Seismologists are closely monitoring what they say is unusually persistent seismic activity at the Xylokastro fault in the Gulf of Corinth following a 4.2 Richter quake Tuesday evening that sparked alarm among local inhabitants, although no damage was reported. The quake, which occurred at 5.59 p.m. in the sea north of the town of Xylokastro, was the largest in a series of tremors which seismologists say are not cause for serious concern. «Such magnitudes are usual in Greece,» Gerasimos Papadopoulos, a researcher at the Geodynamic Institute, told Kathimerini yesterday. «However,the persistence of the phenomenon in this region over the past month is of scientific interest. For the moment, the data are not particularly worrying. But we must use all means at our disposal to evaluate new data on a daily basis.» Xylokastro Mayor Antonis Kladouchos said residents had been upset by the rumbling noise accompanying the tremors, which Papadopoulos said were due to rock falls in underground caves and not activity within the fault line.