Years Of Seismicity Research Articles

Different earthquake nucleation conditions revealed by stress drop and b-value mapping in the northern Chilean subduction zone

Stress drop is an earthquake property indicative for the characteristic relation of slip to fault dimension. It is furthermore affected by fault strength, fault topography, the presence of fluids, rupture size, slip, and velocity. In this article, the stress drop image of an entire subduction zone, namely for the seismically highly active northernmost part of Chile, is combined with mapped b-values and their corresponding magnitude distribution in order to better constrain the conditions under which earthquakes of different provenances may nucleate. The underlying recent earthquake catalog contains over 180,000 events, covering 15 years of seismicity, from which more than 50,000 stress drop estimates were computed. Their spatial average segments the subduction zone into different parts, i.e., average stress drop between seismotectonic areas is different, although this difference is small compared to the natural scatter of stress drop values. By considering stress drop variations, b-value map, magnitude distribution, and thermal models, candidate earthquake nucleation mechanisms are identified which can explain the observed distributions. This is done for two exemplary regions: (1) The plate interface, where principally lower stress drop events are found, while at the same time a high spatial heterogeneity of stress drop values is observed. This indicates relatively smooth or lubricated rupture surfaces, and locally it suggests the existence of alternating regions controlled by strong asperities, weaker material, or creep. (2) The highly active intermediate depth (ID) seismicity region, where the variation of stress drop and b-value point to a gradual change of nucleation mechanism from dehydration embrittlement at the top of the ID cloud, over dehydration driven stress transfer in its central part, to thermal runaway shear mechanisms at its bottom. In both cases, the combination of stress drop and b-value distribution helps to better understand the origin and the differences of the observed seismicity.

Exploiting the legacy of N.N. Ambraseys: known and unknown earthquakes in the Anatolian area

N.N. Ambraseys left us a wealth of papers and volumes on a number of topics; many of them concern the historical earthquake investigation. One of the last works is the 2009 volume (Ambraseys in Earthquakes in the Eastern Mediterranean and the Middle East: a multidisciplinary study of 2000 years of seismicity, Cambridge, Cambridge, UK, 2009), where he summarizes the results of more than thirty years of investigation through archives and libraries, covering earthquakes of a large area, from Albania to Caucasus. For each earthquake, a short summary of the main effects is supplied, together with the list of the sources used. Such information is intended as material for assessing location and size of the earthquakes, task that the author accomplished only in a very preliminary way for a few earthquakes, only. In addition to exhaustive descriptions of the most known earthquakes and the relevant historical sources, the volume contains information on a large number of earthquakes, so far unknown to the current earthquake catalogues. This paper intends to represent a homage to his immense work, partially showing the potential of his volume. We briefly present here some case histories, including the preliminary location and size of the earthquakes – known and unknown—around Anatolia. We add some examples of how he was able to prove that some alleged earthquakes are actually to be considered as fake or very doubtful. We also present the damage information supplied for some known and unknown earthquakes, and how they can be used for assessing location and size of them.

Minimum 1D VP and VP/VS Models and Hypocentral Determinations in the Central Mediterranean Area

Abstract Minimum 1D velocity models and station corrections have been computed for the central Mediterranean area using two main data sets. The first one consists of accurate first arrival-time readings from 103 seismic events with magnitude (ML)≥3.5 recorded by the Italian National Seismic Network (RSN) and the AlpArray Seismic Network (AASN) in the period 2014–2021. Earthquakes were selected on the basis of their spatial distribution, epicentral distance to the nearest seismic station, and maximum distance traveled by Pn and Sn phases. This fine selection of high-quality data combined with the spatial density of the AlpArray seismic stations was decisive in obtaining high resolution for upper mantle velocity, especially in the Alpine belt. To obtain a denser coverage of crustal rays, we extended the first data set with P and S arrivals of local earthquakes from Istituto Nazionale di Geofisica e Vulcanologia (INGV) bulletin data (2016–2018). A total of 75,807 seismic phases (47,183 P phases and 28,264 S phases) have been inverted to calculate best-fit 1D velocity models, at regional and local scales. We then test the performance of the optimized velocity models by relocating the last four years of seismicity recorded by INGV (period 2017–2020). The computed velocity models are very effective for routine earthquake location, seismic monitoring, source parameter modeling, and future 3D seismic tomography.

Comprehensive High‐Precision Relocation of Seismicity on the Island of Hawai‘i 1986–2018

AbstractAbundant seismicity beneath the Island of Hawai‘i from mantle depths to the surface plays a central role in understanding how volcanoes work, grow, and evolve at this intraplate oceanic hotspot. We perform systematic waveform cross‐correlation, cluster analysis, and relative relocation of 347,445 events representing 32 years of seismicity on and around the island from 1986 to 2018. We successfully relocate 275,009 (79%) events using ∼1.7 billion differential times (P and S) from ∼128 million similar‐event pairs. The results reveal a dramatic sharpening of seismicity along faults, streaks, rings, rift zones, magma pathways, and mantle fault zones; seismicity delineating crustal detachments on the flanks of Kīlauea and Mauna Loa is particularly well‐resolved. The resulting high‐precision spatio‐temporal image of seismicity captures almost the entire 1983–2018 Pu‘u ‘Ō‘ō‐Kūpaianaha eruption of Kīlauea with its numerous distinct episodes and wide‐ranging activity, culminating in the 2018 lower East Rift Zone eruption and summit collapse.

Active Tectonics in Tuscany (Central Italy): Ten Years of Seismicity (2009-2019)

Strong earthquakes (moment magnitude MW ≥ 5.5) are uncommon in Tuscany and surroundings (central Italy). The last strong seismic event occurred a century ago (September 7, 1920 Garfagnana, MW = 6.53). The paucity of seismic instrumental recordings hinders the identification of the tectonic regime active in Tuscany. On the other hand, the geological and geomorphological pieces of evidence collected so far, concerning potential active and capable faults, are scarce, fragmentary and ambiguous. In this work I shed light on the active deformation of Tuscany by using two independent approaches: earthquake source mechanisms and GNSS (GPS) geodetic measurements. I have considered 41 small seismic events (MW ≤ 5.1) that occurred in the study area during the last decade. The related source mechanisms (retrieved by the Time Domain Moment Tensor method) define a relatively clear picture of the active deformation: extension along the northern Apennine watershed and strike-slip regime within inner Tuscany, up to the Tyrrhenian coast. This pattern broadly agrees with the horizontal strain field reconstructed by the geodetic velocity field. The latter has been constrained by a network of 840 GPS stations located in Italy and neighboring countries, operating in the last 20 years.

Earthquakes in southern Dalmatia and coastal Montenegro before the large 6 April 1667 event

The fourteenth to seventeenth century seismicity of southern Dalmatia (Croatia) and coastal Montenegro deserved to be fully reappraised because of the ascertained imperfect knowledge offered by modern seismological studies and of the awareness of the smokescreen effect due to the large 6 April 1667 M 6.4 earthquake that impacted exactly the area of study. The investigation consisted of (i) a reconsideration of earthquake records made available by previous studies and (ii) a systematic analysis of historical sources contemporary to the earthquakes, especially those not yet taken into account in seismological studies. The 168 contemporary and independent records collected cast a different light on more than 300 years of seismicity of this area. Records are reckoned to be unevenly distributed among the 39 studied earthquakes, out of which 15 still rely upon a single testimony. Each record has been reevaluated with respect to its content and attributed a level of reliability, which for those reporting other 14 events was so low to prevent us from confirming their real occurrence. Completely unreliable records have been identified and discussed, to conclude that they are at the root of five fake earthquakes. Altogether, 34 intensity values in EMS-98 were assessed related to 15 moderate and five damaging earthquakes. Existing and newly obtained data contributed to putting the pre-1667 seismicity of southern Dalmatia and coastal Montenegro into a substantially different perspective.

Hypocenter Estimation of Induced Earthquakes in Groningen

Induced earthquakes due to gas production have taken place in the province of Groningen in the northeast of The Netherlands since 1986. In the first years of seismicity, a sparse seismological network with large station distances from the seismogenic area in Groningen was used. The location of induced earthquakes was limited by the few and wide spread stations. Recently, the station network has been extended significantly and the location of induced earthquakes in Groningen has become routine work. Except for the depth estimation of the events. In the hypocentre method used for source location by the Royal Netherlands Meteorological Institute (KNMI), the depth of the induced earthquakes is by default set to 3 km which is the average depth of the gas-reservoir. Alternatively, a differential traveltime for P-waves approach for source location is applied on recorded data from the extended network. The epicentre and depth of 87 induced earthquakes from 2014 to July 2016 have been estimated. The newly estimated epicentres are close to the induced earthquake locations from the current method applied by the KNMI. It is observed that most induced earthquakes take place at reservoir level. Several events in the same magnitude order are found near a brittle anhydrite layer in the overburden of mainly rock salt evaporites.

An attempt to reconstruct 2700 years of seismicity using deep-sea turbidites offshore eastern Taiwan

The Taiwan area, where the Philippine Sea Plate collides with Eurasia, is one of the most seismically active areas in the world and has been consequently struck repeatedly by destructive earthquakes. To better constrain the occurrence of large earthquakes, we have conducted two cruises in 2012 and 2013 from which five piston, gravity and box-cores were retrieved from selected sites in targeted areas that contain continuous and accurate turbidite deposition offshore eastern Taiwan. Forty-seven turbidite layers deposited between 675BC and 2012AD have been described in the cores with facies varying from silty clay to coarse sand. We modeled each turbidite layer using the sedimentation rate deduced from radiocarbon age measurements performed on planktonic foraminifers. Precise dating based on 210Pb–137Cs chronology provided ages for the XXth century turbidites. Coring sites' locations, biotic association within turbidites beds and seismic calibration over the instrumental period, suggest that earthquakes are the most likely triggering mechanisms of turbidity currents over the last 2700years. Calibration of correlations between turbidites and instrumental earthquakes allowed us to determine the minimum magnitude of recorded events (Mw=6.8) and the maximum distance between the epicenter and the slope failure. Turbidites' synchroneity between cores has been tested but results showed that this criterion cannot be used in our study area because of the high level of seismicity, i.e., a M>6.8 earthquake recurrence interval much smaller than the uncertainty on radiocarbon ages. The excellent accuracy of dating allows correlating all the turbidites deposited since 1920 with instrumental earthquakes. For each core, we established a return time ranging between 112 and 147years for the period pre-1900 and ~27 and 34years for the period post-1900. The discrepancy between pre-1900 and post-1900 recurrence either suggests, that slope stability varies, with higher slope instability after 1900 possibly due to increased sediment delivery by rivers.

ISC-GEM: Global Instrumental Earthquake Catalogue (1900–2009), II. Location and seismicity patterns

We present the final results of a two-year project sponsored by the Global Earthquake Model (GEM) Foundation. The ISC-GEM global catalogue consists of some 19 thousand instrumentally recorded, moderate to large earthquakes, spanning 110years of seismicity. We relocated all events in the catalogue using a two-tier approach. The EHB location methodology (Engdahl et al., 1998) was applied first to obtain improved hypocentres with special focus on the depth determination. The locations were further refined in the next step by fixing the depths to those from the EHB analysis and applying the new International Seismological Centre (ISC) location algorithm (Bondár and Storchak, 2011) that reduces location bias by accounting for correlated travel-time prediction error structure. To facilitate the relocation effort, some one million seismic P and S wave arrival-time data were added to the ISC database for the period between 1904 and 1970, either from original station bulletins in the ISC archive or by digitizing the scanned images of the International Seismological Summary (ISS) bulletin (Villaseñor and Engdahl, 2005, 2007). Although no substantial amount of new phase data were acquired for the modern period (1964–2009), the number of phases used in the location has still increased by three millions, owing to fact that both the EHB and ISC locators use most well-recorded ak135 (Kennett et al., 1995) phases in the location.We show that the relocation effort yielded substantially improved locations, especially in the first half of the 20th century; we demonstrate significant improvements in focal depth estimates in subduction zones and other seismically active regions; and we show that the ISC-GEM catalogue provides an improved view of 110years of global seismicity of the Earth. The ISC-GEM Global Instrumental Earthquake Catalogue represents the final product of one of the ten global components in the GEM program, and is available to researchers at the ISC (www.isc.ac.uk) website.

Tomography from 26 years of seismicity revealing that the spatial extent of the Yellowstone crustal magma reservoir extends well beyond the Yellowstone caldera

The Yellowstone volcanic field has experienced three of Earth's most explosive volcanic eruptions in the last 2.1 Ma. The most recent eruption occurred 0.64 Ma forming the 60 km long Yellowstone caldera. We have compiled earthquake data from the Yellowstone Seismic Network from 1984 to 2011 and tomographically imaged the three-dimensional P wave velocity (Vp) structure of the Yellowstone volcanic system. The resulting model reveals a large, low Vp body, interpreted to be the crustal magma reservoir that has fueled Yellowstone's youthful volcanism. Our imaged magma body is 90 km long, 5–17 km deep, and 2.5 times larger than previously imaged. The magma body extends ~15 km NE of the caldera and correlates with the location of the largest negative gravity anomaly, a −80 mGal gravity low. This new seismic image provides important constraints on the dynamics of the Yellowstone magma system and its potential for future volcanic eruptions and earthquakes.

A survey of earthquakes and injection well locations in the Barnett Shale, Texas

In August 2012, the Proceedings of the National Academy of Sciences published my article surveying two years of seismicity in the Barnett Shale of Texas. The data showed that virtually all well-located epicenters were situated within 3 km of high-volume injection wells. This article summarizes the results of that publication. Recently there has been concern about earthquakes possibly caused by the disposal of hydrofracture flowback fluids in injection wells. Most investigations of earthquakes caused by human activity take place only after an earthquake occurs that is severe enough to be felt by nearby residents and receive media attention. Such events usually have magnitudes ∼M3 or greater and occur in populated areas. Limiting research only to these events does not help us understand why some injection wells trigger seismic activity and others do not.

Modelling the hydromechanical response in the vicinity of the Koyna reservoir (India): results for the initial filling period

SUMMARY The seismic activity in the Koyna area is clearly related to the water impoundment of the Koyna dam in 1962, and has reached a remarkable level with the occurrence of a major event of magnitude 6.3 in 1967 December 10. We present a homogeneous poroelastic model based on analysis of the first eight years of seismicity, which aims to link the water-level fluctuations of the reservoir with the seismicity. Starting from a discretized lake, we calculate the stress field resulting from the water-level fluctuations and the pore pressure changes due to the undrained and the diffusive responses of the medium. Then, we compare the Coulomb stress variations with a set of relocated seismic events. We find that more than 80 per cent of the relocated events before the M6.3 event are well described by this poroelastic model, leading us to derive a suitable diffusivity cp = 0.2 m 2 s −1 . Then, we model the response of the system after the M6.3 event of 1967 December 10, by comparing the variation of the Coulomb stress field with the spatio-temporal characteristics of the relocated post-seismic events and the decay of aftershocks with time. We find that compared to before the main event a tenfold increase in hydraulic diffusivity is required to satisfactorily describe the aftershock decay with the appropriate Omori exponent. Although this increase in diffusivity may be physically related to the main shock we also note that events later than 9 months after the main shock are not well explained. We therefore propose an alternative hydrological model, which involves two compartments of contrasting diffusivities.

EUROCODE 8 DESIGN RESPONSE SPECTRA EVALUATION USING THE K-NET JAPANESE DATABASE

The Eurocode 8 (EC8) currently proposes two standard shapes for the design response spectra. Type 1 spectra are enriched in long period and are suggested for high seismicity regions. Conversely, Type 2 spectra are proposed for low to moderate seismicity areas (like France), and exhibit both a larger amplification at short period, and a much smaller long period contents, with respect to Type 1 spectra. These propositions, however, were constrained using few events mostly recorded on analogical instruments. In the present study, we use the Japanese high quality digital K-net array in order to evaluate the proposed ECS response spectra. Furthermore, all K-net stations have geotechnical characterisation. We first constructed a database of shallow events, depth less than 25 km, to avoid subduction related records. The database spans six years of seismicity from 1996 until 2003. Thus, 591 events were selected with moment magnitude between 4 and 7.3, recorded at 691 stations, giving a total of 6812 two horizontal components accelerograms. Using these records, we computed spectral ground-motion prediction equations and we used them to review the shape of the proposed EC8 spectra. In particular, we studied the plateau-PGA ratio level, the period interval where this plateau is constant, and site amplification effects. The results show surprisingly that the Type 2 rock better envelope the Japanese data. Another interesting observation is that the K-net data corresponding to all soil classes are rich in short periods around 0.1 s. This characteristic has not been observed in other worldwide databases. Normalised empirical predictions show a widening of the plateau as the soil conditions degrade. This suggests that the Type 2 EC8 spectra do not cover enough the long periods for EC3-soil classes C, D and E. Finally, the computed ground-motion prediction equations show that the peak ground acceleration (PGA) is nearly invariant to the soil conditions. Soil effects are mainly seen in the shape and plateau level.

The Seismogenic Thickness of the Southern California Crust

The average seismogenic thickness, measured from the surface down to maximum depth of earthquake rupture, for the southern California crust is 15.0 km (+1.2/–1.1 km). We determine the seismogenic thickness using the depth distribution of the seismic moment release of ∼19 years of seismicity. We calibrate the depth distribution of moment release from background seismicity by comparing the maximum depth of rupture during moderate- to large-magnitude earthquakes to the premainshock background seismicity of the respective mainshock region. The calibration shows that the depth above which 99.9% of the moment release of background seismicity occurs reliably estimates the maximum depth of rupture during moderate to large earthquakes. Locally, the seismogenic thickness is highly variable, ranging from less than 10 km in the Salton Trough to greater than 25 km at the southwestern edge of the San Joaquin Valley. Similarly, the seismogenic thickness along the major strike-slip faults can vary significantly along strike. Changes in seismogenic thickness along strike do not correspond to the mapped surface segmentation of the major southern California strike-slip fault systems. In the future, such estimates of the seismogenic thickness can be used to refine existing seismic-hazard estimates for southern California.

Nature of seismic coupling along simple plate boundaries of the subduction type

The downdip width of the seismogenic zone is defined for 19 subduction zones. This width is measured from the base of the accretionary prism to the maximum depth of nucleation of thrust events along the plate boundary. Those two points are taken to define the upper and lower depth transitions from stable to unstable frictional sliding. The lower depth transition is found to be between 35 and 70 km. The dip angle of the thrust zone is also reevaluated. We find a linear increase in the dip angle as a function of depth, the slope of which varies between 0.2° and 0.6° km−1. The downdip width obtained, which is generally narrower than previously determined by most other authors, varies from about 50 to 150 km. We also determine the ratio of the rate of slip that occurs in earthquakes to the rate of relative plate motion. This ratio is defined as the seismic coupling coefficient (a). We obtain two different estimates of the seismic coupling coefficient: an average value from 90 years of seismicity and a value obtained using the slip‐predictable recurrence model for large earthquakes. We find a large variation in the computed values of a along and among subduction zones. For most of the subduction zones a is much less than 1.0; for several it is less than a few percent. Worldwide, we find no significant correlation between either the seismic coupling coefficient or the width of the seismogenic zone and subduction parameters such as the age of the oceanic lithosphere that is being subducted, plate convergence rates or absolute velocity of the upper plate in the hot spot reference frame. Such correlation exists only for a few individual subduction zones where other parameters do not vary as much. The observed variations in seismic coupling could be explained as differences in the frictional behavior of materials at the plate interface. Some of these differences may be attributed to the subduction of large bathymetric features, the roughness of topography, the presence of unstable triple junctions and active‐spreading ridges, and sediment composition.

Four thousand years of seismicity along the Dead Sea Rift

A study of major earthquake occurrence along the Dead Sea transform (35.5°–36.5° E; 27.2°–37.5° N) during the past four millenia has been attempted. Geological, archaeological, biblical, historical, and seismological evidence were integrated in an effort to quantify the space‐time distribution of seismicity in the said province. The overall earthquake activity in the conterminous Near East indicates a stable pattern and appeared to have been stationary over the examined time window. About 110 earthquakes in the magnitude range 6.7 ≤ ML ≤ 8.3 affected the area during the past 2500 years. Of these, 42 originated along the Dead Sea fault system itself, while 68 were imported from the Helenic‐Cyprian arcs and the Anatolian‐Elburz‐Zagros fault systems. These events were responsible for the repeated destruction of many cultural centers. In the Dead Sea region proper, the major seismic activity since 2100 B.C.E. (Before Christian Era), has been confined to the vicinity of its eastern shore with extremal seismicity at its southern tip near the prehistorical site of Bab‐a‐Dara'a (31° 15'N, 35° 32'E). This may constitute the first solid evidence that the Biblical “cities of the Plain” (Sodom, Gommorah, etc.) were located there. Recent studies of earthquake deformations in the Lisan deposits near Bab‐a‐Dara'a, agree with our findings. At the present time, a magnitude 6¾ earthquake is pending at the northern edge of the Levant rift, with its average recurrence interval (83 years) exceeded by one standard deviation (32 years).