Implementation of a new real time seismicity detector for the Mayotte crisis

<p>Seismology is one of the main techniques used to monitor volcanic activity worldwide. Seismicity analysis through several seismic sensor deployments has been used to monitor Mayotte volcano crisis since its beginning in May 2018. Because volcanic activity can evolve rapidly, efficient and accurate seismicity detectors are crucial to assess in real-time the activity level of the volcano and, if needed, to issue timely warnings.</p><p> </p><p><span>Traditional real-time seismic processing software, such as EarthWorm or SeisComP, use phase onset pickers followed by a phase association algorithm to declare an event and proceed with its location. Real-time phase pickers usually cannot identify whether the detected phase is a P or S arrival and this decision or assumption is made by the associator. The lack of S arrival has an obvious impact on the hypocentral location quality. S-phases can also help detection on small earthquakes where weak P-phases can be missed.</span></p><p> </p><p><span>We implemented the deep neural network-based method PhaseNet to identify in real-time seismic P and S waves on 3-component seismometers deployed on Mayotte island. We also built an interface to subsequently process PhaseNet results and send pick objects to EarthWorm. We use EarthWorm binder_ew associator module specifically tuned for PhaseNet </span><span><em>a priori</em></span><span> phase identification to detect and locate the events, which are finally archived in a SeisComP database. We implemented this innovative real-time processing system for the REVOSIMA (Reseau de surveillance Volcanologique et Sismologique de Mayotte) hosted at OVPF (Observatoire Volcanologique du Piton de la Fournaise). We assess the robustness of the algorithm by comparing the results to existing automatic and manually detected seismicity catalogs.</span></p><p> </p><p>We show that the existing SeisComP automatic system is outperformed by our new algorithm, both in number of earthquake detections and location reliability. Our implementation also detects more events than the daily manual data screening. While this promising new processing system was first applied to study the Mayotte seismicity, it can be used in any seismic active zone, of volcanic or tectonic origin. Indeed, it will be installed at Martinique volcanic and seismic observatory later this year.</p>

Minimising the computational time of a waveform based location algorithm

<p>Accurate and fast localisation of microseismic events is a requirement for a number of applications, e.g. mining, enhanced geothermal systems. New methods for event localisation have been proposed over the last decades. The waveform-based methods are of the most recent developed ones and their main advantage is the ability to locate weak seismic events. Despite this, these methods are demanding in terms of computational time, making real-time seismic event localisation very difficult. In this work, we further develop a waveform-based method, the Multichannel coherency migration method (MCM), to improve the computational time. The computational time for the MCM algorithm has been reported to linearly depend on several parameters, such as the number of stations, the length of the waveform time window, the computer architecture, and the volume of the area we are searching for the hypocentre. To minimise the computational time we need to decrease one or more of the above parameters without compromising the accuracy of the result. We break the localisation procedure into several steps: (1) we locate the event with a relatively large spatial grid interval which will give less potential hypocentral locations and less calculations as a result. (2) Based on the results of step (1) and the locations of maximum coherencies we decrease the grid volume to a quarter of the original volume and the spatial interval to half the original, focusing only around the area identified in step (1). Step (2) is repeated several times for decreased grid volumes and spatial intervals until the hypocentral location does not significantly change any more. We tested this approach on both synthetic and real data. We find that while the accuracy of the hypocentre is not compromised, the computational time is up to  125,000 times shorter.    </p>

A comprehensive quantification of error location uncertainties for the French earthquake catalog

<p>Locating earthquakes has been a longterm problem in seismology that depends on multiple parameters like station density and spacing, azimuthal gap, velocity models, and phase pick precision. Here, we analyze the current state of the earthquake French catalog for the time period between 2010 until 2018, which we divide into different regions: the Alps, Massif Central, the West, the Pyrenees, the Grand-East and the North. We perform multiple location synthetic tests using as benchmark the earthquake catalog and the evolution of the French seismic network to quantify the improvements in 1) earthquake location through time and 2) the error locations and their uncertainties. For such endeavors, we use NonLinLoc to perform the synthetic tests varying, as input, the stations, the number of stations and phase picks, 1D velocity models and 3D velocity models, and to understand the changes in 1) earthquake hypocenters, 2) ellipsoidal errors and 3) posterior density functions. Then, we relocate the entire catalog using NonLinLoc including 3D velocity models (where available) and compare the hypocentral location differences when we relocate the catalog with 1D velocity models. Additionally, we estimate a quality factor for each of the located earthquakes and report the changes on the quality factor with the temporal evolution of the national seismic network. The resulting catalog and its associated error location will help future seismic hazard estimations in the Metropolitan French area.</p>

Epistemic Uncertainties in Local Earthquake Locations and Implications for Managing Induced Seismicity

ABSTRACT Earthquake hypocentral location is perhaps the most classical problem in seismology, the solution of which is often affected by significant uncertainty. In monitoring the effects of underground anthropogenic activities, the earthquake hypocentral location, magnitude, and ground motions are important parameters for managing induced seismicity (as e.g., for operating traffic-light systems). Such decisional systems define the operative reactions to be enacted once an earthquake, exceeding some magnitude or ground-motion threshold, occurs within a monitoring volume defined in the neighborhood of a certain anthropogenic underground activity. In this case, a reliable evaluation of the hypocentral location, along with its uncertainty, becomes crucial for rational decision making. In this article, we analyze different sources of uncertainty that can be relevant for the determination of earthquake source locations, and introduce a logic-tree-based ensemble modeling approach for framing the problem in a decision-making context. To demonstrate the performance of the proposed approach, we analyze uncertainties in the location of a seismic event that occurred on 22 July 2019 within the perimeter of the monitoring domain defined in the Val d’Agri oil field (southern Italy). We cast the result as a model ensemble that allows us to obtain samples from a parent distribution that better represents both aleatory and epistemic uncertainties of the earthquake location problem. We find that often-neglected epistemic uncertainties (i.e., those that arise when considering alternative plausible modeling approaches or data) can be considerably larger and more representative of the state of knowledge about the source location, than the standard errors usually reported by the most common algorithms. Given the consequential repercussions of decision making under uncertainty, we stress that an objective evaluation of epistemic uncertainties associated with any parameter used to support decisional processes must be a priority for the scientific community.

Children of a Lesser Seismological God: The 1971 Tuscania (Central Italy) “Historical” Earthquake

The 6 February 1971 Tuscania (central Italy) earthquake belongs to a peculiar family of destructive seismic events that have occurred in an area classified as low-seismic hazard, causing heavy damage and tens of casualties. However, this earthquake took place at the dawn of modern seismology in Italy and is far from being fully characterized from an instrumental and macroseismological point of view. This article aims at bridging the gap of information that affects that earthquake, through a twofold research path: (1) with an archival investigation looking for new available sources and with the use of the European Macroseismic Scale-98 (EMS-98) intensity scale, and (2) with the calculation of a more constrained hypocentral location. The results of this investigation can be summarized as follows: the reappraisal of the earthquake in terms of EMS-98 provides a maximum intensity 8 in Tuscania (previously quoted 8–9 Mercalli–Cancani–Sieberg [MCS] in the catalog), and a general decrease of intensity in many damaged localities. The new epicenter location is shifted almost 10 km southeast of the old one, at about 3 km depth. This new location is more robust than the previous one and is consistent with the general distribution of the most damaged localities; however, we cannot exclude that effects of directivity might have played a role in the peculiar pattern of damage caused by the event. Finally, we provide new values of magnitude (MD 4.9 and ML 5.1) that point to an upward scaling of the earthquake. The ultimate lesson of this work is that a deepening of the research can always provide room for an improvement of our knowledge even for significant earthquakes that have occurred relatively recently.

Seismic sources at intermediate depth in the Alboran Sea

<p>The occurrence of moderate magnitude earthquakes in intermediate depth (40<h<150 km) is a characteristic of the seismicity of the Ibero-Magrebian region. The most important concentration of this activity is in the western part of the Alboran Sea, with the epicenters following an N-S direction. In order to improve the knowledge of the geometry of these seismogenic structures, we have carried out a study of the hypocenters distribution and focal mechanisms for earthquakes that occurred in the period 2000-2020 (M>4.0). For the hypocentral location, we have used a non-linear probabilistic approach (NonLinLoc algorithm) jointly with 3-D lithospheric velocity tomography models recently developed for the Alboran-Betic-Rif zone. Focal mechanisms have been obtained from moment tensor inversion of stations at regional distances (Kiwi tools). Maximum likelihood hypocentres confirm a near vertical N-S distribution in a depth range between 50 and 100 km. Focal mechanisms show a different stress pattern, changing from a vertical tension axis for earthquakes located off-shore and western of 4.5ºW to vertical pressure axis for earthquakes inland and at eastern of 4.5ºW.</p>

Rupture characteristics of the 2019 North Peru intraslab earthquake (Mw8.0)

<p>According to GlobalCMT, the 2019/05/26 North Peru earthquake is the largest event since 1976 in the wide depth range between 70km and 550km. Its hypocentral location (at about 130km depth) inside the Nazca slab geometry, together with its normal focal mechanism, favor an origin related to slab bending. Owing to its magnitude and depth, this earthquake generated large coseismic displacements over a broad area, that were geodetically measured by InSAR and GNSS. By combining these observations with regional and teleseismic data, we invert for the rupture process of the event, and first focus on the actual focal plane. Inversion reveals that the steeper plane (dipping 55-60° to the East) is preferred. A clear northward propagation is also imaged, with rupture traveling ~200km in 60s, and with little extent in the dip direction. This narrow rupture aspect implies that the stress drop is significant, even if a simple duration-based measurement would not indicate so. These inversion results obtained at relatively low frequency (below 0.2Hz) are then thoroughly compared with back-propagation images obtained at higher frequency (at 0.5-4Hz), which also highlight the dominantly northward rupture propagation with an average rupture speed of about 3 km/s. Implication in terms of earthquake rupture dynamics and occurrence of such large intermediate depth earthquakes in slabs will finally be discussed.<br>    </p>

The Impact of USArray on Earthquake Monitoring in Alaska

Seismic network coverage in Alaska has fundamentally changed with the presence of the USArray Transportable Array (TA) stations. These new stations provided an unprecedented opportunity to expand earthquake reporting in areas of Alaska that have not previously been instrumented. The Alaska Earthquake Center (AEC) has been incorporating all TA data into its standard earthquake analysis. The TA network is currently the second largest contributor of phase picks in the Alaska earthquake catalog, after the AK network operated by the AEC. Recent increases in reported earthquakes (about 45,000 in 2017 and 55,000 in 2018) are directly attributable to the additional TA stations, especially in the northern and western mainland Alaska. In some regions, the earthquake detection threshold decreased by as much as two units of magnitude. With the TA installation complete in 2017, the detection threshold over the entire mainland Alaska region is M∼1.5. The new stations have also led to a decrease in hypocentral location errors, which are now more uniform over the entire mainland Alaska region. The uniformity of the TA network made it possible, for the first time, to make quantitatively valid comparisons of microseismic activity in different parts of the state. Among other observations, this uniform coverage helped demonstrate that the quiescence that has long been inferred in the central and western Arctic Slope region appears to be real, and not just an artifact of network coverage. This combined network should, with time, provide vastly better data for seismic hazard assessments in an area of increasing national interest.

Seismic Activity in the Central Adriatic Offshore of Italy: A Review of the 1987 ML 5 Porto San Giorgio Earthquake

ABSTRACT On 3 July 1987, a seismic sequence, with a mainshock of ML 5, took place in the offshore Adriatic, close to the coast of Porto San Giorgio (PSG), Italy. We present an accurate relocation of the PSG seismic sequence using a nonlinear probabilistic approach (Lomax et al., 2000). The trade‐off between the hypocentral location and the velocity model was exhaustively explored using six different velocity models available for the area provided by previous studies. Through numerous tests performed by relocating the mainshock, we selected the two best velocity models providing two different depths (2.0 and 18.0 km). To resolve this intrinsic ambiguity, we developed a technique that uses the macroseismic intensity field data based on a grid search of the magnitude–depth space. The results show that the mainshock has a depth of 5.7 km and a magnitude (ML) equal to 5; moreover, the relocated seismic sequence (∼30 events) developed in the upper portion of the crust (at a depth less than 15 km), thus activating thrust faults, which is typical of the main geological features that characterize the outer Apennines thrust belt and the Adriatic foreland folds. Because the Adriatic Sea hosts several hydrocarbon (mainly gas) production fields located near active faults, with some of them in the area of this study, analyzing the instrumental seismicity is necessary to better understand the seismicity generated by these seismogenic faults and improve the assessment of the area’s seismic hazards.