ISMD (INGV Strong Motion Database) provides in quasi real time the waveforms and the related metadata for earthquake with ML ≥ 3.0 occurred in the Italian territory and localized by the INGV (National Institute for Geophysics and vulcanology) surveillance system.
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 fast provide the strong motion data recorded for the most important seismic events.
At first data are published in the automatic version (i.e. check NO in the last column of the earthquake list). The strong motion data published in version NO CHECKED are downloadable in SAC format (counts). From January 2019 the raw data are available also in MisiSeed format (counts).
The Peak Ground Accelerations (PGA) calculated from the NO CHECKED data have purely an approximate meaning and they may contain errors.
Data recorded in case of earthquakes with magnitude ≥ 3.5 are CHECKED (i.e. check YES, in the last column of the earthquake list) by an expert seismologist in the first 24h after the event origin time. In this case, low quality data are removed.
ISMD includes all accelerometric stations registered at FDSN and operated by INGV and provided partners that send data in real time to the National Earthquake Observatory (INGV-ONT). Data and metadata are freely open with the exclusion of the National Accelerometric Network (RAN), FDSN code: IT) for which just the waveforms metadata are available. Data recorded by RAN are downloadable at ITACAweb site.
Data recorded by the strong motion stations included in ISMD are downloadable few minutes after the earthquake occurrence. The origin time (UTC), the hypocentral coordinates and Magnitude (ML=local; Mw=Moment) are provided by the National Earthquake Observatory (INGV-ONT).
In case of a new earthquake with ML ≥ 3.0 the ISMD automatic system downloads 5 minutes of signals, starting from the event origin time. The accelerometric waveforms are selected from the MiniSeed data stream recorded in continuous mode and archived in real time on a seedlink server, implementing the Seiscomp3 client. The accelerometric waveforms are then converted in SAC and ASCII formats.
Data for earthquakes with ML < 3.5 are NO checked and they are available just in raw version (format: SAC, unit: count). From January 2019 raw data are provided also in Miniseed format (unit counts).
Data for earthquakes with ML ≥ 3.5 are CHECKED and then provided in ASCII format in cm/s2 (gal).
The ASCII-files are composed by 50 header-lines (see the example), providing event, station and waveform metadata, followed by the data column in cm/s2 (gal).
In order to analyse again the SAC raw-data, users can convert SAC data in counts to cm/s2 (gal) using for each station the related channel-sensitivity (i.e. obtained considering 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:
gal-data = [ (SAC-counts / channel-sensitivity) * 100 ]
On the basis of the earthquake magnitude, 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 provided by the National Earthquake Observatory (INGV-ONT).
DATA PROCESSING TO OBTAIN CHECKED-DATA
The results above are resumed in the event web page, generated automatically soon after the end of the automatic procedure of 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 version (*.sac, unit counts), ASCII checked acceleration (*.DAT, unit cm/s2), ASCII checked velocity (*.VEL, cm/s), ASCII checked displacement (*.DIS, cm) and ASCII response spectra (acceleration, velocity and displacement).
From January 2019 raw-data are also available in Miniseed format (*.seed, unit counts).
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