ISMD (INGV Strong Motion Database) publishes in quasi real time the waveforms and the related metadata for each earthquake with ML ≥ 3.0 localized in the Italian territory.
During important seismic sequences the minimum magnitude threshold might be set to higher values (usually ranging from 3.5 to 4.0) in order to publish as soon as possible the strong motion data recorded for the most important seismic events.
Data are at first published in the automatic version (see auto in the last column of the earthquake list).
Strong motion data published in version auto are downloadable in SAC-raw format (counts) only.
The Peak Ground accelerations deduced from raw data have purely an approximate meaning and they may contain errors.
For all earthquakes with magnitude ≥ 3.5 (i.e. ≥ 4.0 before October 2016) the automatic results are manually revised (see MAN in the last column of the earthquake list) by an expert seismologist in the first 48h after the event origin time, in order to remove the low quality data.
Data recorded by the INGV strong motion stations and related partners are downloadable few minutes after the earthquake occurrence. The Origin time (UTC), the hypocentral coordinates and the local magnitude are provided by the INGV National Earthquake Center.
Soon after the alert the ISMD automatic system downloads 5 minutes of signals, starting from the event origin time.
Earthquakes strong-motion waveforms are selected from the continuous recordings in MiniSeed format and then archived in real time on a seedlink server, implementing the Seiscomp3 client.
The accelerometric waveforms are then converted in SAC and ASCII formats.
The waveforms are provided both in binary raw SAC (counts) and ASCII (in automatic) processed versions.
The ASCII-files include 50 header-lines (see the example), providing the information of the event and the recording stations, followed by data in cm/s2.
In order to manually reprocessed the SAC raw data, it is possible to convert SAC counts to cm/s2 using for each station the related conversion factor (i.e. a number that consider both the gain of the digitizer and the sensitivity of the sensor) available in the stations instrumentation table.
The simple formula to be used is:
[ raw-SAC-counts * (( 1 / Conversion Factor ) * 100 ) ]
On the basis of the earthquake magnitude, the ISMD automatic system selects data from the recordings stations considering fixed distance cut-off thresholds, empirically determined on the basis of the signal to noise ratio analyses. In particular:
Mag ≤ 3.5 : stations up to 150 km
3.5 < Mag ≤ 5.5 : stations up to 200 km
5.5 < Mag ≤ 6.5 : stations up to 250 km
Mag > 6.5 : stations up to 300 km
Magnitude might be Local (ML) or Moment (MW) in dependence of the information available at INGV National Earthquake Center web site.
AUTOMATIC DATA PROCESSING
- waveforms resampling at 100 Hz;
- application of a first-order baseline operator to the entire record, in order to have a zero-mean of the signal;
- baseline correction (least square method), in order to remove the linear trend;
- tapering of the signal by applying a cosine function (0.01%) at the beginning and at the end of each selected window;
- application of a 4th order Butterworth band-pass acausal filter (Boore and Akkar, 2003) in order to remove the high and low-frequency noise.
The filter cut-off thresholds are automatically selected on the basis of the earthquake magnitude, as indicated below:
ML < 3.5 : 0.5-25 Hz
3.5 ≤ ML < 4.3 : 0.3-35 Hz
4.3 ≤ ML < 5.5 : 0.2-35 Hz
ML ≥ 5.5 : 0.1-40 Hz
- final conversion of the processed acceleration in velocity (cm/s) and displacement (cm).
For details see Pacor et al., 2011 and Massa et al., 2010.
- Peak Ground Acceleration (PGA), Velocity (PGV) and Displacement (PGD);
- 5%-damped elastic acceleration response spectra (SA), for periods up to 4 sec, calculated considering the portion of signal from 5% to 90% of the total energy;
- 5%-damped pseudo-velocity response spectra (PSV);
- 5%-damped relative displacement response spectra (Sd);
- Arias (AI, Arias 1970) and Housner (HI, Housner 1952) intensities;
- directional Horizontal-to-Vertical Spectral Ratio (HVSR) calculated at each station considering 10s of strong-motion phase and coda;
- comparison (in terms of PGA, PGV and acceleration response spectra) between the recorded data and the predicted ones by the Italian GMPEs calibrated by Bindi et al. (2011);
warning for possible biases: Italian GMPEs considers earthquakes with 4.0 ≤ Mw ≤ 6.9 and a maximum (Joyner-Boore or epicentral) distance of 200 km;
- plots of the recorded waveforms (acceleration, velocity, displacement);
- plots of SA, PSV, Sd;
Moreover, considering an area of +/- 1° of latitude [N] and +/- 1.5° of longitude [E] around the epicenter:
- plot of the Italian reference seismic hazard map (Stucchi et al., 2011);
- historical and the instrumental seismicity maps of the epicentral area.
The instrumental seismicity is provided by the INGV official instrumental bulletin, while historical events are provided by the CPTI15 catalogue (Rovida et al., 2016).
The results listed above are resumed in the event web page, generated automatically soon after the end of the analyses.
To view and download the available data, the users have to select one of the three sections available in the right box of the ISMD home page
(i.e. EARTHQUAKES, STATIONS, WAVEFORMS).
After user registration and/or login the waveforms and the response spectra are downloadable both from the event and the station web pages.
Moreover, from the session WAVEFORMS, it is possible to search and download the waveforms (and metadata) associated to different earthquakes, through a selection based on several ground motion parameters and/or site classification.
Data are downloadable as:
SAC raw format (*.sac, counts), ASCII processed acceleration (*.DAT, cm/s2), ASCII processed velocity (*.VEL, cm/s), ASCII processed displacement (*.DIS, cm) and ASCII response spectra (acceleration, velocity and displacement).
In each case the system provided a single "tar" file
(e.g.,ismd_list_event_20160710003706_acc_dw160715135248.tar), including data compressed in "bz2" format
In order to obtain the usable data, please use the commands:
tar -xvf file.tar