Abstract The seismograms of earthquakes, which have closely spaced hypocenters, tend to be similar due to the similarity of the Green’s functions characterizing the source-receiver paths. Based on the λ /4 criterion, it is frequently argued that similar earthquakes may represent repeated slip of the same patch of a fault. Because of the phenomenological nature of waveform similarity, such interpretations are strongly dependent on seismic signal characteristics and on the way, the waveform similarity is defined. In this article, we use two-dimensional synthetic wave-field simulations in lateral heterogeneous media to investigate how the waveform similarity of closely spaced hypocenters changes with interevent separation. We analyze the influence of correlation window length, signal frequency bandwidth, and source-receiver geometry on the waveform similarity and discuss under which conditions the λ /4 criterion can be applied to the synthetic data set. With the correlation window length defined as 2.8 times the travel-time difference between the S - and P -phase onsets, we find a correlation threshold value of 0.95 independent of the signal frequency bandwidth. We use the same threshold value for two field data examples that are similar to the synthetic data in frequency content and waveform complexity, and we discuss the implications of the λ /4 criterion. For three microearthquakes occurring during a fluid-injection experiment at the German deep drilling site (Kontinentale Tiefbohrung [KTB]), the interevent separation constrained by the λ /4 criterion is sufficient to identify these events as a sequence of repeating earthquakes in the sense that at least a fraction of the source area experienced repeated slip. For a second data example of four natural (micro-) earthquakes occurring near the island of Crete, the λ /4 criterion does not sufficiently constrain the hypocenter location to identify these events as repeating earthquakes due to the lack of high-frequency information.
<p>Detecting and notifying ongoing volcanic explosive eruptions can support the activities of the Volcanic Ash Advisory Centres (VAAC) in their contribution to the International Airways Volcano Watch. However, local monitoring systems are missing on many active volcanoes. Here, the use of a global monitoring that, even with lower reliability, can allow a fast response. Many studies have shown so far the utility and potential of long-range infrasound monitoring for this aim, but still open questions remain concerning the real efficiency and reliability of such a system.</p><p>In this study we investigate the potential of the infrasound network of the International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) to detect volcanic explosive eruptions at large distances. We apply a procedure based on the Infrasound Parameter (IP) calculated from a single array to selected volcanoes by accounting for realistic infrasound propagation conditions.</p><p>The procedure was applied to data recorded by the I06AU infrasound array (Cocos Island) between January 2012 and December 2019 and targeting Indonesian volcanoes at source-to-receiver distances ranging between 1000 and 2000 km, where activity from 11 volcanoes was reported in the period of analysis with an energy spanning from mild explosions to VEI4 eruptions.</p><p>The system reliability was evaluated from the ratio between real ones and the total number of notifications provided from I06AU array for each volcano.</p><p>The IP was calculated following previous studies and improved with new constraints accounting for the source strength and signal persistency. These allowed us to improve significantly the system reliability for events VEI3 or greater and strongly reduce the number of false alerts. Still, undetected explosive events remain due to unfavorable propagation conditions and unresolved ambiguity due to short spacing among volcanoes with respect to the array. We propose to solve this last issue by considering volcanic sectors rather than single volcanic edifices. Instead of a notification for a single volcano, an alert for an area of interest could be issued to draw the attention and trigger further analysis of satellite images by the VAACs.</p><p>This study is performed to improve the Volcanic Information System (VIS) proposed and developed in the framework of FP7 and H2020 ARISE projects.</p>
<p>We report on a multi-technique analysis using publicly available data for investigating the huge, accidental explosion that struck the city of Beirut, Lebanon, on August 4, 2020. Its devastating shock wave led to thousands of injured with more than two hundred fatalities and caused immense damage to buildings and infrastructure. Our combined analysis of seismological, hydroacoustic, infrasonic and radar remote sensing data allows us to characterize the source as well as to estimate the explosive yield. The latter ranges between 0.8 and 1.1 kt TNT (kilotons of trinitrotoluene) equivalent and is plausible given the reported 2.75 kt of ammonium nitrate as explosive source. Data from the International Monitoring System of the CTBTO are used for infrasound array detections. Seismometer data from GEOFON and IRIS complement the source characterization based on seismic and acoustic signal recordings, which propagated in solid earth, water and air. Copernicus Sentinel data serve for radar remote sensing and damage estimation. As there are strict limitations for an on-site analysis of this catastrophic explosion, our presented approach based on openly accessible data from global station networks and satellite missions is of high scientific and social relevance that furthermore is transferable to other explosions.</p>
The ability of the International Monitoring System (IMS) infrasound network to detect atmospheric explosions and events of interest strongly depends on station specific ambient noise which includes both incoherent wind noise and real coherent infrasonic waves. To characterize the coherent ambient noise, a broadband array processing was performed with IMS continuous waveform archive from 2007 to 2016 using the Progressive Multi-Channel Correlation algorithm (PMCC). The processing parameters include a new implementation of adaptive frequency dependent window length and a logarithmic band spacing. Such configuration allows to better discriminate between interfering signals with improved accuracy in the wave parameters estimations. Multi-year comparisons between the observed and modeled directional microbarom amplitude variations at several IMS stations using two-dimensional wave energy spectrum ocean wave products are performed to build of a reference database of infrasound oceanic sources in Northwest Pacific. The expected benefits of such studies concern the use of multi-year complementary data to finely characterize coupling mechanisms at the ocean-atmosphere interface. In return, a better knowledge of the source of the ambient ocean noise opens new perspectives by providing additional integrated constraints on the dynamics of the middle atmosphere and its disturbances where data coverage is sparse.
<p>Global scale infrasound observations confirm that the detection capability of the International Monitoring System (IMS) deployed to monitor compliance with the Comprehensive Nuclear-Test ban Treaty (CTBT) is highly variable in space and time. Previous studies estimated the radiated source energy from remote observations using empirical yield-scaling relations accounting for the along-path stratospheric winds. However, these relations simplified the complexities of infrasound propagation as the wind correction applied does not account for an accurate description of the middle atmosphere along the propagation path. In order to reduce the variance in the calculated transmission loss, massive frequency and range-dependent full-wave propagation simulations are carried out, exploring a wide range of realistic atmospheric scenarios. Model predictions are further enhanced by incorporating fine-scale atmospheric structures derived from a two-dimensional horizontal wave number spectrum model. A cost-effective approach is proposed to estimate the transmission losses at distances up to 8,000 km along with uncertainties derived from multiple gravity wave realizations. In the context of the future verification of the CTBT, this approach helps advance the development of network performance simulations in higher resolution and the evaluation of middle atmospheric models at a global scale with limited computational resources.</p>
