16 research outputs found
Development and Testing of Seismic Regional Discriminants
The routine process of the IMS data at the CTBTO International Data Center (IDC) is aimed at identifying in the seismological bulletins all the seismic events the source of which is certainly natural. This process, named screening out, is carried out on several tens of earthquakes per day. It is important that the methods used for the screening process are as reliable and efficient as possible, so as substantially reducing the number of suspicious events without classifying any explosion as natural events.
The principal identification method applied at the CTBTO IDC is based on the difference between the Ms magnitude, computed from the amplitude of the surface Rayleigh waves with period close to 20 s, and the mb magnitude computed from the amplitude of the body waves with period close to 1 s. The applicability of this method is, however, limited by the difficulty of detecting surface waves for events of low mb magnitudes, so as to allow the computation the Ms magnitude for such events, which are very numerous.
For CTBT verification, in case of events of small magnitude, the use of stations at regional distances from the epicenter, is crucial. In this context, another category of seismological screening, the so-called regional discriminants, can be applied. This method is based on the amplitudes of the surface waves (Lg) at much higher frequencies (8-12 Hz). Unfortunately, these high-frequency waves are not easily detectable, and the method requires regional calibration curves for each specific site, calibrations that are not available on a global scale
TRAVEL-TIME SOURCE SPECIFIC STATION CORRECTION IMPROVES LOCATION ACCURACY
Accurate earthquake locations are crucial for investigating seismogenic processes, as well as for applications like verifying compliance to the Comprehensive Test Ban Treaty (CTBT). Earthquake location accuracy is related to the degree of knowledge about the 3-D structure of seismic wave velocity in the Earth. It is well known that modeling errors of calculated travel times may have the effect of shifting the computed epicenters far from the real locations by a distance even larger than the size of the statistical error ellipses, regardless of the accuracy in picking seismic phase arrivals.
We have developed a method of seismic location to minimize this systematic shifts affecting hypocentral locations. These method is based on the fact that such shifts are quite similar for events for which the hypocenters are close to each other with respect to their distance from the recording stations. So this method is based on a set of Ground Truth events recorded by a dense local seismic network in seismically active regions.
The applications concern seismic sequences recorded in Italy and Iran.
We show that mislocations of the order of 10-20 km affecting the epicenters, as well as larger mislocations in hypocentral depths, calculated from a global seismic network can be effectively removed by applying source-specific station corrections applied to the standard IASPEI91 travel times.
This method is valid only for the area where calibration events are located but it is valid for any subset of the station network used for the calibration
A Multidisciplinary Study of the DPRK Nuclear Tests
The Democratic People Republic of Korea announced two underground nuclear tests carried out in their territory respectively on October 9th, 2006 and May 25th, 2009. The scarce information on the precise location and the size of those explosions has stimulated various kinds of studies, mostly based on seismological observations, by several national agencies concerned with the Nuclear Test Ban Treaty verification. We analysed the available seismological data collected through a global high-quality network for the two tests. After picking up the arrival times at the various stations, a standard location program has been applied to the observed data. If we use all the available data for each single event, due to the different magnitude and different number of available stations, the locations appear quite different. On the contrary, if we use only the common stations, they happen to be only few km apart from each other and within their respective error ellipses. A more accurate relative location has been carried out by the application of algorithms such as double difference joint hypocenter determination (DDJHD) and waveform alignment. The epicentral distance between the two events obtained by these methods is 2 km, with the 2006 event shifted to the ESE with respect to that of 2009. We then used a dataset of VHR TerraSAR-X satellite images to detect possible surface effects of the underground tests. This is the first ever case where these highly performing SAR data have been used to such aim. We applied InSAR processing technique to fully exploit the capabilities of SAR data to measure very short displacements over large areas. Two interferograms have been computed, one co-event and one post-event, to remove possible residual topographic signals. A clear displacement pattern has been highlighted over a mountainous area within the investigated region, measuring a maximum displacement of about 45 mm overall the relief. Hypothesizing that the 2009 nuclear test had been carried out close to the area where the displacement has been observed through the DInSAR technique, its relation with the epicenter location obtained through seismological processing has been discussed as a possible alternative hypothesis with respect to the preferred solutions reported by the nuclear explosion database (NEDB). The distance of about 10 km between the two places can be considered acceptable in light of the possible systematic location shifts commonly observed in the seismological practice over a global scale. The difference between the m b magnitudes of the two tests could reflect differences in geological conditions of the two test sites, even if the yield of the two explosions had been the same
Sprofondamenti in aree urbane: il caso del parco “Ytzhak Rabin” (via Panama, Roma)
In Rome urban area, within the garden dedicated
to Ytzhak Rabin (a portion of the Villa Ada park),
occurrence of a few collapses has been observed for the first
time in September 2012. In the surroundings, presence of
underground cavities- mainly catacombs- is conversely well
documented by previous studies.
Local authorities were immediately informed of sinkholes
formation, because collapses occurred very close to a
playground located in a highly frequented sector of the garden.
Propagation processes of the phenomena, periodically
monitored since their first observation, significantly fastened
after a period of heavy rainfalls on Rome area in January-
February 2014. Thus, multidisciplinary geophysical
investigations have been started in agreement with Civil
Protection of Rome Municipality, who has taken preliminary
safety measures (installation of fences and danger signals).
Hitherto, 28 sinkholes have been identified, aligned along a
WNW-ESE trending zone, testifying localized collapses of the
roof of underground cavities, probably belonging to the
palaeocristian catacomb systems dug nearby Via Salaria.
Preliminary geophysical prospections have been performed in
a small test area, in order to verify methods applicability and
thus program a systematic survey of the whole garden.
Drillings, and direct exploration of the accessible galleries, will
complete the reconstruction of underground geometry and
conservation state of vaults and pillars. Once completed the
hazard assessment, the planning of urgent works is expected
in order to restore the complete use of the garden.Published25 - 422TR. Ricostruzione e modellazione della struttura crostaleN/A or not JC
High-resolution aeromagnetic survey of Calabria (Southern Italy)
We present a 1:350,000 high-resolution magnetic anomaly map of Calabria (Southern Italy),
obtained by merging the results from two low-altitude aeromagnetic surveys performed in
southern and northern Calabria. Magnetic anomalies of Calabria are of low intensity, and
mostly range from 11 to –9 nT. Northern Calabria is characterized by positive anomalies in
the Tyrrhenian margin (Coastal Chain) that turn into negative values moving eastward in the
Sila Massif. Southern Calabria is characterized by slightly positive anomaly values, interrupted
by a null magnetic anomaly corridor roughly corresponding to the eastern margin of the
Gioia Tauro basin. Finally, anomaly values turn systematically negative in the Messina Straits.
Due to the unprecedented resolution (low flying height, spatial sampling along the flight line
of ∼5 m and 1–2 km flight line spacing), the new map highlights, in detail, the geometry and
setting of the upper crustal features. As Calabria is one of the most seismically active regions
in Italy, hit by several high-magnitude earthquakes in recent centuries, the interpretation of
this new map will hopefully contribute to new insights into the crustal geological setting,
location and dimension of the main seismogenic sources.Published116-1231A. Geomagnetismo e Paleomagnetismo3SR. AMBIENTE - Servizi e ricerca per la SocietàJCR Journa
Indagini geofisiche e geochimiche di un sinkhole in formazione nell’area di Guidonia (Lazio)
The Plio-Quaternary Acque Albule Basin is already
known for the occurrence of sinkholes; since the
spring 2014, it has been ongoing the study of an active subsidence
process, which formed a depressed area near the
Guidonia village. During the last year, the phenomenon has
become more intense giving rise to two collapses along the
eastern rim of the depression. Thus, geophysical and geogeochemical
investigation campaigns started in order to understand
the genesis and evolution of the phenomenon. Survey
results allowed a geological and structural characterization,
showing the presence of “travertino” at the depression margins
and its absence therein. It has been recognized, by the
geophysics, features with NW-SE and NNE-SSW direction
ascribable to a possible depression fracturing and consistent
with regional structural trends. Thus, it has been possible to
plan and perform geognostic investigation consisting in two
drillings aimed to define the stratigraphy of the marginal and
inner part of the depression.Published7 - 162TR. Ricostruzione e modellazione della struttura crostaleJCR Journa
Travel time source-specific station corrections related to lithospheric structures in the Mediterranean
We compare the locations obtained from arrival times collected by the International Seismological Centre from a network of regional and teleseismic stations for a cluster of Italian earthquakes with the locations of the same events obtained by the dense national seismic network operated by the Istituto Nazionale di Geofisica e Vulcanologia. We find mislocations on the order of 15 km for epicentral coordinates and on the order of 25 km for depths calculated from the regional and teleseismic network and using the standard IASP91 travel times. These mislocations are generally larger than the sizes of the respective error ellipse semi-axes. We then show that systematic shifts of hypocentral coordinates can be substantially reduced by applying source-specific station corrections. Moreover, we find that the size of error ellipses characterizing the teleseismic locations is significantly reduced by the application of such corrections. Our travel time corrections are compared and found fairly consistent with information available in the literature on tomographic studies on the crust and upper mantle in the European-Mediterranean region.Published3-194T. Sismologia, geofisica e geologia per l'ingegneria sismicaJCR Journa
Relocation of Earthquakes by Source‐Specific Station Corrections in Iran
Accurate earthquake locations are crucial for investigating seismogenic processes, as well as for applications like verifying compliance to the Comprehensive Nuclear‐Test‐Ban Treaty. Modeling errors of calculated travel times, in addition to the density of the stations, their epicentral distances, and their azimuthal coverage, may have the effect of shifting the computed epicenters far from the real locations, regardless of the accuracy in picking seismic phase arrivals.
In the present study, we compare the regional locations for one set of earthquakes obtained by arrival times reported by the Iranian Seismological Center with teleseismic locations obtained by arrival times reported by the International Seismological Center. We found location differences on the order of 10–20 km or larger, affecting both epicentral coordinates and depths. Average travel‐time residuals to each station of the global network were computed for a set of sources located in the study area. We show that systematic shifts of hypocentral coordinates, as well as the sizes of their error ellipses, can be substantially reduced by applying source‐specific station corrections. Finally, the validity of the calibration method was confirmed by a test carried out on a dataset different from that used for computing the travel‐time corrections.
This study includes an analysis of the effect of removing arrival times of critical stations from the dataset used for the locations, showing that this effect is largely reduced by the application of travel‐time corrections.Published2498-25094T. Sismologia, geofisica e geologia per l'ingegneria sismicaJCR Journa
TRAVEL-TIME SOURCE SPECIFIC STATION CORRECTION IMPROVES LOCATION ACCURACY
Accurate earthquake locations are crucial for investigating seismogenic processes, as well as for applications like verifying compliance to the Comprehensive Test Ban Treaty (CTBT). Earthquake location accuracy is related to the degree of knowledge about the 3-D structure of seismic wave velocity in the Earth. It is well known that modeling errors of calculated travel times may have the effect of shifting the computed epicenters far from the real locations by a distance even larger than the size of the statistical error ellipses, regardless of the accuracy in picking seismic phase arrivals.
We have developed a method of seismic location to minimize this systematic shifts affecting hypocentral locations. These method is based on the fact that such shifts are quite similar for events for which the hypocenters are close to each other with respect to their distance from the recording stations. So this method is based on a set of Ground Truth events recorded by a dense local seismic network in seismically active regions.
The applications concern seismic sequences recorded in Italy and Iran.
We show that mislocations of the order of 10-20 km affecting the epicenters, as well as larger mislocations in hypocentral depths, calculated from a global seismic network can be effectively removed by applying source-specific station corrections applied to the standard IASPEI91 travel times.
This method is valid only for the area where calibration events are located but it is valid for any subset of the station network used for the calibration.UnpublishedHofburg Palace Vienna, Austria5.7. Consulenze in favore di istituzioni nazionali e attività nell'ambito di trattati internazionaliope