Papua New Guinea

The first publication on the seismicity of Papua New Guinea was probably Sieberg (1910), which was based largely on reports of earthquakes from the church missions established there close to the turn of the century. He listed twenty-four earthquakes that occurred in the period 1900–1906 (fig. 2). He even attempted to interpret the seismicity in terms of the tectonics of the region. However, apart from recognizing that "New Guinea and the Bismarck Archipelago form part of the innermost arc of young folded mountains which approach the rigid Australian platform," his general conclusion was that "our knowledge is very deficient in this respect, because only sporadic observations are available."

By the time Gutenberg and Richter's Seismicity of the Earth was published in 1954 the data set had improved considerably, and most epicenters were computed using arrival times recorded at seismographic stations. They used a time window from 1904 to 1952 and came close to relating earthquakes to a plate-tectonic type of activity. For example, they recognized (pp. 97–101) that "Most of the earth's surface is partitioned among a number of comparatively stable blocks, separated by active belts" and "the foci of deep shocks seem to be restricted to the vicinity of a nearly plane surface, which is probably related to a thrust surface between two different structures, usually dipping towards the continent." However, they mistakenly suggested that the midocean ridges "can hardly be young structures" even though they recognized that in the case of the mid-Atlantic ridge "Its parallelism with the continental coasts is so close that it practically demonstrates a mechanical connection with them." It is interesting to note that in the Papua New

Figure 1
The Australian plate and its boundaries. Area 1 defines the New
Guinea region and Area 2 the Australian continent, the two
regions discussed in the text. Arrows indicate the directions
of plate motion relative to the Antarctic plate.

Figure 2
New Guinea epicenters for 1900–1906 (Sieberg, 1910).

Guinea region the main features of the shallow, intermediate (70–300 km), and deep (> 300 km) earthquakes had by then been determined, which is quite remarkable when one considers the extent of the global network in the pre-1950s, and also the absence of any regional stations at that time.

In Japan, meanwhile, Wadati (1934), using data from regional stations, had been able to define quite precisely the zone of earthquakes dipping beneath Honshu, and had also suggested that "This tendency seems to be observable in many volcanic regions in the world. Of course, we cannot say decisively but if the theory of continental drift suggested by A. Wegener be true, we may perhaps be able to see its traces of the continental displacement in the neighbourhood of Japan"—not bad for 1934.

In New Guinea, the next study after Gutenberg and Richter was made by Brooks (1965) who applied data from 1906 to 1962 to earthquake risk assessments. His results do not represent a significant improvement over Gutenberg and Richter's earlier work. However, Denham (1969) used data from the period 1958–1966 in an attempt to interpret the hypocentral patterns in terms of the fledgling plate-tectonic theories being developed at the time. Figure 3 shows his results. There is a significant improvement in the definition of the lineations, and it is clear that the northern boundary of the Australian plate is not a simple boundary. The reasons for the improvements in the data set were mainly due to the establishment of the Worldwide Standardized Seismographic Network (WWSSN) in the late 1950s and early 1960s and the computing facilities of the U.S. Coast and Geodetic Survey (USCGS). Some indication of the improvements may be gauged from the fact that only 256 events from the region were listed by Brooks (1965) for the period 1906–1962, whereas in 1966 alone 309 earthquakes were located in the same region by the USCGS.

Denham (1969) was able to identify the zones of deep and intermediate-depth earthquakes associated with the volcanic arcs and also the Bismarck Sea Seismic Zone (BSSZ). This is defined by the line of epicenters extending from 143° E to 152º E at 3° S. However, the northern boundary of the Aus-

Figure 3
New Guinea epicenters for 1958–1966 earthquakes with  M  ³  5 (Denham, 1969). BSSZ indicates the Bismarck Sea Seismic Zone, and SS indicates the Solomon Sea.

tralian plate was probably misidentified as being along the northern coast of the island of New Guinea and the northern margin of the Solomon Sea (SS).

As shown in figure 4, compiled from more recent data, this interpretation is probably not correct. The northern margin of the Australian plate is situated farther to the south. In figure 4 all epicenters located by twenty or more stations since 1970 have been plotted. The improved regional seismographic coverage since that date has revealed a zone of seismicity in the central part of the main island of New Guinea (A-Á in fig. 4a) and another zone near the East Papuan peninsula (see Ripper and McCue, 1983). These diagrams indicate the details that can be revealed with a good-quality data base. Each lineation can be interpreted in terms of plate tectonics and the interpreta-

tions checked by analyzing fault-plane solutions and observations of recent crustal movements.

At depths between 70 and 300 km the hypocentral lineations are equally well determined and can be interpreted in terms of subducting lithosphere (fig. 4c). Similarly, for earthquakes deeper than 300 km (fig. 4d) the slabs (or what is left of them) are also clearly identified.

Therefore, in a plate margin situation, even though the tectonics are complicated it is possible to use the spatial distributions of earthquake hypocenters to unravel the current dynamics of the region. Such inferences are not possible in an intraplate environment where no suitable model for earthquake occurrences has as yet been developed.

Figure 4
New Guinea epicenters for 1970–1984 where twenty or more
stations have been used to locate the hypocenters.
(a) Shallow (0–50 km) earthquakes with coastlines. A–Á
indicates the northern margin of the Australian plate
in the center of New Guinea.
(b) Shallow earthquakes without coastlines.
(c) Intermediate Depth (70–300 km) earthquakes
without coastlines.
(d) Deep (> 300 km) earthquakes without coastlines.
Notice how the shallow earthquakes define the plate and
subplate boundaries, and the intermediate and deep
earthquakes define the subducted slabs.