[1] We studied the distribution of asperities for large recent earthquakes along the New Britain trench, Papua New Guinea, to investigate if they are the same for repeated ruptures of the subduction boundary. We determined the slip distributions of two earthquakes (M w $ 8) in 1971 using P diff waveforms, and an earthquake (M w 7.9) in 1995 using direct teleseismic P waves. Combining these findings with previous results for two earthquakes (M w $ 7.5) in 2000, we compared the source areas and asperity distributions for this region of the New Britain Trench. Our results show that the locations of the asperities for the individual earthquakes did not significantly overlap, although the same portion of the subduction zone seems to have reruptured. This fact supports the idea that asperities are not persistent features when portions of the New Britain subduction zone slip in large earthquakes.

A sequence of large earthquakes occurred in the New Britain/New Ireland region of Papua New Guinea in late 2000. The sequence started with a Mw 6.8 earthquake along the New Britain Trench on 29 October. About 20 days later a Mw 8.2 earthquake occurred in the New Ireland region on 16 November and produced large strike‐slip surface displacements and tsunamis. Following the Mw 8.2 event, two large earthquakes (Mw ∼ 7.5) occurred along the nearby New Britain Trench on 16 and 17 November. Furthermore, small triggered events were observed over a wide area outside of the rupture zones of the large earthquakes, with different focal mechanisms from those of the major events. There is likely some mechanism(s) that triggered this remarkable sequence, and we document the details of the spatial and temporal patterns of the events. We investigated if static stress changes can explain the initiation of the large earthquakes and also some groups of triggered smaller events. There are mixed results. The occurrences of the two M ∼ 7.5 major events may be explained by the static stress changes; however, there are also some earthquakes that are not consistent with triggering by static stress changes. There may be multiple mechanisms, including static and dynamic stress changes, that are needed to explain the complicated sequence of earthquakes.

Shores of eastern Hokkaido rose by perhaps 1 m a few centuries ago. The uplifted area extended at least 50 km along the southern Kuril Trench. It included the estuaries Akkeshi-ko and Hichirippu, on the Pacific coast, and Fūren-ko and Onnetō, which open to the Okhotsk Sea. At each estuary, intertidal and subtidal flats rose with respect to tide level; wetland plants colonized the emerging land; and peaty wetland deposits thereby covered mud and sand of the former flats. Previous work at Akkeshi-ko and Onnetō showed that such emergence occurred at least three times in the past 3000 years. Volcanic-ash layers date the youngest emergence to the seventeenth century AD. New evidence from Akkeshi-ko, Hichirippu and Fūren-ko clarifies the age and amount of this youngest emergence. Much of it probably dates from the century's middle decades. Some of the newly emerged land remained above high tides into the middle of the eighteenth century or later. The emergence in the last half of the seventeenth century probably exceeded 0.5 m (inferred from stratigraphy and diatom palaeoecology) without far exceeding 1 m (estimated by comparing seventeenth-and eighteenth-century descriptions of Akkeshi-ko). The stratigraphy and palaeoecology of the emergence are better explained by tectonic uplift than by bay-mouth blockage, tidal-flat accretion or sea-level fall. Eastern Hokkaido needs occasional uplift, moreover, to help reconcile its raised marine terraces with its chronic twentieth-century subsidence. Because it took place above forearc mantle, eastern Hokkaido's seventeenth-century uplift probably lacks analogy with coseismic uplift that occurs above typical plate-boundary ruptures at subduction zones.

We examine the spatial and temporal relationships of the sequence of strong earthquakes that occurred off the Kii Peninsula, Japan, on 5 September 2004. The first event (Mj 7.1) occurred at 10:07:08 (UTC) on a northward dipping plane within the subducting Philippine Sea plate. From 10:16 to 14:47 the seismicity shows a group of earthquakes (Mj 3.2 to 4.8) 35 km to the east which are regarded as foreshocks to the second large earthquake. At 14:57:17, a Mw 6.1 strike-slip event occurred on a northwest trending plane. Some 14 seconds later, a large (Mj 7.4) thrust earthquake started 4.2 km southeast of the initial epicenter of the second earthquake. This largest earthquake is thought to have occurred on a southward dipping plane with the strike in an east-southeastly direction. Using the geometry of faults determined in this study, calculations of the Coulomb failure function show that simple static stress changes do not provide a good explanation for the triggering of the subsequent earthquakes.

[1] Rupture velocity is an important source parameter, which is often difficult to determine, especially for deep-focus earthquakes where there is usually limited near-source information. To help overcome this problem, we developed a new method to estimate rupture velocities of deep-focus earthquakes with better resolution. We first carry out teleseismic P waveform inversions to determine slip distributions for a range of rupture velocities on the two nodal planes. Then forward modeling of regional data is performed using the slip distributions determined in the teleseismic inversions to estimate the rupture velocity. Using this method, we attempted to determine the rupture velocities of large deep-focus earthquakes surrounding Japan, which are well recorded on teleseismic and regional networks. Empirical Green functions are used for both the teleseismic and regional analyses. Although it is difficult to determine the rupture velocity from only the teleseismic data, the analyses including regional data show clear difference which can resolve the rupture velocity and fault geometry. For three deep earthquakes, we obtained rupture velocities of about 1$2 km/s, which correspond to 20$40% of the shear wave velocity and are much slower than typical values for shallow earthquakes.Citation: Park, S.-C., and J. Mori (2008), Rupture velocity estimation of large deep-focus earthquakes surrounding Japan,