Mainshock Magnitude Research Articles

Aftershock Triggering and Spatial Aftershock Zones in Fluid‐Driven Settings: Discriminating Induced Seismicity From Natural Swarms

AbstractAftershock cascades play an important role in forecasting seismicity in natural and human‐made situations. While their behavior including the spatial aftershock zone has been the focus of many studies in tectonic settings, this is not the case when fluid flows are involved. Using high‐quality seismic catalogs, we probe aftershocks dynamics in five settings influenced by fluids: (a) induced seismicity in Oklahoma and Kansas, (b) natural swarms in California and Nevada, and (c) suspected swarms in the Yuha Desert (California). All settings exhibit significant aftershock behavior highlighting the importance of event‐event triggering processes. The spatial aftershock zones scale with mainshock magnitude as expected based on the rupture length. While (a) and (b) show a rapid decay beyond their rupture length, (c) exhibits long‐range behavior suggesting that fluid migration might not be the dominant mechanism. We also find that the scaling of aftershock productivity with mainshock magnitude together with the Gutenberg‐Richter ‐value might allow to distinguish between natural swarms and induced seismicity.

Campi Flegrei, Vesuvius and Ischia Seismicity in the Context of the Neapolitan Volcanic Area

Studying seismicity in a volcanic environment provides important information on the state of activity of volcanoes. The seismicity of the Neapolitan volcanoes, Campi Flegrei, Vesuvius, and Ischia, shows distinctive characteristics for each volcano, covering a wide range of patterns and types. In this study we relocated some significant volcano-tectonic earthquake swarms that occurred in Campi Flegrei and Vesuvius. Moreover, we compared the earthquake occurrence evolution, the magnitude and the seismic energy release of the three volcanoes. Also, we considered the results of seismic analysis in the light of geochemical and ground deformation data that contribute to defining the state of activity of volcanoes. In Campi Flegrei, which is experiencing a long term unrest, we identified a seismogenic structure at shallow depth in Pisciarelli zone that has been activated repeatedly. The increasing seismicity accompanies an escalation of the hydrothermal activity and a ground uplift phase. At Vesuvius a very shallow seismicity is recorded, which in recent years has shown an increase in terms of the number of events per year. Earthquakes are usually located right beneath the crater axis. They are concentrated in a volume affected by the hydrothermal system. Finally, Ischia generally shows a low level of seismicity, however, in Casamicciola area events with a moderate magnitude can occur and these are potentially capable of causing severe damage to the town and population, due to their small hypocentral depth (typically < 2.5 km). After the seismic crisis of August 21, 2017 (mainshock magnitude M = 4), the seismicity returned to a low level in terms of occurrence rate and magnitude of earthquakes. The seismicity of these three different volcanic areas shows some common aspects that highlight a relevant role of hydrothermal processes in the seismogenesis of volcanic areas. However, while the main swarms in Campi Flegrei and most of the Vesuvian earthquakes are distributed along conduit-like structures, the seismicity of Ischia is mainly located along faults. Furthermore, the temporal evolution of seismicity in Neapolitan volcanic area suggests a concomitant increase in the occurrence of earthquakes both in Campi Flegrei and Vesuvius in recent years.

Characterization of injection-induced seismicity at north central Oklahoma, USA

The characteristics (foreshock-aftershock patterns, main shock to the largest aftershock magnitude ratio, and difference in magnitude) of two moderate injection-induced seismicity (IIS) earthquake sequences from Oklahoma, USA, namely, Prague (M 5.7, 2011) and Pawnee (M 5.8, 2016) earthquake sequences, are investigated to comprehend the shallow crustal heterogeneity. The analysis of temporal variation of “b” value from 2002 to 2018 suggests an increase in b-value after 2009. A reduction in b-value after 2016, in response to the reduced injection volumes is noted. A sharp fall in b-value usually precedes the main shocks of magnitude M ≥ 3.5. The foreshock b-values are lower than the regional b-value and aftershock b-values are higher than the regional b-value within the error limits. The investigated earthquake sequences fall under Type 2 of Mogi’s model. The largest aftershock to main shock magnitude difference of 2, observed for the Pawnee earthquake, could be a response to drastic reduction in injection rates in May 2016. The characteristics of ISS observed at Oklahoma are similar to the observations from the reservoir-triggered seismicity (RTS). However, with multiple injection wells operating from time to time in the region with varying amount of fluids injected, the entire IIS at Oklahoma has an appearance of a swarm.

Determination of a unique aftershock spectra

Aftershock seismic hazard analysis depends on both mainshock magnitude and elapsed time after the main earthquake. Aftershocks can result in an increment in the seismic demand of the structures. The objective of the current study is to determine a final aftershock spectrum as an applicable outcome in seismic design and assessment of buildings. As a numerical example, aftershock probabilistic hazard analysis (APSHA) was calculated at a location in southern California. The proposed method was applied for four different periods. Furthermore, a three-story existing concrete building was selected and analyzed regarding aftershock spectrum. It was shown that the amount of spectrum follows an upward trend by increasing the amount of main shock magnitude. The second finding was that the spectral value decreases with growth in elapsed time after the mainshock. While both beams and columns did not have enough strength, by applying aftershock to the building, 20% of the columns were required to be retrofitted again, and the figure for strengthened column experienced a rise of about 38%. However, the number of retrofitted beams did not change considerably.

Stress‐Dependent b Value Variations in a Heterogeneous Rate‐and‐State Fault Model

AbstractThe magnitudes of earthquakes are known to follow a power law distribution referred to as the Gutenberg‐Richter empirical law. Seismological observations and laboratory experiments suggest a decrease of the decay exponent (b value) with differential stress. The physical mechanism controlling this decrease, however, remains unclear. The present study is dedicated to the origin of relative b value variations with stress obtained in a 2‐D rate‐and‐state planar fault model considering a population of asperities with size‐dependent fracture energy. The simulations show that both b value in the intermediate magnitude range and mainshock magnitude increase with normal stress. Analytical relationships are derived, showing that the increase of b value is related to the decrease of critical nucleation length with normal stress, enhancing the productivity of small‐magnitude events and partial ruptures. The theoretical formulas also show how the increase of mainshock magnitude is a consequence of normal stress dependence of stress drop.

Mainshock Anticipated by Intra-Sequence Ground Deformations: Insights from Multiscale Field and SAR Interferometric Measurements

The 2016 Central Italy seismic sequence was characterized by two main events: 24 August, Mw 6, and 30 October, Mw 6.5. We carried out high-resolution field sampling and DInSAR analysis of the coseismic and intra-sequence ground deformations along the Mt Vettore-Mt Bove causative fault (VBF). We found that during the intra-sequence period (24 August–30 October), the ground experienced some deformations whose final patterns seemed to be retraced and amplified by the following mainshock. We interpreted that (i) immediately after the 24 August earthquake, the deformation observed in the southern VBF expanded northwards and westwards over a Length of Deforming Ground (LDG) ranging between 28.7 and 36.3 km, and (ii) it extended to the whole portion of the hanging wall that was later affected by mainshock coseismic deformation. Assuming the LDG to be an indicator for an expected (=coseismic) surface rupture length and using known scaling functions, we obtained 6.4 ≤ Mw ≤ 6.7 for a possible incoming earthquake, which is consistent with the mainshock magnitude. We suggest that the evolution of the ground deformations after a significant seismic event might provide insights on the occurrence of new earthquakes with magnitudes comparable to or larger than the former.

Forecasting strong aftershocks in earthquake clusters from northeastern Italy and western Slovenia

In this study, we propose an analysis of the earthquake clusters that occurred in North-Eastern Italy and western Slovenia from 1977 to today. Given a mainshock generating alarm in the population, we are interested in forecasting if a similar magnitude earthquake will follow. We classify the earthquake clusters associated with mainshocks of magnitude Mm into two classes: if the strongest aftershock has a magnitude ≥Mm-1 (swarms or large aftershock seismic sequences) as type A, otherwise (smaller aftershocks seismic sequences) as type B. A large aftershock following a main shock can cause significant damages to already weakened buildings and infrastructures, so a timely advisory information to the civil protection is of great interest for effective decision-making. For the first time, we applied to a new catalogue a pattern recognition algorithm for cluster type forecasting that we developed for all Italy (Gentili and Di Giovambattista, 2017). Thanks to the lower completeness magnitude of the local OGS catalogue, compared to the national one, and to a new version of the algorithm, we were able to lower the threshold of the clusters mainshocks magnitude from 4.5 to 3.7. The method has been validated by rigorous statistical tests. We tested the algorithm on the 1976 highly destructive earthquake cluster (mainshock magnitude 6.5 – the strongest in the last 80 years in the region) and we retrospectively forecasted it as an A cluster. Successful results were obtained also on other three smaller earthquake clusters in 2019.

A probabilistic framework for the economic loss estimation of structures considering multiple aftershocks

The strong aftershocks have the potential to induce the additional economic loss of structures. This manuscript proposes a framework to estimate the additional economic loss induced by multiple aftershocks, and meanwhile to determine the necessary number of aftershocks that should be considered in the economic loss estimation. Four analysis models (hazard model, structural model, damage model, and loss model) are contained in the proposed framework, which is applied to a 4-story reinforced concrete structure. The result indicates that the additional economic loss induced by a given aftershock is lower when more aftershocks occur before this one. From the perspective of probability, at least two aftershocks should be considered in the loss estimation for M6.0 mainshock, while this number increases to 10 and 28 for M7.0 and M8.0 mainshocks respectively. For low-rise reinforced concrete frame structure, the impact of aftershocks on the economic loss increases with the increase of mainshock magnitude, and it can exceed 40% for strong mainshock from the perspective of probability. The proposed framework and results in this study would be helpful for the decision making in the pre- and post-earthquake environment.

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.

Development of an aftershock occurrence model calibrated for Turkey and the resulting likelihoods

This paper presents the calibration of Omori’s aftershock occurrence rate model for Turkey and the resulting likelihoods. Aftershock occurrence rate models are used for estimating the probability of an aftershock that exceeds a specific magnitude threshold within a time interval after the mainshock. Critical decisions on the post-earthquake safety of structures directly depend on the aftershock hazard estimated using the occurrence model. It is customary to calibrate models in a region-specific manner. These models depend on rate parameters (a, b, c and p) related to the seismicity characteristics of the investigated region. In this study, the available well-recorded aftershock sequences for a set of Mw ≥ 5.9 mainshock events that were observed in Turkey until 2012 are considered to develop the aftershock occurrence model. Mean estimates of the model parameters identified for Turkey are a = -1.90, b = 1.11, c = 0.05 and p = 1.20. Based on the developed model, aftershock likelihoods are computed for a range of diff erent time intervals and mainshock magnitudes. Also, the sensitivity of aftershock probabilities to the model parameters is investigated. Aftershock occurrence probabilities estimated using the model are expected to be useful for post-earthquake safety evaluations in Turkey.

Can Precursory Moment Release Scale With Earthquake Magnitude? A View From the Laboratory

AbstractToday, earthquake precursors remain debated. While precursory slow slip is an important feature of earthquake nucleation, foreshock sequences are not always observed, and their temporal evolution remains poorly constrained. We report on laboratory earthquakes conducted under upper‐crustal stress and fluid pressure conditions. The dynamics of precursors (slip, seismicity, and fault coupling) prior to the mainshock are dramatically affected by slight changes in fault conditions (fluid pressure and slip history). A relationship between precursory moment release and mainshock magnitude is systematically observed, independent of fault conditions. Based on nucleation theory, we derive a semiempirical scaling relationship which explains this trend for laboratory earthquakes. Several natural observations of earthquakes ranging from ~Mw 6.0–9.0, where precursory moment release could be estimated, seem to follow the extrapolation of the laboratory‐derived scaling law. Notwithstanding spatiotemporal complexity in natural seismicity, some moderate to large earthquake magnitudes might be estimated through integrated seismological and geodetic measurements of both seismic and aseismic slips during nucleation.

Aftershock deficiency of induced earthquake sequences during rapid mitigation efforts in Oklahoma

Induced seismicity provides a rare opportunity to study earthquake triggering and underlying stress perturbations. Triggering can be a direct result of induced stress changes or indirect due to elastic stress transfer from preceding events leading to aftershocks. Both of these processes are observable in areas with larger magnitude induced events, such as Oklahoma. We study aftershock sequences of M2.5 to M5.8 earthquakes and examine the impact of targeted injection rate reductions. In comparing aftershock productivity between California and Oklahoma, we find similar exponential scaling statistics between mainshock magnitude and average number of aftershocks. For events with M≥4.5 Oklahoma exhibits several mainshocks with total number of aftershocks significantly below the average scaling behavior. The sequences with deficient aftershock numbers also experienced rapid, strong mitigation and reduced injection rates, whereas two events with M4.8 and M5.0 with weak mitigation exhibit normal aftershock productivity. The timing of when aftershock activity is reduced correlates with drops in injection rates with a lag time of several days. Large mainshocks with significantly reduced aftershocks may explain decreasing seismicity rates while seismic moment release was still increasing in Oklahoma in 2016. We investigate the expected poroelastic stress perturbations due to injection rate changes within a layered axisymmetric model and find that stresses are lowered by 10s to 100s kPa within the injection-affected zone. For earthquakes induced by poroelastic stress-increase at several kilometers from wells, the rapid shut-in of wells may lead to elastic stress reductions sufficiently high to arrest unfolding aftershock sequences within days after mitigation starts.

Patterns of Reservoir‐Triggered Seismicity in a Low‐Seismicity Region of France

We analyzed the impact of the 26 largest impounded reservoirs on reservoir‐triggered seismicity (RTS) patterns in the low‐seismicity region of continental France. We treat reservoir‐triggered earthquakes as tectonic earthquakes and apply similar concepts in our analysis. Generally, the spatial extent of an aftershock zone is controlled by the mainshock rupture length. In a similar manner, we use reservoir length as an equivalent length to the rupture length to assess the spatial extent of reservoir‐triggering earthquakes and one to three reservoir lengths as a proxy for the near‐field distance where the stress change induced by reservoir impoundment may trigger seismicity. Accordingly, we define the 1Lr distance as the near‐field reservoir effect on seismicity and the 10Lr distance as the far field, null effect of reservoir stress change on background seismicity. We find that (1) about a quarter of the reservoirs trigger Mmax=2.5–4.7 within the 1Lr distance in a 15 yr space time window, and (2) as tested against a randomized series, superposed epoch analysis demonstrates a robust increase in the average seismicity rate within 2 yrs for the 1–3Lr distance from reservoirs. The reservoirs that trigger in the (⁠ 1Lr ⁠) near‐field distance are significantly larger than the nontriggering ones. While considering the distance of triggering of earthquakes from the reservoir, it is more appropriate to consider the normalized distance (the distance normalized by the reservoir length) to identify earthquake triggering reservoirs at a 1Lr distance. While considering reservoir dimensions, the reservoir length appears to be a more important parameter than the reservoir depth, as the length is proportional to the area of significant stress change. Our results suggest that the RTS mimics the aftershock sequence of a slow reservoir‐impoundment loading, with a corresponding M∗reservoir=M(Lr) mainshock magnitude. Further, when considering mainshock–aftershock interactions, our analysis and observations support that the Mmax for RTS for a given reservoir remains, on average, smaller than the reservoir magnitude equivalent.

On the possible effect of round-the-world surface seismic waves in the dynamics of repeated shocks after strong earthquakes

Based on the observation data for hundreds of the main shocks and thousands of aftershocks, the existence of effect of round-the-world surface seismic waves is demonstrated (let us conditionally refer to them as a round-the-world seismic echo) and the manifestations of this effect in the dynamics of the repeated shocks of strong earthquakes are analyzed. At the same time, we by no means believe this effect has been fully proven. We only present a version of our own understanding of the physical causes of the observed phenomenon and analyze the regularities in its manifestation. The effect is that the surface waves excited in the Earth by the main shock make a full revolution around the Earth and excite a strong aftershock in the epicentral zone of the main shock. In our opinion, the physical nature of this phenomenon consists in the fact that the superposition leads to a concentration of wave energy when the convergent surface waves reach the epicentral zone (cumulative effect). The effect of the first seismic echo is most manifest. Thus, the present work supports our hypothesis of the activation of rock failure under the cumulative impact of an round-the-world seismic echo on the source area which is releasing (“cooling”) after the main shock. The spatial regularities in the manifestations of this effect are established, and the independence of the probability of its occurrence on the main shock magnitude is revealed. The effect of a round-the-world seismic echo can be used to improve the reliability of the forecasts of strong aftershocks in determining the scenario for the seismic process developing in the epicentral zone of a strong earthquake that has taken place.

Longer aftershocks duration in extensional tectonic settings

Aftershocks number decay through time, depending on several parameters peculiar to each seismogenic regions, including mainshock magnitude, crustal rheology, and stress changes along the fault. However, the exact role of these parameters in controlling the duration of the aftershock sequence is still unknown. Here, using two methodologies, we show that the tectonic setting primarily controls the duration of aftershocks. On average and for a given mainshock magnitude (1) aftershock sequences are longer and (2) the number of earthquakes is greater in extensional tectonic settings than in contractional ones. We interpret this difference as related to the different type of energy dissipated during earthquakes. In detail, (1) a joint effect of gravitational forces and pure elastic stress release governs extensional earthquakes, whereas (2) pure elastic stress release controls contractional earthquakes. Accordingly, normal faults operate in favour of gravity, preserving inertia for a longer period and seismicity lasts until gravitational equilibrium is reached. Vice versa, thrusts act against gravity, exhaust their inertia faster and the elastic energy dissipation is buffered by the gravitational force. Hence, for seismic sequences of comparable magnitude and rheological parameters, aftershocks last longer in extensional settings because gravity favours the collapse of the hangingwall volumes.

SOME PRELIMINARY RESULTS ON THE DISTRIBUTION OF AFTERSHOCK SEQUENCES IN JAPAN-KURIL ISLANDS AND KAMCHATKA

study on the aftershock sequences distributed along the subductions in Japan and Kuril islands, as well as in Kamchatka is undertaken. Aftershock sequences, having a main shock magnitude Mw >7.0, during the time period 1973-2013 are taken into account. The data used (mainshocks, aftershocks and foreshocks if there are any) are restricted in shallow focal depths. A large earthquake in Japan Trench (11 March 2011 / Mw=9.0) occurred and for this reason the investigated area is of particular interest. Our study is concentrated on the spatial distribution of some parameters [Mc, a, b (Gutenberg-Richter distribution) and p, c, k (Omori’s law)] closely associated with the seismic sequences statistics.

How variable is the number of triggered aftershocks?

AbstractAftershock activity depends at first order on the main shock magnitude but also shows important fluctuations between shocks of equal magnitude. We here investigate these fluctuations, by quantifying them and by relating them to the main shock stress drop and other variables, for southern California earthquakes. A method is proposed in order to only count directly triggered aftershocks, rather than secondary aftershocks (i.e., triggered by previous aftershocks), and to only quantify fluctuations going beyond the natural Poisson variability. Testing of the method subjected to various model errors allows to quantify its robustness. It is found that these fluctuations follow a distribution that is well fitted by a lognormal distribution, with a coefficient of variation of about 1.0 to 1.1. A simple model is proposed to relate this observed dependence to main shock stress drop variability.

Lessons learned from the total evacuation of a hospital after the 2016 Kumamoto Earthquake.

The 2016 Kumamoto Earthquakes were a series of earthquakes that included a foreshock earthquake (magnitude 6.2) on April 14 and a main shock (magnitude 7.0) on April 16, 2016. A number of hospitals in Kumamoto were severely damaged by the two major earthquakes and required total evacuation. The authors retrospectively analyzed the activity data of the Disaster Medical Assistance Teams using the Emergency Medical Information System records to investigate the cases in which the total evacuation of a hospital was attempted following the 2016 Kumamoto Earthquake. Total evacuation was attempted at 17 hospitals. The evacuation of one of these hospitals was canceled. Most of the hospital buildings were more than 20 years old. The danger of collapse was the most frequent reason for evacuation. Various transportation methods were employed, some of which involved the Japan Ground Self-Defense Force; no preventable deaths occurred during transportation. The hospitals must now be renovated to improve their earthquake resistance. The coordinated and combined use of military and civilian resources is beneficial and can significantly reduce human suffering in large-scale disasters.

Statistical Features of Foreshocks in Instrumental and ETAS Catalogs

We study the spatial distribution of earthquakes in temporal intervals before and after the occurrence of large shocks (mainshocks) in the magnitude range $$m \in [2,5]$$ for four different regional catalogs. We find that the spatial organization of pre-shock seismicity depends on the mainshock magnitude and is independent of the lower magnitude threshold. These properties are found to be a stable feature of regional catalogs and cannot be reproduced by Epidemic Type Aftershock Sequence models. Our findings suggest that the area fractured during the mainshock is encoded in the foreshock spatial organization and, therefore, enhance the prognostic value of foreshocks.

Site effect studies following the 2016 Mw 6.0 Amatrice Earthquake (Italy): the Emersito Task Force activities

<em>On August 24, 2016, at 01:36 UTC a M<sub>W </sub>6.0 earthquake struck an extensive area of the Central Apennines (Italy). It was followed by a large aftershock (M<sub>W </sub>5.3, August 24, 02:33 UTC) and about 20 earthquakes with magnitude greater than 4.0, located between the towns of Norcia and Amatrice. Due to the mainshock magnitude and the widespread damaging level of buildings in the epicentral area, the Emersito task force has been mobilized by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). The aim of Emersito is to carry out and coordinate the monitoring of local site effects, caused by geological and geomorphological settings. During the first days of the seismic emergency, Emersito installed a temporary seismic network for site effect studies at 4 municipalities close to the epicentral area (Amandola, Civitella del Tronto, Montereale and Capitignano), using 22 stations equipped with both velocimetric and accelerometric sensors. The selection of the sites where stations have been installed was mainly driven by the proximity to the epicentral area (without interfere with the rescue operation) and by peculiar geologic and geomorphologic settings (topographic irregularities, fault zones, alluvial plains).</em>