A seismological study of shallow weak micro-earthquakes in the urban area of Hamburg city, Germany, and its possible relation to salt dissolution

Abstract

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.

Stress evolution during caldera collapse

“…They are hence important for deformation monitoring and hazard assessment at not only volcanoes, but also other natural or man-made instances of depletion-induced subsidence (Cesca et al, 2011;Dahm et al, 2011;Lenhardt and Pascher, 1996;Segall, 1989). Anderson, 1936).…”

Part 1: Contemporary Challenges and Current Solutions in Sinkhole Occurrence and Mitigation

“…solubility and strength [14] Landscape evolution along coasts, e.g. inlets and bays [15] Known to occur more in evaporite than carbonate karsts due to higher solubility and lower strength [6] Seismic events/earthquakes [16] Groundwater flow-affected by rainfall (recharge) [17] Human Climatic change [6] Changes to drainage patterns-sinkhole frequency increases near drainages, fault, etc. [18] Subsurface drainage also needs consideration-e.g.…”

“…Eastern part of Saudi Arabia-land subsidence problems [29] Eastern Dead Sea shoreline in Jordan-old water channels and water table effects, plus active tectonism [30] Apulia in southern Italy-coastal plains [15,31] Plains in carbonatic ridges of bedrock outcrops in the Apennines [20] Rock type: evaporites Britain-evaporite karst causing subsidence and building damage [32] Hamburg, Germany-salt diaper; seismically affected [16,33] Rock type: calcareous Co. Durham, UK-gypsum dissolution below town of Darlington [34] NE Spain-evaporite dissolution (gypsum, halite, Na-sulphates) under alluvial deposits; groundwater flow accelerates dissolution [17] In turn, sinkholes affect human structures. For example, the Madrid-Barcelona high-speed railway is affected by human-induced sinkholes; land usage in Hamburg, Germany; and causing damage to the built environment, as with mining geohazards in Reading [35][36][37].…”

Sinkholes in the city of Hamburg—New urban shear-wave reflection seismic system enables high-resolution imaging of subrosion structures

“…The depth and deep causes of the sinkholes in Hamburg are unresolved. They may be controlled by salt or by caprock dissolution, and it is unknown whether larger systems of gypsum caves exist (see Dahm et al, 2010Dahm et al, , 2011.…”

“…The latest earthquake series occurred in April 2009 and were recorded by three broadband seismometers. Dahm et al (2011) located the largest of three recorded events and estimated its source mechanism, which has a prominent normal faulting component (see Fig. 2) and a moment magnitude of Mw 0.6.…”