surface waves is about 30 bars, but the local stress drop for the individual events may be significantly higher than this. (6) The complex multiple event is a manifestation of a heterogeneous distribution of the mechanical properties along the fault, which may be caused by either asperities, differences in strength, differences in pore pressure, differences in slip characteristics (stable sliding versus stick slip), or combinations of these factors. (7) This complexity has important bearing on the state of stress along transform faults and is important in assessing the effect of large earthquakes along other transform faults like the San Andreas.

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The Thessaloniki (northern Greece) earthquake sequence appears to have occurred along faults forming a graben structure. Ths graben, situated in the border region between the Serbomacedonian massif and the Vardar zone, is bounded to the south-west by clearly exposed north-west striking north-east dipping normal faults.Relative hypocentre determinations, fault-plane solutions, surface faulting and the aftershock distribution suggest that some of these faults have been reactivated during the 1978 earthquakes.The source parameters of the mainshock (mb = 6.1, Ms = 6.4) were determined by computing body-wave synthetic seismograms in the time domain and comparing them with the observed. This modelling constrained the orientation of faulting determined by the P-wave fault-plane solution. It also constrained the source depth to 6 f 2 km. Similar depths were calculated by a relative relocation method for the other three large events of the sequence. The dislocation time-function required for the far-field had a total duration of 9 f 1.5 s, the body-wave moment was 5.2 ? 1.8 x loz5 dyne cm. For a fault length of 35 km and a fault width of 17 km (both estimated from the aftershock distribution) the static stress drop was found to be 4 bar.An accelerograph record of the mainshock shows two distinct events, 3-4 s apart. These two events were unresolved by the long-period data in the far-field but the unusually long duration of the mainshock time-function suggests an overall slow energy release which probably occurred as a sequence of events close in space and time. Seismic energy released in this fashion can account for waveform complexities observed at some stations.

Several large shallow earthquakes (Ms > 7.0) have occurred along the Middle American Trench since the installation of the WWSSN network. Included are the 1965, 1968, and 1978 Oaxaca events, the 1970 Chiapas event, the 1973 Colima event, and the 1979 Petatlan event. These earthquakes have been studied in an attempt to identify similarities and differences between them that may lead to a better understanding of fracture and subduction processes. The events have seismic moments ranging from 1.0×1027 dyne cm for the 1968 event to 3.2×1027 dyne cm for the 1978 event. All events are of predominantly thrust type, consistent with subduction to the northeast of the Cocos plate. Body waves for the 1965, 1968, 1978, and 1979 events along the trench indicate rather simple faulting processes. These events all had focal depths of 15 to 20 km and stress drops of the order of 10 bars. The 1970 and 1973 events, the easternmost and westernmost, respectively, of the events studied here, are located close to triple junctions for the Cocos–North America–Caribbean plates (1970) and the Cocos–North America–Rivera plates (1973). These two events generated more complex body waves than the Oaxaca and Petatlan quakes. This source complexity may have been related to the more complicated tectonics near the 1970 and 1973 epicenters. This may offer a new perspective to viewing source complexity in relation to subduction zones.

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