I. Selwyn Sacks

Sediment incorporation in island-arc magmas: Inferences from 10Be

Abstract The radioisotope 10Be has been used as a tracer to evaluate subduction and recycling of sediments in island arcs. As a cosmogenic isotope strongly enriched in oceanic sediments, it is especially suitable for monitoring sediment subduction. We report here 10Be results for 106 arc volcanic rocks and 33 basalts from mid-ocean ridges, oceanic islands, continental rifts, and continental flood basalt provinces. The 33 basalts from non-arc environments all contain Conceptually, four models may be used to explain the incorporation of 10Be in island arcs. Model 1 is the limiting case calculation presented in Brown et al. (1982). Physically, it corresponds to an assumption that all 10Be in the uppermost sediment layers is carried to depth, the 10Be is mechanically decoupled from the sediment pile and is added to the arc magma source region. This model results in the highest calculated 10Be contents in arc lavas. In the more physically reasonable Model 2, the 10Be-rich upper sediments are assumed to mix with the deeper, 10Be-poor, sediments and the sediments are subsequently incorporated into the source region. Model 3 assumes 10Be to be incorporated from sedimentary layers encountered during magma ascent. In Model 4 only sediments contained within grabens in the downgoing slab may be subducted. As yet insufficient data exist to permit conclusive evaluation of these models, but correlations between 10Be contents in arc volcanic rocks and recent sedimentation rates and sediment thickness, measured outboard of the trench, suggest that Model 2 may be the most important mechanism for 10Be incorporation in many island arcs.

Thermal and dynamical evolution of the upper mantle in subduction zones

We present results from two-dimensional (2-D) numerical experiments on the thermal and dynamical evolution of the subducting slab and of the overlying mantle wedge for a range in subduction parameters. These include subduction rate and the age and rheology of both subducting and overriding plates. Experiments also consider the influence of slab forcing conditions (from purely kinematic to purely dynamic) on the evolution of both the slab and mantle wedge. One goal is to determine how different parameters control thermal evolution of the slab-wedge interface, from just after subduction initiation up through roughly 500–600 km of subduction, where temperatures are approaching steady state. An additional goal is to define optimal conditions for the melting of slab sediments and crust. Results show slab surface temperatures (SSTs) depend strongly on subduction velocity, plate thermal structure, and upper mantle (or wedge) viscosity structure. Fast subduction beneath a thick (>70 km) overriding plate results in the coolest SSTs. Maximum SSTs are recorded as an early transient event for cases of slow subduction ( 100 km) which deflects the zone of maximum shear away from slab-wedge interface.

Triggering of volcanic eruptions

Although earthquakes and volcanic eruptions are each manifestations of large-scale tectonic plate and mantle motions, it is usually thought that the occurrences of these events are not directly related. There have been some studies, however, in which triggering of volcanic eruptions by earthquakes (remote from the volcano) has been proposed,. The 1992 Landers (southern California) earthquake caused triggered seismicity at very large distances, including the magmatically active Long Valley caldera region which also experienced a significant coincident deformation transient. Motivated by this demonstration of the ability of a distant earthquake to disturb a volcanic system, and the earlier studies of specific cases of eruption triggering, we examine here the historical record of eruptions and earthquakes to see if there are indeed significantly more eruptions immediately following large earthquakes. We find that within a day or two of large earthquakes there are many more eruptions within a range of 750 km than would otherwise be expected. Additionally, it is well known that volcanoes separated by hundreds of kilometres frequently erupt in unison; the characteristics of such eruption pairs are also consistent with the hypothesis that the second eruption is triggered by earthquakes associated with the first.

Stress Triggering of the 1999 Hector Mine Earthquake by Transient Deformation Following the 1992 Landers Earthquake

The M 7.3 June 28, 1992 Landers and M 7.1 October 16, 1999 Hector Mine earthquakes, California, both right lateral strike-slip events on NNW-trending subvertical faults, occurred in close proximity in space and time in a region where recurrence times for surface-rupturing earthquakes are thousands of years. This sug- gests a causal role for the Landers earthquake in triggering the Hector Mine earth- quake. Previous modeling of the static stress change associated with the Landers earthquake shows that the area of peak Hector Mine slip lies where the Coulomb failure stress promoting right-lateral strike-slip failure was high, but the nucleation point of the Hector Mine rupture was neutrally to weakly promoted, depending on the assumed coefficient of friction. Possible explanations that could account for the 7-year delay between the two ruptures include background tectonic stressing, dissi- pation of fluid pressure gradients, rate- and state-dependent friction effects, and post- Landers viscoelastic relaxation of the lower crust and upper mantle. By employing a viscoelastic model calibrated by geodetic data collected during the time period between the Landers and Hector Mine events, we calculate that postseismic relaxa- tion produced a transient increase in Coulomb failure stress of about 0.7 bars on the impending Hector Mine rupture surface. The increase is greatest over the broad surface that includes the 1999 nucleation point and the site of peak slip further north. Since stress changes of magnitude greater than or equal to 0.1 bar are associated with documented causal fault interactions elsewhere, viscoelastic relaxation likely con- tributed to the triggering of the Hector Mine earthquake. This interpretation relies on the assumption that the faults occupying the central Mojave Desert (i.e., both the Landers and Hector Mine rupturing faults) were critically stressed just prior to the Landers earthquake.

Viscosity structure beneath northeast Iceland

The dynamics of crustal rifting in Iceland depend strongly on the lower crustal rheology, which controls the intensity of upper crustal stress concentration and scale time of heat diffusion from the underlying mantle plume. While magnetotelluric surveys suggest the presence of a pervasive hot and highly ductile lowermost crust with possibly high fraction of partial melt, observations of low seismic attenuation and strong shear wave transmission suggest a much cooler lower crust and upper mantle. Since viscosity is also sensitive to the degree of partial melt present, viscosity estimates for these regions could shed light on the factors responsible for these observations. In this study we utilize horizontal and vertical displacement vectors determined in GPS campaigns in northeast Iceland since 1986. These are modeled in terms of steady state tectonic loading plus postseismic/postdiking relaxation following the 1975–1984 Krafla rifting episode, as first proposed by Foulger and others. With the elastic part of the model fixed by external constraints, these data have a high sensitivity to the viscosity structure beneath Iceland. Lower crust and upper mantle viscosities of about 3 × 1019 Pa s and 3 × 1018 Pa s, respectively, yield the closest agreement with the data. Our lower crustal viscosity estimate is consistent with the low attenuation and low (subsolidus) temperature for the lower crust inferred in recent studies. Inversions for fissure opening during the Krafla rifting episode yield about 7 m of opening centered on the Krafla rift, as is observed. Allowing for contemporaneous deep rifting on vertical faults along the Askja segment partially accounts for the observed increase in separation across the rift during 1987–1992 but does not account for large displacements in the southeastern part of the network or the large relative subsidence around the Askja rift during 1987–1990. Recent deep normal faulting beneath the Askja rift and further south might explain all of these remaining features.

A comparison of the anelasticity structure beneath western South America and Japan

Similar spectral ratio techniques were used for Q-determinations in two regions. In South America, QP in the (somewhat seismically active) wedge bounded by the seismic plane, the trench and volcanoes, is 1000–2000. Beneath Japan for the corresponding region (which is a seismic) QP is 400–500; this value is identical to that in the asthenosphere below the trench, i.e., there is no evidence for extra low Q-values beneath the volcanoes. In South America, the high QP-values (up to 3000) extend to 350 km, but there do not seem to be any paths from deep earthquakes (600 km) with QP greater than 1000, i.e., there must be a Q-inversion below 350 km. In Japan, however, the high QP-values in the Benioff zone (≈3000) persist down to the deep earthquakes. Above the deep earthquake zone in Japan and Fiji, very low QP-values (100) were found, in contrast to a minimum value of 350 in South America. The tectonic implications of these structures are that lithospheric plates have varying thicknesses; continental plates are several times thicker than oceanic plates.

Slow earthquakes triggered by typhoons

The first reports on a slow earthquake were for an event in the Izu peninsula, Japan, on an intraplate, seismically active fault. Since then, many slow earthquakes have been detected. It has been suggested that the slow events may trigger ordinary earthquakes (in a context supported by numerical modelling), but their broader significance in terms of earthquake occurrence remains unclear. Triggering of earthquakes has received much attention: strain diffusion from large regional earthquakes has been shown to influence large earthquake activity, and earthquakes may be triggered during the passage of teleseismic waves, a phenomenon now recognized as being common. Here we show that, in eastern Taiwan, slow earthquakes can be triggered by typhoons. We model the largest of these earthquakes as repeated episodes of slow slip on a reverse fault just under land and dipping to the west; the characteristics of all events are sufficiently similar that they can be modelled with minor variations of the model parameters. Lower pressure results in a very small unclamping of the fault that must be close to the failure condition for the typhoon to act as a trigger. This area experiences very high compressional deformation but has a paucity of large earthquakes; repeating slow events may be segmenting the stressed area and thus inhibiting large earthquakes, which require a long, continuous seismic rupture.

Beryllium-10 in continental sediments

Abstract The concentration of 10 Be has been measured in 10 samples taken from a transect of surface sediments beginning in the Atchafalaya River and extending across the Bay 136 km into the Gulf of Mexico. If corrected for a lower retentivity of sand for Be, they have a concentration that is constant within 13%. This concentration is about an order of magnitude smaller than that of deep ocean sediments. For comparison, measurements of 10 Be in rainwater, in a sample of soil and in a deep ocean core were made.

Episodic aseismic earthquake precursors

Shallow earthquakes are generally believed to be brittle fractures in a stressed medium with rupture velocity at a speed close to that for shear waves. We know, however, that the Earth allows failure over a wide range of timescales. Creep events occur on the San Andreas fault system, and for some earthquakes the high-speed rupture is accompanied by a slower process1–5. Here we report on episodic slow events which occurred before the Japan Sea earthquake of magnitude 7.7 on 26 May 1983. A Sacks–Evertson borehole strain meter6 90 km from the earthquake recorded about 100 aseismic strain events in a five-month period before and immediately following the earthquake; none has been detected after the large aftershocks. The observed signals are consistent with a precursory redistribution of stress through aseismic slip on a deep extension of the main-shock fault plane. Such episodic aseismic events may provide a mechanism for relatively rapid stress concentration before earthquakes.

Asthenospheric viscosity inferred from correlated land–sea earthquakes in north-east Japan

The viscosity of the Earths mantle has been estimated from studies of post-glacial rebound1,2, post-seismic deformations of the ground following large earthquakes3,4, and aftershock sequences5–7. Here we derive a value for the viscosity of the asthenosphere from a correlation found in the historical catalogue of subduction-induced seismicity between the intraplate (land) and interplate (sea) earthquakes in north-east Japan. The correlation persists since the time of reliably reported earthquakes in AD 1600; land events precede sea events by ∼36 yr, with a mean distance between land–sea pairs of ∼200 km. Because of the visocoelastic coupling of the lithosphere to the asthenosphere, a plausible mechanism to explain the correlation is stress migration, governed by the viscosity of the asthenosphere. Large land shocks generate diffuse-like stress pulses which sweep past, and unlock, the thrust fault in the subduction zone, thus triggering the sea events. The correlation time and distance provide a measure of the speed of diffusion (5.6 km yr−1) and hence an estimate of the viscosity (7 × 1018 Pa s).