Highlights Archiv 2014

Off-rift volcanism in rift zones determined by crustal unloading (23.03.2014)
Francesco Maccaferri, Eleonora Rivalta, Derek Keir, Valerio Acocella

Volcanoes often occur in rift valleys, within the rift itself or on the rift flanks as e.g. in East Africa. A team of scientists from the GFZ German Research Centre for Geosciences, University of Southampton and University Roma Tre (Italy) have shown that the pattern of stresses in the crust changes when the crust thins due to stretching and becomes gravitationally unloaded. As a consequence of this stress pattern, the path of the magma pockets ascending from the ponding zone is deviated diagonally to the sides of the rift.

| Nature Geoscience | | DOI: 10.1038/NGEO2110 |

Groundwater level changes induced by the 2011 Tohoku earthquake in China mainland
Rui Yan; Heiko Woith; Rongjiang Wang (2014)

Groundwater level changes induced by the 2011 Mw = 9.0 Tohoku earthquake were observed in China mainland at epicentral distances between 1300 and 5400 km. Out of 216 wells, 73 showed sustained changes that are evenly distributed between those experiencing water level rises and falls. Water level oscillations during the passage of the seismic waves were recorded at another 85 wells. At the remaining 58 wells, no response to the earthquake was observed. No spatial pattern is evident. About 80 per cent of the well-aquifer systems are sensitive to Earth tides. Wells that showed a response to the Tohoku earthquake are characterized by admittance factors with respect to the major tidal constituents M2 and O1 above 0.5 m of water level change per microstrain. Postseismic phase shifts in the M2 tide were observed at 31 sites. Such phase shifts may indicate changes in the aquifer permeability. Thus, earthquake-induced temporal variations in the permeability might have occurred at about 43 per cent of those wells that displayed sustained water level changes, but less than 15 per cent of all observed wells.

| Geophysical Journal International | DOI: 10.1093/gji/ggu196 | PDF |

Velocity ratio variations in the source region of earthquake swarms in NW Bohemia obtained from arrival time double-differences
Dahm, T. and T. Fischer (2014)

The origin of energetic, non-volcanic midcrustal earthquake swarms in NW Bohemia / Vogtland in the Czech Republic / German border region is still unsolved.  Earthquake swarms since 1985 occur regularly with decreasing interswarm periods in recent years. Single swarms consist of thousands of weak earthquakes with maximal  magnitudes up to M4.5, and may last several month up to years.  The release and migration of deep magmatic fluids and gases in the middle crust are suggested as a possible cause to explain the seismic swarm activity. The paper analyses for the first time the change of elastic properties resolved with  high temporal resolution directly in the earthquake swarm focal zone.  For this, a new method has been applied which considers double differential times of P and S waves from clustered swarm earthquakes.  Thus, the possible influence of the long travelpath from the earthquakes to the seismic stations at the surface is cancelled out, and only  changes in the source volume of the swarms is considered.Unexpectedly, we found for the three swarms analyzed very strong and consistent temporal variations, indicating a strong drop of P-wave velocity in the beginning of a swarm phase and a continues recovery during the final stage of the activity. The observations might be explained by the instantaneous degassing of fluids under high overpressure, and  the slow upward migration of the gaseous fluids directly after the initial phase.The findings support the hypothesis that gas is controlling the earthquake swarms, and the methodical approach may be used in future to better monitor and predict the duration and strength of earthquake swarms in NW Bohemia.

| Geophysical Journal International | DOI:10.1093/gji/ggt410 |

The 2013 September–October seismic sequence offshore Spain: a case of seismicity triggered by gas injection?
Simone Cesca, Francesco Grigoli, Sebastian Heimann, Alvaro González, Elisa Buforn, Samira Maghsoudi, Estefania Blanch and Torsten Dahm

These are the most accurate epicentral locations, obtained using a 3D velocity model. All events are above Ml=2.0.
These are the most accurate epicentral locations, obtained using a 3D velocity model. All events are above Ml=2.0.

This paper timely investigated a very significant seismic sequence offshore Spain in Autumn 2013, culminating in a maximal magnitude Mw 4.3 on October 1, 2013. The epicentral region is located close to the Castor project platform, where gas was injected in a depleted oil reservoir at 2 km depth, an operation which rarely induced significant seismicity in the past. Through a full waveform seismological analysis we could locate and characterise the seismic sources, and additionally evaluate statistical parameters of the seismic sequence. Our study suggests that seismicity triggered by gas  injection occurred along a low-dipping angle fault.

| Geophysical Journal International | DOI:/10.1093/gji/ggu172 | PDF |

Gradual unlocking of plate boundary controlled initiation of the 2014 Iquique earthquake
Bernd Schurr, Günter Asch, Sebastian Hainzl, Jonathan Bedford, Andreas Hoechner, Mauro Palo, Rongjiang Wang, Marcos Moreno, Mitja Bartsch, Yong Zhang, Onno Oncken, Frederik Tilmann, Torsten Dahm, Pia Victor, Sergio Barrientos & Jean-Pierre Vilotte (2014)

The Pisagua earthquake, which took place on 1st of April 2014 offshore the coast of Chile, broke part of the so-called northern Chile seismic gap, the last segment of the South American plate boundary that had not ruptured in the last century. For this reason, the region has been monitored closely with various instruments for several years now. The Integrated Plate boundary Observatory Chile (IPOC), in which GFZ is the coordinating partner, comprises broadband seismometers, accelerometers, GNSS (Global Navigation Satellite System) stations, and strain meters, amongst others. Evaluation of the data from these instruments enabled not only the development of a model for the spatial and temporal evolution of the main shock and the largest aftershock (white contour lines in the left panel), but also derivation of the long-term coupling state of the tectonic plates (background coloring). Furthermore, statistical analysis of the foreshock activity (blue dots) revealed a decrease of its b-value, which describes the ratio of the frequency between larger and smaller quakes (lower right panel). These observations lead to the conclusion that gradual weakening of the central part of the seismic gap, accentuated by the foreshock activity in a zone of intermediate seismic coupling, was instrumental in causing failure of the plate boundary. This insight distinguishes the Pisagua earthquake from most great earthquakes. Finally, it is noteworthy that the earthquake ruptured only part of the previously-locked segment; the remaining locked part now poses a significant and increased seismic hazard, with the potential to host an earthquake of magnitude larger than 8.5.

| Nature | DOI: /10.1038/nature13681 |

Satellite radar data reveal short-term pre-explosive displacements and a complex conduit system at Volcán de Colima, Mexico

Jacqueline T. Salzer, Mehdi Nikkhoo, Thomas R. Walter, Henriette Sudhaus, Gabriel Reyes-Dávila, Mauricio Bretón and Raúl Arámbula (2014)

The geometry of the volcanic conduit is a main parameter controlling the dynamics and the style of volcanic eruptions and their precursors, but also one of the main unknowns. Pre-eruptive signals that originate in the upper conduit region include seismicity and deformation of different types and scales. However, the locality of the source of these signals and thus the conduit geometry often remain unconstrained at steep sloped and explosive volcanoes due to the sparse instrumental coverage in the summit region and difficult access. Here we infer the shallow conduit system geometry of Volcán de Colima, Mexico, based on ground displacements detected in high resolution satellite radar data up to 7 h prior to an explosion in January 2013. We use Boundary Element Method modeling to reproduce the data synthetically and constrain the parameters of the deformation source, in combination with an analysis of photographs of the summit. We favor a two-source model, indicative of distinct regions of pressurization at very shallow levels. The horizontal location of the upper pressurization source coincides with that of post-explosive extrusion. The pattern and degree of deformation reverses again during the eruption; we therefore attribute the displacements to transient (elastic) pre-explosive pressurization of the conduit system. Our results highlight the geometrical complexity of shallow conduit systems at explosive volcanoes and its effect on the distribution of pre-eruptive deformation signals. An apparent absence of such signals at many explosive volcanoes may relate to its small temporal and spatial extent, partly controlled by upper conduit structures. Modern satellite radar instruments allow observations at high spatial and temporal resolution that may be the key for detecting and improving our understanding of the generation of precursors at explosive volcanoes.

|Frontiers in Earth Science| doi: 10.3389/feart.2014.00012 |

Static stress triggering explains the empirical aftershock distance decay.
Hainzl, S., Moradpour, J., Davidsen, J. (2014)

The shape of the spatial aftershock decay is sensitive to the triggering mechanism and thus particularly useful for discriminating between static and dynamic stress triggering. The empirically observed rather small decay exponent has been previously declared to indicate dynamic stress triggering. We show by comprehensive simulations of a simple clock-advance model that static stress triggering can explain the empirical observations with a remarkable agreement with the observation over the whole distance range.

Geophysical Research Letters, 41, 24, p. 8818-8824.| doi:10.1002/2014GL061975 | pdf |