Institute of Geophysics, School of Earth Sciences, Victoria University of Wellington, Wellington, New Zealand.

Recent strong (M 6.6) earthquakes in Greece are examined from the point of view of two current, but disparate, approaches to long-term seismogenesis. These are the evolving stress field (ESF) approach, in which earthquakes are considered to be triggered by accumulated stress changes from past earthquakes and tectonic loading on the major faults, and the precursory scale increase (Y) approach, in which a major earthquake is preceded in the long term by an increase in minor earthquake occurrences, with the magnitude of the precursory earthquakes, and the precursor time and area all scaling with the major earthquake magnitude. The strong earthquakes are found to be consistent with both approaches, and it is inferred that both approaches have a relevant role to play in the description of the long-term generation process of major earthquakes. A three-stage faulting model proposed previously to explain the Y phenomenon involves a major crack, which eventually fractures in the major earthquake, being formed before the onset of precursory seismicity. Hence we examine whether ESF can account for the formation of the major crack by examining the accumulated stress changes at the time of the onset of Y for each strong earthquake. In each case, the answer is in the affirmative; there is enhanced stress in the vicinity of the main shock at the time of the onset. The same is true for most, but not all, of the locations of precursory earthquakes.

A three-stage faulting model explains the observed quantitative relations between long-term precursory seismicity, mainshocks and aftershocks. Seismogenesis starts with the formation of a major crack, culminates in the corresponding major fracture and earthquake, and ends with healing. Crack formation is a self-organised critical phenomenon, and shear fracture is a delayed sequel to crack formation. It is postulated that the major crack generates a set of minor cracks, just as, later, the major fracture generates a set of minor fractures. Fracturing of the minor cracks raises the average seismicity level. By Mogi’s uniformity criterion, the major earthquake is delayed until the minor fractures have healed and the stress-field has regained relative uniformity. In accord with the scaling principle, the model applies at all magnitude levels. The size of any given initial crack determines the scale of the ensuing seismogenic process. A graphical technique of cumulative magnitude analysis gives a quantitative representation of the seismicity aspects of the model. Examples are given for large earthquakes in a region of continental collision and a subduction region. The principle of hierarchy is exemplified by the seismogenesis of a M 5.9 mainshock occurring entirely within the precursory stage of a M 7.0 mainshock. The model is capable of accommodating a variety of proposed shorter-term precursory phenomena.

The Hellenic subduction region displays the same precursory swarm phenomenon as has been found in comparable regions of New Zealand and Japan. In the earthquake catalogue of the Aristotle University of Thessaloniki, 10 past sequences of precursory swarms and related major mainshock events have been identified. These correlate, in respect of location, magnitude and time, with the 9 sequences previously identified in New Zealand, and 9 in Japan, bringing the total of sequences to 28, and the totals of related events (allowing for clustering) to 56 precursory swarms and 42 mainshock events. The results add strength to the hypothesis that swarms are long-range predictors of mainshock events. A close similarity between the swarm and aftershock magnitudes in a given sequence is also confirmed in Greece, supporting the proposal that swarms are an integral part of the seismogenic process in subduction regions. Further, the modelling of swarms as part of an overall increase in seismicity, the onset of which marks the onset of seismogenesis, is well illustrated from past sequences in Greece. Formal tests are being carried out in Greece, in parallel with New Zealand and Japan, to ascertain the performance of the hypothesis as a basis for long-range synoptic forecasting.

A sudden increase in the scale of seismicity has occurred as a long-term precursor to twelve major earthquakes in California and Northern Mexico. These include all earthquakes along the San Andreas system during 1960-2000 with magnitude M 6.4. The full list is as follows: Colorado Delta, 1966, M 6.3; Borrego Mt., 1968, M 6.5; San Fernando, 1971, M 6.6; Brawley, 1979, M 6.4; Mexicali, 1980, M 6.1; Coalinga, 1983, M 6.7; Superstition Hills, 1987, M 6.6; Loma Prieta, 1989, M 7.0; Joshua Tree, 1992, M 6.1; Landers, 1992, M 7.3; Northridge, 1994, M 6.6; Hector Mine, 1999, M 7.1. Such a Precursory Scale Increase () was inferred from the modelling of long-term seismogenesis as a three-stage faulting process against a background of self-organised criticality. The location, onset-time and level of are predictive of the location, time and magnitude of the future earthquake. Precursory swarms, which occur widely in subduction regions, are a special form of ; the more general form is here shownto occur frequently in a region of continental transform. Other seismicity precursors, including quiescence and foreshocks, contribute to or modulate the increased seismicity that characterises . The area occupied by is small compared with those occupied by the seismicity precursors known as AMR, M8 and LURR. Further work is needed to formulate as a testable hypothesis, and to carry out the appropriate forecasting tests.

Earthquake prediction based on precursors can aim to provide fully quantified, time-varying, synoptic forecasts, which do not depart from physical and geological principles, and are amenable to formal testing. These features are in contrast to the traditional occultist or soothsayer style of prediction. The recently-advanced, pre-emptive hypothesis that earthquakes are intrinsically unpredictable, and precursors non-existent, is also amenable to testing: it is refuted by the well-known relations between mainshocks and aftershocks. These relations show that a set of aftershocks is to a high degree predictable from the mainshock, so that, as a matter of principle, the mainshock is a precursor to its aftershocks. This result is compatible with the power-law property of seismicity, on which the unpredictability hypothesis is based. Empirical research on most precursors is difficult because of the scarcity of data, and is still largely at the anecdotal stage. Additional difficulties at the experimental stage are exemplified by the failure of the Tokai and Parkfield experiments to advance the study of precursors as planned. A comparative abundance of data is available on seismicity anomalies, and research on this type of precursor is progressing towards the operational stage.

Time-invariant, long-range, and short-range forecasting models were fitted to the earthquake catalogue of Greece for magnitudes 4.0 and greater to optimize their ability to forecast events of magnitude 6.0 and greater in the period 1966–1980. The models considered were stationary spatially uniform and spatially varying Poisson models, a long-range forecasting model based on the precursory scale increase phenomenon with every earthquake regarded as a precursor according to scale, and epidemic type short-range forecasting models with spatially uniform and spatially varying spontaneous seismicity. Each of the models was then applied to the catalogue for 1981–2002, and their forecasting performance was compared using the log likelihood statistic. The long-range forecasting model performed substantially better than the time-invariant models, and the short-range forecasting models performed substantially better again. The results show that the information value to be gained from modeling temporal and spatial variation of earthquake occurrence rate, at both long and short range, is much greater than can be gained from modeling spatial variation alone.