100 Years of Paper Seismograms from Denmark and Greenland, 1907–2008

The paper seismograms from 100 years of observations in Denmark and Greenland has since October 2021 been made available through the Danish National Archives. Five case stories illustrate the quality and variation of the seismograms, and the historical context of operation of the stations. (1) The earliest recorded earthquake in the archive is recorded at GDH station in Greenland, where the 1907 Mw 7.2 earthquake in Tajikistan is recorded on smoked paper. (2) The first Danish earthquake is a local event close to Copenhagen in 1930. (3) We have illustrated the 50 megaton nuclear explosion in Novaya Zemlya in 1961—the largest nuclear test explosion ever. (4) The M 9.2 earthquake in Alaska in 1964 recorded on several instruments at COP. (5) A local earthquake in northeast Greenland recorded both on paper on World-Wide Standard Seismographic Network instruments and digitally on a modern broadband instrument.

A Review of Timing Accuracy across the Global Seismographic Network

The accuracy of timing across a seismic network is important for locating earthquakes as well as studies that use phase-arrival information (e.g., tomography). The Global Seismographic Network (GSN) was designed with the goal of having reported timing be better than 10 ms. In this work, we provide a brief overview of how timing is kept across the GSN and discuss how clock-quality metrics are embedded in Standard for Exchange of Earthquake Data records. Specifically, blockette 1001 contains the timing-quality field, which can be used to identify time periods when poor clock quality could compromise timing accuracy. To verify the timing across the GSN, we compare cross-correlation lags between collocated sensors from 1 January 2000 to 1 January 2020. We find that the mean error is less than 10 ms, with much of the difference likely coming from the method or uncertainty in the phase response of the instruments. This indicates that timing across the GSN is potentially better than 10 ms. We conclude that unless clock quality is compromised (as indicated in blockette 1001), GSN data’s timing accuracy should be suitable for most current seismological applications that require 10 ms accuracy. To assist users, the GSN network operators have implemented a “gsn_timing” metric available via the Incorporated Research Institutions for Seismology Data Management Center that helps users identify data with substandard timing accuracy (the 10 ms design goal of the GSN).

Rayleigh-Wave Amplitude Uncertainty across the Global Seismographic Network and Potential Implications for Global Tomography

ABSTRACT The Global Seismographic Network (GSN) is a multiuse, globally distributed seismic network used by seismologists, to both characterize earthquakes and study the Earth’s interior. Most stations in the network have two collocated broadband seismometers, which enable network operators to identify potential metadata and sensor issues. In this study, we investigate the accuracy with which surface waves can be measured across the GSN, by comparing waveforms of vertical-component Rayleigh waves from Mw 6 and larger events between collocated sensor pairs. We calculate both the amplitude deviation and correlation coefficient between waveforms at sensor pairs. In total, we make measurements on over 670,000 event–station pairs from events that occurred from 1 January 2010 to 1 January 2020. We find that the average sensor-pair amplitude deviation, and, therefore, GSN calibration level, is, approximately, 4% in the 25–250 s period band. Although, we find little difference in sensor-pair amplitude deviations as a function of period across the entire network, the amount of useable data decreases rapidly as a function of increasing period. For instance, we determined that just over 12% of records at 250 s period provided useable recordings (e.g., sensor-pair amplitude deviations of less than 20% and sensor-pair correlation greater than 0.95). We then use these amplitude-estimate deviations to identify how data coverage and quality could be limiting our ability to invert for whole Earth 3D attenuation models. We find an increase in the variance of our attenuation models with increasing period. For example, our degree 12 attenuation inversion at 250 s period shows 32% more variance than our degree 12 attenuation model at 25 s. This indicates that discrepancies of deep-mantle tomography between studies could be the result of these large uncertainties. Because these high uncertainties arise from limited, high-quality observations of long-period (>100 s) surface waves, improving data quality at remote GSN sites could greatly improve ray-path coverage, and facilitate more accurate and higher resolution models of deep Earth structure.

Continuous Improvement in the Performance and Operations of the Global Seismographic Network (GSN)

<p>The decades long recordings of high-quality open data from the Global Seismographic Network have facilitated studies of earth structure and earthquake processes, as well as monitoring of earthquakes and explosions worldwide.  These data have also enabled a wide range of transformative, cross-disciplinary research that far exceeded the original expectations and design goals of the network, including studies of slow earthquakes, landslides, the Earth’s “hum”, glacial earthquakes, sea-state, climate change, and induced seismicity. </p><p>The GSN continues to produce high quality waveform data, metadata, and multiple data quality metrics such as timing quality and noise levels.   This requires encouraging equipment vendors to develop modern instrumentation, upgrading the stations with new seismic sensors and infrastructure, implementing consistent and well documented calibrations, and monitoring of noise performance.    A Design Goals working group is convening to evaluate how well the GSN has met its original 1985 and 2002 goals, as well as how the network should evolve in order to be able to meet the requirements for enabling new research and monitoring capabilities.   </p><p>In collaboration with GEOFON and GEOSCOPE the GSN is also reviewing the current global distribution and performance of very broadband and broadband stations that comprise these three networks.  We are working to exchange our expertise and experience about new technologies and deployment techniques, and to identify regions where we could collaborate to make operations more efficient, where current efforts are overlapping or where we have similar needs for relocating stations. </p>

Operational Capabilities during Crisis: The Chilean Seismographic Network

The severe mobility restrictions imposed countrywide by authorities of Chile in response to the pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have impacted all areas of activities. Major difficulties began in March 2020, with partial quarantines in the capital city, which later extended to other cities in the country, and it soon transformed in total confinement. We examine the evolution of the pandemic and its consequences on the field and headquarters operations of the National Seismological Centre (CSN), as remote stations could not be visited to carry out maintenance work. Several indicators, as a function of time, such as station operability percentage, timely reports, and ability to deliver requested information, reveal some negative impact on the uptime of stations but not in the capability of the CSN to fulfill its mission, which is to deliver timely seismic information to emergency services. The largest event in the country within this period took place on 3 June 2020, activating in a timely manner, the new tools on finite-fault modeling being developed within the center.

Improvements in seismic resolution and current limitations in the Global Seismographic Network

SUMMARY Station noise levels play a fundamental limitation in our ability to detect seismic signals. These noise levels are frequency-dependent and arise from a number of physically different drivers. At periods greater than 100 s, station noise levels are often limited by the self-noise of the instrument as well as the sensitivity of the instrument to non-seismic noise sources. Recently, station operators in the Global Seismographic Network (GSN) have deployed several Streckeisen STS-6A very broad-band borehole seismometers. These sensors provide a potential replacement for the no-longer-produced Streckeisen STS-1 seismometer and the GeoTech KS-54 000 borehole seismometer. Along with showing some of the initial observational improvements from installing modern very broad-band seismometers at depth, we look at current limitations in the seismic resolution from earth tide periods 100 000 s (0.01 mHz) to Nyquist at most GSN sites (0.02 s or 50 Hz). Finally, we show the potential for improved observations of continuously excited horizontal Earth hum as well as the splitting of very long-period torsional modes. Both of these observations make use of the low horizontal noise levels which are obtained by installing very broad-band borehole seismometers at depth.

Seasonal and spatial variations in the ocean-coupled ambient wavefield of the Ross Ice Shelf

The Ross Ice Shelf (RIS) is host to a broadband, multimode seismic wavefield that is excited in response to atmospheric, oceanic and solid Earth source processes. A 34-station broadband seismographic network installed on the RIS from late 2014 through early 2017 produced continuous vibrational observations of Earth's largest ice shelf at both floating and grounded locations. We characterize temporal and spatial variations in broadband ambient wavefield power, with a focus on period bands associated with primary (10–20 s) and secondary (5–10 s) microseism signals, and an oceanic source process near the ice front (0.4–4.0 s). Horizontal component signals on floating stations overwhelmingly reflect oceanic excitations year-round due to near-complete isolation from solid Earth shear waves. The spectrum at all periods is shown to be strongly modulated by the concentration of sea ice near the ice shelf front. Contiguous and extensive sea ice damps ocean wave coupling sufficiently so that wintertime background levels can approach or surpass those of land-sited stations in Antarctica.

Seismic Sensors Record a Hurricane’s Roar

Newly installed infrasound sensors at a Global Seismographic Network station on Puerto Rico recorded the sounds of Hurricane Maria passing overhead.