Andaman-Sumatra island arc: II. The December 26, 2004 earthquake as one of the key episodes in seismogenic activation of the arc in the beginning of XXI century

The interpretation of the nature and parameters of the source for the earthquake that occurred in Sumatra on December 26, 2004 is suggested. Our study relies on a variety of data on the geological structure of the region, long-term seismicity, spatial distribution of the foreshocks and aftershocks, and focal mechanisms; and the pattern of shaking and tsunami, regularities in the occurrence of the earthquakes, and the genetic relationship between the seismic and geological parameters inherent in various types of seismogenic zones including island arcs. The source of the Sumatran earthquake is a steep reverse fault striking parallel to the island arc and dipping towards the ocean. The length of the fault is ～450 km, and its probable bedding depth is ～70–100 km. The magnitude of this seismic event corresponding to the length of its source is 8.9–9.0. The vertical displacement in the source probably reached 9–13 m. The fault is located near the inner boundary of the Aceh Depression between the epicenter of the earthquake and the northern tip of the depression. The strike-slip and strike-slip reverse the faults cutting the island arc form the northern and southern borders of the source. The location and source parameters in the suggested interpretation account quite well for the observed pattern of shaking and tsunami. The Aceh Depression and its environs probably also host other seismic sources in the form of large reverse faults. The Sumatran earthquake, which was the culmination of the seismogenic activation of the Andaman-Sumatra island arc in the beginning of XXI century, is a typical tsunamigenic island-arc earthquake. By its characteristics, this event is an analogue to the M _W = 9 Kamchatka earthquake of November 4, 1952. The spatial distribution of the epicenters and the focal mechanisms of the aftershocks indicate that the repeated shocks during the Sumatran event were caused by the activation of a complex system of geological structures in various parts of the island arc and Andaman Sea instead of the slips on a single rupture (a subduction thrust about 1200–1300 km in length).     

Regional focal mechanisms for earthquakes in the Aegean area

The distribution of the focal mechanisms of the shallow and intermediate depth (h>40 km) earthquakes of the Aegean and the surrounding area is discussed. The data consist of all events of the period 1963–1986 for the shallow, and 1961–1985 for the intermediate depth earthquakes, withM _s 5.5. For this purpose, all published fault plane solutions for each event have been collected, reproduced, carefully checked and if possible improved accordingly. The distribution of the focal mechanisms of the earthquakes in the Aegean declares the existence of thrust faulting following the coastline of southern Yugoslavia, Albania and western Greece extending up to the island of Cephalonia. This zone of compression is due to the collision between two continental lithospheres (Apulian-Eurasian). The subduction of the African lithosphere under the Aegean results in the occurrence of thrust faulting along the convex side of the Hellenic arc. These two zones of compression are connected via strike-slip faulting observed at the area of Cephalonia island. TheP axis along the convex side of the arc keeps approximately the same strike throughout the arc (210° NNE-SSW) and plunges with a mean angle of 24° to southwest. The broad mainland of Greece as well as western Turkey are dominated by normal faulting with theT axis striking almost NS (with a trend of 174° for Greece and 180° for western Turkey). The intermediate depth seismicity is distributed into two segments of the Benioff zone. In the shallower part of the Benioff zone, which is found directly beneath the inner slope of the sedimentary arc of the Hellenic arc, earthquakes with depths in the range 40–100 km are distributed. The dip angle of the Benioff zone in this area is found equal to 23°. This part of the Benioff zone is coupled with the seismic zone of shallow earthquakes along the arc and it is here that the greatest earthquakes have been observed (M _s 8.0). The deeper part (inner) of the Benioff zone, where the earthquakes with depths in the range 100–180 km are distributed, dips with a mean angle of 38° below the volcanic arc of southern Aegean.     

The seismic catastrophe of December 26, 2004, in the Indian Ocean as a tsunamigenic earthquake in island arc structures

An interpretation of the occurrence conditions and source parameters is proposed for the catastrophic earthquake of December 26, 2004, in the northwestern part of the Sunda island arc. The interpretation is based on the analysis of spatial distributions of aftershock epicenters and regions subjected to destructive tsunamis, seismicity manifestations in the NW part of the Sunda island arc in the past century, and locations of large tsunami sources of historical earthquakes off the Sumatra Island coast. The source parameters of the December 26, 2004, earthquake are compared with the reliably established main characteristics of sources of the largest tsunamigenic earthquakes in island arcs of the Pacific Ocean. According to the proposed interpretation, the December 26, 2004, earthquake source is a steep reverse fault striking NW and dipping toward the Indian Ocean. The source, ～450 km long, is located in front of the NW termination of Sumatra Island, in the southern part of the Nicobar Islands. Possible positions and sizes of large potential seismic sources in the NW part of the Sunda island arc are suggested.     

Deformation and stress regimes in the Hellenic subduction zone from focal Mechanisms

Fault plane solutions for earthquakes in the central Hellenic arc are analysed to determine the deformation and stress regimes in the Hellenic subduction zone in the vicinity of Crete. Fault mechanisms for earthquakes recorded by various networks or contained in global catalogues are collected. In addition, 34 fault plane solutions are determined for events recorded by our own local temporary network on central Crete in 2000–2001. The entire data set of 264 source mechanisms is examined for types of faulting and spatial clustering of mechanisms. Eight regions with significantly varying characteristic types of faulting are identified of which the upper (Aegean) plate includes four. Three regions contain interplate seismicity along the Hellenic arc from west to east and all events below are identified to occur within the subducting African lithosphere. We perform stress tensor inversion to each of the subsets in order to determine the stress field. Results indicate a uniform N-NNE direction of relative plate motion between the Ionian Sea and Rhodes resulting in orthogonal convergence in the western forearc and oblique (40–50^∘) subduction in the eastern forearc. There, the plate boundary migrates towards the SE resulting in left-lateral strike-slip faulting that extends to onshore Eastern Crete. N110^∘E trending normal faulting in the Aegean plate at this part is in accordance with this model. Along-arc extension is observed on Western Crete. Fault plane solutions for earthquakes within the dipping African lithosphere indicate that slab pull is the dominant force within the subduction process and responsible for the roll-back of the Hellenic subduction zone.     

A long-term earthquake forecast for the Kuril-Kamchatka arc for the period from September 2010 to August 2015 and the reliability of previous forecasts,as well as their applications

We describe results from the ongoing 2008–2010 work on long-term earthquake prediction for the Kuril-Kamchatka arc based on the patterns of seismic gaps and the seismic cycle. We provide a forecast for the next 5 years, September 2010 to August 2015, specified for all segments of the earthquake-generating Kuril-Kamchatka arc zone. For 20 segments we predict the phases of the seismic cycle, the normalized rate of small earthquakes (A₁₀), the magnitudes of moderate earthquakes to be expected with probabilities of 0.8, 0.5, and 0.15, the maximum possible magnitudes, and the probabilities of great (M ≥ 7.7) earthquakes. It is shown that the forecast given for the previous 5 years, from September 2005 to September 2010, was found to be accurate. We report the measures that were taken for seismic safety and retrofitting based on these forecasts.     

A long-term earthquake forecast for the Kuril-Kamchatka arc for the period from September 2011 to August 2016. The likely location,time, and evolution of the next great earthquake with M ≥ 7.7 in Kamchatka

We consider the results from the ongoing 2010–2011 work on long-term earthquake prediction for the Kuril-Kamchatka arc based on the pattern of seismic gaps and the seismic cycle. We develop a forecast for the next 5 years, from September 2011 to August 2016, for all segments of the Kuril-Kamchatka arc earthquake-generating zone. For 20 segments we predict the appropriate phases of the seismic cycle, the normalized rate of small earthquakes (A₁₀), the magnitudes of moderate earthquakes to be expected with probability 0.8, 0.5, and 0.15, and the maximum possible magnitudes and probability of occurrence for great (M ≥ 7.7) earthquakes. This study serves as another confirmation that it is entirely necessary to continue the work in seismic retrofitting in the area of Petropavlovsk-Kamchatskii.     

Characterization of the seismogenic process in the Aleutian island arc: III. Earthquakes at the western and eastern margins of the arc

Parameters of the focal mechanisms of earthquakes, as well as their relations to the characteristics of seismicity and geological structure are analyzed in the regions of the Komandorskie Islands in the west of the Aleutian arc, the Fox Islands, and the Alaska Peninsula coast in the east of the arc. Different types of ruptures are revealed in the western and eastern parts of the Aleutian arc. The leading type of ruptures at the southern slope of the Komandorskie Islands is steep reverse faults crossing the arc at azimuths from submeridional to northeastern. A similar type of rupture occurs in abundance on the Rat Islands and is predominant on the Near Islands. Steep strike-slips with small components of the normal or reverse fault manifest themselves at the northern side of the block uplift of the Komandorskie Islands. Seismogenic ruptures in the region of the Komandorskie Islands do not contradict geological data on the rupture tectonics on Medny and Bering islands. At the southern slope of the Fox Islands, as well as in the Andreanof Islands, steep reverse faults striking longitudinally (along the arc) with the dip toward the deep-sea trench are the predominant type of seismogenic ruptures. This type of seismogenic ruptures is the leading type for the structures of island arcs with present-day volcanism; an example is the Kurile-Kamchatka island. Different types of predominant seismogenic ruptures in the western and eastern parts of the Aleutian island arc probably reflect different stages of the tectonic development of these regions of the arc. Possible positions and sizes of sources of the largest historical earthquakes in the eastern part of the Aleutian island arc are considered     

Temporal change of characteristics of shallow volcano-tectonic earthquakes associated with increase in volcanic activity at Kuchinoerabujima volcano, Japan

Shallow volcano-tectonic (VT) earthquakes recorded at the Kuchinoerabujima island volcano in southwest Japan are analyzed in order to clarify the role of hydrothermal activity in the development of volcanic seismicity. From analysis of shallow VT earthquakes in 2006, two specific episodes of elevated seismicity are observed in April and November 2006. The VT earthquakes have hypocenters at depths of 0–0.4 km beneath the summit crater, and normal fault focal mechanisms with WNW–ESE extension consistent with the tensional stress field indicated by the alignment of craters and fissures. Although the hypocenters and focal mechanisms are found to be largely invariant during these episodes, the corner frequencies of the VT earthquakes underwent a pronounced increase and decrease accompanying the changes in seismicity rates. The corner frequencies increased to 20–25 Hz approximately one month prior to the onset of elevated seismicity, and then decreased to 10–15 Hz in the period of peak seismicity. The rupture length also decreased at the onset of seismicity, thereafter increasing as the seismicity continued. The peak seismicity in terms of the daily number of VT events was accompanied by inflation around the crater, suggestive of a pressure increase in the volcanic system. It is inferred that the increase in shallow VT seismicity and rupture length is related to the development of a fractured zone. The pressure increase in the volcanic system is attributed to the intrusion of hydrothermal fluids, which is supported by an observed increase in fumarolic temperature and activity. The preceding monochromatic events are thus considered to be generated by the effect of fluid-filled cracks. The shortening of rupture length is then inferred to be related to the closing of non-fluid-filled cracks in the fracture zone under the increasing pressure field, leading to a transition from monochromatic events to low-frequency and shallow VT seismicity.     

Regional centroid moment tensor solutions in Cyprus from 1977 to the present and seismotectonic implications

We analyzed digital seismic records in order to extend back in time the catalog of regional centroid moment tensors (RCMTs) for the Cyprus region. We applied the analysis and inversion methodology also used for the present-day seismicity on seismograms recorded at regional distance. We computed 18 new regional CMTs of earthquakes of moderate magnitude (4.8 ≤ M ≤ 5.5) which occurred in the Cyprus region for the time span 1977–1996. These new focal mechanisms improved the knowledge given by the previously computed solutions carried out by other institutions, as well as the dataset of available earthquake source parameters. The complete focal mechanism database contributed to better define the deformation styles in the study area and to obtain a detailed characterization of the geodynamics of the Cyprus area. New RCMTs support the hypothesis that Cyprus is located in the middle of the transition area from subduction to continental collision along the Africa–Arabian–Eurasian boundary. In particular, data confirm (a) this transition zone is strictly located west of Cyprus, probably related to a tear in the subduction system, and (b) the still active compression in the Cyprus Arc can be seen as a starting point of the continental collision eastward.     

The possibility of estimating the coseismic deformation from water level observations in wells

A method for point estimation of coseismic volume deformation from amplitudes of earthquake-induced jumps in the water level and from the tidal sensitivity of the water level is presented. Application of this method is illustrated by six Kamchatka earthquakes of 1997–2004 with M _w = 6.2–7.8 that occurred 128–316 km from the YuZ-5 well. Estimates of coseismic deformation provided by the level gage observations and by the model agree in amplitude and sign. The model reproduces an extended dislocation source in a homogeneous elastic isotropic half-space. Source rupture parameters correspond to focal mechanisms from the CMT international catalog. Consistence in experimental and theoretical estimates of the volume coseismic deformation indicates the high strain-resistance properties of YuZ-5 borehole and the possibility of quantitative estimation of seismotectonic deformation from high-precision water level measurements with time resolution not lower than 10 minutes.     

The April 9, 2001 Juan Fernández Ridge (M w 6.7) Tensional Outer-Rise Earthquake and its Aftershock Sequence

On April 9, 2001 a M _w 6.7 earthquake occurred offshore of the Chilean coast close to the intersection of the subducting Juan Fernández Ridge (JFR) and the trench near 33°S. The mainshock as well as an unprecedented number of aftershocks were recorded on regional broad-band and short-period seismic networks. We obtained a regional moment tensor solution of the mainshock that indcates a tensional focal mechanism consistent with the Harvard CMT solution. Based on waveform modeling and relocation, the depth of the mainshock was found to be 10–12 km. We relocated 142 aftershocks, which are strongly clustered and restricted to 10–30 km in depth. The seismicity distribution indicates a conjugate normal fault system extending into the lithospheric mantle that correlates with ridge-parallel fractures observed by previous seismic and bathymetric surveys. In conjunction with the historic regional distribution of outer-rise and large interplate seismicity, our results indicate that, with the exception of anomalously large thrust events, preexisting fractures associated with large bathymetric features like ridges have to exist to allow the generation of outer-rise seismicity along the Chilean margin. Hence, flexural bending and time-dependent interplate earthquakes can locally affect the nucleation of outer-rise events. The occurrence of the outer-rise seismicity in the oceanic mantle suggests the existence of lithospheric scale faults which might act as conduits to hydrate the subducting slab.Robert Fromm-Rhim passed away July 31st, 2004.     

The seismotectonics of western Skagerrak

The seismically active Skagerrak region in the border area between Denmark and Norway has traditionally been associated with uncertain earthquake locations due to the limited station coverage in the region. A new seismic station in southern Norway and a recent update of the earthquake database of the Danish National Network have led to a much more complete and homogeneous data coverage of the Skagerrak area, giving the possibility of improved earthquake locations in the region. In this study, we relocate earthquakes in the Skagerrak area to obtain a more exact picture of the seismicity and investigate well-recorded events to determine the depth distribution. Hypocenter depths are found to be generally in the range 11–25 km. Furthermore, new composite focal mechanisms are determined for clusters of events with similar waveforms. Results indicate that the Skagerrak seismicity is associated with shallow, crustal faults oriented in the NS direction south of the Sorgenfrei–Tornquist Zone (STZ) as well as with the STZ itself. Mainly reverse faulting mechanisms along NE–SW oriented faults indicate maximum horizontal compression in the NW–SE direction. This is in agreement with World Stress Map generalizations, most likely associated with ridge push forces from the mid-Atlantic ridge, though modified probably by local crustal weaknesses.     

Diurnal periodicity of earthquakes and its seasonal variations

Analysis of earthquake catalogues on 14 world regions has revealed a distinct diurnal periodicity of seismic events in all of them. The amplitude of the diurnal variations usually decreases with an increase in earthquake energy, although in some regions, the time series of strong earthquakes also demonstrate diurnal periodicity. Earthquakes are more frequent at night. The acrophase of the course of diurnal seismicity correlates with geographic longitude. The Rayleigh — Schuster hodographs of diurnal periodicity demonstrate sharp changes (kinks) in the vicinity of the equinox and solstice moments. The annual hodograph of the diurnal periodicity of earthquakes is distinctly divided by the equinox moments into segments with different slopes. The defined segments differ in the amplitude and acrophase of the course of diurnal seismicity. The data imply influence of the mutual positions of the Sun and Earth on seismicity in different world regions. Possible mechanisms responsible for such influence are discussed.     

Earthquake focal mechanisms and stress inversion in the Irpinia Region (southern Italy)

The goal of this study was to estimate the stress field acting in the Irpinia Region, an area of southern Italy that has been struck in the past by destructive earthquakes and that is now characterized by low to moderate seismicity. The dataset are records of 2,352 aftershocks following the last strong event: the 23 November 1980 earthquake (M 6.9). The earthquakes were recorded at seven seismic stations, on average, and have been located using a three-dimensional (3D) P-wave velocity model and a probabilistic, non-linear, global search technique. The use of a 3D velocity model yielded a more stable estimation of take-off angles, a crucial parameter for focal mechanism computation. The earthquake focal mechanisms were computed from the P-wave first-motion polarity data using the FPFIT algorithm. Fault plane solutions show mostly normal component faulting (pure normal fault and normal fault with a strike-slip component). Only some fault plane solutions show strike-slip and reverse faulting. The stress field is estimated using the method proposed by Michael (J Geophys Res 92:357–368, 1987a) by inverting selected focal mechanisms, and the results show that the Irpinia Region is subjected to a NE–SW extension with horizontal σ ₃ (plunge 0°, trend 230°) and subvertical σ ₁ (plunge 80°, trend 320°), in agreement with the results derived from other stress indicators.     

A model for the magmatic–hydrothermal system at Mount Rainier,Washington, from seismic and geochemical observations

 Mount Rainier is one of the most seismically active volcanoes in the Cascade Range, with an average of one to two high-frequency volcano-tectonic (or VT) earthquakes occurring directly beneath the summit in a given month. Despite this level of seismicity, little is known about its cause. The VT earthquakes occur at a steady rate in several clusters below the inferred base of the Quaternary volcanic edifice. More than half of 18 focal mechanisms determined for these events are normal, and most stress axes deviate significantly from the regional stress field. We argue that these characteristics are most consistent with earthquakes in response to processes associated with circulation of fluids and magmatic gases within and below the base of the edifice.Circulation of these fluids and gases has weakened rock and reduced effective stress to the point that gravity-induced brittle fracture, due to the weight of the overlying edifice, can occur. Results from seismic tomography and rock, water, and gas geochemistry studies support this interpretation. We combine constraints from these studies into a model for the magmatic system that includes a large volume of hot rock (temperatures greater than the brittle–ductile transition) with small pockets of melt and/or hot fluids at depths of 8–18 km below the summit. We infer that fluids and heat from this volume reach the edifice via a narrow conduit, resulting in fumarolic activity at the summit, hydrothermal alteration of the edifice, and seismicity. Received: 10 February 1999 / Accepted: 26 June 1999     

Consistent CMT solutions from Harvard University before great earthquakes in Kurile Islands and its significance for earthquake prediction

Introduction In the development of seismic science,the improvement of observational instruments is prone to produce new parameters and promote rapid growth of seismology.For example,after the global digital seismograph network is established,some studies that could not be made or easily made in the past are now being carried out successively.As a result,analytical studies based on digital data have been made one after another and digital seismology has formed.Up to now,Harvard University has …     

Mechanical features of Shanxi Datong earthquake swarm sequence and their numerical simulation by using finite element method

The seismicity characteristics of the earthquake swarm occurred in Datong area in Shanxi Province during 1989–1993 period are studied in detail. The focal mechanisms and rupture processes of the principal earthquakes in it are also estimated in this paper. A mechanical model of fracture system of asymmetrical conjugate array is suggested from the view point of fracture propagation in order to simulate the mechanical features in the seismic source respect of the Datong swarm sequence. The computation results of displacement field, stress field and strain energy distribution from this model by using finite element method are presented. In addition, some recorded data of geothermal precursors before these three climaxes of the Datong earthquake swarm sequence are discussed mainly in this paper. Contribution No. 94A0093, Institute of Geophysics, SSB, China.     

Quantitative studies of global seismicity (2)

In this paper, the seismicity indexes of global earthquakesM ≥ 6 during 1964–1983 were calculated, using data of ISC and USGS. The authors suggested a method suitable to make a set of regression formulas betweenm _bandM _s. Calculation showed that the level of global seismicity of shallow earthquakes during the years 1964–1965, 1968, 1971, 1975–1976 was higher than normal, especially the peak for the years 1975–1976 was more conspicuous. The year 1984 took the place of the year 1954 in the 20th century as the year of lowest global shallow focus seismicity. According to the actual value ofA(b) calculated, the level of deep focus seismicity reached the highest point in 1971 and dropped to the lowest point in 1977. In the time interval of 1977–1983 the global shallow focus seismicity decreased continuously whereas the deep focus seismicity increased with fluctuations.     

Maximum entropy spectral characteristics of seismic activity for great earthquakes in China

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A comprehensive analysis of Kamchatka seismicity for the period 2005–2007 using the regional catalog

We characterize the Kamchatka seismicity for the period 2005–2007. Regional catalogs of Kamchatka earthquakes were used to develop 2D distributions of parameters of background seismicity. The characteristics we consider include the activity A ₁₀, the slope of the recurrence curve γ, the parameters involved in the methods RTL, ΔS, and the “Z-function”, as well as the control of earthquake clustering. We have detected the space-time agreement between the anomalies exhibited by several parameters.