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| JOURNAL HOME | HELP | CONTACT PUBLISHER | SUBSCRIBE | ARCHIVE | SEARCH | TABLE OF CONTENTS |
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INTRODUCTION |
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 TOP INTRODUCTION GEOLOGICAL SETTING, HISTORICAL...THE IRPINIA SEISMIC NETWORK...ISNet UPGRADE FOR SEISMIC...EXAMPLES OF RECORDED WAVEFORMSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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Even more than 20 years after the main event, the seismotectonic environment that contains the fault system on which the 1980 earthquake occurred shows continued background seismic activity including moderate-sized events such as the 1996 (M 5.1), 1991 (M 5.1) and 1990 (M 5.4) events. Moreover, the locations of the microearthquakes (taken from the database of the Istituto Nazionale di Geofisica e Vulcanologia, INGV) define an epicentral area with a geometry and extent surprisingly similar to that of the 1980 earthquake and its aftershocks (figure 1A). These simple observations suggest that it may be possible to study the preparation cycles of strong earthquakes on active faults by studying the microseismicity between seismic events. With this in mind, a seismic network of large dynamic range was planned and is now in an advanced phase of completion in the southern Apennines. Called ISNet (Irpinia Seismic Network), it is equipped with sensors that can record high-quality seismic signals from both small-magnitude and strong earthquakes, from which it will be possible to retrieve information about the rupture process and try to understand the scaling relationships between small and large events.
Due to its high density, wide dynamic range, and advanced data-acquisition and data-transmission technologies, the network is being upgraded to become the core infrastructure of a prototype system for seismic early warning and rapid post-event ground-shaking evaluation in the Campania region, which has seismic hazard that ranks among the highest in Italy (Cinti et al. 2004). ISNet will be devoted to real-time estimation of earthquake location and magnitude and to measuring peak ground-motion parameters so as to provide rapid ground-shaking maps for the whole of the Campania region. The information provided by ISNet during the first seconds of a potentially damaging seismic event can be used to activate several types of security measures, such as the shutdown of critical systems and lifelines (Iervolino et al. 2006).
The implementation of a modern seismic network involves many different research and technological aspects related to the development of sophisticated data management and processing. The communication systems need to rapidly generate useful, robust, and secure alert notifications. Here we provide a general technical and seismological overview of ISNet's complex architecture and implementation.
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GEOLOGICAL SETTING, HISTORICAL AND RECENT SEISMICITY OF THE CAMPANIA-LUCANIA APENNINES |
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TOPINTRODUCTIONGEOLOGICAL SETTING, HISTORICAL...THE IRPINIA SEISMIC NETWORK...ISNet UPGRADE FOR SEISMIC...EXAMPLES OF RECORDED WAVEFORMSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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5.5
earthquakes, thus making it a region with a high seismic risk level. Figure 1A shows the recent instrumental seismicity for M > 2.5 recorded by the INGV seismic network in the area indicated by the rectangle for the period 1981-2002. Note that the seismicity is mainly concentrated around the three fault segments associated with the Irpinia earthquake, or along their continuation toward the northeast and the southwest, which shows high seismic activity. On the other hand, using macro-seismic data integrated with geological, geomorphological, and geophysical data, it has been possible to retrieve location and fault geometry for a limited number of historical events (Fracassi and Valensise 2003). The locations and magnitude of the historic earthquakes retrieved from the Catalogo dei Forti Terromoti in Italia (CFTI) (Boschi et al. 1997) are shown in figure 1B. For the best-documented historic events, the dates of occurrence are also given.
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THE IRPINIA SEISMIC NETWORK (ISNet) |
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TOPINTRODUCTIONGEOLOGICAL SETTING, HISTORICAL...THE IRPINIA SEISMIC NETWORK...ISNet UPGRADE FOR SEISMIC...EXAMPLES OF RECORDED WAVEFORMSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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Subnets of Seismic Stations and LCCs
ISNet is composed of 29 seismic stations grouped in six subnets, each
composed of a maximum of six to seven stations
(figure 3 and
table 1). The stations of each
subnet are connected with real-time communications to a specific LCC.
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The six LCCs collect and store the incoming data from the seismic stations of the subnet to which they are connected via digital radio. The LCCs are positioned near small towns (in a shelter) or in existing buildings with an AC power supply and fast communication connections. At some sites, the LCC also hosts a seismic station. In such cases, the sensors are outside in a shallow hole, at a depth of 1 m to 1.5 m, with the data-logger and other equipment located inside an adjacent building. Each LCC has gel batteries for 320 Ah, a GSM remote-control system, a Cisco router, and an HP Proliant server with a 320-Gbyte hard disk. All of the instruments are connected to the batteries and 72-h standby power is guaranteed. All the LCCs use the Earthworm system for data collection and processing (Johnson et al. 1995).
Sensors and Data-loggers
Inside each seismic site, the sensors are installed on a 1-m3
reinforced concrete base at least 0.8 m inside the soil. To ensure a high
dynamic range, each station is equipped with two types of three-component
sensors: strong-motion accelerometers and velocity instruments. Twenty-four
sites are equipped with a Guralp CMG-5T accelerometer and a set of short
period (T0 = 1 s) Geotech S13-Js. The remaining sites have
a Guralp CMG-5T and broadband Nanometrics Trillium (0.025-50 Hz band) sensors.
To minimize the thermal effect for the last sites to be built, the
accelerometer was placed in a 30-cm deep hole and covered with sand. Before
installation, the sensor/data-logger pairs are fully calibrated for
single-channel responses by an automated process. This calibration covers the
entire frequency spectrum using LabVIEW/MatLab software package that provides
the transfer function in graphical mode and in terms of poles and zero.
Data acquisition at the seismic stations is performed by an innovative
data-logger produced by Agecodagis, the Osiris-6 model
(http://www.agecodagis.com).
Some characteristics of the Osiris-6 are a 
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24-bit A/D converter,
a 100-MHz ARM® processor with embedded Linux and open-source software,
onsite data storage (through one removable 5-Gbyte microdrive or a compact
flash card), serial and TCP/IP connectivity, global positioning system (GPS)
time tagging, an integrated SeedLink server, and simple/flexible configuration
via a Web interface (HTTP). The data-loggers have six physical and up to 24
logical channels, and each waveform can be analyzed with different sampling
rates at the same time, for different purposes. (See
Romano and Martino 2005 for an
overview of the Osiris-6 data-loggers).
The external GPS receiver (RS-232) guarantees a timing accuracy that is better than 1 µs. A complete health status is available that assists in the diagnosis of station component failure or data-logger malfunction. The data-loggers store the data locally on their microdrives or send it via SeedLink to Earthworm in the nearest LCC in 1-s packets. The real-time analysis system performs event detection and location based on triggers coming from the data-loggers and parametric information such as arrival-time picks or not-yet-triggered stations provided by the other LCCs. A PostgreSQL developed database (ISNet Devices Manager) tracks the general configuration of the seismic network, such as the recorded channels, sampling rates for each channel, gain, sensor type, data-loggers, and other network devices, with IP addresses, station positions, and serial numbers for each device installed.
Current Data-communication Configuration
ISNet presently uses several different transmission systems. The seismic
stations are connected via spread-spectrum radio links to the LCCs
(figure 3). To transmit
waveforms in real time from the seismic sites to the LCCs, two outdoor 1310
Cisco Wireless local area network (LAN) bridges that operate in the 2.4 GHz
industrial, scientific, and medical (ISM) band are used for each link. Each
LCC is connected through different technology and media types to the RISSC in
Naples, as shown in table
2.
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Each seismic site has a real-time data flow of 18.0 kbps (at a 125-Hz sampling rate for each physical channel), and the overall data communication bandwidth that is needed is around 540 kbps for 30 stations. ISNet supports this throughput under the worst conditions seen, and it has been designed to permit further developments, such as additional seismic or environmental sensors, without greater economic and technological investment. The currently used data transmission protocol is TCP/IP, but for early-warning-application data acquisition, we intend to adopt the connectionless user datagram protocol/Internet protocol (UDP/IP) to avoid unwanted overheads and "handshaking" between sending and receiving transport-layer entities before sending data segments. In early-warning waveform analysis, where single-packet error/missing does not influence decisions in a critical manner, this protocol is much faster and simpler to handle than TCP/IP.
The Network Control Centre (RISSC)
The seismic waveforms are stored locally on the data-loggers and in
real-time at the nearest LCC. At present, only selected signals are
transmitted to the RISSC in Naples, and this is done manually and only for
research purposes. We are developing a storage system at the RISSC that acts
after the triggering of an LCC (through Earthworm) or a station, which will
have fully automated capabilities. For redundancy, we are also considering
storing the complete datasets coming from each station at the RISSC, following
a central site model. For this reason, we have installed a large storage
cluster (6 terabyte HP storage server) at the RISSC on which all of the
waveforms/events can be loaded. The RISSC tracks the seismic events and
monitors the entire network, including the data-loggers and radio links,
through commercial network and bandwidth monitoring software.
ISNet Data Flow and Management
As shown in figures 3 and
4, the ISNet data and
information flow can be managed on three different levels:
The LCCs run the Earthworm real-time seismic processing system and keep a complete local database of waveforms from the seismic stations directly connected to them. The system will perform real-time event detection and location based on the triggers coming from data-loggers and parametric information such as arrival-time picks coming from the other LCCs. Once an event is detected, the system will perform automatic magnitude and focal mechanism estimations. The results of these analyses are used to build a local event database, and at the same time, they are sent to the other LCCs and to the RISSC. Immediately after an event, the RISSC will calculate ground-shaking maps using data provided by LCCs and/or from the event database. Finally, the recorded earthquake data are stored in an event database, where they are available for distribution and visualization for further offline analyses (figure 4).
As indicated in the previous section, ISNet has a complex infrastructure and must be accurately managed for proper real-time functionality. The most important factor is station and transmission network availability. To check the availability status of the network, we need a real-time cross-check of different information sources. A large network is made up of many vendor devices for which the health status cannot necessarily be monitored by commercial software. To overcome these limitations, we are developing the ISNet Devices Manager (ISNM). The ISNM will be a complete real-time monitoring suite with a notification system to report failure or critical status of any of the network elements (figure 5). The collected data from each device will be stored in a database. Considering the network complexity, various information sources will be used: SMS, e-mail, simple network management protocol (SNMP), Internet control message protocol (ICMP), proprietary commands (Osiris), and netflow (Cisco network flow-control protocol).
Today, in the first phase of development, the network manager handles two kinds of data about the devices: user-inserted data and automatically collected data. ISNM is made up of four fundamental components:
The ISNM has a server/client architecture. The data collected at each of the LCCs will be sent in parameterized form to the server, which will then analyze the incoming information from all of the network elements.
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ISNet UPGRADE FOR SEISMIC EARLY-WARNING AND POST-EVENT APPLICATIONS |
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TOPINTRODUCTIONGEOLOGICAL SETTING, HISTORICAL...THE IRPINIA SEISMIC NETWORK...ISNet UPGRADE FOR SEISMIC...EXAMPLES OF RECORDED WAVEFORMSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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Data Communication Enhancements for Early-warning Purposes
A reliable and effective data communications system is a main concern for
all early-warning applications. Based on our experience in developing ISNet,
we think that complete control and management of the telecommunication
systems, from the data-loggers to the RISSC, is necessary for effective
experimentation with early-warning applications. Without it one cannot hope to
understand latency, delay, failure, and weak points in complex
data-communications structures designed to provide rapid information; identify
elements where essential time can be gained; and determine what future
technological improvements of the entire early-warning system are
indicated.
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Combining different technologies, such as satellite, radio, and digital wire lines, will make it easier and faster to accomplish these high availability rates. We will evaluate the use of these multiple communication technologies in three LCCs. We have also taken into account the following constraints when planning the new data communication system: system reliability and redundancy, low or no system damage during a strong earthquake, overall transmission delays less than 100-200 ms, and data security.
Integrated Seismic Stations
An important element in managing an emergency in the minutes after a strong
earthquake is predicting the distribution of the damage in the region. Rapid
ground-shaking calculation in terms of one or more strong ground-motion
parameters such as peak ground acceleration (Pga), peak ground velocity (Pgv),
spectral ordinates (Sa(T)) or instrumental intensity
(IMM) are key elements in this effort
(Wald et al. 1999;
Goltz 2003). Ground-shaking
maps are usually generated by integrating data recorded during the earthquake
with estimates performed by using pre-existing attenuation relationships. For
the Campania region, we retrieved an ad hoc attenuation relationship for peak
ground-motion quantities applying the stochastic simulation method of Boore
(1983) and source and
attenuation parameters retrieved from the INGV earthquake waveform catalog
(Convertito et al.
2007).
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EXAMPLES OF RECORDED WAVEFORMS |
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TOPINTRODUCTIONGEOLOGICAL SETTING, HISTORICAL...THE IRPINIA SEISMIC NETWORK...ISNet UPGRADE FOR SEISMIC...EXAMPLES OF RECORDED WAVEFORMSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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As an example showing the network sensitivity at a very low-magnitude level, we report in figure 6 the vertical acceleration records of a sequence of microearthquakes, with Mw computed by using the Hanks and Kanamori (1979) relationship in the range 1.2 to 1.9, that occurred in a 20-min time window and were located approximately at the network center (see locations in figure 2). In figure 6, the records have been arranged from top to bottom as a function of the epicentral distance so as to display the variation in the signal-to-noise levels as a function of seismic moment and distance from the earthquake source, whose depths range between 11 km and 23 km (as reported in the INGV earthquake database).
The seismic moment and the corresponding magnitude moment of these events
has been calculated by measuring the low-frequency DC level on the
S-wave displacement spectrum after the constant
QS attenuation correction and applying the Brune
(1970) model:
 | (1) |
= 2.7 g/cm3 (density)
c = 3.6 km/s (S-wave velocity)
Rc = 0.6 (radiation pattern of S waves)
Fs = 2 (free-surface factor)
r = hypocentral distance
Qs = 120 (Convertito et al. 2007)
0 is low frequency displacement spectrum amplitude in
m·s.
A time window of 5 s bracketing the handpicked S arrival has been selected for this analysis. The estimated M0 values are reported in correspondence with each event detected, as shown in figure 6. The values of M0 greater than 1 x 1018 dyne-cm have been estimated as the averages calculated from the data recorded by four stations. By visual inspection of the accelerograms in figure 6, we can roughly estimate a maximum detection distance of about 10 km for events with seismic moment as small as M0 = 7.5 x 1017 dyne-cm or 1.2 moment magnitude. Since the half-distance spacing between ISNet stations in its central area is about 10 km, we would expect to have at least two records for such small-magnitude events when they occur inside the network. However, more refined analyses based on the recorded noise levels are needed to better quantify the distance/magnitude detection thresholds. The example shown may provide an approximate indication of the acceleration network sensitivity.
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It is interesting to note the presence of coherent secondary arrivals along the coda of seismograms, probably generated by reflection and conversion of primary waves at shallow crustal discontinuities. The quality of recordings and the excellent signal-to-noise ratio for such a small-magnitude set of events suggests that it may be possible to use these earthquake records to obtain detailed information on the propagation medium.
During the period that the network has been recording, an event occurred in
the Gargano area at a distance of about 150 km
(figure 2) from the center of
the network. This earthquake was recorded by the 13 stations of the
accelerometric network (figure
8), and the S-wave displacement spectra from different
ISNet stations are plotted in figure
9. The spectra have been corrected for the hypocentral distances
and coefficients from equation 1 to obtain the seismic moment units on the
spectrum-amplitude axis. We saw that despite an amplitude correction based on
a simplified source model, both the Gargano event and the Irpinia seismic
sequence show coherent spectral shapes among the different stations, implying
an accurate calibration of the seismological instruments and lack of dominant
spectral distortions due to site/propagation effects. As expected, a high
corner frequency is observed for the smaller event (around 6-7 Hz) relative to
the larger one (around 3-4 Hz), but the observed corner frequency ratio
(around 2) is considerably smaller than the expected value (about 18) for
constant stress-drop scaling (
=(7/16) x
Mo/r3 Keilis-Borok
[1959]). The spectral analysis
of events with smaller seismic moments shows that a low-pass cut-off frequency
generally occurs at about 8-10 Hz, thus indicating that the observed violation
of stress-drop scaling law is probably related to a high-frequency,
earth-filtering attenuation effect, as suggested by Anderson and Hough
(1984).
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DISCUSSION |
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TOPINTRODUCTIONGEOLOGICAL SETTING, HISTORICAL...THE IRPINIA SEISMIC NETWORK...ISNet UPGRADE FOR SEISMIC...EXAMPLES OF RECORDED WAVEFORMSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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This seismic alert management system is conceived as having levels of analysis and decision distributed over the seismic network nodes. This objective is realized through the implementation of virtual subnets managed by data concentrators (the LCCs). Each node of the network has to be able to process and analyze the first P-wave signal in real-time, providing the measured quantities (e.g., arrival time, frequency, amplitude) to its closest LCC. As more stations record the seismic signal, the new measurements are sent to and processed by the LCC, which cross-checks the information from the different stations and outputs a progressively refined estimate of earthquake location and magnitude, along with uncertainty. Given this architecture, it is critical that specific procedures are in place to perform efficient, rapid, and robust data analysis.
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For real-time magnitude estimates, we are investigating the advantages of using near-source, strong-motion records at close source distances. Based on a massive analysis of the European Strong Motion Database (Ambraseys et al. 2004), Zollo et al. (2006) have demonstrated that the low-pass-filtered, peak amplitudes of initial P- and S-wave seismic signals recorded in the vicinity of a current earthquake source correlate with the earthquake magnitude and can be used for real-time estimation of the event size in seismic early-warning applications. The earthquake size can therefore be estimated using only a couple of seconds of signal from the P- or S-wave onsets. Results of Zollo et al. (2006) suggest that estimations of earthquake magnitude in real-time procedures can be obtained by combining such measurements from initial P- and S-wave signals as a function of time from the first P-wave detection. The use of S-wave data for early-warning applications is feasible in cases where a dense strong-motion network is deployed around the potential earthquake source area (hypocentral distance less than 20-30 km), so that first S-P times are smaller than 2-3 seconds, which is true of events occurring inside the ISNet. The regression law proposed by Zollo et al. (2006) that relates early P- and S-peak amplitudes to magnitude can be usefully adopted to obtain real-time estimates of magnitude if the hypocentral distance can be determined using real-time location procedures such as, for instance, the method proposed by Horiuchi et al. (2005) or the new approach developed by Satriano et al. (forthcoming).
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ACKNOWLEDGMENTS |
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TOPINTRODUCTIONGEOLOGICAL SETTING, HISTORICAL...THE IRPINIA SEISMIC NETWORK...ISNet UPGRADE FOR SEISMIC...EXAMPLES OF RECORDED WAVEFORMSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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1 Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano,
Naples, Italy 
2 Università degli Studi di Napoli "Federico II,"
Dipartimento di Scienze Fisiche, Naples, Italy
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TOPINTRODUCTIONGEOLOGICAL SETTING, HISTORICAL...THE IRPINIA SEISMIC NETWORK...ISNet UPGRADE FOR SEISMIC...EXAMPLES OF RECORDED WAVEFORMSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
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Istituto Nazionale di Geofisica e Vulcanologia
Osservatorio
Vesuviano
Via Diocleziano 328, 80124
Naples,
Italy
iannaccone{at}ov.ingv.it
(G.I.)
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