Multifractal Analysis of the Arnea,Greece Seismicity with Potential Implications for Earthquake Prediction

Two-dimensional multifractal analysis is performed in a seismic area of Northern Greece responsible for recent strong earthquakes, including the Arnea sequence of May 1995, culminating in a M_w 5.3 event on 4/5/1995. It is found that multifractality gradually increases prior to the major seismic activity and that declusterization replaces clusterization not long before its initialization. The fractal dimensions D(q) (q > 0) abruptly drop for aftershocks, reflecting their very strong spatial clustering. The observed seismicity patterns seem to be compatible with a percolation process. Before the main sequence, the fractal dimension is consistently in the range 1.67–1.96 (standard deviation included). Percolation theory predicts 1.9 for 2D percolation clusters and 1.8 for the backbone of 3D percolation clusters. If the observed gradual increase in multifractality is due to multifractality reaching a maximum prior to the major slip (percolation), this may enable us to roughly estimate its time of occurrence.     

Uncertainties in the estimation of earthquake magnitudes in Greece

Instrumental magnitudes in Greece have been reported as: a) Mmagnitudes based on the records of the Wiechert or Mainka seismographs,b) M_LGR magnitudes based on the records of the Wood-Anderson(WA) seismographs (T_o = 0.8 sec, V_effective 1000) or othershort period seismographs calibrated against WA records and,c) M_LSM magnitudes based on strong motion records(accelerograms). Comparison of such magnitudes with momentmagnitudes, M_w, for 329 earthquakes, with epicenters in thebroader Aegean area, performed in this study, showedthat M, M_LGR+0.5 and M_LSM are practically equalto M_w, with a small overall standard error ( = 0.23).Therefore, equivalent moment magnitudes, M_w ^*,estimated from these magnitudes and reported in the catalogues of theGeophysical Laboratory of the University of Thessaloniki are equal tomoment magnitudes for all practical purposes with reasonable uncertainties.It has been further shown that surface wave magnitudes, M_s,for M_s <6.0, can be also transferred into momentmagnitudes, M_w ^*, but the larger uncertaintiesencountered make its use rather problematic.     

New observational information on the precursory accelerating and decelerating strain energy release

Recent reliable data are used to study the behavior of seismic activity before 46 strong shallow earthquakes (M ≥ 6.0), which correspond to five complete samples of mainshocks. These samples include 6 mainshocks (M = 6.0–7.1) that occurred in western Mediterranean since 1980, 17 mainshocks (M = 6.0–7.2) which occurred in the Aegean (Greece and surrounding area) since 1980, 5 mainshocks (M = 6.4–7.5) that occurred in Anatolia since 1980, 12 mainshocks (M = 6.0–7.3) that occurred in California since 1980 and 6 mainshocks (M = 7.0–8.3) that occurred in Japan since 1990. In all 46 cases, a similar precursory seismicity pattern is observed. Specifically, it is observed that accelerating Benioff strain (square root of seismic energy) release caused by preshocks occurs in a broad circular region (critical region), with a radius about eight times larger than the fault length of the mainshock, in agreement with results obtained by various research groups during the last two decades. However, in a much smaller circular region (seismogenic region), with a radius about four times the fault length, the corresponding preshock strain decelerates with the time to the mainshock. The time variation of the strain follows in both cases a power law but the exponent power is smaller than unit (m ¯ = 0.3) in the case of the accelerating preshock strain and larger than unit (m ¯ = 3.0) in the case of the decelerating preshock strain. Predictive properties of this “Decelerating In–Accelerating Out Strain” model are expressed by empirical relations. The possibility of using this model for intermediate-term earthquake prediction is discussed and the relative model uncertainties are estimated.