In this paper we test whether or not structural and morphological features inherited from the Eurasian continental margin are affecting the contemporary stress and strain fields in south-central Taiwan. Principal stress directions (sigma(1), sigma(2) and sigma(3)) are estimated from the inversion of clustered earthquake focal mechanisms and the direction of the maximum compressive horizontal stress (S-H) is calculated throughout the study area. From these data the most likely fault plane orientations and their kinematics are inferred. The results of the stress inversion are then discussed together with the directions of displacement, compressional strain rate and maximum shear strain rate derived from GPS data. These data show that there is a marked contrast in the direction of SH from north to south across the study area, with the direction of SH remaining roughly subparallel to the relative plate motion vector in the north, whereas in the south it rotates nearly 45 degrees counter-clockwise. The direction of the horizontal maximum compression strain rate (epsilon(H)) and associated maximum shear planes, together with the displacement field display an overall similar pattern between them, although undergoing a less marked rotation. We interpret the southward change in the SH, eH and the dextral maximum shear plane directions, together with that of the horizontal displacement field to be related to the reactivation of east-northeast striking faults inherited from the rifted Eurasian margin and to the shelf/slope break. Inherited faults in the basement are typically reactivated as strike-slip faults, whereas newly formed faults in the fold-and-thrust belt are commonly thrusts or oblique thrusts. Eastwards, the stress inversions and strain data show that the western flank of the Central Range is undergoing extension in the upper crust. SH in the Central Range is roughly parallel to the relative plate convergence vector, but in southwestern Taiwan it undergoes a marked counter-clockwise rotation westwards across the Chaochou fault. Farther north, however, there is no significant change across the Lishan fault. This north to south difference is likely due to different margin structures, although local topographic effects may also play a role.

An intraplate earthquake doublet, with 11-min delay between the events, devastated the city of Varzeghan in northwestern Iran on August 11, 2012. The first Mw 6.5 strike-slip earthquake, which occurred after more than 200 years of low seismicity, was followed by an Mw 6.4 oblique thrust event at an epicentral separation of about 6 km. While the first event can be associated with a distinct surface rupture, the absence of a surface fault trace and no clear aftershock signature makes it challenging to identify the fault plane of the second event. We use teleseismic body wave inversion to deduce the slip distribution in the first event. Using both P and SH waves stabilize the inversion and we further constrain the result with the surface rupture extent and the aftershock distribution. The obtained slip pattern shows two distinct slip patches with dissimilar slip directions where aftershocks avoid high-slip areas. Using the estimated slip for the first event, we calculate the induced Coulomb stress change on the nodal planes of the second event and find a preference for higher Coulomb stress on the N-S nodal plane. Assuming a simple slip model for the second event, we estimate the combined Coulomb stress changes from the two events on the focal planes of the largest aftershocks. We find that 90% of the aftershocks show increased Coulomb stress on one of their nodal planes when the N-S plane of the second event is assumed to be the correct fault plane.

The effective elastic thickness (T-e) of the lithosphere provides geophysical information about long-term flexural strength and can be used to constrain thermorheological properties of the lithosphere. T-e is typically calculated from the spectral analysis of gravity and topography data; variations in T-e are, however, not well resolved in Greenland due to poor constraints on crustal structure (including crustal thickness) and complications due to ice loading. In addition, geological and geophysical constraints on the tectonic history of Greenland are sparse due to the thick ice cover. Here we use the global gravity model EIGEN-6C4 together with a new model of the crust-mantle boundary to obtain a high-resolution T-e map of Greenland. The distribution of T-e indicates reduced strength in the lower crust and lithospheric mantle beneath southern and central Greenland, which may be due to the passage of the Iceland hot spot during the last 100Ma. In contrast, the northern part of Greenland shows a large T-e, implying mechanical coupling between crust and uppermost mantle and suggesting the existence of a cold and strong tectonic unit. In a relative sense, the distribution of T-e values is consistent with estimates of lithospheric thickness based on seismic velocity models, indicating a dominantly thermal control on lithospheric structure and evolution.

Plain Language Summary Greenland is covered by a large ice sheet; its geodynamic history is therefore mostly unknown as only the coastal areas are accessible for direct geological sampling. However, geophysical data can be used to investigate the lithospheric structure to infer its geodynamic history. Here we use satellite gravity data together with elevation data and a crustal density model to look at the distribution of the effective elastic thickness, which provides information about the variations in strength of the lithosphere. We find that most of Greenland's lithosphere is weaker than expected given its age of formation (approximate to 1 billion years), suggesting that it was affected by a thermal event within the last approximate to 400 million years. We interpret the weakness in the lithosphere as a result of the movement of Greenland with respect to hot mantle material, which is now located beneath Iceland leading to large volcanism there. The results of this study show for the first time the effect of the hot mantle material on the lithosphere of Greenland, which can help us identify the different possible tracks of the moving hot mantle material and understand the stability of the ice sheet.

A peculiar cluster of seismicity near the tip of Sandfellsjokull on the eastern flank of Katla volcano in southern Iceland has been analyzed in detail using data from a temporary seismic network. A total of 300 events were detected between July 2011 and August 2013, most of them from a swarm between December 4th and 12th, 2011. The sparser permanent network detected a small fraction of these events, but also a larger swarm in November 2010. When seismic activity started in this area is uncertain because of changes in the detection capability of the network over time. The events are of low magnitude (-0.5 < ML < 0.5) and the b-value of their magnitude distribution is high (1.6 +/- 0.1). Based on their frequency content (4-25 Hz) and clear P and S arrivals, the events are classified as volcano-tectonic. Two multiplets probably with different source mechanism are identified in their population. The events locate at approximately 3.5 km depth. Most of them are tightly clustered according to double difference relative locations in a volume that is only about 400 m in diameter in all directions. Several events are scattered up to 800 m beneath this volume. There is some suggestion of elongate structure in the cluster with a NNE/SSW strike and a dip of 60 degrees. We argue that these events cannot be due to a glacial or a broad tectonic process. Possibly, a localized source of fluid pressure, e.g., a small magma body at depth may be the source of these events.

We investigate the performance of a seismic event detection algorithm using migration and stacking of seismic traces. The focus lies on determining optimal data dependent detection parameters for a data set from a temporary network in the volcanically active Hengill area, southwest Iceland. We test variations of the short-term average to long-term average and Kurtosis functions, calculated from filtered seismic traces, as input data. With optimal detection parameters, our algorithm identified 94 per cent (219 events) of the events detected by the South Iceland Lowlands (SIL) system, that is, the automatic system routinely used on Iceland, as well as a further 209 events, previously missed. The assessed number of incorrect (false) detections was 25 per cent for our algorithm, which was considerably better than that from SIL (40 per cent). Empirical tests show that well-functioning processing parameters can be effectively selected based on analysis of small, representative subsections of data. Our migration approach is more computationally expensive than some alternatives, but not prohibitively so, and it appears well suited to analysis of large swarms of low magnitude events with interevent times on the order of seconds. It is, therefore, an attractive, practical tool for monitoring of natural or anthropogenic seismicity related to, for example, volcanoes, drilling or fluid injection.

The crust-mantle boundary (the Moho) is a first order interface in the Earth and the depth to the Moho is therefore well studied in most regions. However, below regions which are covered by large ice sheets, such as Greenland and Antarctica, the Moho is only partly known and seismic data are difficult to obtain. Here, we take advantage of the global gravity model EIGEN-6C4, together with the Parker-Oldenburg algorithm, to estimate the depth to the Moho beneath Greenland and surroundings. The available free-air gravity data are corrected for the topographic effect and the effect of sedimentary basins. We also correct for the effect on gravity due to the weight of the ice sheet and the accompanying deflection of the Earth's surface, which has not previously been taken into account in gravity studies of currently glaciated regions. Our final Moho depth model for Greenland has an associated uncertainty of +/- 4.5 km for areas with sedimentary basins and 4 km for areas without sedimentary basins. The model shows maximum Moho depths below east Greenland of up to 55 km and values less than 20 km offshore east Greenland. There is a marked increase in Moho depth of 10-15 km from northern to central Greenland, indicating a significant change in geology. A deep Moho at the northern coast of Greenland towards Ellesmere Island might be related to the location of the hot-spot track. Our Moho model is consistent with previous models, but has a higher lateral resolution of 0.1 degrees and covers the entire area of on- and offshore Greenland.

Much of the seismic activity of northern Sweden consists of micro-earthquakes occurring near postglacial faults. However, larger magnitude earthquakes do occur in Sweden, and earthquake statistics indicate that a magnitude 5 event is likely to occur once every century. This paper presents dynamic analyses of the effects of larger earthquakes on an upstream tailings dam at the Aitik copper mine in northern Sweden. The analyses were performed to evaluate the potential for liquefaction and to assess stability of the dam under two specific earthquakes: a commonly occurring magnitude 3.6 event and a more extreme earthquake of magnitude 5.8. The dynamic analyses were carried out with the finite element program PLAXIS using a recently implemented constitutive model called UBCSAND. The results indicate that the magnitude 5.8 earthquake would likely induce liquefaction in a limited zone located below the ground surface near the embankment dikes. It is interpreted that stability of the dam may not be affected due to the limited extent of the liquefied zone. Both types of earthquakes are predicted to induce tolerable magnitudes of displacements. The results of the postseismic slope stability analysis, performed for a state after a seismic event, suggest that the dam is stable during both the earthquakes.

Katla is a threatening volcano in Iceland, partly covered by the Myrdalsjokull ice cap. The volcano has a large caldera with several active geothermal areas. A peculiar cluster of long-period seismic events started on Katla's south flank in July 2011, during an unrest episode in the caldera that culminated in a glacier outburst. The seismic events were tightly clustered at shallow depth in the Gvendarfell area, 4 km south of the caldera, under a small glacier stream at the southern margin of Myrdalsjokull. No seismic events were known to have occurred in this area before. The most striking feature of this seismic cluster is its temporal pattern, characterized by regular intervals between repeating seismic events, modulated by a seasonal variation. Remarkable is also the stability of both the time and waveform features over a long time period, around 3.5 years. We have not found any comparable examples in the literature. Both volcanic and glacial processes can produce similar waveforms and therefore have to be considered as potential seismic sources. Discerning between these two causes is critical for monitoring glacier-clad volcanoes and has been controversial at Katla. For this new seismic cluster on the south flank, we regard volcano-related processes as more likely than glacial ones for the following reasons: 1) the seismic activity started during an unrest episode involving sudden melting of the glacier and a jokulhlaup; 2) the glacier stream is small and stagnant; 3) the seismicity remains regular and stable for years; 4) there is no apparent correlation with short-term weather changes, such as rainstorms. We suggest that a small, shallow hydrothermal system was activated on Katla's south flank in 2011, either by a minor magmatic injection or by changes of permeability in a local crack system.

Glacially induced intraplate faults are conspicuous in Fennoscandia where they reach trace lengths of up to 155 km with estimated magnitudes up to 8 for the associated earthquakes. While they are typically found in northern parts of Fennoscandia, there are a number of published accounts claiming their existence further south and even in northern central Europe. This study focuses on a prominent scarp discovered recently in lidar (light detection and ranging) imagery hypothesized to be from a post-glacial fault and located about 250 km north of Stockholm near the town of Bollnas. The Bollnas scarp strikes approximately north-south for about 12 km. The maximum vertical offset in the sediments across the scarp is 4-5m with the western block being elevated relative to the eastern block. To investigate potential displacement in the bedrock and identify structures in it that are related to the scarp, we conducted a multidisciplinary geophysical investigation that included gravity and magnetic measurements, high-resolution seismics, radio-magnetotellurics (RMT), electrical resistivity tomography (ERT) and ground-penetrating radar (GPR). Results of the investigations suggest a zone of low-velocity and high-conductivity in the bedrock associated with a magnetic lineament that is offset horizontally about 50m to the west of the scarp. The top of the bedrock is found similar to 10m below the surface on the eastern side of the scarp and about similar to 20m below on its western side. This difference is due to the different thicknesses of the overlying sediments accounting for the surface topography, while the bedrock surface is likely to be more or less at the same topographic level on both sides of the scarp; else the difference is not resolvable by the methods used. To explain the difference in the sediment covers, we suggest that the Bollnas scarp is associated with an earlier deformation zone, within a wide (> 150 m), highly fractured, water-bearing zone that became active as a reverse fault after the latest Weichselian deglaciation.