Limitations of rupture forecasting exposed by instantaneously triggered earthquake doublet

Earthquake hazard assessments and rupture forecasts are based on the potential length of seismic rupture and whether or not slip is arrested at fault segment boundaries. Such forecasts do not generally consider that one earthquake can trigger a second large event, near-instantaneously, at distances greater than a few kilometers. Here we present a geodetic and seismological analysis of a magnitude 7.1 intra-continental earthquake that occurred in Pakistan in 1997. We find that the earthquake, rather than a single event as hitherto assumed, was in fact an earthquake doublet: initial rupture on a shallow, blind 2 reverse fault was followed just 19 seconds later by a second rupture on a separate reverse fault 50 km away. Slip on the second fault increased the total seismic moment by half, and doubled both the combined event duration and the area of maximum ground shaking. We infer that static Coulomb stresses at the initiation location of the second earthquake were probably reduced as a result of the first. Instead, we suggest that a dynamic triggering mechanism is likely, although the responsible seismic wave phase is unclear. Our results expose a flaw in earthquake rupture forecasts that disregard cascading, multiple-fault ruptures of this type

Stimulated X-ray emission for materials science

Resonant inelastic X-ray scattering and X-ray emission spectroscopy can be used to probe the energy and dispersion of the elementary low-energy excitations that govern functionality in matter: vibronic, charge, spin and orbital excitations(1-7). A key drawback of resonant inelastic X-ray scattering has been the need for high photon densities to compensate for fluorescence yields of less than a per cent for soft X-rays(8). Sample damage from the dominant non-radiative decays thus limits the materials to which such techniques can be applied and the spectral resolution that can be obtained. A means of improving the yield is therefore highly desirable. Here we demonstrate stimulated X-ray emission for crystalline silicon at photon densities that are easily achievable with free-electron lasers(9). The stimulated radiative decay of core excited species at the expense of non-radiative processes reduces sample damage and permits narrow-bandwidth detection in the directed beam of stimulated radiation. We deduce how stimulated X-ray emission can be enhanced by several orders of magnitude to provide, with high yield and reduced sample damage, a superior probe for low-energy excitations and their dispersion in matter. This is the first step to bringing nonlinear X-ray physics in the condensed phase from theory(10-16) to application

FemtoSpeX a versatile optical pump soft X ray probe facility with 100 fs X ray pulses of variable polarization

Here the major upgrades of the femtoslicing facility at BESSY II Khan et al., 2006 are reviewed, giving a tutorial on how elliptical polarized ultrashort soft X ray pulses from electron storage rings are generated at high repetition rates. Employing a 6 kHz femtosecond laser system consisting of two amplifiers that are seeded by one Ti Sa oscillator, the total average flux of photons of 100 fs duration FWHM has been increased by a factor of 120 to up to 106 photons s 1 0.1 bandwidth 1 on the sample in the range from 250 to 1400 eV. Thanks to a new beamline design, a factor of 20 enhanced flux and improvements of the stability together with the top up mode of the accelerator have been achieved. The previously unavoidable problem of increased picosecond background at higher repetition rates, caused by halo photons, has also been solved by hopping between different camshaft bunches in a dedicated fill pattern 3 1 camshaft fill of the storage ring. In addition to an increased X ray performance at variable linear and elliptical polarization, the sample excitation in pump probe experiments has been considerably extended using an optical parametric amplifier that supports the range from the near UV to the far IR regime. Dedicated endstations covering ultrafast magnetism experiments based on timeresolved X ray circular dichroism have been either upgraded or, in the case of time resolved resonant soft X ray diffraction and reflection, newly constructed and adapted to femtoslicing requirements. Experiments at low temperatures down to 6 K and magnetic fields up to 0.5 T are supported. The FemtoSpeX facility is now operated as a 24 h user facility enabling a new class of experiments in ultrafast magnetism and in the field of transient phenomena and phase transitions in solid