Moving Forward by Looking Back: Utilization of Legacy Seismic Data in the Modern Age

Prior to the arrival of modern-day digital seismometers, instruments used analog media such as paper helicorders. Where 40 years-worth of digital data collection exist, analog instruments recorded for nearly a century. Many of these irreplaceable records are still extant and stored by various operators around the world. These collections of analog seismograms represent vast untapped data resources with great potential. Accessing this longer time span provides for unique opportunities to study unusual events and time-dependent phenomena that are too gradual to be constrained by 40-years of digital data.

Since the analog era, our use of seismograms has moved from limited segments of earthquake recordings to continuous waveforms of ground motion. These approaches allow investigation of non-traditional targets such as subsurface changes, glacial calving events, and tracking storm systems. The vast majority of analog recordings are dominated by these signals, once considered noise. However, before any scientific investigation can happen, analog data must be converted into digital waveforms digestible to modern techniques.

Like many others around the world, the Harvard seismic station has played host to a collection of analog records since 1933. Over the past years, efforts have been made to rescue, preserve, scan, and digitize the HRV records. In the course of these efforts a MATLAB-based software, DigitSeis, has been developed. DigitSeis converts high-resolution scans of analog seismograms into digital time series, and it is currently the only existing digitization software which fully considers the timemarks present in analog seismograms, both re-incorporating them into the waveforms, and using them to define timing.

The software has been used for both "in-house" efforts to digitize the HRV collection, and a citizen science project involving hundreds of students from high schools throughout Japan. The students learn the DigitSeis software with an example image, and once their analysis has been reviewed and passed, they move on to digitize images that have never been digitized. The project produces digital data for seismologists to use, but is also a great opportunity to engage students in science and research at an early stage. The time series digitized by these students are made openly available online.