Adrien Oth

Bed load transport monitoring using seismic observations in a low‐gradient rural gravel bed stream

The characterization of bed load transport in rivers is critical for the fundamental understanding and management of fluvial systems. Bed load monitoring based on seismological observations has recently emerged as a viable noninvasive measurement technique. However, applications of this new approach have been hitherto restricted to the case of sediment transport in steep mountain rivers. Here we further develop and evaluate the approach for a lower gradient gravel bed stream in a rural catchment using seismic observations, in situ hydroacoustic measurements of bed load motion (impact-plate-type device), and 3-D hydromorphodynamic modeling. The results of this joint analysis of seismic measurements, hydroacoustic records, and sediment transport simulations show that the seismic monitoring technique for bed load transport characterization is applicable for a broader range of river systems than previously investigated.

Spectral Analysis of K-NET and KiK-net Data in Japan, Part I: Database Compilation and Peculiarities

Abstract The wealth of accelerometric recordings collected by the K-NET and KiK-net networks in Japan since 1996 provides a unique opportunity to improve our understanding of many important seismological research questions. Subsets of these data have been used for many case studies, most of them, however, not focusing specifically on the best practices for data selection and giving relatively little attention to the properties and peculiarities directly observable from the data. Yet for many applications, these steps are an important prerequisite for successful and reliable analysis. For this reason, we devote this article to the extraction of a large data set of surface and borehole recordings from the K-NET and KiK-net databases with strong emphasis on data quality and reliability. The final data set available for subsequent work consists of 78,840 records from 2201 earthquakes covering the Japan Meteorological Agency (JMA) magnitude range 2.7–8, observed at 1681 sites throughout Japan. We explain how this data set has been compiled, including automatic phase picking and relocation of events. We also present an overview of the general features of the data set, providing important information for subsequent analysis. Strong amplification effects at high frequencies are immediately visible on the surface recordings. Furthermore, there is a clear presence of downgoing waves in the borehole records, as deconvolution of borehole/surface recording pairs indicates.

Evaluation and optimization of seismic networks and algorithms for earthquake early warning - the case of Istanbul (Turkey)

Earthquake early warning (EEW) systems should provide reliable warnings as quickly as possible with a minimum number of false and missed alarms. Using the example of the megacity Istanbul and based on a set of simulated scenario earthquakes, we present a novel approach for evaluating and optimizing seismic networks for EEW, in particular in regions with a scarce number of instrumentally recorded earthquakes. We show that, while the current station locations of the existing Istanbul EEW system are well chosen, its performance can be enhanced by modifying the parameters governing the declaration of warnings. Furthermore, unless using ocean bottom seismometers or modifying the current EEW algorithm, additional stations might not lead to any significant performance increase.

Designing efficient earthquake early warning systems: case study of Almaty, Kazakhstan

Rapidly expanding urban areas in Central Asia are increasingly vulnerable to seismic risk; but at present, no earthquake early warning (EEW) systems exist in the region despite their successful implementation in other earthquake-prone areas. Such systems aim to provide short (seconds to tens of seconds) warnings of impending disaster, enabling the first risk mitigation and damage control steps to be taken. This study presents the feasibility of such a system for Almaty, Kazakhstan. Genetic algorithms are used to design efficient EEW networks, computing optimal station locations and trigger thresholds in recorded ground acceleration. Factors like the possibility of station failure, elevation and access difficulty to a potential site, and the potential usefulness of existing stations in the region are considered. We present a large set of possible efficient networks, to which further selection criteria can be applied by both the installation teams and the end user, such as authorities in Almaty.

KivuSNet: The First Dense Broadband Seismic Network for the Kivu Rift Region (Western Branch of East African Rift)

ABSTRACT The Kivu rift is located in the bordering region of the Democratic Republic of Congo and Rwanda, in the western branch of the East African rift. Here, the active volcanoes Nyamulagira (the most active in Africa) and Nyiragongo (host to the largest persistent lava lake on Earth) threaten the city of Goma and neighboring agglomerations, and destructive earthquakes can also affect the region. Despite this high level of hazard, modern seismic monitoring infrastructure was lacking in the area until very recently, leaving many aspects about the volcanic activity and seismicity up to speculation. In order to remedy this unsatisfactory situation, the first dense real‐time telemetered broadband seismic network, KivuSNet, was deployed in the region, with the first two stations in 2012/2013 followed by six additional ones in 2014. Since October 2015, a network of 13 stations is running in the Kivu rift, and with currently seven additional stations in the process of installation, this network is under continuous development. KivuSNet opens a new window for the seismological knowledge in this highly active rifting region. It allows for unprecedented insights into tectonic and volcanic seismicity, tremor patterns, and Earth structure as well as for sustainable real‐time monitoring of the volcanoes. Together with the often collocated KivuGNet geodetic stations, KivuSNet closes a dramatic observational gap in this region. This article presents the key features of the network, discusses technical aspects, and provides an overview of first results obtained using the thus far acquired data, showing KivuSNet’s wide potential.

Long-term monitoring of long-period seismicity and space-based SO2 observations at African lava lake volcanoes Nyiragongo and Nyamulagira (DR Congo)

Magma ascent that may lead to an eruption is commonly accompanied by variations of long-period seismic activity and SO2 degassing. Space-based measurements of SO2 emission rates represent a rapidly emerging and highly convenient approach for volcano monitoring; however, combining these long-term remote sensing observations with seismic data is still rare and, in particular, the potential of such a multidisciplinary approach as volcano monitoring tool remains largely unexplored. Here, shallow magmatic activity and magma migration patterns at the two closely located African volcanoes Nyiragongo and Nyamulagira are inferred from a nearly three-years-long SO2 emissions record and seismic observations between April 2014 and February 2017. The discrimination of magma movements into shallow plumbing systems allows for signs of volcanic unrest to be deciphered on a daily time scale, even with limited instrumentation on site.

Accurate estimation of seismic source parameters of induced seismicity by a combined approach of generalized inversion and genetic algorithm: Application to The Geysers geothermal area, California

The accurate determination of stress drop, seismic efficiency and how source parameters scale with earthquake size is an important issue for seismic hazard assessment of induced seismicity. We propose an improved non-parametric, data-driven strategy suitable for monitoring induced seismicity, which combines the generalized inversion technique together with genetic algorithms. In the first step of the analysis the generalized inversion technique allows for an effective correction of waveforms for attenuation and site contributions. Then, the retrieved source spectra are inverted by a non-linear sensitivity-driven inversion scheme that allows accurate estimation of source parameters. We therefore investigate the earthquake source characteristics of 633 induced earthquakes (Mw 2-3.8) recorded at The Geysers geothermal field (California) by a dense seismic network (i.e., 32 stations, more than 17.000 velocity records). We find a non-self-similar behavior, empirical source spectra that require an ωγ source model with γ > 2 to be well fit and small radiation efficiency ηSW. All these findings suggest different dynamic rupture processes for smaller and larger earthquakes, and that the proportion of high frequency energy radiation and the amount of energy required to overcome the friction or for the creation of new fractures surface changes with earthquake size. Furthermore, we observe also two distinct families of events with peculiar source parameters that in one case suggests the reactivation of deep structures linked to the regional tectonics, while in the other supports the idea of an important role of steeply dipping faults in the fluid pressure diffusion.

Reply to “Comment on ‘Attenuation, source parameters and site effects in the Irpinia–Basilicata region (southern Apennines, Italy)’ by I.B. Morozov”

We thank Igor B. Morozov for his interest in our article and for his comment (Morozov 2011) regarding the non-parametric attenuation curves for the Irpinia–Basilicata region obtained by generalized spectral inversion (Cantore et al. 2011). Morozovs comment has its root in a new model proposed by Morozov (2008, 2010) for the interpretation of seismic attenuation data, where the author comes to the conclusion that the typically used geometrical spreading terms are oversimplified and argues in favor of a new geometrical spreading model basically including an additional corrective term decaying exponentially with time (respectively distance). Morozov (2008, 2010) reanalyzed a range of datasets presented by other authors in the past using this model and, contrary to previous studies, concludes that all these datasets can be well explained using a frequencyindependent Qe (which he terms as “effective Q”), in contrast to the classical power-law model often implying a significant frequency-dependence of Q. The publication of his new model has led to a major controversy, and other authors provided a range of weighty counterarguments to Morozovs point of view (Xie 2010; Li 2010; Li and Lu 2010). In his comment on our article, Morozov (2011) essentially repeats the arguments that he has already discussed in great detail in his previous papers (Morozov 2008, 2010). In view of this open discussion, we acknowledge that there are certainly different ways of interpreting our non-parametric attenuation results (see also the paper Oth et al. 2011), but also note that a detailed discussion of the differing viewpoints was beyond the scope of our article, for which we refer the interested reader to the cited publications. Nevertheless, we would like to provide a brief discussion of the key points of Morozovs (2011) comment in the following. First of all, we would like to recall that the attenuation results obtained from our spectral inversions are the non-parametric attenuation functions themJ Seismol (2012) 16:91–93 DOI 10.1007/s10950-011-9243-x

Single-Station Seismo-Acoustic Monitoring of Nyiragongo's Lava Lake Activity (D.R. Congo)

Since its last effusive eruption in 2002, Nyiragongo has been an open-vent volcano characterized by the worlds largest persistent lava lake. This lava lake provides a unique opportunity to detect pressure change in the magmatic system by analyzing its level fluctuations. We demonstrate that this information is contained in the seismic and infrasound signals generated by the lava lake’s activity. The continuous seismo-acoustic monitoring permits quantification of lava lake dynamics, which is analyzed retrospectively to identify periods of volcanic unrest. Synchronous, high-resolution satellite SAR (Synthetic Aperture Radar) images are used to constrain lava lake level by measuring the length of the SAR shadow cast by the rim of the pit crater where the lava lake is located. Seventy-two estimations of the lava lake level were obtained with this technique between August 2016 and November 2017. These sporadic measurements allow for a better interpretation of the continuous infrasound and seismic data recorded at the closest station (~6 km from the crater). Jointly analyzed seismo-acoustic and SAR data reveal that slight changes in the spectral properties of the continuous cross-correlated low-frequency seismo-acoustic records (and not solely the single LP events) can be used to track fluctuations of the lava lake level on a daily and hourly basis. We observe that drops of the lava lake and the appearance of significant LP “lava lake” events are a consequence of deep magma intrusion, which induces changes in the shallow magmatic system. This study highlights the potential to continuously monitor Nyiragongo’s lava lake activity (and subsequent information about pressure changes within the magmatic system) using a single seismo-acoustic station located several kilometers from the vent.

On the relation of earthquake stress drop and ground motion variability: Stress Drop and Ground Motion Variability

One of the key parameters for earthquake source physics is stress drop since it can be directly linked to the spectral level of ground motion. Stress drop estimates from moment-corner frequency analysis have been shown to be extremely variable, and this to a much larger degree than expected from the between-event ground motion variability. This discrepancy raises the question whether classically determined stress drop variability is too large, which would have significant consequences for seismic hazard analysis. We use a large high-quality dataset from Japan with well-studied stress drop data to address this issue. Non-parametric and parametric reference ground motion models are derived and the relation of between-event residuals for JMA equivalent seismic intensity and peak ground acceleration with stress drop is analyzed for crustal earthquakes. We find a clear correlation of the between-event residuals with stress drops estimates; however, while the island of Kyushu is characterized by substantially larger stress drops than Honshu, the between-event residuals do not reflect this observation, leading to the appearance of two event families with different stress drops levels yet similar range of between-event residuals. Both the within-family and between-family stress drop variations are larger than expected from the ground motion between-event variability. A systematic common analysis of these parameters holds the potential to provide important constraints on the relative robustness of different groups of data in the different parameter spaces and to improve our understanding on how much of the observed source parameter variability is likely to be true source physics variability.