We present source models derived from geodetic data for the four major Canterbury earthquakes of 2010Á2011. The September 2010 Darfield earthquake was largely right-lateral, but with several other fault segments active. The February 2011 Christchurch earthquake was mixed right-lateral and reverse with a left-stepping offset interrupting an ENE-striking rupture. The June 2011 earthquake included left-lateral slip on a NNW-striking fault. The December 2011 earthquakes were characterised by offshore reverse slip on an ENE-striking plane. Displacements of GPS sites define small but clearly detectable postseismic deformation east of the September 2010 earthquake, near the February 2011 earthquake and following the June 2011 earthquake. There has been no major moment release in a 15-km-long region between the eastern end of the September 2010 faulting and the western end of the February 2011 faulting. We recommend careful monitoring of this region for the next several years.

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Great earthquakes rarely occur within active accretionary prisms, despite the intense long-term deformation associated with the formation of these geologic structures. This paucity of earthquakes is often attributed to partitioning of deformation across multiple structures as well as aseismic deformation within and at the base of the prism (Davis et al., 1983). We use teleseismic data and satellite optical and radar imaging of the 2013 M w 7.7 earthquake that occurred on the southeastern edge of the Makran plate boundary zone to study this unexpected earthquake. We first compute a multiple point-source solution from W-phase waveforms to estimate fault geometry and rupture duration and timing. We then derive the distribution of subsurface fault slip from geodetic coseismic offsets. We sample for the slip posterior probability density function using a Bayesian approach, including a full description of the data covariance and accounting for errors in the elastic structure of the crust. The rupture nucleated on a subvertical segment, branching out of the Chaman fault system, and grew into a major earthquake along a 50°north-dipping thrust fault with significant along-strike curvature. Fault slip propagated at an average speed of 3:0 km=s for about 180 km and is concentrated in the top 10 km with no displacement on the underlying décollement. This earthquake does not exhibit significant slip deficit near the surface, nor is there significant segmentation of the rupture. We propose that complex interaction between the subduction accommodating the Arabia-Eurasia convergence to the south and the Ornach Nal fault plate boundary between India and Eurasia resulted in the significant strain gradient observed prior to this earthquake. Convergence in this region is accommodated both along the subduction megathrust and as internal deformation of the accretionary wedge.

Flank instability and sector collapses, which pose major threats, are common on volcanic islands. On 22 Dec 2018, a sector collapse event occurred at Anak Krakatau volcano in the Sunda Strait, triggering a deadly tsunami. Here we use multiparametric ground-based and space-borne data to show that prior to its collapse, the volcano exhibited an elevated state of activity, including precursory thermal anomalies, an increase in the island’s surface area, and a gradual seaward motion of its southwestern flank on a dipping décollement. Two minutes after a small earthquake, seismic signals characterize the collapse of the volcano’s flank at 13:55 UTC. This sector collapse decapitated the cone-shaped edifice and triggered a tsunami that caused 430 fatalities. We discuss the nature of the precursor processes underpinning the collapse that culminated in a complex hazard cascade with important implications for the early detection of potential flank instability at other volcanoes.

The increasing demands upon groundwater resources due to expanding metropolitan and agricultural areas are a serious challenge, particularly in semiarid and arid regions. In Iran, decades of unrestrained groundwater extraction for domestic, agricultural, and industrial use have resulted in a precipitous depletion of this valuable resource. Here we show that the decline in groundwater levels is associated with land‐surface deformation on local and regional scales. Combining water‐level data with satellite radar observations provides evidence for the prevalence of compacting aquifers in the country. Groundwater level decline is often associated with destruction of the aquifers, which appears to be a common problem in the groundwater basins of central and northeast Iran. Global warming and future climate change will affect arid and semiarid areas in the coming decades, further augmenting hazards associated with groundwater‐induced land subsidence.

Abstract:Beijing is one of the most water-stressed cities in the world. Due to over-exploitation of groundwater, the Beijing region has been suffering from land subsidence since 1935. In this study, the Small Baseline InSAR technique has been employed to process Envisat ASAR images acquired between 2003 and 2010 and TerraSAR-X stripmap images collected from 2010 to 2011 to investigate land subsidence in the Beijing region. The maximum subsidence is seen in the eastern part of Beijing with a rate greater than 100 mm/year. Comparisons between InSAR and GPS derived subsidence rates show an RMS difference of 2.94 mm/year with a mean of 2.41˘1.84 mm/year. In addition, a high correlation was observed between InSAR subsidence rate maps derived from two different datasets (i.e., Envisat and TerraSAR-X). These demonstrate once again that InSAR is a powerful tool for monitoring land subsidence. InSAR derived subsidence rate maps have allowed for a comprehensive spatio-temporal analysis to identify the main triggering factors of land subsidence. Some interesting relationships in terms of land subsidence were found with groundwater level, active faults, accumulated soft soil thickness and different aquifer types. Furthermore, a relationship with the distances to pumping wells was also recognized in this work.

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