Tectonic Seismicity Research Articles

Heterogeneous slab thermal dehydration driving warm subduction zone earthquakes

Changing thermal regime is one of the key mechanisms driving seismogenic behaviors at cold megathrusts, but it is difficult to interpret warm subduction zones such as Vanuatu for the temperatures are higher than that accommodates shallow brittle failures. We construct a 3-D thermomechanical model to clarify the thermal structure that controls tectonic seismicity in Vanuatu and predict a warm circumstance associated with abundant seismicity. Results reveal a heterogeneous slab ranging from 300 °C to over 900 °C from the Moho to subvolcanic depth. The subduction seismicity corresponds well to the plate interface where dynamic thermal dehydration is focused. The transformation from hydrated basalts to eclogites along the slab facilitates the occurrence of intense earthquakes and slips. Multistage mineralogical metamorphism affects the dynamic stability of megathrusts and favors the generation of active interplate large events. Therefore, slab thermal dehydration plays a greater role than slab temperature condition in influencing the subduction earthquake distribution in warm subduction systems.

Volcanic and Tectonic Sources of Seismicity Near the Tanaga Volcanic Cluster, Alaska

AbstractTanaga Island in the Central Aleutian Islands includes four stratovolcanoes: Sajaka, Tanaga, and East Tanaga in the northwest, and Takawangha in the central part of the island. Of these volcanoes, only Tanaga has a confirmed record of historical eruptive activity. We use double‐difference methods to relocate Tanaga Island earthquakes from the period 2003–2017 to gain insight into volcanic and tectonic activity in the region. High precision relative relocations show subsurface structures in the Tanaga region related to volcanism and crustal tectonics. In 2005, a swarm of ∼600 volcano‐tectonic events located below the NW portion of the island culminated with an episode of volcanic tremor. Although there was no known eruption associated with this swarm, we suggest that this activity is associated with fluids beneath Takawangha volcano. Overall, relatively little seismicity appears to be associated with volcanism: of 5,680 earthquakes relocated in this study, only ∼700 took place within 10 km of the volcanoes themselves. Other regions appear to experience primarily, if not exclusively, tectonic seismicity. Two regions on Tanaga Island became seismically active following a M6.6 tectonic earthquake east of the island on 2 May 2008, suggesting the triggering of tectonic earthquakes by the M6.6. Seismic activity recorded offshore and below the southern part of the island is interpreted as due to the clockwise rotation of the Delarof Block in the forearc, and bookshelf faulting along the volcanic arc. This suggests a complex pattern of earthquake hypocenters governed by both volcanic and tectonic processes surrounding Tanaga and Takawangha volcanoes.

The coulomb stress changes for correlation analysis of tectonic earthquake activity and volcanic of the 2001–2010 Merapi eruption based on focal mechanism data

The incident of increased seismic activity of the Merapi volcano was initiated by several tectonic earthquake phenomena. Analysis of the mechanism of tectonic earthquakes can be studied based on the combination of static stress on the rock. This paper describes the distribution of the Coulomb stress changes due to the interaction that occurs in tectonic earthquakes with volcanic activity. The data in the calculation of the Coulomb stress distribution are secondary in the form of tectonic seismicity data downloaded from the ISC (International Seismological Center), USGS (United States Geological Survey), and Global CMT (Global Centroid Moment Tensor) catalogs between 2001 and 2010. Calculation results and data analysis is the occurrence of an increase in Coulomb stress response in the Merapi Volcano area with a value range of 0.2–10 ​kPa at a depth of 2–4 ​km below the peak of Merapi Volcano. The Coulomb stress changes distribution is thought to indicate the accumulation of stress due to tectonic earthquakes that downward at the top of the volcano, thereby affecting the stability of the magma chamber and causing volcanic activity at the Merapi volcano.

Separating Primary and Secondary Mechanisms for Seismicity Induced by Hydraulic Fracturing

ABSTRACT Hydraulic fracturing operations are generally associated with induced seismic events with magnitudes that are typically lower than those of tectonic-based earthquakes. Although fluid migration and the resulting pore-pressure increase appear to be the primary driving mechanisms, one must also understand fault activation and the secondary interevent triggering process(es) leading to the generation of aftershock sequences. This requires distinguishing between seismic events that are a direct consequence of fluid injections and those that predominantly arise because of secondary mechanisms such as static or dynamic stress triggering caused by preceding earthquakes. Focusing on a case study in the Montney area in western Alberta, Canada, where both surface array and borehole geophone data are available, we (1) establish the importance of secondary mechanisms and aftershock sequences in this area and (2) show that the statistical properties of the aftershock sequences can allow one to establish whether significant fault activation is at play and whether the activated fault zone was critically prestressed. Specifically, we find evidence for aftershock sequences using different statistical methodologies, although the presence of aftershocks in the studied hydraulic-fracturing-induced context is significantly less dominant compared with tectonic seismicity. The identified aftershock sequences are characterized by long-range spatial aftershock zones, setting them apart from other cases in fluid-driven settings but comparable to tectonic aftershock sequences and indicative of fault activation. Despite this observation, we find overall a strong dominance of smaller triggers for aftershock sequences and an aftershock production associated with large-magnitude events that is significantly lower than what has been observed in tectonic settings. This suggests that the activated fault zone as a whole was not critically prestressed, and the associated initial seismic hazard was significantly lower compared with tectonic cases.

P and S wave detection for the Nevados de Chillan Volcanic Complex (Chile), based on spectro-temporal features

The localization of seismic events is a relevant step for volcano monitoring as it provides information of physical parameters that support the forecasting of volcanic activity. Automating this step is necessary to process the enormous amount of signals received by the observatories, improving relevant and opportune information retrieval at the early stages of monitoring. Localisation of seismic events is based on identifying the arrival time of the P and S waves in the raw signals, in a procedure called P and S picking. This is mainly performed manually or semi-automatically, usually applying the STA/LTA algorithm. The literature shows that the automation research on this process has been focused mainly on tectonic seismicity since the P and S waves are more difficult to detect in volcanic seismicity. This work aims to automate this process, proposing a novel and simple technique for volcano seismic events, using spectro-temporal energy features extracted from the spectrograms. The energy accumulated by an event over time, its second derivative, as well as a modification of the STA/LTA method were applied to detect the P and S waves for volcano tectonic (VT) events and only the P wave for long period (LP) events. The proposed method was adjusted and tested with the multi-station events recorded at the active Nevados de Chillan Volcanic Complex (Chile) and the results were compared with the reference manual picking provided by the observatory and with the traditional STA/LTA approach. The spectro-temporal approach presented lower errors and standard deviations than STA/LTA, in particular for the P detection for LP and the S detection, which are more difficult to detect. The median time error for the P wave detection was 0.0430 ± 0.21s for VT events and 0.0870 ± 0.35s for LP. For the S wave detection the error was 0.05 ± 0.49s. 80% of the P wave detection absolute errors were below 0.13[s] for VT and below 0.48[s] for LP. In turn, for the S wave detection, 80% of the errors were below 0.47[s]. In addition to the time errors, the distance errors were also evaluated, thus the localization was calculated for each picking method. The results presented a longitudinal error of 0.3° for both methods and a latitude error of 0.01° for the spectro-temporal method and 0.03° for STA/LTA method, once again demonstrating the good performance of the proposal. These results are promising despite the complexity of the P and S wave detection in volcanic events and the simplicity of the method compared to the methods proposed in the literature.

Rapid Near-Field Attenuation of Ground Motion from Shallow Induced Earthquakes, Case Study: Preston New Road, United Kingdom

Abstract Ground motions from shallow induced earthquakes and tectonic seismicity were investigated in this study by directly modeling the seismic attenuation quality factor (Q) using spectral fitting and coda envelope decay methods. We use data from the Preston New Road (PNR) shale gas induced seismicity sequences near Blackpool, United Kingdom, in 2018 and 2019, in addition to regional tectonic events in the United Kingdom. Our results show that the local Q obtained from the induced seismic sequences at PNR, attributed to shallower layers in the crust, leads to a rapid rate of near-field decay (sudden loss in amplitude of earthquake signal over a short distances), with significantly stronger attenuation than observed for regional events. We furthermore find that estimates of Q are nonunique to a given record, differing both with the method and the analysis windows used, particularly at high frequency. These differences can be attributed to the different modeling methodologies (e.g., different assumptions) or to fundamental differences in physical attenuation processes within the seismic wavefield itself, which traverses multiple ray paths and comprises various phases. Our results indicate that to model ground motions for shallow earthquakes, it is important to consider the composite Q along a specific path rather than an average regional Q. To this extent, a depth-dependent attenuation model is considered crucial to bridge the gap between shallow induced earthquakes and tectonic seismicity.

Visibility Graph Analysis of Reservoir-Triggered Seismicity: The Case of Song Tranh 2 Hydropower, Vietnam.

In this study, the visibility graph analysis of seismicity triggered by Song Tranh 2 hydropower (Vietnam) is performed. The relationship between the seismic (the Gutenberg-Richter b-value) and topological (the k-M slope) parameters of seismicity is analysed. Our findings indicate that the relationship between the Gutenberg-Richter b-value and the k-M slope of the investigated seismicity is in agreement with that characterising the tectonic seismicity. The results obtained from analysing the reservoir-triggered seismicity of Song Tranh 2 area could contribute to better characterisation of the relationship between the seismological and topological parameters of seismicity, strengthening the universal character of the relationship between the b-value and the k-M slope.

Sheared granular matter and the empirical relations of seismicity.

The frictional instability associated with earthquake initiation and earthquake dynamics is believed to be mainly controlled by the dynamics of fragmented rocks within the fault gauge. Principal features of the emerging seismicity (e.g., intermittent dynamics and broad time and/or energy scales) have been replicated by simple experimental setups, which involve a slowly driven slider on top of granular matter, for example. Yet these setups are often physically limited and might not allow one to determine the underlying nature of specific features and, hence, the universality and generality of the experimental observations. Here, we address this challenge by a numerical study of a spring-slider experiment based on two-dimensional discrete element method simulations, which allows us to control the properties of the granular matter and of the surface of the slider, for example. Upon quasistatic loading, stick-slip-type behavior emerges which is contrasted by a stable sliding regime at finite driving rates, in agreement with experimental observations. Across large parameter ranges for damping, interparticle friction, particle polydispersity, etc., the earthquake-like dynamics associated with the former regime results in several robust scale-free statistical features also observed in experiments. At first sight, these closely resemble the main empirical relations of tectonic seismicity at geological scales. This includes the Gutenberg-Richter distribution of event sizes, the Omori-Utsu-type decay of aftershock rates, as well as the aftershock productivity relation and broad recurrence time distributions. Yet, we show that the correlations associated with tectonic aftershocks are absent such that the origin of the Omori-Utsu relation, the aftershock productivity relation, and Båth's relation in the simulations is fundamentally different from the case of tectonic seismicity. This, we believe, is mainly due to a lack of macroscale relaxation processes that are closely tied to the generation of real aftershocks. We argue that the same is true for previous laboratory experiments.

Biases in estimating b-values from small earthquake catalogues: how high are high b-values?

SUMMARY The Gutenberg–Richter (GR) b-value describes the relative proportion of small to large earthquakes in a scale-free population and is a critical parameter for probabilistic estimation of seismic hazard. At low magnitudes, the scale-free behaviour breaks down below the magnitude of completeness mc due to censoring of the data, when the instrumentation used to construct the catalogue is incapable of completely recording all earthquakes in the study region above the background noise. At high magnitudes, it must also break down because natural tectonic and volcanic processes are incapable of an infinite release of energy. This breakdown at large magnitudes is commonly modelled as an exponential roll-off to either the incremental or cumulative GR distribution. This introduces an extra parameter and hence requires relatively more data to justify the additional model complexity. For tectonic seismicity, the estimated b-value is commonly close to unity. In contrast, studies of volcanic and induced seismicity often report significantly higher estimates of the b-value, albeit using relatively small data sets—both in sample size and dynamic (magnitude) range for data above mc. Here, using synthetic data, we show that when we have low dynamic range, it is statistically challenging to test whether the sample is representative of the scale-free GR behaviour or whether it is controlled primarily by the finite size roll-off. We then explore the potential biases that arise when the data quality does not allow this distinction to be made and what the implications are for interpreting studies that have high estimated b-values. We find that systematically higher b-values than those used to generate the synthetic data are regularly obtained when assuming the wrong model and when having a too high mc, resulting in too small catalogues. This is important because it changes our understanding of the accuracy of elevated or variable b-values in catalogues of different dynamic ranges, and quantifies the likely bias in the inferred b-value compared to the underlying true distribution and its associated uncertainty. Finally, we recommend steps to minimize this bias.

Hydrogeochemistry of Hot Springs and the 2018 Mojiang M 5.9 Earthquake-Related Chemical Changes in the Simao Basin, China

The Simao Basin is characterized by strong tectonic activity and frequent seismicity. This study investigated the hydrochemical characteristics of 21 thermal springs in the Simao Basin from 2018 to 2020. In this study period, the 2018 Mojiang M5.9 earthquake caused several hydrochemical changes. The results indicate that the Simao Basin contained saline spring waters, HCO3−-rich spring waters, and SO42−-rich spring waters. In the study area, the water chemistry types were controlled by stratum lithology. Saline springs flowed through red beds and dissolved large amounts of halite, which is a rich source of Cl−and Na+ ions. In the hot spring waters, Ca2+ (Mg2+) and HCO3− were mainly derived from the dissolution of carbonate minerals, gypsum, and anhydrite of Triassic rocks. The higher SO42- content in the hot spring waters was caused by the pyrite present in Ailaoshan metamorphic rocks. The reservoir temperatures (121–289 °C) in the Simao Basin were estimated by the silica-enthalpy mixing model equation and the silica-enthalpy diagram. The hot springs had higher reservoir temperatures (>250 °C) and were mainly located at the edges of the basin. Metamorphic rocks exposed in the region had low permeabilities and these springs was close to nearby deep faults that provided deep heat. In most springs, the concentrations of Ca2+ and HCO3− ions increased obviously before the 2018 Mojiang M5.9 earthquake; however, the concentrations of these ions decreased after the earthquake. The hydrogeochemical variations might be attributed to the vigorous water-rock interactions and the mixing of secondary fluids. The entry of cold shallow groundwater caused changes in the reservoir temperatures of some spring samples.

Raton Basin Induced Seismicity Is Hosted by Networks of Short Basement Faults and Mimics Tectonic Earthquake Statistics

AbstractThe Raton Basin has been an area of injection induced seismicity for the past two decades. Previously, the reactivated fault zone structures and spatiotemporal response of seismicity to evolving injection have been poorly constrained due to sparse publicly available seismic monitoring. The application of a machine‐learning phase picker to 4 years of continuous seismic data from a local array enables the detection and location of ∼38,000 earthquakes. The events from 2016 to 2020 are ∼2.5–6 km below sea level and range from ML < −1 to 4.2. Most earthquakes occur within previously identified ∼N‐S zones of seismicity, however our new catalog illuminates that these zones are composed of many short faults with variable orientations. The two most active zones, the Vermejo Park and Tercio zones, are potentially linked by small intermediate faults. In total, we find ∼60 short (<3 km long) basement faults with strikes from WNW to NNE. Faulting mechanisms are predominantly normal but some variability, including reverse dip‐slip and oblique‐slip, is observed. The Trinidad fault zone, which previously hosted a Mw 5.3 earthquake in 2011, is quiescent during 2016–2020, likely in response to both slow accumulation of tectonic strain after the 2011 sequence, and the significant decrease (80% reduction) in nearby wastewater injection from 2012 to 2016. Unlike some other regions, where induced seismicity was triggered in response to higher injection rates, the Raton Basin's frequency‐magnitude and spatiotemporal statistics are not distinguishable from tectonic seismicity. The similarity suggests that seismicity in the Raton Basin is predominantly releasing tectonic stress.

Analytical Models to Determine In-Plane Damage Initiation and Force Capacity of Masonry Walls with Openings

Masonry panels consisting of piers and spandrels in buildings are vulnerable to in-plane actions caused by seismicity and soil subsidence. Tectonic seismicity is a safety hazard for masonry structures, whereas low-magnitude induced seismicity can be detrimental to their durability due to the accumulation of light damage. This is particularly true in the case of unreinforced masonry. Therefore, the development of models for the accurate prediction of both damage initiation and force capacity for masonry elements and structures is necessary. In this study, a method was developed based on analytical modeling for the prediction of the damage initiation mode and capacity of stand-alone masonry piers; the model was then expanded through a modular approach to masonry walls with asymmetric openings. The models account for all potential damage and failure modes for in-plane loaded walls. The stand-alone piers model is applicable to all types of masonry construction. The model for walls with openings can be applied as is to simple buildings but can also be extended to more complex structures with simple modifications. Model results were compared with numerous experimental cases and exhibited very good accuracy.

Seismicity and seismic hazard assessment in West Africa

No in-depth seismic hazard study for West Africa (WA) has ever been conducted, as the regional earthquake catalogues are incomplete. Such lack of comprehensive seismic hazard study for the region has negatively affected the planning and development of critical infrastructure and disaster risk management. Therefore, this study aims to bridge the knowledge gap by applying modern techniques to updating the existing catalogues and assessing the seismic hazards for the region. We updated the current earthquake catalogue for WA using information from the International Seismological Centre, published information, and data from seismic stations in the region. The seismotectonic setting of WA is considered stable continental crust despite several recorded earthquakes occurring in the region, the largest being the June 22, 1939 earthquake of magnitude M6.8 with epicentre in Ghana. However, WA has also been classified as a region of shallow crustal seismicity. Therefore, we investigated both these research schools of thought and compared their results. For each scenario, three different ground-motion models (GMMs) were applied and combined to produce each hazard map using logic tree formalism with equal weights. The region was divided into five seismic source zones for computation of the earthquake recurrence parameters, with the same parameters computed for the entire WA region. The computed Gutenberg–Richter b-value, activity rates λ, and regional maximum possible magnitudes mmax for the five zones ranged from 0.84 to 1.0, 0.3–2.1, and 5.2–7.0, respectively. The calculated b-value, λ, and mmax for the entire region were 0.77, 4.1, and 7.2, respectively. The estimated b-value of 0.77 falls within the generally accepted range for tectonic seismicity. The seismic hazard predicted by GMMs for stable continental areas was higher than that predicted for shallow crustal seismicity in the investigated region. Therefore, our results confirmed that WA is characterised by stable continental crust. The highest hazard levels were observed in parts of Ghana, Togo, Guinea-Bissau, Guinea, and the Cameroon Volcanic Line region (CVL), ranging between 0.02 g and 0.03 g.

Probabilistic Forecasting of Hydraulic Fracturing-Induced Seismicity Using an Injection-Rate Driven ETAS Model

Abstract The development of robust forecasts of human-induced seismicity is highly desirable to mitigate the effects of disturbing or damaging earthquakes. We assess the performance of a well-established statistical model, the epidemic-type aftershock sequence (ETAS) model, with a catalog of ∼93,000 microearthquakes observed at the Preston New Road (PNR, United Kingdom) unconventional shale gas site during, and after hydraulic fracturing of the PNR-1z and PNR-2 wells. Because ETAS was developed for slower loading rate tectonic seismicity, to account for seismicity caused by pressurized fluid, we also generate three modified ETAS with background rates proportional to injection rates. We find that (1) the standard ETAS captures low seismicity between and after injections but is outperformed by the modified model during high-seismicity periods, and (2) the injection-rate driven ETAS substantially improves when the forecast is calibrated on sleeve-specific pumping data. We finally forecast out-of-sample the PNR-2 seismicity using the average response to injection observed at PNR-1z, achieving better predictive skills than the in-sample standard ETAS. The insights from this study contribute toward producing informative seismicity forecasts for real-time decision making and risk mitigation techniques during unconventional shale gas development.

A case study for determination of seismic risk priorities in Van (Eastern Turkey)

Lake Van Basin, located in Eastern Turkey, is worth examining in terms of seismicity due to large-scale losses of property and life during the historical and instrumental period. The most important and largest province in this basin is Van. Recent indicators of the high seismicity risk in the province are damage occurring after devastating earthquakes in 2011 (Mw=7.2 and Mw=5.6) and lastly in 2020 Khoy (Mw=5.9). The seismic hazard analysis for Van and its districts in Eastern Turkey was performed in probabilistic manner. Analyses were made for thirteen different districts in Van. In this study, information is given about the tectonic setting and seismicity of Van. The probabilistic seismic hazard curves were obtained for a probability of exceedance of 2%, 10% and 50% in 50-year periods. The PGA values in the Van province vary from 0.24 g – 0.43 g for earthquakes with repetition period of 475 years. Risk priorities were determined for all districts. The highest risk was calculated for Caldiran and the lowest risk was found for Gurpinar. Risk priorities for buildings in all districts were also determined via rapid seismic assessment for reinforced-concrete and masonry buildings in this study.

Indicators of Volcanic Eruptions Revealed by Global M4+ Earthquakes

AbstractDetermining whether seismicity near volcanoes is due primarily to tectonic or magmatic processes is a challenging but critical endeavor for volcanic eruption forecasting and detection, especially at poorly monitored volcanoes. Global statistics on the occurrence and timing of earthquakes near volcanoes both within and outside of eruptive periods reveal patterns in eruptive seismicity that may improve our ability to discern magmatically driven seismicity from purely tectonic seismicity. In this paper, we catalog magnitude four and greater (M4+) earthquakes near volcanoes globally and compute statistics on their occurrence with respect to various eruptive and volcanic attributes, evaluating their utility as diagnostic indicators of eruptions. Using a 2‐week time window and a 30 km radius around the volcanoes, we find that 11% of eruptions are preceded by at least one M4+ earthquake, but only 1% of such earthquakes is followed by eruption. However, earthquakes located 5–15 km from the volcano, those with normal faulting mechanisms and/or large nondouble‐couple components, and those occurring as groups are more commonly associated with eruptions, providing significant forecasting utility in some cases. Similarly, certain volcanoes are more likely to exhibit such precursors, such as those with long repose periods. We illustrate the use of these data in eruption forecasting scenarios, including rapid identification of analogous earthquake sequences at other volcanoes. When integrated within the context of multiparametric, multidisciplinary probabilistic assessments of volcanic activity, global earthquake statistics can improve eruption forecasts, and our work provides a model for use on other rapidly expanding global volcanological databases.

Time-dependent and spatiotemporal statistical analysis of intraplate anomalous seismicity: Sarria-Triacastela-Becerreá (NW Iberian Peninsula, Spain)

SUMMARY The Sarria-Triacastela-Becerreá seismicity is located in an intraplate region considered seismically stable, but in 1995 started an unprecedented earthquake activity in the area. Since then an anomalous long-term continuous seismicity remains until now in the same location. Despite the long-term seismicity and the large magnitudes for the region standards (5.1 Mw), the origin and mechanisms of this seismicity remains unclear. The isolation of background from the main shock–aftershock contribution by means of the application of the Epidemic Type Aftershock Sequences (ETAS) model, the spatial depiction and the resemblance to near seismic regions allow us to identify several seismicity behaviours. From 1995 to 1998/99, the seismicity in this location mainly consists of intensive and deeper earthquake sequences linked to the 1995 and 1997 main shocks. Our results suggest that the main shocks triggered aftershocks and additionally initiated or facilitated aseismic processes. A likely scenario is that the main shocks broke a sealed source at depth allowing a subsequent intrusion of high-pressurized fluids from depth. The resemblance of this period with Zamora seismic characteristics proposes that Triacastela seismicity was also initiated by tectonic activity. From 1998/99 to 2018, a change in seismicity is observed, the background contribution took control and swarm-type activity is predominant. While the earthquake rate decreases, the relative background contribution goes up. Actually, after 2013 the clusters almost disappear and background contribution achieves 55 per cent of the total activity. The spatial migration to the southeast and the upward trend to shallower depth support fluid migration as possible driving mechanism responsible for the transient seismicity in this period. The swarm-type activity in Triacastela in later periods and the resemblance of b-values with Ponte Caldelas and Ventaniella seismicity suggest that the seismicity in Triacastela is related to fluid migration and the reactivation of fractured areas. We propose that the mechanism of this anomalous and long-term seismicity in Triacastela is the mix of different mechanisms, starting with the tectonic seismicity, generated during the 1995 and 1997 seismic sequences, which initiated a fluid upward migration through fractured crustal fault patches, observed after 1998–1999, and responsible for the seismicity during the following 20 yr.

The b-value of the Kelud Volcano in the Last Three Decades

Kelud volcano is located in the Kediri sub-district, East Java Province, Indonesia. This volcano is still active, with total population, in the radius of 10 km, is around 10 thousand people. Kelud volcano is a popular tourist destination. On the weekend, total visitor can reach 5,000 people per-day. These people are at high risk when the Kelud volcano erupts. The last eruption of the Kelud volcano occurred in 2014 and was explosive eruption. Previously, there was an effusive eruption in 2007. These two types of eruption have its own geo hazard risk. Thus, predict the eruption type could help hazard mitigation. In this study, two data sets of earthquakes, 1990-2007 and 2008-2020, were analysed to determine the b-value and its relationship to the eruption type of the Kelud volcano. The calculation of the b-value uses the Gutenberg-Richter relationship. Calculation of the b-value in 2007, when there was an effusive eruption, showed a value of 2.27, while in 2014 (when there was an explosive eruption) was 1.85. After 2009, the curve of the b-value against time shows decrease. As a long term precursor of the Kelud activity, this b-value curve should be analysed continuously, besides volcano tectonic seismicity monitoring.

Earthquakes Induced by Wastewater Injection, Part I: Model Development and Hindcasting

ABSTRACTIn the past decade, several parts of central United States, including Oklahoma, have experienced unprecedented seismicity rates, following an increase in the volumes of wastewater fluids that are being disposed underground. In this article, we present a semi-empirical model to hindcast the observed seismicity given the injection time history. Our proposed recurrence model is a modified version of the Gutenberg–Richter relation, building upon the seismogenic index model, which predicts a linear relationship between the number of induced events and the injected volume. Our methodology accounts for the effects of spatiotemporal pore-pressure diffusion, the stressing-rate dependency of the time lag between injection and seismicity rate changes, and the rapid cessation of seismicity upon unloading. We also introduced a novel multiscale regression, which enabled us to produce grid-independent results of increased spatial resolution. Although the model is generic to be applicable in any region and has essentially only two free parameters for spatial calibration, it matches the earthquake time history of Oklahoma well across various scales, for both increasing and decreasing injection rates. In the companion paper (Grigoratos, Rathje, et al., 2020), we employ the model to distinguish the disposal-induced seismicity from the expected tectonic seismicity and test its forecasting potential.

A comparison of characteristic parameters of mining related and tectonic seismic aftershock sequences

The temporal decay pattern of seismicity following rock bursts and large magnitude events is studied for aftershock sequences in several mines in Ontario, Canada. The decay phenomenon is described by the so – called Modified Omori’s law. The dependency of the parameters K, c and p with the magnitude of large mine seismic events is analyzed. A total of eleven mining related seismic sequences, with mainshock magnitudes ranging from 0.5 to 4.0 Mn (Nuttli magnitude scale) and from four different mine environments are examined. Results, including the calculation of the Gutenberg - Richter’s and Reasenberg – Jones parameters b and a’, are compared with previous research in tectonic seismicity using available data from New Zealand, Italy and U.S.A. Further parameters were derived and applied in a correlation study of mining related and tectonic events. The study revealed a good correlation between the productivity parameter a’ and b – values. Particularly, the b – values show significant different number ranges for mining related and tectonic data, which was applied to derive a direct numerical relationship between mining related and tectonic parameters and the aftershock number estimation. Finally, numerical values for the b – values in mining related seismic activity were obtained.