Relative relocation of earthquakes without a predefined velocity model: an example from a peculiar seismic cluster on Katla volcano's south-flank (Iceland)

Relative relocation methods are commonly used to precisely relocate earthquake clusters consisting of similar waveforms. Repeating waveforms are often recorded at volcanoes, where, however, the crust structure is expected to contain strong heterogeneities and therefore the 1D velocity model assumption that is made in most location strategies is not likely to describe reality. A peculiar cluster of repeating low-frequency seismic events was recorded on the south flank of Katla volcano (Iceland) from 2011. As the hypocentres are located at the rim of the glacier, the seismicity may be due to volcanic or glacial processes. Information on the size and shape of the cluster may help constraining the source process. The extreme similarity of waveforms points to a very small spatial distribution of hypocentres. In order to extract meaningful information about size and shape of the cluster, we minimize uncertainty by optimizing the cross-correlation measurements and relative-relocation process. With a synthetic test we determine the best parameters for differential-time measurements and estimate their uncertainties, specifically for each waveform. We design a relocation strategy to work without a predefined velocity model, by formulating and inverting the problem to seek changes in both location and slowness, thus accounting for azimuth, take-off angles and velocity deviations from a 1D model. We solve the inversion explicitly in order to propagate data errors through the calculation. With this approach we are able to resolve a source volume few tens of meters wide on horizontal directions and around 100 meters in depth. There is no suggestion that the hypocentres lie on a single fault plane and the depth distribution indicates that their source is unlikely to be related to glacial processes as the ice thickness is not expected to exceed few tens of meters in the source area

Characteristics and Geological Origin of Earthquakes and Tremor at Katla Volcano (S-Iceland)

Katla is a hazardous volcano in south Iceland, hosting a large caldera covered by the Mýrdalsjökull glacier. The last phreatomagmatic eruption occurred in 1918 and the present repose time is the longest known in history. The 2010 eruption of the neighbouring Eyjafjallajökull volcano prompted scientists’ concerns because the two volcanoes are tectonically connected. No visible eruption occurred, but in July 2011 a 23 hour tremor burst was associated with a glacial flood which caused damage to infrastructure. Deepening of the geothermally fed ice cauldrons, increased earthquake activity within the caldera and new seismicity on the south flank were also observed. Analysis of seismic data, including development of new location strategies, and a geological field study of the south flank were conducted to interpret the seismic sources. The tremor burst consisted of two volcano-related phases originated at the active cauldrons and a third phase generated by the flood. The increased seismicity inside the caldera and evidence of rapid ice melting may indicate that the volcano-related tremor was caused by a subglacial eruption. Alternatively, tremor may have been generated by hydrothermal boiling induced by the flood. The seismicity on the south flank consists of long-period repeating events occurring with regular time intervals, modulated by seasons (higher occurrence in summer). Because of the temporal evolution, hypocentre depth distribution and coincidence with the 2011 unrest, a volcano-related source is considered more likely than a glacial one. Hydrothermal processes may be easier to reconcile with the seasonal pattern than magmatic, although no direct indication of hydrothermal activity was found. A field survey revealed previously unknown flank eruption sites within the south flank. A magmatic source for the seismicity should therefore not be discarded. This observation is of major importance for hazard assessment of the south flank of Katla

Characteristics and geological origin of earthquakes and tremor at Katla (S-Iceland)

Katla is a hazardous volcano in south Iceland, in large part covered by the Mýrdalsjökull glacier. It hosts a large caldera with several active geothermal areas and is characterised by persistent seismicity. Katla is one of the most active volcanoes in Iceland with at least 20 phreatomagmatic eruptions in the last 1100 years associated with catastrophic jökulhlaups. The last occurred in 1918. The present repose time is the longest known in history and the 2010 eruption of the neighbouring Eyjafjallajökull volcano prompted scientists’ concerns because the two volcanoes are tectonically connected. The seismic network around Katla was therefore densified. No visible eruption occurred, but in July 2011 a 23 hour tremor burst was associated with a glacial flood which caused damage to infrastructure. This event was accompanied by deepening of the geothermally fed ice cauldrons, increased earthquake activity within the caldera and new seismicity on the south flank. The question arose whether or not a subglacial eruption occurred, i.e. whether the tremor was generated by magmatic processes or by the flood. Analysis of seismic data, including development of new location strategies, and a geological field study of the south flank were conducted to interpret the seismic sources and the volcanological significance of the 2011 unrest. July 2011 marked a change in the seismicity pattern at Katla that suggests a modification of the volcanic system. The tremor burst consisted of two phases originated at the active cauldrons and associated with hydrothermal or magmatic processes and a third phase generated by the flood. The increased seismicity rate inside the caldera and evidence of rapid melting of the glacier may indicate that the tremor was caused by a subglacial eruption. Alternatively, tremor may have been generated by hydrothermal boiling and/or explosions induced by the flood. An increase of heat released by the volcano is required in any case. The seismicity at the southern edge of the glacier in the Gvendarfell area consists of long-period repeating events occurring with regular time intervals, modulated by seasons (higher occurrence in summer). Because of the temporal evolution of the seismicity, hypocentre depth distribution, features of the glacier and coincidence with the 2011 unrest, volcano-related processes are considered more likely than glacial to generate this seismicity. A hydrothermal source may be easier to reconcile with the seasonal pattern than a magmatic one, although no direct indication of hydrothermal activity has been observed. A field survey revealed previously unknown flank eruption sites within the Gvendarfell area. Renewed volcano-related processes are therefore plausible and a magmatic source for the Gvendarfell events should not be discarded. This observation is of major importance for hazard assessment of the south flank of Katla. This work highlights the difficulty of discerning glacial and volcanic signals for the study and monitoring of subglacial volcanoes

Combining single-station microtremor and gravity surveys for deep stratigraphic mapping

Any stratigraphic reconstruction by means of surface geophysical methods is affected by the nonuniqueness of data inversion and by the resolution-depth trade-off. The combination of different geophysical techniques can reduce the number of degrees of freedom of the problem. We have focused on two low-impact single-station geophysical techniques: microtremor and gravity. These have been used by previous authors for stratigraphic mapping only by comparing the results independently. We suggest a procedure to combine microtremor and gravity data into a unique subsoil model and explore to what extent their combined use can overcome their individual weaknesses and constrain the final result. We apply the procedure to the Bolzano sedimentary basin, Northern Italy, to derive a 3D bedrock model of the basin. We use microtremor data to map the ground resonance frequencies and derive an initial 3D bedrock depth model by assuming a VS profile for the sediment fill. Then, we define a density model for rock and sediments and perform 3D gravity forward modeling. We then perturb the VS and density models and find the parameters that best fit the observed gravity anomalies. Data uncertainties are examined to explore the significance of the results. Joint use of the two techniques successfully helps interpret the stratigraphic model: Ground resonance frequencies guarantee the spatial resolution of the bedrock geometry model, whereas gravity data help constrain the frequency to depth conversion

Detecting 1-D and 2-D ground resonances with a single-station approach

The vibration modes of the ground have been described both in the 1D and 2D case. The 1D resonance is found on geological structures whose aspect ratio is low, that is on layers with a lateral width much larger than their thickness. A typical example is that of a horizontal soft sediment layer overlying hard bedrock. In this case, the 1D resonance frequency, traditionally detected by means of the microtremor H/V technique, depends on the bedrock depth and on the shear wave velocity of the resonating cover layer. The H/V technique is thus used both to map the resonance frequencies in seismic microzonation studies and for stratigraphic imaging. When 2D resonance occurs, generally on deep and narrow valleys, the whole sedimentary infill vibrates at the same frequency and stratigraphic imaging can no longer be performed by means of the 1D resonance equation. Understanding the 1D or 2D resonance nature of a site is therefore mandatory to avoid wrong stratigraphic and dynamic interpretations, which is in turn extremely relevant for seismic site response assessment. In this paper we suggest a procedure to address this issue using single station approaches, which are much more common compared to the multi-station synchronized approach presented by research teams in earlier descriptions of the 2D resonances. We apply the procedure to the Bolzano sedimentary basin in Northern Italy, which lies at the junction of 3 valleys, for which we observed respectively 1D-only, 1D and 2D, and 2D-only resonances. We conclude by proposing a workflow scheme to conduct experimental measurements and data analysis in order to assess the 1D or 2D resonance nature of a site using a single station approach

An experimental approach to unravel 2D ground resonances: application to an alluvial-sedimentary basin

Abstract The study of ground resonances is important to assess seismic site amplification and to infer information on the geometrical and mechanical properties of the resonating structures. 1D- and 2D-type resonances imply different dynamic behavior that can be distinguished by inspecting the individual spectral components of single-station microtremor measurements. Typically, 2D resonance modes develop along cross-sections of deep sediment-filled valleys and consist of longitudinal, transverse and vertical modes that can be identified as spectral peaks when ground motion is recorded parallel to the axes of the valley. In the case of more complex geometries, such as sedimentary basins, resonance modes are more difficult to predict and depend on the unknown complexity of the buried bedrock geometry. We show how a simple signal rotation procedure applied to single-station microtremor recordings reveals the underlying 2D resonance pattern. The method allows assessing the axes of motion of buried geological structures and identifying 2D resonance modes along these axes. Their directionality, frequency and amplitude features are then analyzed to extract information on the bedrock geometry. We test our method in the Bolzano alluvial-sedimentary basin and we observe that apparently complicated resonance patterns may be simplified by locally referring to the simplest description of the phenomenon as 2D resonance of a valley slice. The bedrock morphology can be decomposed into 2D-like geometries, i.e., excavated channels, and the observed resonances develop within cross-sections of these channels. Graphical Abstrac

A peculiar cluster of microearthquakes on the eastern flank of Katla volcano, southern Iceland

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

HVSR Measurements of the Pescara and Manfredonia Late Quaternary paleovalleys

This dataset contains microtremor horizontal-to-vertical spectral ratio (mHVSR) measurements taken along the Adriatic coastal plain in the two paleovalley systems of Pescara and Manfredonia. The measurements were acquired to stay near as possible to pre-existing geological data and, at the same time, far from every possible noise source. The H/V curve was processed the data with the Grilla software. Each three-component time series was split into non-overlapping windows of equal length (30 s). Fourier spectra were computed for each time window and smoothed with triangular functions having a width equal to 10% of the central frequency. We obtained the mHVSR curves by averaging the mHVSR ratios computed for each window, H being the geometric average of the instrumental N-S and E-W components. The individual spectral components and mHVSR were calculated in terms of average ± standard deviation; transient perturbations were carefully removed by manual selection

A double-correlation tremor-location method

A double-correlation method is introduced to locate tremor sources based on stacks of complex, doubly-correlated tremor records of multiple triplets of seismographs back projected to hypothetical source locations in a geographic grid. Peaks in the resulting stack of moduli are inferred source locations. The stack of the moduli is a robust measure of energy radiated from a point source or point sources even when the velocity information is imprecise. Application to real data shows how double correlation focuses the source mapping compared to the common single correlation approach. Synthetic tests demonstrate the robustness of the method and its resolution limitations which are controlled by the station geometry, the finite frequency of the signal, the quality of the used velocity information and noise level. Both random noise and signal or noise correlated at time shifts that are inconsistent with the assumed velocity structure can be effectively suppressed. Assuming a surface wave velocity, we can constrain the source location even if the surface wave component does not dominate. The method can also in principle be used with body waves in 3-D, although this requires more data and seismographs placed near the source for depth resolution

Locating tremor using stacked products of correlations

We introduce a back-projection method to locate tremor sources using products of cross-correlation envelopes of time series between seismic stations. For a given subset of n stations, we calculate the (n − 1)th-order product of cross-correlation envelopes and we stack the back-projected products over combinations of station subsets. We show that compared to existing correlation methods and for realistic signal and noise characteristics, this way of combining information can significantly reduce the effects of correlated (spurious or irrelevant signals) and uncorrelated noise. Each back-projected product constitutes an individual localized estimate of the source locations, as opposed to a hyperbola for the existing correlation techniques, assuming a uniform velocity in two dimensions. We demonstrate the method with synthetic examples and a real-data example from tremor at Katla Volcano, Iceland, in July 2011. Despite very complex near-surface structure, including strong topography and thick ice cover, the method appears to produce robust estimates of tremor location