Induced seismicity during the construction of the Gotthard Base Tunnel, Switzerland: hypocenter locations and source dimensions

A series of 112 earthquakes was recorded between October 2005 and August 2007 during the excavation of the MFS Faido, the southernmost access point of the new Gotthard Base Tunnel. Earthquakes were recorded at a dense network of 11 stations, including 2 stations in the tunnel. Local magnitudes computed from Wood–Anderson-filtered horizontal component seismograms ranged from −1.0 to 2.4; the largest earthquake was strongly felt at the surface and caused considerable damage in the tunnel. Hypocenter locations obtained routinely using a regional 3-D P-wave velocity model and a constant Vp/Vs ratio 1.71 were about 2 km below the tunnel. The use of seismic velocities calibrated from a shot in the tunnel revealed that routinely obtained hypocenter locations were systematically biased to greater depth and are now relocated to be on the tunnel level. Relocation of the shot using these calibrated velocities yields a location accuracy of 25 m in longitude, 70 m in latitude, and 250 m in focal depth. Double-difference relative relocations of two clusters with highly similar waveforms showed a NW–SE striking trend that is consistent with the strike of mapped faults in the MFS Faido. Source dimensions computed using the quasidynamic model of Madariaga (Bull Seismo Soc Am 66(3):639–666, 1976) range from 50 to 170 m. Overlapping source dimensions for earthquakes within the two main clusters suggests that the same fault patch was ruptured repeatedly. The observed seismicity was likely caused by stress redistribution due to the excavation work in the MFS Faido.     

International tourism and civil aviation in the South Pacific: issues and innovations

This paper examines recent trends in tourism in the South Pacific islands and looks in particular at the way in which airline links between markets and destinations are forged, maintained and developed through time. Geography, politics and organisational dynamics combine to present island nations in the South Pacific with numerous dilemmas. The interdependencies between tourism and aviation cannot be over-stressed. Tourism is no panacea to cure the economic problems of Pacific island microstates, but tourism and aviation are complementary industries which offer better prospects than standing in the dole queue for aid. Given the right political and managerial climate, there is a chance to trade with dignity.     

Gravity anomalies and dynamics of the Swiss Alps

An interpretation of the gravity observations in Switzerland is presented. The gravity anomalies are mainly caused by crustal effects. Taking 0.5 g/cm³ as the average density contrast between crust and upper mantle, the remaining positive residual field of + 50 mGal can be explained as the effect of the so-called “lithospheric root” underneath the Swiss Alps. This “root” must be considered as a relatively cold body with increased density. In order to calculate the thermally induced density distribution in the upper mantle, a kinematic and geothermal model has been constructed simulating the Alpine history for a time span covering the last 40 m.y.At the same time the moving lithospheric body (crustal uplift and subsidence of the lower lithosphere) is analysed from a dynamical point of view in order to test the kinematic conditions. The calculation leads to a mantle subsidence rate of 1 mm/y for the present period.     

Regional minimum 1-D P-wave velocity model for a new seismicity catalogue with precise and consistent earthquake locations in southern Iran

For faster and more robust ray tracing in 1-D velocity models and also due to the lack of reliable 3-D models, most seismological centers use 1-D models for routine earthquake locations. In this study, as solution to the coupled hypocenter-velocity problem, we compute a regional P-wave velocity model for southern Iran that can be used for routine earthquake location and also a reference initial model for 3-D seismic tomography. The inversion process was based on travel time data from local earthquakes paired reports obtained by merging the catalogues of Iranian Seismic Center (IRSC, 6422 events) and by the Broadband Iranian National Seismic Network (BIN, 4333 events) for southern Iran in the period 2006 through July 2017. After cleaning the data set from large individual reading errors and by identifying event reports from both networks belonging to same earthquake (a process called event pairing), we obtained a data set of 1115 well-locatable events with a total number of 24,606 P-wave observations. This data set was used to calculate a regional minimum 1-D model for southern Iran as result of an extensive model search by trial-and-error process including several dozens of inversions. Significantly different from previous models, we find a smoothly increasing P-velocity by depth with velocities of 5.8 km/s at shallow and velocities of 6.4 km/s at deepest crustal levels. For well-locatable events, location uncertainties are estimated in the order of ±?3 km for epicenter and double this uncertainty for hypocentral depth. The use of the minimum 1-D model with appropriate station delays in routine hypocenter location processing will yield a high-quality seismic catalogue with consistent uncertainty estimates across the region and it will also allow detection of outlier observations. Based on the two catalogues by IRSC and BIN and using the minimum 1-D model and station delays for all stations in the region, we established a new combined earthquake catalogue for southern Iran. While the general distribution of the seismicity corresponds well with that of the two individual catalogues by IRSC and BIN, the new catalogue significantly enhances the correlation of seismicity with the regional fault systems within and between the major crustal blocks that as an assembly build this continental region. Furthermore, the unified seismic catalogue and the minimum 1-D model resulting from this study provide important ingredients for seismic hazard studies.     

Intracontinental subduction and Palaeozoic inheritance of the lithosphere suggested by a teleseismic experiment across the Chinese Tien Shan

Teleseismic tomography across the Chinese Tien Shan shows that seismic wave speeds in the lithosphere beneath central Tien Shan are high and therefore the lithosphere is not weaker than that beneath the adjacent undeformed Tarim and Junggar basins. There is evidence for significant velocity contrasts within the lithosphere that are presumably inherited from the Palaeozoic collision history. The high-velocity, thick Yili block observed underneath the northern Tien Shan is a clue for shortening by a intracontinental subduction. The observed geometry is consistent with a simple model of intracontinental subduction and suggests that, during orogeny, the lithosphere has remained heterogeneous and has deformed along existing planes of weakness rather than by homogeneous thickening of a particularly weak lithosphere.