Earthquake multiplets in the southeastern Solomon Islands (original) (raw)
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Geophysical Journal International, 2003
The Solomon Islands arc area is a complex plate convergence zone. At the North Solomon Trench on the northern side of the arc, it is believed that the Pacific Plate was subducting before coming into collision with the Ontong Java Plateau, the world's largest oceanic plateau. After the collision about 5 Ma, northeastward subduction initiated along the southern side of the arc at the San Cristobal Trench, another trench on the south side. GPS observations and crustal seismic structure surveys confirm that convergence occurs at both trenches. Without detailed and accurate seismicity, it is difficult to characterize the plate subduction to reveal the tectonics of such a complex zone where a key mechanism of continental growth may also exist. In 1994, an ocean-bottom seismometer (OBS) experiment was carried out for the first time in the area around the Solomon Islands arc to locate microearthquakes. Observations started in late August and continued until early September. Five digital recording OBSs were deployed around the Russell Islands west of Guadalcanal Island. OBS spacing was about 20 km. All the OBSs were recovered and yielded data with a good signal-to-noise ratio. 40 earthquakes, with magnitudes in the range 1.5-4.4 were located over 8 days. The seismicity clearly images the two subducting plates. Though the seismicity beneath the arc side slope of the San Cristobal Trench is relatively high, we can see the seismicity which is related to the subducting Pacific Plate beneath Santa Isabel Island. In addition, earthquakes occur within the crust beneath the southern part of the New Georgia Basin and the Russell Islands. An aseismic area extending 40 km inward from the San Cristobal trench axis implies initial aseismic slip of the India-Australia Plate at a small dip angle.
Source process of the great 1971 Solomon Islands doublet
Physics of the Earth and Planetary Interiors, 1989
Large underthrusting earthquakes in the Solomon Islands tend to occur as multiplets, separated by a few hours to several days in time and 30-100 km in space. The largest sequence, a doublet on July 14, 1971 (M~= 8.0) and July 26, 1971 (M~= 8.1), is particularly unusual in that it occured at the junction of two trenches, with the July 14 event in the Solomon Islands Trench preceding the July 26 rupture in the New Britain Trench. The location and large moment release of this doublet as well as an incompatibility of published body wave and surface wave focal mechanisms for the July 14 event, motivated a detailed study of the source process of the 1971 sequence. To satisfy both the surface wave and body wave data, we explored the possibility of a change in the faulting geometry during rupture for the July 14 event. A fault model that changes orientation from a northwest-southeast striking, shallow-dipping plane, similar to the mechanisms of neighboring underthrusting events in the Solomon Islands Trench, to a more north-south striking plane-70 s after rupture initiation is consistent with the observed Rayleigh and Love wave amplitude patterns. This change in mechanism is suggested by systematic variations in the mechanisms of smaller earthquakes near the cusp in the Solomon Islands plate. A simultaneous inversion of WWSSN P wave seismograms for the time, location, seismic moment and focal mechanism of individual subevents gives results in agreement with the surface wave model, although the precise change in mechanism is not well resolved. The spatiotemporal distribution of subevents reveals two major pulses of moment release. The first occurred on the initial fault near the epicenter at the origin time of the earthquake. The second occurred on the north-south striking fault, 50-70 km northwest of the epicenter-70 s after the origin time. The change in mechanism appears to reflect contortion of the slab near the trench junction. Inversion of WWSSN P wave seismograms for the rupture history of the July 26 event reveals a westerly propagating rupture extending-60 km along the New Britain Trench. For both of the 1971 earthquakes, regions of high moment release are located in the vicinity of the Solomon Islands-New Britain Trench junction. Contortion of the subducting lithosphere as it bends around the arc-arc junction may have enhanced the degree of mechanical coupling between the subducting and overriding plates, concentrating stress in this region, and in turn, promoting efficient triggering across the trench junction.
Geophysical Research Letters, 2017
The 17 December 2016 Solomon Islands earthquake (M w 7.9) initiated ~103 km deep in the subducting Solomon Sea slab near the junction of the Solomon Islands and New Britain trenches. Most aftershocks are located near the Solomon Islands plate boundary megathrust west of Bougainville, where previous large interplate thrust faulting earthquakes occurred in 1995 (M w 7.7) and 1971 (M w 8.0). Teleseismic body wave modeling and aftershock relocations indicate that the initial 30 s of the 2016 rupture occurred over depths of 90 to 120 km on an intraslab fault dipping ~30° to the southwest, almost perpendicular to the dipping slab interface. The next 50 s of rupture took place at depths of 32 to 47 km in the deeper (Domain C) portion of the overlying megathrust fault dipping ~35° to the northeast. High susceptibility to triggering in the region accounts for this compound rupture of two separate fault planes.
An extensive region of off-ridge normal-faulting earthquakes in the southern Indian Ocean
Journal of Geophysical Research, 1984
An unusually large number of off-ridge earthquakes have occurred within a broad region astride the Southeast Indian Ridge. The source mechanisms of nine of these earthquakes, in lithosphere from 4 to 27 m.y. old, have been determined with a formal inversion technique based on matching observed and synthetic P and SH waves. All events occur in the oceanic mantle, from 8 to 22 km below the seafloor.
The distribution of earthquake multiplets beneath the southwest Pacific
Earthquakes beneath the southwest Pacific occur from the surface down to 700 km depth. Teleseismic waveforms created by some of these earthquakes are almost identical. We investigate Tonga-Kermadec and Vanuatu subduction zone earthquake P-coda waveforms using a cross-correlation technique and hierarchical clustering algorithm in order to determine the origin of waveform similarity and the distribution of earthquakes producing similar waveforms.We show that scatterers forming the majority of power in the P-wave coda are localised around the receiver. As a result, waveform similarity provides a much weaker constraint on source separation than in local studies. Waveform similarity can provide stronger constraints on focal mechanism.Most earthquake multiplets within the Tonga-Fiji-Kermadec Wadati-Benioff zone are found at depths between 0-60 km and 520-620 km. A significant proportion of all deep-focus events in south Pacific subduction zones have waveforms similar to those of at least one other event. Relative relocation of events within the largest identified multiplet reveals a planar zone of seismicity sub-parallel to the nodal plane of a related centroid moment tensor solution.Groups of earthquakes with similar waveforms remain active on at least the 14-year recording timescale. We equate this to repeated rupture on single or closely related shear systems within the subducting slabs.
Bulletin of the Seismological Society of America, 2007
Analysis of the earth's longest period normal modes shows that the December 2004 Sumatra-Andaman earthquake was much larger (M w 9.3) than initially inferred from surface-wave data and involved slip on a much longer fault than initially inferred from body-wave data. The seismic moment and relative excitation of the normal modes indicate that the entire aftershock zone ruptured, consistent with the large tsunami amplitudes in Thailand, Sri Lanka, and India. An apparent increase in seismic moment with period results from interference between parts of the fault. The earthquake resulted from subduction of the Indian plate beneath the Burma microplate, a sliver plate between the Indian and Sunda plates. Hence, the rate and direction of convergence depends on the motion of the Burma plate, which is not well known. Convergence would be highly oblique if the rate of motion between Burma and Sunda is that inferred from spreading in the Andaman Sea, and less if a slower rate is inferred from the Sagaing fault. The December earthquake was much larger than expected from a previously proposed relation, based on the idea of seismic coupling, in which such earthquakes occur only when young lithosphere subducts rapidly. Moreover, a global reanalysis finds little support for this correlation. Hence, we suspect that much of the apparent differences between subduction zones, such as some trench segments but not others being prone to M w Ͼ8.5 events and hence oceanwide tsunamis, may reflect the short earthquake history sampled. This possibility is supported by the variability in rupture mode at individual trench segments.