Abundance Of Earthquakes Research Articles

When slabs collide: A tectonic assessment of deep earthquakes in the Tonga-Vanuatu region

An unusual group of more than 100 earthquakes is located ∼600 km beneath the North Fiji Basin in the southwest Pacific Ocean. These earthquakes are attributed to seismicity within detached segments of the subducted Australian plate. One of these detached slab segments has collided with, and now impinges on, the subducted Pacific plate at a depth of ∼500 km. The region in the mantle where the two slab segments meet is also characterized by an unparalleled abundance of large-magnitude (Mw >7.0) earthquakes for this depth. The folded shape of the Pacific slab beneath Fiji, as well as the abundance of earthquakes, is interpreted to result from deformation and deformation-enhanced phase transformations as the two slabs collide and settle on the 660 km discontinuity. Detachment of the slab segments is interpreted to have occurred at ca. 5 Ma, but collision between the eastern segment and the west-dipping Pacific slab is interpreted to have occurred at ca. 4 Ma, coincident with initial opening of the overriding Lau Basin.

Earthquakes and strain in subhorizontal slabs

Using an extensive database of fault plane solutions and precise locations of hypocenters, we show that the classic patterns of downdip extension (DDE) or downdip compression (DDC) in subduction zones deteriorate when the dip of the slab is less than about 20°. This result is depth‐independent, demonstrated by both intermediate‐focus (depths from 70 to 300 km) and deep‐focus (depths greater than 300 km) earthquakes. The absence of pattern in seismic strain in subhorizontal slabs also occurs locally over scales of about 10 km, as evident from a detailed analysis of a large (Mw 7.1) earthquake sequence beneath Fiji. Following the paradigm that a uniform strain of DDE/DDC results from sinking of the cold, dense slab as it encounters resistance from the highly viscous mantle at depth, breakdown of DDE/DDC in subhorizontal slabs reflects waning negative buoyancy (“slab pull”) in the downdip direction. Our results place a constraint on the magnitude of slab pull that is required to dominate over localized sources of stress and to align seismic strain release in dipping slabs. Under the condition of a vanishing slab pull, eliminating the only obvious source of regional stress, the abundance of earthquakes in subhorizontal slabs indicates that a locally variable source of stress is both necessary and sufficient to sustain the accumulation of elastic strain required to generate intermediate‐ and deep‐focus seismicity. Evidence is growing that the process of seismogenesis under high pressures, including localized sources of stress, is tied to the presence of petrologic anomalies.

Origin of the West Taiwan basin by orogenic loading and flexure of a rifted continental margin

Seismic and well data suggest that the West Taiwan basin developed by orogenic loading and flexure of a rift‐type continental margin. The most likely source of the loading is Taiwan, where oblique convergence between the Eurasian and Philippine Sea plates has produced an orogenic belt up to 300 km in length, 100 km in width, and 4 km in height. Flexure modeling shows that surface loading is unable to explain the depth of the West Taiwan basin. Other, subsurface or buried loads are required. Combined surface and buried loading explains the depth and width of the basin. It also accounts for a Bouguer gravity anomaly “high” and flanking “low” over the orogenic belt, a lateral offset of 20–30 km between the peak topography and the maximum depth to the seismic Moho, and evidence for tectonic uplift in the Penghu Islands. The depth of the base of the foreland sequence in the northern part of the West Taiwan basin can be explained well by an elastic plate model with an effective elastic thickness, Te, of 13 km. While this value is low when compared to most other foreland basins, it is within the range of values derived from rifted continental margins. The northern part of the West Taiwan basin unconformably overlies a passive margin sequence and therefore appears to have inherited the long‐term (>1 Myr) flexural properties of the margin. In the southern part of the basin, however, the depth to the base of the foreland sequence dips too steeply to be explained by elastic plate models. This part of the basin therefore appears to be yielding rather than flexing. Differences in the flexural behavior along strike of the West Taiwan foreland basin lithosphere are reflected in seismicity patterns west of the thrust front. The northern part of the basin is associated with a low level of seismic activity, while the south correlates with an abundance of earthquakes, especially at shallow (<25 km) depths. There is a cluster of earthquakes along two extensional faults that were active during rifting of the underlying margin. Therefore lithospheric flexure and fault reactivation may be important contributors to the seismicity of the Taiwan region.

Correlation between seismicity and the distribution of thermal and carbonate water in southern and Baja California, United States and Mexico

A comparison of the distribution of thermal and thermal-related springs and wells in southern California, United States, and Baja California, Mexico, with the abundance of earthquakes of magnitude 4 or greater shows as close a relationship between thermal waters and the distribution of seismicity as to the distribution of active faults. It appears that the distribution of thermal water variations in the geothermal gradient in turn influences the stress accumulation capability of the rocks at depth. Thus, areas with abundant thermal waters (and hence steep geothermal gradients) release stress by frequent moderate earthquakes; areas lacking thermal waters, such as the central Transverse Ranges, accumulate stresses that are released by infrequent large earthquakes.