Pseudo-prospective testing of 5-year earthquake forecasts for California using inlabru

Probabilistic earthquake forecasts estimate the likelihood of future earthquakes within a specified time-space-magnitude window and are important because they inform planning of hazard mitigation activities on different timescales. The spatial component of such forecasts, expressed as seismicity models, generally rely upon some combination of past event locations and underlying factors which might affect spatial intensity, such as strain rate, fault location and slip rate or past seismicity. For the first time, we extend previously reported spatial seismicity models, generated using the open source inlabru package, to time-independent earthquake forecasts using California as a case study. The inlabru approach allows the rapid evaluation of point process models which integrate different spatial datasets. We explore how well various candidate forecasts perform compared to observed activity over three contiguous five year time periods using the same training window for the seismicity data. In each case we compare models constructed from both full and declustered earthquake catalogues. In doing this, we compare the use of synthetic catalogue forecasts to the more widely-used grid-based approach of previous forecast testing experiments. The simulated-catalogue approach uses the full model posteriors to create Bayesian earthquake forecasts. We show that simulated-catalogue based forecasts perform better than the grid-based equivalents due to (a) their ability to capture more uncertainty in the model components and (b) the associated relaxation of the Poisson assumption in testing. We demonstrate that the inlabru models perform well overall over various time periods, and hence that independent data such as fault slip rates can improve forecasting power on the time scales examined. Together, these findings represent a significant improvement in earthquake forecasting is possible, though this has yet to be tested and proven in true prospective mode.

Evaluation of the seismogenic (M6+) nodes for the territory of Bulgaria and adjacent areas

In this study seismogenic nodes capable to generate earthquakes with magnitudes M ≥ 6 are identified for the territory of Bulgaria and adjacent areas. Definition of nodes is based on a morphostructural zonation. Pattern recognition algorithm Cora-3 is applied to identify the seismogenic nodes, characterized by specific geological and geophysical data. The pattern recognition method is trained on information for 30 seismic events with M ≥ 6 for the period 29 BC–2020, selected from historical and instrumental Bulgarian earthquake catalogues. As a result, 56 seismogenic nodes are recognized, most of them in southwestern Bulgaria.

Comprehensive Survey of Seismic Hazard at Geothermal Sites by a Meta-Analysis of the Underground Feedback Activation Parameter afb

Global efforts to tame CO2 emissions include the use of renewable energy sources, such as geo-energy harnessing. However, injecting pressurised fluids into the deep underground can induce earthquakes, hence converting CO2-related risk into seismic risk. Induced seismicity hazard is characterised by the overall seismic activity afb that is normalised by the injected fluid volume V and the parameter b of the Gutenberg–Richter law. The (afb,b) set has so far been estimated for a dozen of reservoir stimulations, while at least 53 geothermal fluid stimulations are known to exist, based on our survey. Here, we mined the induced seismicity literature and were able to increase the number of estimates to 39 after calculating afb from related published parameters and by imputing b with its expectation where this parameter was missing (0.65 ≤ b ≤ 2.9, with mean 1.16). Our approach was a two-step procedure: we first reviewed the entire literature to identify seismic hazard information gaps and then did a meta-analysis to fill those gaps. We find that the mean and median afb estimates slightly decrease from afb ≈ −2.2 to afb = −2.9 and −2.4, respectively, and that the range of observations expands from −4.2 ≤ afb ≤ 0.4 to −8.9 ≤ afb ≤ 0.4, based on a comprehensive review unbiased towards high-seismicity experiments. Correcting for potential ambiguities in published parameters could further expand the range of possibilities but keep the mean and the median relatively close to original estimates, with afb ≈ −2.3 and −2.4, respectively. In terms of the number of earthquakes induced (function of 10afb), our meta-analysis suggests that it is about half the number that could previously be inferred from published afb estimates (i.e., half the seismic hazard). These results are hampered by high uncertainties, demonstrating the need to re-analyse past earthquake catalogues to remove any ambiguity and to systematically compute afb in future geothermal projects to reduce uncertainty in induced seismicity hazard assessment. Such uncertainties are so far detrimental to the further development of the technology.

Seismic Envelopes of Coda Decay for Q-coda Attenuation Studies of the Gargano Promontory (Southern Italy) and Surrounding Regions

Here, we describe the dataset of seismic envelopes used to study the S-wave Q-coda attenuation quality factor Qc of the Gargano Promontory (Southern Italy). With this dataset, we investigated the crustal seismic attenuation by the Qc parameter. We collected this dataset starting from two different earthquake catalogues: the first regarding the period from April 2013 to July 2014; the second regarding the period from July 2015 to August 2018. Visual inspection of the envelopes was carried out on recordings filtered with a Butterworth two-poles filter with central frequency fc = 6 Hz. The obtained seismic envelopes of coda decay can be linearly fitted in a bilogarithmic diagram in order to obtain a series of single source-receiver measures of Qc for each seismogram component at different frequency fc. The analysis of the trend Qc(fc) gives important insights into the heterogeneity and the anelasticity of the sampled Earth medium.

A revised image of the instrumental seismicity in the Lodi area (Po Plain, Italy)

We analysed the instrumental seismicity in a sector of the Po Plain (Italy) to define the baseline for seismic monitoring of a new underground gas storage plant that will use the depleted gas reservoir of Cornegliano Laudense, near Lodi. The target area – a square approximately 80 km × 80 km wide – is commonly considered aseismic. The analysed period, 1951–2019, includes all available instrumental data. We gathered the P- and S-phase readings collected by various agencies for more than 300 events, approximately located inside the target area. We processed the earthquakes uniformly, using absolute location algorithms and velocity models adopted by the regional and national monitoring networks. The relocated earthquake dataset depicts an image of weak and deep seismicity for this central sector of the Po Plain, which is quite different from the initial one derived from the existing earthquake catalogues. Within a distance of approximately 30 km from Lodi, earthquakes are extremely rare (on average 0.5 earthquakes per year, assuming a completeness magnitude Mc = 2.7 from the 1980s); only two weak events fall at less than 15 km distance from the reservoir in the whole period 1951–2019. The strongest events instrumentally recorded are related to the seismic sequence of Caviaga in 1951 that represent the first instrumental recordings for that area. Confirming the hypocentral depths recently proposed by Caciagli et al. (2015), the events are far from the gas reservoir; we suggest common tectonic stress of the main shock of 1951 and the M4.2 earthquake of 17 December 2020, based on the similarities in depth, location, and focal mechanism. While it is clear that the deep seismicity corresponds to the collision between the Northern Apennines and the Southern Alps, the characterization of the geological structures that generate earthquakes appears uncertain. Our results are a preliminary benchmark for the definition of seismogenic zones in the Lodi area, whose definition can be improved with the existing observational capabilities now available in the surroundings.

Revealing 60 years of Earthquake Swarms in the Southern Red Sea, Afar and the Gulf of Aden

Earthquake swarms occur sporadically at divergent plate boundaries but their recurrence over multiple decades and relation to magmatic spreading activity remain poorly understood. Here we study more than 100 earthquake swarms over a 60-year period in the southern Red Sea, Afar, and Gulf of Aden region. We first compiled an earthquake-swarm catalogue by integrating reexamined global and local earthquake catalogues from 1960 to 2017. This yielded 134 earthquake swarms that mainly cluster in 19 different areas in the study region, showing that in most cases swarms recur every few decades in the same area. The swarms exhibit a range of earthquake magnitudes and often include multiple M3 to M5 events with some swarms having occasional larger earthquakes over M6, primarily in southern Afar. Many of the earthquake swarms were clearly associated with rifting events, consisting of magmatic intrusions, surface faulting, and in some cases volcanic eruptions. Together, the swarms suggest that extension at these divergent plate boundaries occurs episodically along <100 km long segments, some of which were previously unrecognized. Within the study region, the Gulf of Aden shows the most frequent swarm activity, followed by Afar and then the southern Red Sea. The results show that the three areas were subject to an increase of earthquake-swarm activity from 2003 to 2013 in the form of three rifting episodes and at least seven volcanic eruptions. We interpret that the most likely controls on temporal variations in earthquake swarm activity are either temporal variations in magma supply, or rifting-induced stress change that trigger clusters of swarms.

Konversi Empiris Summary Magnitude, Local Magnitude, Body-Wave Magnitude, Surface Magnitude, dan Moment Magnitude Menggunakan Data Gempabumi 1922-2020 di Nusa Tenggara Barat

The existence of magnitude type variation from existing earthquake catalogue sources show that uniforming process is necessary. Beside that these type of magnitude will saturates in certain value, which are different with moment magnitude (Mw) which is not saturated and can describe earthquake process better. Our research initially did compatibility test between summary magnitude which is largely used by BMKG with other magnitude type. Furthermore, the purpose of our research is determination of empirical relation between magnitude type summary magnitude (M), local magnitude (ML), body-wave magnitude (mb), dan surface magnitude (Ms) which are usually used by earthquake catalogues to Mw. Method used in this research is linear regression using data set from BMKG, ISC-EHB, USGS, and Global CMT catalogues with are limited in West Nusa Tenggara and surrounding area. Data used in this research contains of 24.703 earthquake events during period May 9th 1922 until June 27th 2020. The result of this research shows there was good relation between M magnitude type with others magnitude type. Our research also found a conversion formula of M, ML, MLv, mb, and Ms to Mw with well-defined correlation.

Complementing regional moment magnitudes to GCMT: a perspective from the rebuilt International Seismological Centre Bulletin

Seismologists and geoscientists often need earthquake catalogues for various types of research. This input usually contains basic earthquake parameters such as location (longitude, latitude, depth, and origin time), as well as magnitude information. For the latter, the moment magnitude Mw has become the most sought after magnitude scale in the seismological community to characterize the size of an earthquake. In this contribution we provide an informative account of the Mw content for the newly rebuilt Bulletin of the International Seismological Centre (ISC, http://www.isc.ac.uk, last access: May 2021), which is regarded as the most comprehensive record of the Earth's seismicity. From this data, we extracted a list of hypocentres with Mw from a multitude of agencies reporting data to the ISC. We first summarize the main temporal and spatial features of the Mw provided by global (i.e. providing results for moderate to great earthquakes worldwide) and regional agencies (i.e. also providing results for small earthquakes in a specific area). Following this, we discuss their comparisons, by considering not only Mw but also the surface wave magnitude MS and short-period body wave magnitude mb. By using the Global Centroid Moment Tensor solutions as an authoritative global agency, we identify regional agencies that best complement it and show examples of frequency–magnitude distributions in different areas obtained both from the Global Centroid Moment Tensor alone and complemented by Mw from regional agencies. The work done by the regional agencies in terms of Mw is fundamental to improve our understanding of the seismicity of an area, and we call for the implementation of procedures to compute Mw in a systematic way in areas currently not well covered in this respect, such as vast parts of continental Asia and Africa. In addition, more studies are needed to clarify the causes of the apparent overestimation of global Mw estimations compared to regional Mw. Such difference is also observed in the comparisons of Mw with MS and mb. The results presented here are obtained from the dataset (Di Giacomo and Harris, 2020, https://doi.org/10.31905/J2W2M64S) stored at the ISC Dataset Repository (http://www.isc.ac.uk/dataset_repository/, last access: May 2021).

Spatial Variations of B-Value and Fundamental Parameters of The Earthquake Occurrences in The Eastern Mediterranean and Caucasus

The Gutenberg-Richter (GR) law is a well-known empirical relation in seismology, which describes the frequency of earthquake occurrence as a function of the magnitude. The \(b\)-value anomalies may indicate the high or low-stress levels in the heterogeneity or the crust's thermal gradient. Some researchers have examined the spatial and temporal anomalies of the \(b\)-value before the mainshock and the aftershocks' spatial variability. The variations of the magnitude completeness (\({M}_{c}\)) have estimated from the different earthquake catalogues. The high-resolution map of the GR \(b\)-value, \({M}_{c}\) and stress variance have analysed in the different seismic regions in the Eastern Mediterranean and Caucasus. This study considered the spatial anomalies and correlation models between the \(b\)-value, faulting styles, and stress regime and moment release. Lower \(b\)-values (\(b\le 1\)) were observed along with the Main Marmara Fault (MMF), eastern Turkey, western Alborz, northern Zagros, southeast Iran and the northeast Caucasus, which indicates the active seismic region. The \({M}_{c}\) level in most of Turkey is in and around 2.8, and in the Caucasus is \({M}_{c}\)=3, while Iran has \({M}_{c}\)=3.5 value. This work includes a stress inversion map in the region based on the focal mechanisms. The normal, strike-slip and a few thrust fault solutions were observed in the research area. Consequently, the spatial pattern of the \(b\)-values and stress regime can be used as a tool for predicting the forthcoming seismic hazard regions.

A revised image of the instrumental seismicity in the Lodi area (Po Plain, Italy)

We analyse the instrumental seismicity in a sector of the Po Plain (Italy) with the aim of defining the baseline for seismic monitoring of a new underground gas storage plant that will use the depleted gas reservoir of Cornegliano Laudense, near Lodi. The target area – a square approximately 80 x 80 km wide – is commonly considered aseismic. The analysed period, 1951–2019, includes all available instrumental data. We gathered the P- and S-phase readings collected by various agencies for more than 300 events, approximately located inside the target area. We processed the earthquakes in a uniform way, using absolute location algorithms and velocity models adopted by the regional and national monitoring networks. The relocated earthquake dataset depicts an image of weak and deep seismicity for this central sector of the Po Plain, which is quite different from the initial one derived from the existing earthquake catalogues. Within a distance of approximately 30 km from Lodi, earthquakes are extremely rare (on average 0.5 earthquake/yr, assuming a completeness magnitude Mc = 2.7 from the 1980s); only 2 weak events fall at less than 15 km distance from the reservoir in the whole period 1951–2019. The strongest events instrumentally recorded are related to the seismic sequence of Caviaga in 1951 that represent the first instrumental recordings for that area. Confirming the hypocentral depths recently proposed by Caciagli et al., 2015, the events are far from the gas reservoir; we suggest a common tectonic stress of the main shock of 1951 and the M4.2 earthquake of Dec 17, 2020, on the basis of the similarities in depth, location and focal mechanism. While it is clear that the deep seismicity corresponds to the collision between the Northern Apennine and Southern Alps, it is much less clear, however, which geological structures are capable of generating earthquakes. Our results and the improvement in the observational capabilities of the very last years will help refining the seismogenic sources hypothesized for this area.