SummaryIn the night from 8/9 April 2009, shortly after midnight on Maundy Thursday before Easter, several people in Gross-Flottbek, Hamburg, felt unusual strong ground shocks so that some of them left their houses in fear of earthquake shaking. Police and Fire Brigade received phone calls of worried residents. A few days later Internet pages were published where people reported their observations. On 21 April 2009 at about 8 p.m. local time a second ground shaking event was felt. Damage to buildings or infrastructure did not occur to our knowledge. The Institute of Geophysics, University of Hamburg, installed from 22 April to 17 May 2009 three temporal seismic stations in the epicentral area. Seismological data from two nearby stations at the Deutsches Elektron-Synchrotron (DESY) at 1 km and the Geophysical Institute at 7 km distance were collected and integrated to the temporal network. The events occurred above the roof of the shallow Othmarschen Langenfelde salt diapir (OLD), in an area known for active sinkhole formation and previous historic ground shaking events. The analysis of the seismological data shows that three shallow microearthquakes occurred from 8 to 21 April at a depth of about 100 m, the largest one with a moment magnitude of about M W 0.6. Depth location of such shallow events is difficult with standard methods, and is here constrained by waveform modeling of surface waves. Earthquakes occurring in soft sediments within the uppermost 100 m are a rare phenomena and cannot be explained by standard models. Rupture process in soft sediments differ from those on faults in more competent rock. We discuss the rupture and source mechanism of the events in the context of previous historic shocks and existing sinkhole and deformation data. Although the event was weak, the rupture duration of 0.3 s was unusual long. Three possible models for the generation of repeated ground shaking events in Gross Flottbek are developed and discussed, implying quit different hazards for subsidence, ground motion and sinkhole formation. Our favored model postulates that roof failure occurs in an existing soil cavity beneath the epicenter at a depth of about 100 m. Other models bearing a smaller geo-hazard cannot be disproved with the data available, but future geophysical experiments may be planned to resolve this question.

Both the European X-ray Free Electron Laser (XFEL) [1] and the proposed International Linear Collider (ILC)[2] will rely on superconducting accelerating modules (cryomodules) for their linear accelerator (linac) section/s. In this paper, vibration stability studies of a type III cryomodule with relation to its mechanical design, at room temperature, have been presented. Results of this study are relevant for the design of both the XFEL and the ILC module prototypes.

A new method for low level low frequency vibration measurements in the cryogenic environment is presented. Commercial moving coil seismometers (geophones) have been used to investigate the mechanical stability of the quadrupole of a third generation (so-called Type-III) FLASH cryomodule (named Module 6) in fully operating conditions. Geophones were able to operate at 4.5 K without any loss of performance, providing nanometer level resolution even in the 1-10 Hz frequency band, region not covered by existing data because of the reduced sensitivity of the cooled piezoelectric accelerometers [1]. A preliminary evaluation of the impact of the refrigeration system and of the high power RF on the vibration level of the quadrupole is also presented. The results are of interest for the design of linear accelerator (linac) cryomodules of the International Linear Collider (ILC) [2] and of the European X-ray Free Electron Laser (XFEL) [3], for which the design is a further evolution of the FLASH Type III.

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