Sequence Of Strong Earthquakes Research Articles

Assessing the living environment of persons displaced following a strong earthquake sequence in Puerto Rico, 2020.

In the public health portfolio of disaster tools, rapid needs assessments are essential intelligence data mining resources that can assess immediate needs in almost all hazard scenarios. Following prolonged and unusual seismic activity that caused significant structural damage, mainly in the southwest part of the island of Puerto Rico, thousands of area residents were forced to leave their homes and establish improvised camps. The austere environmental exposure and limited access to safety and hygiene services prompted public health authorities to request assistance with conducting a rapid needs assessment of those encampments. This report summarizes the design, organization, and execution of a rapid needs assessment of improvised camps following a strong sequence of earthquakes in Puerto Rico.

The Ionospheric Plasma Perturbations before a Sequence of Strong Earthquakes in Southeast Asia and Northern Oceania in 2018

From August to October 2018, a series of strong earthquake (EQ) events occurred in southeast Asia and northern Oceania (22°S to 0°N, 115°E to 170°E) within 50 days. In this paper, we analyze the features of ionospheric plasma perturbations, recorded by the Plasma Analyzer Package (PAP) and Langmuir probe (LAP) onboard the China Seismo-Electromagnetic Satellite (CSES-01), before four EQs with magnitudes of Ms 6.9 to Ms 7.4. The ion parameters such as the oxygen ion density (No+), the ion drift velocity in the vertical direction (Vz) under the conditions of geomagnetic storms, and strong EQs are compared. The results show that within 1 to 15 days before the strong EQs, the No+ and the electron density (Ne) increased while the electron temperature (Te) decreased synchronously. Meanwhile, the Vz significantly increased along the ground-to-space direction. The relative variation of No+ and Vz before the strong EQs is more prominent, and the Vz is not easily influenced by the geomagnetic storm but is susceptible to the seismic activities. Our results suggest that the anomaly of ionospheric plasma perturbations occurring in this area is possibly related to the pre-EQ signatures.

Seismic and Geodetic Imaging (DInSAR) Investigation of the March 2021 Strong Earthquake Sequence in Thessaly, Central Greece

Three strong earthquakes ruptured the northwest Thessaly area, Central Greece, on the 3, 4 and 12 March 2021. Since the area did not rupture by strong earthquakes in the instrumental period of seismicity, it is of great interest to understand the seismotectonics and source properties of these earthquakes. We combined relocated hypocenters, inversions of teleseismic P-waveforms and of InSAR data, and moment tensor solutions to produce three fault models. The first shock (Mw = 6.3) occurred in a fault segment of strike 314° and dip NE41°. It caused surface subsidence −40 cm and seismic slip 1.2–1.5 m at depth ~10 km. The second earthquake (Mw = 6.2) occurred to the NW on an antithetic subparallel fault segment (strike 123°, dip SW44°). Seismic slip of 1.2 m occurred at depth of ~7 km, while surface subsidence −10 cm was determined. Possibly the same fault was ruptured further to the NW on 12 March (Mw = 5.7, strike 112°, dip SSW42°) that caused ground subsidence −5 cm and seismic slip of 1.0 m at depth ~10 km. We concluded that three blind, unknown and unmapped so far normal fault segments were activated, the entire system of which forms a graben-like structure in the area of northwest Thessaly.

Estimation of Absolute Stress in the Hypocentral Region of the 2019 Ridgecrest, California, Earthquakes

AbstractStrength of the upper brittle part of the Earth's lithosphere controls deformation styles in tectonically active regions, surface topography, seismicity, and the occurrence of plate tectonics, yet it remains one of the most debated quantities in geophysics. Direct measurements of stresses acting at seismogenic depths are largely lacking. Seismic data (in particular, earthquake focal mechanisms) have been used to infer orientation of the principal stress axes. I show that the focal mechanism data can be combined with information from precise earthquake locations to place constraints not only on the orientation, but also on the magnitude of absolute stress at depth. The proposed method uses relative attitudes of conjugate faults to evaluate the amplitude and spatial heterogeneity of the deviatoric stress and frictional strength in the seismogenic zone. Relative fault orientations (dihedral angles) and sense of slip are determined using quasi‐planar clusters of seismicity and their composite focal mechanisms. The observed distribution of dihedral angles between active conjugate faults in the area of Ridgecrest (California, USA) that hosted a recent sequence of strong earthquakes suggests in situ coefficient of friction of 0.4–0.6, and depth‐averaged shear stress on the order of 25–40 MPa, intermediate between predictions of the “strong” and “weak” fault theories.

Strong Earthquake Sequences in Greece during 2008-2014: Moment Tensor Inversions and Fault Plane Discrimination

As is well known, Greece has a significant number of earthquakes each year. Ιn recent years, several earthquakes have occurred in Greece. For this scope, a methodology was used to determine the source parameters. This methodology is based on minimizing the difference between the observed and the synthetic waveforms, using the method Source Parameters Calculation—SPCa [1]. The source parameters, using the proposed methodology, are calculated by comparing observed seismograms and synthetic by inverting data. The synthetics are calculated using the reflectivity method (Kennett, 1983) as implemented by Randall et al. (1994) for a given earth structure. This study includes inversion results for the strongest events that occurred in Greece from 2008 to 2014. For the same events calculated the main fault plane, using the method of Hypocenter Centroid-plot (HC-plot) [2] [3]. This methodology is a simple geometrical method based on the combination between the hypocentral position and the two possible fault planes.

The relationship between stress changes and strong earthquakes since 1900 around the Bayan Har block in northern Tibet, China

A 3D viscoelastic finite-element model of the Qinghai-Tibet Plateau, including the Bayan Har block, is established in this article. This model takes into consideration differences in the regional geological structure, the main active fault zone, the active tectonic block (hereafter referred to as a block) and boundary fault zone, the irregular topography and layered lithospheric structure, and the seismic velocity structure of the crust and mantle. Moreover, the present tectonic background stress field in the research area is reconstructed using the constraints of the observed values for the rate of crustal horizontal movement and the direction of maximum principal compressive stress. On these bases, the historical strong earthquake sequences with Ms ≥ 7 that have occurred around the Bayan Har block since 1900 AD are simulated. Based on the Coulomb failure stress and equivalent stress data, the relationships between stress evolution and strong earthquakes, the interactions between strong earthquakes are investigated. In addition, we have also explored the changes in stress on the Longmenshan fault zone before and after the 2008 Wenchuan earthquake, as well as the relationships between stress evolution and regional seismicity. The results show that the occurrence of strong earthquakes around the Bayan Har block may be related to the increase in the total stress in the source area. The influence of long-term tectonic stress loading on strong earthquakes cannot be ignored. The 2008 Wenchuan earthquake induced an increase in stress in the southern section of the Longmenshan fault zone, indicating that the 2008 Wenchuan earthquake has an obvious promoting effect on the 2013 Lushan earthquake. The seismicity of the Bayan Har block and its adjacent areas may be affected by the regional total stress state.

Elastic-plastic behavior of AP1000 nuclear island structure under mainshock-aftershock sequences

This paper studies dynamic responses of AP1000 nuclear island structure in strong earthquake sequences. A numerical model to simulate nuclear structural behaviors in earthquake is validated by comparison with data from a previous study on a nonlinear dynamic analysis of a reinforced concrete shield building. The validated numerical model is then used to carry out a series of parametric analyses with 112 computational cases so as to determine influence of strong aftershocks on structural elastic-plastic behavior considering input of three-dimensional ground motions. The results indicate that the influence of aftershocks on structural horizontal/vertical dynamic responses is very small in design basis earthquake sequences. However, the influence must be considered seriously in beyond-design basis earthquake sequences as values of RMVs (Ratio of Mean Value) deviating IPRs (Input Peak Ratio) obviously, which means structural dynamic responses are greatly changed in strong aftershocks. Damage aggravating effect induced by strong aftershocks can cause severe damage of structural members and it is found the greater the magnitude of aftershocks, the severer the aggravation effect. Although earthquake input energy is mostly dissipated by damping energy, plastic damage energy plays considerable role in strong aftershocks as it shares beyond 8 percent of the total input energy, which is 10 times more compared to design basis earthquake sequences.

Iconic crumbling of the clock tower in Amatrice after 2016 central Italy seismic sequence: advanced numerical insight

The present paper investigates from an advanced numerical point of view the progressive damage of the Amatrice (Rieti, Italy) civic clock tower, after a long sequence of strong earthquakes that struck central Italy in 2016. Two advanced numerical models are here utilised to have an insight into the modalities of progressive damage and the behaviour of the structure under strong non-linear dynamic excitations, namely a Non-Smooth Contact Dynamics (NSCD) and a FE Concrete Damage Plasticity (CDP) models. In both cases, a full 3D detailed discretization is adopted. From the numerical results, both the role played by the actual geometries and the insufficient resistance of the constituent materials are envisaged, showing a good match with actual crack patterns observed after the seismic sequence.

Thermal infrared anomaly indicating unformed strong earthquake sequences

By processing and analyzing many satellite remote sensing images in infrared channels, we found visual evidence indicating earthquake sequences. In a sequence, large earthquakes could migrate thousands of kilometers along a linear route from one land mass to another, and infrared anomalies also cover these land masses. The sequence has at least two types: (1) quakes alternately occurring at two ends of a belt, or (2) quakes progressively migrating along a far-flung route. The former can be foreshowed in a single infrared belt with a legible directed structure and higher brightness temperature; the latter can be identified in advance from the shift, veer, and deformation of more areas with a higher brightness temperature. The thermal infrared anomalies with a shorter duration are used to predict the quake-sequence evolving in a longer period. Two sorts of sequences can meet and form a more complex structure that can also be shown in advance by an anomalous infrared area. These studies on the seismic sequences will provide a potent means for analyzing earthquake-pregnant fields and estimating the location of unformed earthquake sequences.

The Cephalonia, Ionian Sea (Greece), sequence of strong earthquakes of January-February 2014: a first report

On 26.1.2014 and 3.2.2014 two strong earthquakes of M<sub>w</sub>6.0 and M<sub>w</sub>5.9 ruptured the western Cephalonia Isl., Ionian Sea (Greece), at the SSW-wards continuation of the Lefkada segment of the Cephalonia Transform Fault Zone (CTFZ), causing considerable damage and a variety of ground failures. High-precision relocation of the aftershocks implies that the seismogenic layer was of 35 km in length (L) striking NNE-SSW, of 10 km maximum in width and 15 km in thickness. Two aftershock spatial clusters were revealed at north (<em>L<sub>1</sub></em>~10 km) and at south (<em>L<sub>2</sub></em>~25 km). However, no time correlation was found between the two clusters and the two strong earthquakes. Fitting the temporal evolution of aftershocks to the Omori-law showed slow aftershock decay. Fault plane solutions produced by moment tensor inversions indicated that the strong earthquakes as well as a plenty of aftershocks (M<sub>w</sub>≥4.0) were associated with dextral strikeslip faulting with some thrust component and preferred fault planes striking about NNE-SSW. Average fault plane parameters obtained for the three largest events are: strike 21(±2)<sup>0</sup>, dip 65.5(±3)<sup>0</sup>, slip 173(±3)<sup>0</sup>. Broadband P-wave teleseismic records were inverted for understanding the rupture histories. It was found that the earthquake of 26.1.2014 had a complex source time function with c. 62 cm maximum slip, source duration of ~12 s and downwards rupture. Most of the slip was concentrated on a 13x9 km fault rupture. The earthquake of 3.2.2014 had a relatively simple source time function related with one big patch of slip with maximum slip c. 45 cm, with 10 s source duration. The rupture was directed upwards which along with the shallow focus (~5 km) and the simple source time function may explain the significantly larger (0.77 g) PGA recorded with the second earthquake with respect to the one recorded (0.56 g) with the first earthquake. Most of the slip was concentrated on a 12x6 km fault rupture. Maximum seismic intensity (<em>I<sub>m</sub></em>) of level VII and VIII to VIII+ was felt in Lixouri town and the nearby villages from the first and the second earthquake, respectively. The rupture histories and the increased building vulnerability after the damage caused by the first shock may account for the larger <em>I<sub>m</sub></em> caused by the second shock. However, the ground failures area of the second earthquake was nearly half of that of the first earthquake, which is consistent with the faster attenuation of ground acceleration away from the meizoseismal area caused by the second earthquake with respect to the first one. From that the 2014 earthquakes ruptured on land western Cephalonia we suggested to revise the CTFZ geometry in the sense that the Lefkada CTFZ segment does not terminates offshore NW Cephalonia but extends towards SSW in western Cephalonia.

Single-station standard deviation analysis of 2010–2012 strong-motion data from the Canterbury region, New Zealand

A systematic analysis was conducted of the different variability components that affect the prediction of \(\text{ log }_{10}(PSA)\) (i.e., Pseudo-Spectral Acceleration) ordinates on (mostly) deep sedimentary soil sites using a sizable set of strong motion data recorded in the strong earthquake sequences of 2010 and 2012 in the Canterbury region of New Zealand. Following recent, well established approaches of residual analysis of ground motion predictions, as well as recent GMPEs based on a global dataset, it was found that the event-corrected single-station standard deviation (“sigma”) is strongly decreased, for all selected stations, with respect to the uncorrected sigma. Likewise, the event-corrected intraevent sigma estimated for the entire dataset is significantly reduced compared to the standard deviation associated to ground motion prediction models, i.e. the “ergodic” sigma, for all spectral periods. The event-corrected sigma values for the present dataset are surprisingly consistent with those recently derived using KiK-Net strong motion data from Japan and those by Boore and Atkinson (Earthq Spectra 34(1):99–138, 2008) GMPE, and remain fairly constant with respect to the spectral period at about \(0.15\sim 0.2\). An interpretation was provided of the physical meaning of the site correction term (\({\delta }S2S)_{s}\) indicating a plausible correlation with prevailing geological conditions in the site area.

Stress Interactions within the Strong Earthquake Sequence from 2001 to 2010 in the Bayankala Block of Eastern Tibet

Abstract Four major earthquakes of M s >7.0 occurred in the Tibetan Plateau during the last decade. They were the 2001 M s 8.1 Kunlun Mountains (Hoh Xil) earthquake in Qinghai, the 2008 M s 7.3 Yutian earthquake in Xinjiang, the 2008 M s 8.0 Wenchuan earthquake in Sichuan, and the 2010 M s 7.1 Yushu earthquake in Qinghai. All four occurred in relative proximity to the Bayankala block. By the integration of coseismic effects and viscoelastic relaxations, we studied the stress interactions among those four earthquakes. Then, the epidemic‐type aftershock sequence (ETAS) model and stochastic declustering method were used to detect the local seismic signals of each succeeding major earthquake. This analysis considered increases or decreases with the Coulomb failure stress (CFS) variations induced by its preceding major earthquakes. The results showed that the stress interactions among these four major earthquakes are too weak to trigger each other and that the preceding major earthquakes did not obviously impact the local seismicity of the succeeding major earthquakes. It demonstrates that the occurrences of these four major earthquakes that clustered around the Bayankala block might not have been caused by the triggering effects. Instead these might have resulted from the southeastward movement of the Bayankala block, which related to the compression and penetration of the Indian–Australian Plate into the Eurasian Plate.

Tectonic dynamics and correlation of major earthquake sequences of the Xiaojiang and Qujiang-Shiping fault systems, Yunnan, China

The N-S trending Xiaojiang fault zone and the NW-SE trending Qujiang-Shiping fault zone are adjacent active fault systems and seismogenic zones associated with strong and major earthquakes in Yunnan, China. To understand the interaction of the two fault systems, and its probable influence on earthquake occurrences, this paper conducts a synthetic study based on data of active tectonics, historical earthquakes, relocated small earthquakes, GPS station velocities and focal mechanism resolutions. The study makes several conclusions. (1) The active southward motion of the western side of the Xiaojiang fault zone (i.e. the side of the Sichuan-Yunnan block) has a persistent and intensive effect on the Qujiang-Shiping fault zone. The later fault zone has absorbed and transformed the southward motion of the western side of the former fault zone through dextral strike-slip/shearing as well as transverse shortening/thrusting. (2) Along the Xiaojiang fault zone, the present sinistral strike-slip/shearing rate decreases from 10 and 8 mm/a on the northern, central and central-southern segments to 4 mm/a on the southern segment. The decreased rate has been adjusted in the area along and surrounding the Qujiang-Shiping fault zone through reverse-dextral faulting and distributed shearing and shortening. (3) The tectonic-dynamic relation between the Xiaojiang fault zone and the Qujiang-Shiping fault zone is also manifested by a close correlation of earthquake occurrences on the two fault zones. From 1500 to 1850 a sequence of strong and major earthquakes occurred along the Xiaojiang fault zone and its northern neighbor, the Zemuhe fault zone, which was characterized by gradually accelerating strain release, gradually shortening intervals between M⩾7 events, and major releases occurring in the mid to later stages of the sequence. As a response to this sequence, after an 88-year delay, another sequence of 383 years (from 1588 to 1970) of strong and major earthquakes occurred on the Qujiang-Shiping fault zone, and had the same features in accelerating strain release and its temporal course. (4) Since there has been no M⩾7 event for 177 years on the Xiaojiang fault zone, the potential risk of a strong or major earthquake occurring on this fault zone in the future should be noticed and studied further.

Study on the forecast effects of PI method to the North and Southwest China

AbstractIn this paper, the codes of Pattern Informatics (PI) method put forward by Rundle et al. have been worked out according to their algorithm published, and the retrospective forecast of PI method to North China (28.0°–42.0° N, 108.0°–125.0° E) and to Southwest China (22.0°–28.3° N, 98.0°–106.0° E) has been tested. The results show that the hit rates in different regions show a great difference. In Southwest China, 32 earthquakes with ML5.0 or larger have occurred during the predicted time period 2000–2007, and 26 out of the 32 earthquakes occurred in or near the hot spots. In North China, the total number of ML5.0 or larger was 12 during the predicted time period 2000–2007, and only 3 out of the 12 earthquakes occurred in or near the hot spots. From our results, we hold that if the PI method could be applied to all kinds of regions, the parameters associated with time points and time windows should be chosen carefully to obtain the higher hit rate. We also found that the aftershocks in a strong earthquake sequence affect the PI results obviously. Copyright © 2009 John Wiley & Sons, Ltd.

Static stress triggering effects related with M s8.0 Wenchuan earthquake

In this article, firstly, we calculated and analyzed the patterns of Coulomb stress changes induced by a sequence of strong earthquakes that occurred in Songpan (松潘), Sichuan (四川) Province in 1973 and 1976, and discovered that the Ms8.0 Wenchuan (6C76;川) earthquake of 2008 was epicentered in a relevant Coulomb stress triggering zone. This suggests that the Coulomb stress on the middle and southern segments of the Longmenshan (龙门山) fault zone increased after the Songpan sequence of strong earthquakes, and the stress increment might cause the 2008 Wenchuan earthquake having already occurred somewhat ahead of time. Further, we calculated and analyzed Coulomb stress changes coinduced by both the Songpan sequence and the Ms8.0 Wenchuan mainshock. The result shows that the Ms6.4 Qingchuan (青川) earthquake of May 25, 2008 on the northeastern segment of the Longmenshan fault zone was triggered by the Wenchuan mainshock, and that the southwestern segment of the fault zone is also in the stress triggering zone. Besides, the Maoxian (茂县)-Wenchuan fault (i.e., the back-range fault of the Longmenshan fault zone), which extends parallel to the seismogenic fault of the Wenchuan earthquake, is in a shadow zone of the Coulomb stress changes, and therefore, its potential hazard for producing a strong or large earthquake in the near future could be reduced relatively.

Stress transfer and its implication for earthquake hazard on the Kunlun Fault, Tibet

The 1600-km-long Kunlun Fault striking E–W to WNW–ESE had long been recognized as one of the major left-lateral strike-slip faults bounding the Tibetan Plateau, and ranked one of the most active faults in China continent. During the past hundred years, over twenty strong earthquakes occurred along and near the Kunlun Fault, including six large earthquakes ( M > 7). Since some major highly-populated and industrialized cities are close to the Kunlun Fault, understanding of stress transfer and earthquakes migration along the Kunlun Fault is most important for assessing seismic hazard in this region. In this study, by integrating coseismic effect, viscoelastic relaxation and tectonic loading, we studied the evolution of the regional Coulomb stress field by analyzing a sequence of strong earthquakes along the Kunlun Fault. We studied the stress evolution over one century by analysing a sequence of five earthquakes ( M ≥ 7) that occurred along the Kunlun Fault since 1937. The model of dislocation sources embedded in a mixed elastic/inelastic layered half-space was used, and the layered model and relevant parameters were constrained by seismic studies. Fault rupture locations and geometry, as well as slip distribution of earthquakes were taken from field observations and seismic studies. Numerical results showed a good correlation between stress transfer, accumulation and earthquakes occurrence. All four studied earthquakes occurred after the 1937 Tuosuo Lake quake were encouraged by the preceding earthquakes with positive stress loading. In subject to the choice of the earthquake source parameter two or three out of four events occurred in regions that experienced previous coseismic and postseismic stress changes of at least 0.01 MPa, suggesting that earthquake triggering due to stress transfer has occurred along the Kunlun Fault. The total stress change since 1937 of the Kunlun Fault region has lead to high levels of stress accumulated on the Xidatan–Dongdatan segment and Maqin–Maqu segment, which have not experienced any significant large earthquake over at least several hundred or several thousand years. The accumulated stress raises the potential earthquake hazard in these areas. Our study demonstrated the crucial importance of postseismic viscoelastic relaxation in the stress transfer and accumulation following large earthquakes.

The East Aegean Sea strong earthquake sequence of October–November 2005: lessons learned for earthquake prediction from foreshocks

Abstract. The seismic sequence of October–November 2005 in the Samos area, East Aegean Sea, was studied with the aim to show how it is possible to establish criteria for (a) the rapid recognition of both the ongoing foreshock activity and the mainshock, and (b) the rapid discrimination between the foreshock and aftershock phases of activity. It has been shown that before the mainshock of 20 October 2005, foreshock activity is not recognizable in the standard earthquake catalogue. However, a detailed examination of the records in the SMG station, which is the closest to the activated area, revealed that hundreds of small shocks not listed in the standard catalogue were recorded in the time interval from 12 October 2005 up to 21 November 2005. The production of reliable relations between seismic signal duration and duration magnitude for earthquakes included in the standard catalogue, made it possible to use signal durations in SMG records and to determine duration magnitudes for 2054 small shocks not included in the standard catalogue. In this way a new catalogue with magnitude determination for 3027 events was obtained while the standard catalogue contains 1025 events. At least 55 of them occurred from 12 October 2005 up to the occurrence of the two strong foreshocks of 17 October 2005. This implies that foreshock activity developed a few days before the strong shocks of 17 October 2005 but it escaped recognition by the routine procedure of seismic analysis. The onset of the foreshock phase of activity is recognizable by the significant increase of the mean seismicity rate which increased exponentially with time. According to the least-squares approach the b-value of the magnitude-frequency relation dropped significantly during the foreshock activity with respect to the b-value prevailing in the declustered background seismicity. However, the maximum likelihood approach does not indicate such a drop of b. The b-value found for the aftershocks that followed the strong shock of 20 October 2005 is significantly higher than in foreshocks. The significant aftershock-foreshock difference in b-value is valid not only if the entire aftershock sequence is considered but also if only the segment of aftershocks that occurred within the first 24-h or the first 48-h after the mainshock of 20 October 2005 are taken into account. This difference in b-value should be examined further in other foreshock-aftershock sequences because it could be used as a diagnostic of the mainshock occurrence within a few hours after its generation.

Deflection Process for Stress Field of Jiashi Strong Earthquake Sequence

The Central Moment Tensor (CMT) solution of Jiashi strong earthquake sequence issued by Harvard University is analyzed in the paper to invert for the regional basic stress field. The systematic cluster analysis of CMT solution shows the time-sequence features in 3 periods. Further analysis on the stress field in the focal region discovers the variations taken place in the period from March 1, 1997 to Jan. 4, 2003. Since Feb. 24, 2003, the direction of basic stress field is recovered. Thus a complete process from the initial variation in the basic stress field to the final recovery of the basic stress field is finished. Combining with the force condition and tectonic background in the region, the genesis for stress field variation of strong earthquake sequence is also discussed in the paper. We suggest that Jiashi strong earthquake sequence has the characteristics of body fracture.

The 2004 sequence of triggered earthquakes off the Kii peninsula, Japan

AbstractWe examine the spatial and temporal relationships of the sequence of strong earthquakes that occurred off the Kii Peninsula, Japan, on 5 September 2004. The first event (Mj 7.1) occurred at 10:07:08 (UTC) on a northward dipping plane within the subducting Philippine Sea plate. From 10:16 to 14:47 the seismicity shows a group of earthquakes (Mj 3.2 to 4.8) 35 km to the east which are regarded as foreshocks to the second large earthquake. At 14:57:17, a Mw 6.1 strike-slip event occurred on a northwest trending plane. Some 14 seconds later, a large (Mj 7.4) thrust earthquake started 4.2 km southeast of the initial epicenter of the second earthquake. This largest earthquake is thought to have occurred on a southward dipping plane with the strike in an east-southeastly direction. Using the geometry of faults determined in this study, calculations of the Coulomb failure function show that simple static stress changes do not provide a good explanation for the triggering of the subsequent earthquakes.

Stress Changes Modelled for the Sequence of Strong Earthquakes in the South Iceland Seismic Zone Since 1706

The South Iceland seismic zone is, roughly speaking, situated between two sections of the mid-Atlantic ridge, i.e., the Reykjanes Ridge southwest of Iceland and the Eastern Volcanic Zone on the island. It is a transform zone, where earthquakes are expected to occur on E-W-trending left-lateral shear faults, equivalent to conjugate, N-S-oriented right-lateral, rupture planes. In fact, earthquakes take place on en-echelon N-S-oriented faults, which is indicated by the distribution of main shock intensities, aftershocks as well as by surface fault traces. The stress field continuously generated in the fault zone by opening of the adjacent ridges is computed and superimposed on the stress field changes induced by a series of 13 earthquakes (M ≥ 6) between 1706 and 2000. The level of the pre-seismic stress field is analysed as well as the size of the area under high stress. Finally, the post-seismic stress field of June 2000 is analysed, to see where high stresses might have accumulated. The modelling indicates that the rupture planes located on separated parallel N-S-striking zones are dense enough to lead to an area-wide stress release by the series of events. The obtained pre-seismic stress level for most events is high and stable with the exception of situations when several strong shocks occur over a time span of several days, i.e., display typical main shock-aftershock patterns. The size of areas under high stress aside from of the rupture plane, i.e., where no event occurs at the specific time, is of medium to small size.