm Events in the June 1987 Earthquake Sequence from the Lake Aswan Area in Egypt

--We have examined the digital waveform data and relocated a number of events within the June 1987 earthquake swarm, which occurred beneath the northern part of Lake Aswan, 70 km southwest of the Aswan High Dam in Egypt. This swarm occurred between June 17th and 19th with a maximum magnitude event of "M_L"=3.5.¶Cross correlation between a chosen master and the analyzed events has been carried out on seismograms from stations of the Aswan network. The cross correlation demonstrates the presence of a difference in both the P wave ((t_p) and the S wave ((t_s) arrival times at each station in the network relative to the arrival times of the master event at the same stations. (t_p ranges between т.15 and 0.11 second, while (t_s ranges between т.17 and 0.11 second.¶The primary interpretation is that the se time differences represent an error in the manually picking arrival times. Then, (t_p and (t_s values for each event result from a change in the hypocentral parameters from those of the master event, assuming the P- and S-wave velocity distribution remains constant during the swarm activity. This interpretation enables us to determine the relative distribution of hypocenters with respect to the hypocentral location of the master event. We present the results from a swarm of 9 events demonstrating they originate from a nearly unique location, rather than the zone identified from the preliminary locations which used manually picked onset times.     

A study on relation between acoustic emission and characteristic displacement field on the sample with multi en echelon structures——The theoretic and experimental explorations of strain gap

Introduction ZHOU, et al (1994) collected and investigated periods of level deformation data from 1954 to 1992, and gained sequence of a picture per year (or several years) that vividly showed the varia-tion of the deformation field of Tangshan earthquake source area around the Tangshan diamond block which was 21.3. They indicated that the Tangshan earthquake source area showed de-formation gap and the hard solid characteristic of relative stabilization or relative closedown. In the investig…     

Changes in the photosynthetic coefficients α and Pm of Planktothrix rubescens grown on light-dark cycles

The relationship between photosynthesis and irradiance was investigated in a strain of the cyanobacterium Planktothrix rubescens isolated from Lake Zürich, using cultures grown on a 12:12-h light-dark cycle. From the photosynthesis-irradiance (P/I) curves, values of the light-affinity coefficient, f, and the maximum rate of photosynthesis, P_m, were determined: in different cultures f ranged from 0.02-0.08 7mol mg^-1 h^-1 (7mol m^-2 s^-1)^-1 and P_m from 0.6-2.6 7mol mg^-1 h^-1 (rates of O₂ production related to dry biomass). Comparisons made at different phases of the light-dark cycle in individual cultures showed that in the first hour of the light phase f rose by an average of 21 % and P_m by 7 %. In cultures that had been grown in many light-dark cycles, however, the rise occurred only if light was given at the beginning of the notional light phase; cultures kept in darkness showed no rise. It is concluded that the observed increases in f and P_m after dawn were in response to the exogenous irradiance rather than due to endogenous rhythms. These findings are discussed in relation to diel variations in the photosynthetic coefficients of natural populations of P. rubescens in Lake Zürich.     

A study on relation between acoustic emission and characteristic displacement field on the sample with multi en echelon structures — The theoretic and experimental explorations of strain gap

Based on the phenomena that the deformation gap was observed before the great Tangshang earthquake, this paper discusses the strain gap according to test and theory. The (strain) patterns were recorded photographically by realtime holographic interferometry and shadow optical method of caustics, as soon as the loading process started. In the meantime, the AE (acoustic emission) signals were recorded by a micro crack information storage-analysis system. According to damage theory and location of micro fracture, we have studied the stain gap and gained: a) It is necessary that strain gap appears under the condition of linear elasticity theory, and its situation is relatively stable, corresponding to stress concentration. b) Micro fractures, which appear initially at area of high stress, occur rarely at the strain gap, and their locations are finally in the zone between the stress concentration area and the strain gap, which indicate the clusters or groups. However, the major macro fracture (final rupture) started from the shadow areas, and then grew quickly towards the strain gaps, which resulted in failure of sample. Foundation item: The Dual Project of China Seismological Bureau (9691309020301), the Specialized Funds for National Key Basic Study (G1998040704), the project for the MOST under contract (2001BA601B02) and Youth Funds for applied basic study of the Science and Technology Bureau of Yunnan Province (98D019Q).     

Geophysical Applications of Multidimensional Filtering with Wavelets

--We present imaging results in geophysics based on using multidimensional Gaussian wavelets as a filter in a 2-D Cartesian domain. Besides decomposing the field into various distinct lengthscales, we have also constructed the 2-D maps describing the spatial distributions of the maximum of the wavelet-transformed L₂-norm E_max (x,y) and its corresponding local wavenumber k_max (x,y), where x and y are the Cartesian coordinates. For geoid anomalies, using a wavelet filter extending to 90 degrees, we have discerned the distinct outlines of convergent and divergent tectonic zones and have conducted a quantitative comparison of the short-wavelength gravitational anomalies at those wavelengths between two different geographical locations. We have also compared the wavelet results with a nonlinear bandpass filter in the spectral domain where a Gaussian filter with the logarithm of the degree l acting as the argument has been employed. A wavelet solution, with a length-scale corresponding to 256 degrees, would need a filter with over 400 spherical harmonics centering around l=157 for an optimal spatial fit. The computational effort with the bandpass filter technique greatly exceeds those associated with wavelets. We have also shown the ability of the wavelets to analyze the vastly different scales present in high Rayleigh number convection and the mixing of passive heterogeneities driven by thermal convection. Wavelets will be a useful tool for rapid analyzing of the large multidimensional fields to be captured in many other geophysical endeavors, such as the upcoming gravity satellite missions and satellite radar interferometry images.     

On the Generation of ULF Magnetic Variations by Conductivity Fluctuations in a Fault Zone

v--vPrior to the October 18, 1989 Loma Prieta M_s 7.1 earthquake, Fraser-Smith et al. (1990) recorded a 10-100 fold increase in ultra-low frequency (ULF) magnetic fields near the earthquake epicenter. Several mechanisms for generation of these ULF fields by fluid flow in the earth have been advanced, but all appear to require unrealistic fluid velocities or hydraulic permeabilities to match the observations. As an alternative explanation, Merzer and Klemperer (1998) proposed that the increase in ULF magnetic fields could result from induced electric currents flowing in a fault-zone made temporarily much more electrically conductive by stress-induced reorganization of pore geometry. Using a numerical model we show that while this mechanism could produce a significant increase in ULF variations, mutual induction between the fault zone and the surrounding crust would probably limit the amplitude increase to levels well below those observed at Loma Prieta. We consider a variant on this quasi-static conductive fault zone model in which low frequency telluric currents are modulated by small higher frequency variations of bulk fault zone conductivity. We show that because the spectrum of natural EM variations is red, substantially larger relative increases in ULF magnetic fields could be produced by this mechanism with even small conductivity fluctuations at these frequencies. These variations would be easy to detect with a well-designed experiment, if they occurred. In principle this mechanism could explain the Loma Prieta ULF observations, however the magnitude of conductivity fluctuations that would be required to match the very large reported amplification factors still appears to be too large to be physically plausible.     

Characteristics of Cloud Drop Spectra at Different Levels and with Respect to Cloud Thickness

--Measurements on drop size spectra were made in cumulus clouds over Pune (inland) region on many days during the summer monsoon seasons. In this paper, the measurements in non-raining cumulus clouds made in the years 1984, 1985 and 1986 at different levels and for different cloud thickness have been studied. In general, the drop size spectra broadened with height and the concentration of drops with diameter > 50 wm (N_L), mean volume diameter (MVD), liquid water content (LWC) and dispersion increased with height while the concentration of drops with diameter < 20 wm (N_S) and the total concentration of drops (N_T) decreased with height. The average drop size distributions were unimodal at the lower levels while they were bimodal at the higher levels. High water contents were confined to drops in the size range 5-25 wm at both higher and lower levels. The average drop size spectra were broader and N_L, LWC, MVD and dispersion greater while N_T and N_S smaller for thicker clouds (range of vertical extent 1.1-2.1 km) as compared to those for thinner clouds (range of vertical extent 0.3-1.1 km). Water contents for the drops > 28 wm were higher while those for the drops > 28 wm lower in thicker clouds than in thinner clouds. The average drop size distributions were bimodal in the former case, while they were unimodal in the other case.     

Optimized Seismic Threshold Monitoring – Part 1: Regional Processing

v--v Continuous seismic threshold monitoring is a technique that has been developed over the past several years to assess the upper magnitude limit of possible seismic events that might have occurred in a geographical target area. The method provides continuous time monitoring at a given confidence level, and can be applied in a site-specific, regional or global context.¶In this paper (Part 1) and a companion paper (Part 2) we address the problem of optimizing the site-specific approach in order to achieve the highest possible automatic monitoring capability of particularly interesting areas. The present paper addresses the application of the method to cases where a regional monitoring network is available. We have in particular analyzed events from the region around the Novaya Zemlya nuclear test site to develop a set of optimized processing parameters for the arrays SPITS, ARCES, FINES, and NORES. From analysis of the calibration events we have derived values for beam-forming steering delays, filter bands, short-term average (STA) lengths, phase travel times (P and S waves), and amplitude-magnitude relationships for each array. By using these parameters for threshold monitoring of the Novaya Zemlya testing area, we obtain a monitoring capability varying between m_b 2.0 and 2.5 during normal noise conditions.¶The advantage of using a network, rather than a single station or array, for monitoring purposes becomes particularly evident during intervals with high global seismic activity (aftershock sequences), high seismic noise levels (wind, water waves, ice cracks) or station outages. For the time period November-December 1997, all time intervals with network magnitude thresholds exceeding m_b 2.5 were visually analyzed, and we found that all of these threshold peaks could be explained by teleseismic, regional, or local signals from events outside the Novaya Zemlya testing area. We could therefore conclude within the confidence level provided by the method, that no seismic event of magnitude exceeding 2.5 occurred at the Novaya Zemlya test site during this two-month time interval.¶As an example of particular interest in a monitoring context, we apply optimized threshold processing of the SPITS array for a time interval around 16 August 1997 m_b 3.5 event in the Kara Sea. We show that this processing enables us to detect a second, smaller event from the same site (m_b 2.6), occurring about 4 hours later. This second event was not defined automatically by standard processing.     

Observation of Double-mixing Line Structure in the Convective Boundary Layer During the Summer Monsoon Season

v--vThe special aerological observations carried out as a part of Land Surface Processes Experiment (LASPEX) were used to investigate the thermodynamic structure of the convective boundary layer during the summer monsoon of 1997. The analysis suggested that the convective boundary layer top was found at 700 hPa which was associated with Š_e minimum and Š_es maximum values. Double-mixing line structure was noticed in the conserved variable diagrams which was possibly attributed to the radiative warming/evaporation of falling precipitation.     

Slug Flow--A Possible Explanation for Hydrogeochemical Earthquake Precursors at Bad Brambach, Germany

--Extensive hydrogeological, geochemical, radiometric and hydro-isotope investigations in the Vogtland region, Germany, since 1989 suggest a fluid connection between a special epicentral area (focal depth: 3-15 km; M_L < 5) and a mineral spring at Bad Brambach. Twenty-six hydrogeochemical anomalies are related to earthquakes/swarmquakes of that epicentral area near Novy Kostel (CZ) during the last 9 years. The anomalies were originated by a slug-flow process in the fluid-filled fracture system near the surface. The gas component of the observed fluid (99 vol. % CO₂) is of upper mantle/crustal origin. The fluid transport pathway to the surface is the seismically active Mariánské Lázné fault zone. The interpretation suggests an influence of the fluid system due to earthquake preparation processes.     

Calibration of the Regional Crustal Waveguide and the Retrieval of Source Parameters Using Waveform Modeling

v--vRegional crustal waveguide calibration is essential to the retrieval of source parameters and the location of smaller (M < 4.8) seismic events. This path calibration of regional seismic phases is strongly dependent on the accuracy of hypocentral locations of calibration (or master) events. This information can be difficult to obtain, especially for smaller events. Generally, explosion or quarry blast generated travel-time data with known locations and origin times are useful for developing the path calibration parameters, but in many regions such data sets are scanty or do not exist. We present a method which is useful for regional path calibration independent of such data, i.e. with earthquakes, which is applicable for events down to M_w = 4 and which has successfully been applied in India, central Asia, western Mediterranean, North Africa, Tibet and the former Soviet Union. These studies suggest that reliably determining depth is essential to establishing accurate epicentral location and origin time for events. We find that the error in source depth does not necessarily trade-off only with the origin time for events with poor azimuthal coverage, but with the horizontal location as well, thus resulting in poor epicentral locations. For example, hypocenters for some events in central Asia were found to move from their fixed-depth locations by about 20 km. Such errors in location and depth will propagate into path calibration parameters, particularly with respect to travel times. The modeling of teleseismic depth phases (pP, sP) yields accurate depths for earthquakes down to magnitude M_w = 4.7. This M_w threshold can be lowered to four if regional seismograms are used in conjunction with a calibrated velocity structure model to determine depth, with the relative amplitude of the P_nl waves to the surface waves and the interaction of regional sPmP and pPmP phases being good indicators of event depths. We also found that for deep events a seismic phase which follows an S-wave path to the surface and becomes critical, developing a head wave by S to P conversion is also indicative of depth. The detailed characteristic of this phase is controlled by the crustal waveguide. The key to calibrating regionalized crustal velocity structure is to determine depths for a set of master events by applying the above methods and then by modeling characteristic features that are recorded on the regional waveforms. The regionalization scheme can also incorporate mixed-path crustal waveguide models for cases in which seismic waves traverse two or more distinctly different crustal structures. We also demonstrate that once depths are established, we need only two-stations travel-time data to obtain reliable epicentral locations using a new adaptive grid-search technique which yields locations similar to those determined using travel-time data from local seismic networks with better azimuthal coverage.     

3-D Simulation of Earthquake Generation Cycles and Evolution of Fault Constitutitve Properties

--The earthquake generation cycle consists of tectonic loading, quasi-static rupture nucleation, dynamic rupture propagation and stop, and subsequent stress redistribution and fault restrengthening. From a macroscopic point of view, the entire process of earthquake generation cycles should be consistently described by a coupled nonlinear system of a slip-response function, a fault constitutive law and a driving force. On the basis of such a general idea, we constructed a realistic 3-D simulation model for earthquake generation cycles at a transcurrent plate boundary by combining the viscoelastic slip-response function derived for a two-layered elastic-viscoelastic structure model, the slip- and time-dependent fault constitutive law that has an inherent mechanism of fault restrengthening, and the steady relative plate motion as a driving force into a single closed system. With this model we numerically simulated the earthquake generation cycles repeated in a seismogenic region on a plate interface, and examined space-time changes in shear stress, slip deficits and fault constitutive properties during one complete cycle in detail. The occurrence of unstable dynamic slip brings about decrease both in fault strength and shear stress to a constant residual level. After the arrest of dynamic slip, the breakdown strength drop j†_p of fault is restored rapidly and the process of stress accumulation resumes in the seismogenic region. On the other hand, the restoration of the critical weakening displacement D_c proceeds gradually with time through the interseismic period. The restoration of D_c can be regarded as the macroscopic manifestation of the microscopic recovery process of fractal fault surface structure. Through numerical simulation with a multi-segmented fault model, we examined the effects of viscoelastic fault-to-fault interaction. The effect of transient viscoelastic stress transfer through the asthenosphere is significant as well as the direct effect of elastic stress transfer, and it possibly explains the time lag of the sequential occurrence of large events along a plate boundary.     

Application of Regional Phase Amplitude Tomography to Seismic Verification

v--vWe have applied tomographic techniques to amplitude data to quantify regional phase path effects for use in source discrimination studies. Tomography complements interpolation (kriging) methods by extending our ability to apply path corrections into regions devoid of calibration events, as well as raising levels of confidence in the corrections because of their more physical basis. Our tomography technique solves for resolvable combinations of attenuation, source-generation, site and spreading terms. First difference regularization is used to remove singularities and reduce noise effects.¶In initial tests the technique was applied to a data set of 1488, 1.0 Hz, P_g/L_g amplitude ratios from 13 stations for paths inside a 30° by 40° box covering western China and surrounding regions. Tomography reduced variance 60%, relative to the power-law distance correction traditionally applied to amplitude ratios. Relative P_g/L_g attenuation varied with geologic region, with low values in Tibet, intermediate values in basins and high values for platforms and older crust. Spatial patterns were consistent with previous path effect studies in Asia, especially local earthquake coda-Q. Relative spreading was consistent with expected values for P_g and L_g. Relative site terms were similar to one another, yet some tradeoff with attenuation was evident.¶Tomography residuals followed systematic trends with distance, which may result from the evolution from direct to coda phases, focusing, model tradeoff or data windowing effects. Examination of the residuals using a kriging interpolator showed coherent geographical variations, indicating unmodeled path effects. The residual patterns often follow geological boundaries, which could result from attenuating zones or minor blockages that are too thin to be resolved, or that have anisotropic effect on regional phases. These results will guide efforts to reparameterize tomography models to more effectively represent regional wave attenuation and blockage. The interpolated residuals also can be combined with predictions of the tomographic model to account for path effects in discrimination studies on a station by station basis.     

Experimental Study of the Earthquake Recurrence Period and the Trend of Post-seismic Development

In order to study the earthquake recurrence and the characteristics of earthquake series, rupture tests of rock samples and plexiglass samples were made. On rock samples, a number of acoustic emission (AE) and strain measuring points were deployed; the load was one side direct shear. The variation characteristics of AE and strain at different detecting points around the extra large fracture were observed and studied. On plexiglass samples, a series of inclined cracks were prefabricated by a small-scale compressive testing machine. The samples were then loaded on a shockproof platen, when the samples were loaded, the stress intensity factor (SIF) was determined by the laser interferometric technique and shadow optical method of caustics. The fracture conditions such as material toughness around the extra large fracture were also studied. From those experimental results and the theory of fracture mechanics, the earthquake recurrence period and the trend of post-seismic development were studied.     

Earthquake rupture complexity due to dynamic nucleation and interaction of subsidiary faults

We numerically study the dynamic interaction of propagating cracks. It is assumed that propagating cracks can nucleate and drive subsidiary cracks because of shear strain enhancement near the propagating crack tips. The critical strain fracture criterion is assumed in the analysis. Intense interaction is expected to occur among the cracks. All the cracks are assumed to be parallel and antiplane strain deformation is assumed in the computation.In the interaction of two non-coplanar cracks, a strain shadow is formed in the neighborhood of each crack because of the strain release by the introduction of the crack. The growth of each crack is accelerated when the propagating tips of each crack are outside of the strain shadow of the other crack. In general, the crack tips enter the strain shadow, and the crack tips decelerate. The calculation shows that only one of the two cracks can continue to grow, and the other's growth is decelerated and arrested. If we can assume that the suite of cracks interact in a pairwise manner only, then this may suggest that only a limited number of cracks can continue to grow during the final stage of the rupture process. Hence the crack interaction causes complexity in dynamic earthquake faulting. The concepts of barrier and asperity have been employed by many researchers for the interpretation of complex seismic wave data. However, the physical realities of such concepts are obscure. Our calculations show that dynamic crack interactions can produce barriers and asperities in some cases; the crack tip deceleration or arrest due to the interactions among non-coplanar cracks can be interpreted as being due to a barrier. The dynamic coalescence among the coplanar cracks can be regarded as an asperity.Umeda found a localized area that strongly radiates high-frequency seismic waves in the epicentral areas of some large shallow earthquakes. He defined this as an earthquake bright spot. Our analysis implies that only a limited number of cracks continue to grow when many interactive cracks nucleate, and that all other cracks stop extending soon after nucleation. Hence, if the nucleation and termination of several cracks occur in a localized area, it will be observed seismologically as an earthquake bright spot. This is because it is theoretically known that the sudden termination of crack growth and dynamic crack coalescence efficiently emits high-frequency elastic waves.     

The Problem of Regionalization in the Study of Magnitude-intensity Correspondences in the Mediterranean Area: A Distribution-free Approach

--A distribution-free statistical approach is proposed for tackling the problem of regionalization devoted to the study of magnitude-intensity relationships in the Mediterranean area. The training-set constitutes a compilation of more than 2000 earthquakes which occurred in the Mediterranean region since the end of the nineteenth century, when instrumental data became available, with epicentral or maximum intensity S VI and M_S magnitude values. Firstly an empirical magnitude-intensity correspondence has been computed for each intensity class by using the entire data set in the assumption of homogeneity at the regional scale. Residuals of such relation have been analyzed by a distribution-free statistical approach in order to evaluate the opportunity of a regionalization able to locally improve the performances of magnitude-intensity relationships. The analysis indicates that data concerning larger earthquakes (intensity S VII) do not suggest the opportunity of zonation, and that unbiased estimates of macroseismic magnitude can also be obtained in the assumption that magnitude-intensity correspondences are uniform over the entire Mediterranean area. Therefore, better constrained relations determined for the entire Mediterranean region should be preferred to ill-defined local ones. As concerns smaller events (intensity VI), the procedure suggests that medium/small-scale lateral variations (on a wavelength lower than 10² km) should be taken into account if an efficient estimate of magnitudes from maximum observed intensity is pursued, but that data presently available are not sufficient to suggest any reliable zonation of the area under study.