Determining earthquake epicentre from a single 3-component seismological station

An attempt has been made in this study to determine the epicentres by trigonometrical method from a single station using the three components of the ground's motion recorded by the Benioff Short Period Seismograph of the World Wide Standardized Seismograph Network (WWSSN) at Central Seismological Observatory (C.S.O.), Shillong. The trigonometrical method is based on the angle of bearing obtained from the ratio of the amplitudes of waves recorded on the horizontal components of the WWSSN seismograms. A comparison of recorded epicentres from the I.M.D. Seismological Network of Seismo, New Delhi with that of a single station i.e., C.S.O., Shillong has been made to estimate the accuracy of the method. The characteristics of the epicentral variations obtained from the Seismological Network of Seismo, New Delhi with that of C.S.O., Shillong are relatively small for local earthquakes compared to regional earthquakes with C.S.O., Shillong as the reference station. The characteristics of the epicentral variations particularly in the Shillong plateau are observed to be minimum. Strong correlations of epicentres are observed for both local as well as regional earthquake events.

Crustal Fault Connectivity of the Mw 7.8 2016 Kaikōura Earthquake Constrained by Aftershock Relocations

©2019. American Geophysical Union. All Rights Reserved. The 14 November 2016 Mw7.8 Kaikōura earthquake in the northern South Island, New Zealand, involved highly complex, multifault rupture. We combine data from a temporary network and the permanent national seismograph network to repick and relocate ~2,700 aftershocks of M≥3 that occurred between 14 November 2016 and 13 May 2017. Automatic phase-picking is carried out using REST, a newly developed hybrid method whose pick quality is assessed by comparing automatic picks for a subset of 138 events with analysts' picks. Aftershock hypocenters computed from high-quality REST picks and a 3-D velocity model cluster almost exclusively in the shallow crust of the upper plate and reveal linkages at depth between surface-rupturing fault segments. Only eight aftershocks are relocated on a deeper structure positioned between patches of geodetically detected afterslip. This indicates that afterslip has not triggered significant earthquake activity on the subduction interface during the period of aftershock activity analyzed.

Crustal Fault Connectivity of the Mw 7.8 2016 Kaikōura Earthquake Constrained by Aftershock Relocations

©2019. American Geophysical Union. All Rights Reserved. The 14 November 2016 Mw7.8 Kaikōura earthquake in the northern South Island, New Zealand, involved highly complex, multifault rupture. We combine data from a temporary network and the permanent national seismograph network to repick and relocate ~2,700 aftershocks of M≥3 that occurred between 14 November 2016 and 13 May 2017. Automatic phase-picking is carried out using REST, a newly developed hybrid method whose pick quality is assessed by comparing automatic picks for a subset of 138 events with analysts' picks. Aftershock hypocenters computed from high-quality REST picks and a 3-D velocity model cluster almost exclusively in the shallow crust of the upper plate and reveal linkages at depth between surface-rupturing fault segments. Only eight aftershocks are relocated on a deeper structure positioned between patches of geodetically detected afterslip. This indicates that afterslip has not triggered significant earthquake activity on the subduction interface during the period of aftershock activity analyzed.

Statistical analysis of fireballs: Seismic signature survey

Fireballs are infrequently recorded by seismic sensors on the ground. If recorded, they are usually reported as one-off events. This study is the first seismic bulk analysis of the largest single fireball data set, observed by the Desert Fireball Network (DFN) in Australia in the period 2014–2019. The DFN typically observes fireballs from cm-m scale impactors. We identified 25 fireballs in seismic time series data recorded by the Australian National Seismograph Network (ANSN). This corresponds to 1.8% of surveyed fireballs, at the kinetic energy range of $10^6$ – $10^{10}$ J. The peaks observed in the seismic time series data were consistent with calculated arrival times of the direct airwave or ground-coupled Rayleigh wave caused by shock waves by the fireball in the atmosphere (either due to fragmentation or the passage of the Mach cone). Our work suggests that identification of fireball events in the seismic time series data depends on both physical properties of a fireball (such as fireball energy and entry angle in the atmosphere) and the sensitivity of a seismic instrument. This work suggests that fireballs are likely detectable within 200 km direct air distance between a fireball and seismic station, for sensors used in the ANSN. If each DFN observatory had been accompanied by a seismic sensor of similar sensitivity, 50% of surveyed fireballs could have been detected. These statistics justify the future consideration of expanding the DFN camera network into the seismic domain.

O Instituto Geofísico da Universidade do Porto no contexto da Guerra Fria (1960-1963)

This article aims to explore the role of the Geophysical Institute of the University of Porto (IGUP) in the context of the Cold War, between the years of 1960 and 1963. Through the analysis of the documents that are part of the IGUP archive, I intend to understand its participation in the network International Scientific World-Wide Standard Seismograph Network (WWSSN), a program created by the North-American governmental cell, the US Department of Commerce Coast and Geodetic Survey (C&GS), in the first two decades of the second half of the 20th century, designed to obtain measurements seismological calibrated and standardized at a global level, and a collection of data, subsequently made available for unclassified scientific research. Given the relevance that this program had for the scientific area of seismology, this investigation was an attempt to understand how the project developed over time, and what were the forms of its establishment, operation, as well as its direct impact for IGUP itself, for the University of Porto, for national and international science and for the context in which Portugal was inserted in the Cold War.

An earthquake catalogue for seismic events in the Norman Wells region of the central Mackenzie Valley, Northwest Territories, using waveform data from local seismic stations

The development of unconventional hydrocarbon resources in the Norman Wells region of the Central Mackenzie Valley, Northwest Territories, has been explored by the energy industry. In early 2014, Conoco-Philips Canada conducted two multi-stage test operations of hydraulic fracturing (HF) in the region. In this study, we combine seismic data from the Canadian National Seismograph Network, four new stations established by the Northwest Territories Geoscience Office in collaboration with Natural Resources Canada in the Norman Wells region, and a local dense array installed by Conoco-Philips Canada to study the seismicity distribution during the pre-HF, HF and post-HF periods. We have identified and located 130 earthquakes within 100 km of the geographic centre of the local seismic network near Norman Wells for the pre-HF period (11 September 2013 - 7 February 2014). In comparison, 231 events are located during the HF period (8 February 2014 - 10 March 2014), and for the two post-HF periods, 11 March 2014 - 31 July 2014 and 27 February 2015 - 31 December 2015, we have catalogued 255 and 138 events, respectively. Source parameters and detailed phase pickings of each earthquake are given in the Appendices.

High-resolution source imaging based on time-reversal wave propagation simulations using assimilated dense seismic records

Summary This paper describes an efficient approach to high-resolution time-reversal source imaging simulation. Dense seismograph network records are back-propagated from stations to the hypocenter through a 3D subsurface structure model to estimate the initial source wavefield at the earthquake initiation time. By assimilating high-density observational data into the time-reversal wave propagation, a clearer source image can be determined, even for deep and distant earthquakes, than is achievable with conventional source imaging. The effectiveness of data-assimilation-based source imaging by a time-reversal wave propagation simulation with a 3D heterogeneous structural model was demonstrated using recordings from a nationwide strong-motion seismograph network during the 2007 Off Niigata, Japan, Mw 6.6 earthquake, and the 2007 Off Ibaraki, Japan, Mw 6.8 earthquake. Such data-assimilated-based simulations are also effective for early forecasting of strong ground motions caused by large earthquakes through fast time-advancing simulations based on the current assimilated wavefield. We will discuss the feasibility of a disaster prevention system for the early forecasting of strong motion disasters due to large earthquakes, based on repeatedly estimating source parameters and forecasting strong motions in future time based on the current assimilated wavefields.

A Critical Assessment of Canadian Earthquake Monitoring and Alerting Practice versus the Initial Challenges of the 2020 COVID-19 Experience

We describe the regular pre-COVID mode of operations for the Canadian National Seismograph Network and the associated monitoring, alerting, and analysis for earthquakes in Canada; we describe how the current operational posture evolved and discuss the ways in which the posture was and was not suitable to respond to the challenges and constraints of the COVID-19 situation in Canada. We find that many of the design and operation decisions that have been taken over the last several decades for earthquake monitoring in Canada, collectively driven largely by considerations of resilience and cost-effectiveness and further refined after the experience of the H1N1 pandemic, resulted in a system that continued to function effectively under lockdown conditions. There were many earthquakes in Canada that required seismologist response during the lockdown, all of which were handled remotely without issue. Specific challenges and lessons learned from the first few months of the pandemic are noted.

Seismoving App: Development of an early warning system for earthquakes

This work presents the development of an early warning system for earthquakes using the school seismograph network created by the Geodynamic Institute of the National Observatory of Athens. This work was conducted under the scientific oversee of Dr. Gerasimos Chouliaras, Research Director of the Geodynamic Institute of the National Observatory of Athens and the educational supervision of Prof. Dr. Norbert Frank, Professor in Physics and Earth Sciences at Heidelberg University.

Analog Seismogram Archives at the Earthquake Research Institute, the University of Tokyo

The Earthquake Research Institute (ERI) of the University of Tokyo maintains archives of analog seismograms and mareograms. The main collection is ∼236,000 Japanese historical seismograms recorded at the University of Tokyo (at various buildings and using various instruments around Hongo [Tokyo] with a total of 189,000 records from 1881 to 1993), at the Tsukuba observatory (∼11,000 records from 1921 to 1986), and at the Wakayama seismological network (∼12,650 records from 1928 to 1968). Seismograms recorded by temporal stations at various locations in Japan for several years, typically following large earthquakes, are also included. Different types of instruments were used to record the data. The oldest record from a large earthquake is from the 1891 Nobi earthquake recorded at Hongo on a circular seismogram using an Ewing-type seismograph. Teleseismic seismograms include those from the 1899 Alaska earthquake at Hongo on an Omori-type seismograph. Imamura-type and Omori-type tremometers and strong-motion seismographs had also been used for a long time. While these seismograms were microfilmed by the 1990s, the original smoked paper records have also been archived. Foreign seismogram collections include those from earthquakes in Taiwan between 1904 and 1917 recorded in both Japan and Taiwan and those from the Canadian Seismograph Network between 1981 and 1989. For the Worldwide Standardized Seismograph Network stations, almost all (∼5,000,000) microfilm records at 167 stations from 1963 to 1988 are archived. High-resolution image scanning of analog daily seismograms at the Wakayama microearthquake network is currently being performed, and the scans are provided using Leaflet software so that the users can easily access and enlarge parts of seismograms. The tsunami waveform archive contains ∼3100 records on Japanese tide gauges from large earthquakes between 1911 and 1996. The available data, with dates and types of instruments, can be searched from the database through the website of the ERI.