Izu-Bonin-Mariana Arc - Seismicity

Seismicity

Spatial patterns of seismicity are essential for locating and understanding the morphology and rheology of subducting lithospheric slabs, and this is particularly true for the IBM Wadati-Benioff zone (WBZ). Katsumata & Sykes 1969 first outlined the most important features of the IBM WBZ. Their study detected a zone of deep earthquakes beneath the southern Marianas and provided some of the first constraints on the deep, vertical nature of subducting Pacific lithosphere beneath southern IBM. They also found a region of reduced shallow seismicity (≤70 km) and an absence of deep (≥ 300 km) events beneath the Volcano Islands adjacent to the junction of the Izu Bonin and Mariana trenches, where the trench trends nearly parallel to the convergence vector.

More recently, Engdahl, van der Hilst & Buland 1998 provided an earthquake catalog containing improved locations (Figure 10). This data set shows that, beneath northern IBM, the dip of the WBZ steepens smoothly from ~40° to ~80° southwards, and seismicity diminishes between depths of ~150 km and ~300 km (Figures 11a c). The subducted slab beneath central IBM (near 25°N; Fig. 11c) is delineated by reduced seismic activity that nevertheless defines a more vertical orientation that persists southward (Figures 11d f). Deep earthquakes, here defined as seismic events ≥300 km deep, are common beneath parts of the IBM arc system (Figures 10, 11). Deep events in the IBM system are less frequent than for most other subduction zones with deep seismicity, such as Tonga/Fiji/Kermadec and South America. Beneath northern IBM, deep seismicity extends southward to ~27.5°N, and a small pocket of events between 275 km and 325 km depth exists at ~22°N. There is narrow band of deep earthquakes beneath southern IBM between ~21°N and ~17°N, but south of this there are extremely few deep events. Although early studies assumed that seismicity demarcated the upper boundary of the slab, more recent evidence has shown that many of these earthquakes occur within the slab. For instance, a study by Nakamura et al. 1998 showed that a region of events beneath northernmost IBM region occur ~20 km beneath the top of the subducting plate. They propose that transformational faulting, which occurs when metastable olivine changes to a more compact spinel structure, produces this zone of seismicity. Indeed, the faulting mechanism for deep earthquakes is a hotly debated topic (e.g., Green & Houston 1995), and has yet to be resolved. Double seismic zones (DSZs) have been detected in several parts of the IBM subduction zone, but their locations within the slab as well as interpretations for their existence vary dramatically. Beneath southern IBM, Samowitz & Forsyth 1981 found a DSZ lying 80 km and 120 km deep, with the two zones separated by 30 35 km. Earthquake focal mechanisms indicate that the upper zone, where most events occur, is in downdip compression, while the lower zone is in downdip extension. This DSZ is located at a depth where the curvature of slab is greatest; at greater depths it unbends into a more planar donfiguration. Samowitz & Forsyth 1981 suggested that unbending or thermal stresses in the upper 150 km of the slab may the primary cause of the seismicity. For northern IBM, Iidaka & Furukawa 1994 used a refined earthquake relocation scheme to detect a DSZ between depths of 300 km and 400 km, which also has a spacing of 30 35 km between the upper and lower zones. They interpreted data from S to P converted phases and thermal modeling to propose that the DSZ results from transformational faulting of a metastable olivine wedge in the slab. Recent work suggests that compositional variations in the subducting slab may also contribute to double seismic zone (Abers 1996), or that DSZs represent the locus of serpentine dehydration in the slab (Peacock 2001).