1,179 research outputs found
Detecting 1-D and 2-D ground resonances with a single-station approach
No full textThe 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
Combining single-station microtremor and gravity surveys for deep stratigraphic mapping
No full textAny 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
A peculiar cluster of microearthquakes on the eastern flank of Katla volcano, southern Iceland
No full textA 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
Joint relative location of earthquakes without a pre-defined velocity model: an example from a peculiar seismic cluster on Katla volcano's south-flank (Iceland)
No full textRelative locationmethods are commonly used to precisely locate 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 1-D 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-location 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 location strategy to work without a
pre-defined 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 1-D 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 metres
wide in horizontal directions and around 100 metres 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 metres in the source area. Our method is designed for a very small source region,
allowing us to assume a constant slowness for the whole cluster and to include the effects
of 3-D heterogeneity such as refraction. Similar circumstances may arise in other volcanic
regions with a high level of heterogeneity and where densely clustered earthquakes are often
recorded
Locating tremor using stacked products of correlations
No full textWe 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
Long-period seismic events with strikingly regular temporal patterns on Katla volcano's south flank (Iceland)
No full textKatla is a threatening volcano in Iceland, partly covered by the Mýrdalsjökull 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 Mýrdalsjökull. 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 jökulhlaup; 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
Giulia Veronica Varisco
No full textThe headword explains the biography and the contribution of the author Giulia Varisco to the children's literatur
A double-correlation tremor-location method
No full textA 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
Imaging buried anticlines in the Po Plain, northern Italy, based on HVSR frequency and amplitude analyses
No full textThe use of the HVSR (Horizontal-to-Vertical Spectral Ratio) method on single-station microtremor measurements is well documented in small alluvial plains for bedrock mapping. In large sedimentary basins, like the Po Plain, its application is still debated. To shed some light on this issue, we investigated two seismogenic structures buried below the Po Plain Quaternary deposits: the Mirandola and Casaglia anticlines. We acquired and analysed a dense distribution of HVSR data covering the two areas and mapped the frequency and amplitude values of the observed resonance peaks. The top of both anticlines is highlighted by high amplitude peaks picturing E-W elongated sectors with high-impedance contrast, where Quaternary deposits are reduced in thickness to about 60-130 m and directly overlay the Pliocene (Mirandola) and Miocene (Casaglia) marine units. In Mirandola, the high-amplitude peaks also correspond to higher resonance frequencies, while in Casaglia, the distribution of resonance frequencies is relatively uniform suggesting a flatter crestal region and the lateral continuity of the resonance surface. The combination of peak frequency and amplitude information on a dense grid of measurement points is thus confirmed to be useful for identifying and mapping buried geological structures such as structural highs. Further modelling is being carried out to estimate the depth of the surface responsible for the observed resonances, through calibration with borehole information
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