G. F. Karakaisis

Evidence for transform faulting in the Ionian sea: The Cephalonia island earthquake sequence of 1983

Accurate locations of aftershocks of the January 17, 1983 (Ms=7.0) main shock in the Ionian islands have been determined, as well as fault plane solutions for this main shock and its largest aftershock, which are interpreted as a right-lateral, strike-slip motion with a thrust component, on a fault striking in about a NE-SW direction.This is considered as a transform fault in the northwesternmost part of the Hellenic arc.

The 2001 Skyros, Northern Aegean, Greece, earthquake sequence: off - fault aftershocks, tectonic implications, and seismicity triggering

was identified as a possible site for the occurrence of a strong event by Papadimitriou and Sykes [2001] who applied an evolutionary stress model in the Northern Aegean area. [4] The paper analyzes the details of the earthquakes in the Skyros sequence, aiming to contribute to the understanding of the seismotectonic properties in this area where the western termination of the north Aegean strike slip faulting against the mainland of Greece takes place. The co-seismic stress changes associated with the main shock are computed and the areas of static stress increases are correlated with the aftershock spatial distribution.

Seismic source-parameter relations for earthquakes in Greece

A data set of nineteen, mainly shallow, moderate to large earthquakes, which occurred in the Aegean and the surrounding area, has been used to derive empirical relations for kinematic fault parameters. Thus the relations between seismic momentM0 and magnitudeMs andmb and betweenM0 andMs and fault dimensionsS andL have been determined. From these relations and theoretical ones it was deduced that earthquakes in the Aegean and the surrounding events, chiefly interplate, are characterized by low average stress drop values. Values of Δσ ranging from 1 to 30 bar are consistent with the data. It was also found that, in general terms, most of the data obey the geometrical similarity conditionL=2w, whereL is the fault length measured along the strike andw is its width measured along the dip. For strike-slip faults, however, the conditionL=4w seems to hold.

Accelerating Seismic Crustal Deformation in the Southern Aegean Area

A region of intense accelerating seismic crustal deformation has been identified in the southwestern part of the Hellenic arc (broader area of Cythera island). The identification is performed using a detailed parametric grid search of the broader southern Aegean area for accelerating energy release behavior. The identified region has similar properties with past preshock (critical) regions, which have been identified for strong mainshocks in the Aegean area. Based on such observations, which suggest that this region is at a critical state that can lead to a critical point, that is, to the generation of a mainshock, an estimation is made of the possible epicenter coordinates, magnitude, and origin time of this oncoming large (M ∼7.0) earthquake. The estimation procedure is validated on the basis of retrospective analysis of strong events in the Aegean area, as well as by appropriate application on synthetic random catalogs. These results, the existence of similar observations of accelerating seismic deformation in eastern part of southern Aegean and independent information on the time distribution of large earthquakes (M ≥6.8) for the whole southern Aegean indicate that the generation of strong earthquakes in this area in the next few years must be considered as very probable.

Global Observational Properties of the Critical Earthquake Model

The preshock (critical) regions of 20 mainshocks with magnitudes between 6.4 and 8.3, which occurred recently (since 1980) in a variety of seismotectonic regimes (Greece, Anatolia, Himalayas, Japan, California), were identified and investigated. All these strong earthquakes were preceded by accelerating time-to-mainshock seismic crustal deformation (Benioff strain). The time variation of the cumulative Benioff strain follows a power law with a power value ( m = 0.3) in very good agreement with theoretical considerations. We observed that the dimension of the critical region increased with increasing mainshock magnitude and with decreasing long-term seismicity rate of the region. An increase of the duration of this critical (preshock) phenomenon with decreasing long-term seismicity rate was also observed. This spatial and temporal scaling expresses characteristics of the critical earthquake model, which are of importance for earthquake prediction research. We also showed that the critical region of an oncoming mainshock coincides with the preparing region of this shock, where other precursory phenomena can be observed.

Subcrustal microearthquake seismicity and fault plane solutions beneath the Hellenic Arc

Subcrustal seismicity recorded in the southern Aegean sea during a 7-week microearthquake study was low compared with shallow seismicity. Most intermediate depth seismicity occurred beneath the western and eastern ends of the Hellenic arc. This distribution confirms that a slab of lithosphere is being subducted at a very shallow (<15°) angle for 200 km beneath the western end (Peloponnese) but more steeply beneath the eastern end (Dodecanese). We could locate only one intermediate depth event beneath the central pan of the arc, where teleseimically located intermediate depth earthquakes also are infrequent. T axes for most of the 22 focal mechanisms of subcrustal earthquakes are roughly parallel to the local dip direction of the seismic zone. Between depths of 40 and 80 km, the mechanisms are more confused than at greater depth, perhaps because some of these earthquakes did not occur within the downgoing slab. Earthquakes deeper than 80 km, and within the subducted slab, have nearly horizontal P axes that trend NNE-SSW in the eastern part and NNW-SSE in the western part of the arc. These deeper mechanisms show horizontal P axes along strike, perhaps in response to the contortion of the slab or to the westward motion of Turkey, as well as lengthening downdip, probably in response to gravity acting on excess mass in the slab. Thus the short slab, both downdip and along strike, subducting beneath the Aegean is subjected to a more complex set of forces than the long slabs of the Pacific.

Long-term earthquake prediction in the circum-pacific convergent belt

AbstractInvestigation of the time-dependent seismicity in 274 seismogenic regions of the entire continental fracture system indicates that strong shallow earthquakes in each region exhibit short as well as intermediate term time clustering (duration extending to several years) which follow a power-law time distribution. Mainshocks, however (interevent times of the order of decades), show a quasiperiodic behaviour and follow the ‘regional time and magnitude predictable seismicity model’. This model is expressed by the following formulas

Long-term earthquake prediction in Iran based on the time- and magnitude-predictable model

\begin{gathered} \log T_t = 0.19 M_{\min } + 0.33 M_p - 0.39 \log m_0 + q \hfill \\ M_f = 0.73 M_{\min } - 0.28 M_p + 0.40 \log m_0 + m \hfill \\ \end{gathered}