Daniele Spallarossa

A three-dimensional crustal velocity model of the southwestern Alps from local earthquake tomography

A temporary network of 65 short-period seismological stations was installed in the southwestern Alps during the second half of 1996. It complemented the permanent monitoring networks, obtaining an average interstation distance of ∼10 km. Travel time data from 446 local earthquakes and 104 quarry blasts were inverted simultaneously for hypocenter parameters and three-dimensional velocity structure. The P wave velocity model displays strong lateral contrasts both at shallow and deeper levels. A low-velocity anomaly stands out at shallow depths beneath the Digne and Castellane nappes in the southwestern part of the investigated area. Farther east, the Monviso ophiolitic massif appears to have a much larger extension at depth than previously assumed. The largest and strongest anomaly is located under the Dora Maira massif and the westernmost Po plain. It correlates with the well-known Ivrea body, which is classically interpreted as a wedge of Adriatic upper mantle. At the best resolved depths (10 and 15 km) it appears as a rather thin (10 to 15 km), north-south elongated, high-velocity (7.4 to 7.7 km s−1) anomaly with very sharp edges, extending to the south as far as 10 km north of the surface trace of the Frontal Penninic Thrust. Special care was taken with regard to the quantitative estimation of the resolution for the main anomalies using the inversion of synthetic travel time data.

SITE EFFECTS BY H/V RATIO: COMPARISON OF TWO DIFFERENT PROCEDURES

In this article, H/V ratio are evaluated using different spectral techiniques applied to both earthquake and microtremor data (Nakamura technique). In particular, in order to avoid numerical instability, two different numerical techiniques are taken into account: (a) a smoothing procedure applied to the spectra of the seismogram components and (b) a regularization method applied to the H/V ratio (Landweber scheme). The data set consists of more than 70 earthquake events recorded by three component sensors displaced in the town of Fabriano (Central Italy) during the Umbria-Marche sequence started on September 1997. The local magnitudes range between 2.7 and 4.4, while the epicentral distances range between nearly 30 and 60 km. The stations were set to continuous recording so that a huge amount of microtremors was stored. The results are compared in terms of predominant frequencies and amplification levels in order to point out the influence of the adopted methods. The H/V ratio provides similar results if applied to a smoothed version of both earthquake and microtremor spectra, confirming that Nakamura technique is a cheap and a fast method to collect information on the site amplification effects. Moreover, the results relevant to earthquake data seem not to depend on the method used to stabilize the H/V ratio, whereas those relevant to microtremor data does. The explanation of this fact is suggested by the behaviour of the Landweber filter showing that the predominant frequency detected by means of microtremor data lies in a high instability region of the spectra.

Source Parameters Estimated from the Aftershocks of the 1997 Umbria–Marche (Italy) Seismic Sequence

We derived the source parameters of 563 aftershocks (1.4 ≤ M L ≤ 4.5) of the 1997 Umbria–Marche seismic sequence by analyzing three-component digital recordings of a temporary network and developed a local magnitude scale using the whole available data set (more than 25,000 waveforms) from synthetic Wood–Anderson seismograms. Then, considering a subnetwork composed of nine portable stations, more than 12,000 earthquake spectra were corrected for the near-surface attenuation parameter k and the path attenuation. The corrected spectra were used to compute the seismic moment, the source radius, and stress drop assuming an ω–2 Brune source model. We calculate the relationship log M = 17.46 + 1.12 M L between seismic moment and local magnitude for the studied area. The source dimension versus seismic moment relationship did not show any breakdown in the similarity of the rupture process, even when events of magnitude less than 2.2 (the threshold value for the completeness of the considered catalog.) were considered. The average stress drop obtained analyzing the S -wave spectra was 38 ± 10 bar, and the source dimension ranged between about 40 and 500 m.

Local and Duration Magnitudes in Northwestern Italy, and Seismic Moment Versus Magnitude Relationships

In the present work, we develop some local magnitude scales for northwestern Italy based on vertical short-period records. This study is motivated by the possibility of applying the computed scales to an instrumental catalog of more than 25,000 local earthquakes, as this region has been continuously monitored by 12 short-period vertical-component (1c) stations since the mid-1980s. Furthermore, a digital network of three-component (3c) broadband or 5 second sensors has monitored northwestern Italy since 1996. Today, a significant number of earthquakes have been simultaneously recorded by both networks, allowing the calibration of the 1c local scale by using magnitudes computed according to a scale derived for the 3c digital network. Moreover, because station Sant’ Anna di Valdieri houses both a 3c (code stv2) and 1c (code stv) sensors, the magnitude scales for the two networks can be developed using the same reference station. The magnitude scale M L = log A + log( R /100) + 0.0054( R − 100) + 3 − S is derived for the 3c digital network with the requirement that the correction S of station stv2 is zero. This scale is based on 10,057 maximum amplitudes (2822 earthquakes) computed from horizontal synthesized Wood-Anderson seismograms, in the hypocentral distance 10 to 310 km and in the range 0 ≤ M L ≤ 5. With respect to an carlier magnitude scale derived for the 3c network constraining the sum of all the station corrections to zero, the magnitudes predicted by the previous equations show an average bias of (−0.2 ± 0.1), which can be ascribed to the different constraint applied to the station corrections. The magnitudes predicted by the scale for the 3c network are used to calibrate magnitude scales based on either total duration or maximum amplitude from synthesized Wood-Anderson seismograms computed for each short-period vertical recording. The magnitude scale obtained considering maximum amplitudes from vertical short-period recordings is M L = log A + log( R /100) + 0.0041 ( R − 100) + 3 − S ′. The reliability of the obtained magnitude scales is assessed using 827 earthquakes different from those we considered in the regression analysis. Finally, the following seismic moment versus local magnitude relations are valid in the western Alps in the range 0 where M L3C is the local magnitude computed starting from the horizontal component of broadband (flat frequency response, from 0.033 to 50 Hz) or semibroadband (flat frequency response, from 0.2 to 40 Hz) sensors and M L1C is the magnitude computed starting from the vertical short-period recordings. [1]: /embed/graphic-1.gif

Litho—asthenospheric structures of northern Italy as inferred from teleseismic P-wave tomography

Abstract Regional three-dimensional inversions of teleseismic P-wave travel time residuals recorded by high-frequency regional and local seismic networks operating along the Western Alps and surrounding regions were carried out and lithosphere and upper mantle P-wave velocity models down to 300 km were obtained. Residuals of more than 500 teleseismic events, recorded by 98 fixed and temporary seismic stations, have been inverted. The comparison between real residuals and the ones obtained from tomographic model indicates that the method is able to solve the feature of the regional heterogeneities. Where the resolution is good, coherent lithospheric and upper mantle structures are imaged. In the shallower layers, high- and low-velocity anomalies follow the structural behaviour of the Alpine-Apenninic chains showing the existence of very strong velocity contrasts. In the deepest layers, velocity contrast decreases however two deep-seated high-velocity structures are observed. The most extended in depth and approximately trending NE-SW has been interpreted as a wreck of the oldest subduction responsible of the Alpine orogenesis. The second one, connected to the northwestern sector of the Apenninic chain, appears to vanish at depths greater than 180 km and is probably due to still active Apenninic roots. Cross-sections depict the spatial trend of perturbations and in particular outline the sub-vertical character of the Alpine and Apenninic anomalies. Under the Ligurian Sea, the 3-D inversion confirms the uplift of the asthenosphere in agreement with the tectonic evolution of the basin.

ATTENUATION RELATIONSHIP FOR LOW MAGNITUDE EARTHQUAKES USING STANDARD SEISMOMETRIC RECORDS

Northwestern Italian weak-motion data were used to study attenuation characteristics of horizontal peak ground acceleration (PGA) and horizontal peak ground velocity (PGV) from earthquakes of local magnitudes (M l ) up to 5.1. Data have been provided by the RSNI (Regional seismic network of Northwestern Italy) and RSLG (Regional seismic network of Lunigiana-Garfagnana) waveform database. The database consists of more than 14000 horizontal components recorded in the period 1999-2002 by both broadband and enlarged band seismometers. The accuracy of the procedure used to extract PGA values from the velocity recordings was verified comparing observed and derived PGA values at station STV2, which was equipped with both a temporary K2 Kinemctrtcs accelerometer and Guralp CMG40 broadband sensor. The attenuation of both peak ground acceleration and peak ground velocity was found to be logarithmically distributed with a strong attenuation for low distances (less than 50 km) and low M l values (<3.0). The resulting equations are: Log(PGA)=−3.19+0.87M−0.042M 2−1.92 Log(R)+0.249S, Log(PGA)=−4.23+0.76M−0.018M2−1.56 Log(R)+0.230S, where PGA is expressed in g, PGV is expressed in m/s, M is local magnitude, R is the hypocentral distance in kilometers and S is a dummy variable assuming values of 0 and 1 for rock and soil respectively. For increasing distance and magnitude, both PGA and PGV values show a linear distribution. The validity range of the obtained attenuation relationships is 0–200 km for distances and M l up to 4.5. Sensitivity studies performed by analysis of residuals, showed that predicted PGA and PGV values are stable with respect to reasonable variations of the model and distances providing the data. Comparisons with attenuation relationships proposed for Italian region, derived from strong motion records, are also presented.

An ML Scale in Northwestern Italy

A local-magnitude scale is derived for northwestern Italy from wave-form data recorded at 17 stations from 2600 local earthquakes, ranging in distance from 10 to 300 km. By averaging the horizontal components in a single measure, we used 8127 zero-to-peak amplitudes from synthetic Wood–Anderson seismograms to determine, in a least-squares sense, the appropriate –log A attenuation function, the event local magnitude, and the station corrections. Both a parametric and a non-parametric description of –log A is considered while performing the inversion. In both cases, the constraint of 1-mm motion recorded at 100 km for a magnitude 3.0 earthquake was used. The resulting parametric distance correction is given by –log A = 1.144log( r /100) + 0.00476( r – 100) + 3. The remarkable agreement between the parametric and nonparametric results confirms that the assumptions on the attenuation function that we made for deriving the parametric distance correction are reasonable. Moreover, inversion of bootstrap replications of our data set furnished stable solutions. Station magnitude corrections range between –0.28 and 0.48, suggesting a variable and significant effect of station site properties on recorded amplitudes. Finally, the local-magnitude scale has been used to evaluate the magnitude values for the whole catalog of earthquakes recorded by the IGG–University of Genoa network from 1996 until March 2001 (about 3200 events) and for a subset relevant to the southwestern Alps (about 1900 events). Statistics performed using the new local-magnitude scale and the usual duration magnitude demonstrate the influence of this choice on both completeness and b -value estimation. For example, in the southwestern Alps, the completeness threshold decreases from 2.0 ( M d) to 1.7 ( M L) and the b -value from 1.38 ( M d) to 1.08 ( M L).

The 1997 Umbria-Marche (Italy) earthquake sequence: Tomographic images obtained from data of the GNDT-SSN temporary network

We present some preliminary images of the 3-D P-wavevelocity model and of the relocated seismicityobtained from the data collected by the GNDT-SSNtemporary network installed in the epicentral area ofthe earthquake sequence that followed the 26September, 1997, Central Italy main shock(Mw = 6.0). This network consisted of a total of 15stations, was deployed in the southern part of thearea affected by the earthquake sequence and operatedfor a total of 17 days starting on 10/18/97.Our results indicate that 1) the P-velocity structuredisplays a pattern of lateral variations consistentwith the general NW-SE trend of the Apennines in thearea; 2) the aftershock foci distribute, in thesouthern part of the sequence, on distinct and welldefined SW dipping planes which surface intersectionsmatch previously recognized active normal faults; 3)a distinct zone of aftershock quiescence is observedin correspondence of the 10/12 (ML = 5.3) and10/14/97 (ML = 5.7) hypocenters near Sellano; 4)the seismicity at the southern end is very shallow andit is unclear the relationship between the 1997 andthe 1979 Norcia sequences.

Anomalously deep earthquakes in northwestern Italy

It is usually assumed that earthquakes in intraplate regions occur in the upper crust, and northwestern Italy is generally assigned to this kind of ‘normal’ seismicity. In this work, the depth distribution of the events localized in this area by the Istituto Geofisico Geodetico (IGG) seismic network in the period 1991–1997 is analyzed in detail. In particular, the location capability of the network is discussed, adopting as reference quarry blasts (for the epicentral position) and the locations obtained from a dense temporary network (for the depth estimate). Within the so-obtained error limits, the depth distribution of events show a characteristic pattern: while for most of the area covered by the network the well-located seismicity lies within the first 20 km of depth, in a band following the inner arc of the Western Alps, numerous events have anomalously large focal depths, reaching a maximum of 114 km. These depth determinations cannot be attributed to instabilities of the location procedure: different choices of the propagation models used for the hypocentral determination led to very similar depth values, always significantly larger than the standard values for the surrounding areas. A strong correlation has been found between the 3-dimensional distribution of these foci and the P-wave propagation anomalies obtained from tomographic studies, suggesting a direct link between elastic and rheological properties of lower crust and upper mantle in this area.

Lateral variations of Pn wave velocity in northwestern Italy

The Pn arrival times recorded from seismic networks operating throughout northwestern Italy and surrounding regions were inverted to map the structural variations of the uppermost mantle over the area and to estimate the crustal static delays at each station. By means of careful data selection a quality data set was obtained removing statistical outliers, poorly recorded events, and scantily sampled stations. Moreover, synthetic data were used to evaluate the resolution power of the available data set and the adopted iterative inversion technique. The agreement between synthetic and calculated models is more satisfactory where the path coverage of the rays is quite complete. A low-velocity zone was found beneath the western and northwestern side of the Alps; it may be related to an increase in depth of the Moho, as supported by other geophysical data. The high Pn velocities found in the eastern side of the western Alps indicate the presence of high velocity and density, lower crust rocks of the so-called “Ivrea Body.” The high Pn velocity underlying the Ligurian Sea could be related to a high-velocity structure existing in the upper mantle. The shape of the velocity anomalies matches not only the major tectonic features, but also the Bouguer gravity anomalies of the area.