Evidence for both crustal and mantle earthquakes in the subducting Juan de Fuca plate

We investigate the relationship between high-precision hypocentres in the subducting Juan de Fuca plate beneath southwest British Columbia, and the velocity structure from receiver function analysis and reflection seismic data. 108 earthquakes at depths of 40 to 65 km were relocated using a double-difference algorithm to obtain precise earthquake locations. Correlation of the relocated seismicity with structural information shows a concentration of earthquakes near the top of the subducting oceanic crust, and a deeper layer of seismicity in the uppermost mantle of the subducting Juan de Fuca plate. The strong correlation of the seismicity with the structure of the subducting plate suggests that seismic failure is caused by changes in mechanical strength within the plate, supporting the hypotheses that phase transformations and thermal-petrological conditions play an important role in the seismogenesis of double-seismic zones

Three dimensional interface modelling with two-dimensional seismic data: the Alpine crust-mantle boundary

We present a new approach to determine the 3-D topography and lateral continuity of seismic interfaces using 2-D-derived controlled-source seismic reflector data. The aim of the approach is to give the simplest possible structure consistent with all reflector data and error estimates. We define simplicity of seismic interfaces by the degree of interface continuity (i.e. shortest length of offsets) and by the degree of interface roughness (least surface roughness). The method is applied to structural information of the crust—mantle boundary (Moho) obtained from over 250 controlled-source seismic reflection and refraction profiles in the greater Alpine region. The reflected and refracted phases from the Moho interface and their interpretation regarding crustal thickness are reviewed and their reliability weighted. Weights assigned to each reflector element are transformed to depth errors considering Fresnel volumes. The 2-D-derived reflector elements are relocated in space (3-D migration) and interpolation is performed between the observed reflector elements to obtain continuity of model parameters. Interface offsets are introduced only where required according to the principle of simplicity. The resulting 3-D model of the Alpine crust—mantle boundary shows two offsets that divide the interface into a European, an Adriatic and a Ligurian Moho, with the European Moho subducting below the Adriatic Moho, and with the Adriatic Moho underthrusting the Ligurian Moho. Each sub-interface depicts the smoothest possible (i.e. simplest) surface, fitting the reflector data within their assigned errors. The results are consistent with previous studies for those regions with dense and reliable controlled-source seismic data. The newly derived Alpine Moho interface, however, surpasses earlier studies by its lateral extent over an area of about 600 km by 600 km, by quantifying reliability estimates along the interface, and by obeying the principle of being consistently as simple as possibl

Pulse of the seafloor: Tidal triggering of microearthquakes at 9°50′N East Pacific Rise

Unequivocal evidence of tidal triggering is observed for microearthquakes (−0.4 to 2.0 M_L) recorded between October 2003 to April 2004 near 9°50′N on the East Pacific Rise (EPR). Although semidiurnal tidal stress changes are small (99.9%) nonrandom temporal distribution, with events occurring preferentially near times of peak extension. Due to the proximity of this site to an ocean tidal node, where changes in sea surface height are minimal, periodic stress changes are dominated by the solid Earth tide. In contrast, previous studies on the Juan de Fuca Ridge have shown microearthquake triggering to be a response to seafloor unloading during times of low ocean tide. The modulation of 9°50′N microearthquakes by small-amplitude periodic stresses is consistent with earthquake nucleation within a high stressing rate environment that is maintained near a critical state of failure by on-axis magmatic and hydrothermal processes

A narrowly spaced double-seismic zone in the subducting Nazca plate

High-precision relocations of intermediate-depth earthquakes (80–130 km) below the Central Andes reveal a fine-scale double-layered Wadati-Benioff zone (WBZ). Upper and lower band of seismicity are separated by about 9 km and occur at the top of the oceanic crust and in the uppermost oceanic mantle, respectively. Analysis of focal mechanisms and waveform similarities indicate that fluid processes are causing the events. Earthquakes in the oceanic crust occur on pre-existing normal faults due to hydraulic embrittlement from metamorphic dehydration, and on subvertical faults that connect the two layers in a narrow depth range. Extensional faulting predominates in both layers, indicating that slab pull forces are the dominant stress source superseding possible unbending forces in this segment of the Nazca plate

Radiography of a normal fault system by 64,000 high-precision earthquake locations: The 2009 L’Aquila (central Italy) case study

We studied the anatomy of the fault system where the 2009 L’Aquila earthquake (MW 6.1) nucleated by means of ~64 k high-precision earthquake locations spanning 1 year. Data were analyzed by combining an automatic picking procedure for P and S waves, together with cross-correlation and double-difference location methods reaching a completeness magnitude for the catalogue equal to 0.7 including 425 clusters of similar earthquakes. The fault system is composed by two major faults: the high-angle L’Aquila fault and the listric Campotosto fault, both located in the first 10 km of the upper crust. We detect an extraordinary degree of detail in the anatomy of the single fault segments resembling the degree of complexity observed by field geologists on fault outcrops. We observe multiple antithetic and synthetic fault segments tens of meters long in both the hanging wall and footwall along with bends and cross fault intersections along the main fault and fault splays. The width of the L’Aquila fault zone varies along strike from 0.3 km where the fault exhibits the simplest geometry and experienced peaks in the slip distribution, up to 1.5 km at the fault tips with an increase in the geometrical complexity. These characteristics, similar to damage zone properties of natural faults, underline the key role of aftershocks in fault growth and co-seismic rupture propagation processes. Additionally, we interpret the persistent nucleation of similar events at the seismicity cutoff depth as the presence of a rheological (i.e., creeping) discontinuity explaining how normal faults detach at depth

Systematic along-axis tidal triggering of microearthquakes observed at 9°50′N East Pacific Rise

Hydrothermal fluid circulation at mid-ocean ridges facilitates the exchange of heat and chemicals between the oceans and the solid Earth, and supports chemosynthetic microbial and macro-faunal communities. The structure and evolution of newly formed oceanic crust plays a dominant role in controlling the character and longevity of hydrothermal systems; however, direct measurements of subsurface processes remain technologically challenging to obtain. Previous studies have shown that tidally-induced stresses within the subseafloor modulate both fluid flow and microearthquake origin times. In this study, we observe systematic along-axis variations between peak microearthquake activity and maximum predicted tidal extension beneath the hydrothermal vent site at 9°50′N East Pacific Rise. We interpret this systematic triggering to result from pore-pressure perturbations propagating laterally through the hydrothermal system. Based on our observations and a one-dimensional pore pressure perturbation model, we estimate bulk permeability at ∼10⁻¹³ to 10⁻¹² m² within layer 2B over a calculated diffusive lengthscale of 2.0 km

Anatomy of a microearthquake sequence on an active normal fault

The analysis of similar earthquakes, such as events in a seismic sequence, is an effective tool with which to monitor and study source processes and to understand the mechanical and dynamic states of active fault systems. We are observing seismicity that is primarily concentrated in very limited regions along the 1980 Irpinia earthquake fault zone in Southern Italy, which is a complex system characterised by extensional stress regime. These zones of weakness produce repeated earthquakes and swarm-like microearthquake sequences, which are concentrated in a few specific zones of the fault system. In this study, we focused on a sequence that occurred along the main fault segment of the 1980 Irpinia earthquake to understand its characteristics and its relation to the loading-unloading mechanisms of the fault system

Probing the equation of state in the AGS energy range with 3-d hydrodynamics

The effect of (i) the phase transition between a quark gluon plasma (QGP) and a hadron gas and (ii) the number of resonance degrees of freedom in the hadronic phase on the single inclusive distributions of 16 different types of produced hadrons for Au+Au collisions at AGS energies is studied. We have used an exact numerical solution of the relativistic hydrodynamical equations without free parameters which, because of its 3-d character, constitutes a considerable improvement over the classical Landau solution. Using two different equations of state (eos) - one containing a phase transition from QGP to the Hadronic Phase and two versions of a purely hadronic eos - we find that the first one gives an overall better description of the Au+Au experimental data at $AGS$ energies. We reproduce and analyse measured meson and proton spectra and also make predictions for anti-protons, deltas, anti-deltas and hyperons. The low m_t enhancement in pi- spectra is explained by baryon number conservation and strangeness equilibration. We also find that negative kaon data are more sensitive to the eos, as well as the K-/pi- ratio. All hyperons and deltas are sensitive to the presence of a phase transition in the forward rapidity region. Anti-protons, Omegas and heavy anti-baryons are sensitive in the whole rapidity range.Comment: 25 pages (.tex) and 9 figures (.ps