Characteristics of ring seismicity in different depth ranges before large and great earthquakes in the Sumatra region

Characteristics of the seismicity in depth ranges 0–33 and 34–70 km before ten large and great (M _w = 7.0−9.0) earthquakes of 2000–2008 in the Sumatra region are studied, as are those in the seismic gap zones where no large earthquakes have occurred since at least 1935. Ring seismicity structures are revealed in both depth ranges. It is shown that the epicenters of the main seismic events lie, as a rule, close to regions of overlap or in close proximity to “shallow” and “deep” rings. Correlation dependences of ring sizes and threshold earthquakes magnitudes on energy of the main seismic event in the ring seismicity regions are obtained. Identification of ring structures in the seismic gap zones (in the regions of Central and South Sumatra) suggests active processes of large earthquake preparation proceed in the region. The probable magnitudes of imminent seismic events are estimated from the data on the seismicity ring sizes.     

Great earthquakes,seismicity gaps and potential for earthquake disaster along the Himalaya plate boundary

Analysis of the space-time patterns of seismicity in the Himalaya plate boundary has established the existence of three seismic gaps:

• 1.(1) The “Kashmir gap” lying west of the 1905 Kangra earthquake;
• 2.(2) the “Central gap”, situated between the 1905 Kangra and the 1934 Bihar earthquakes;
• 3.(3) the “Assam gap” between the 1897 and 1950 Assam earthquakes.

This study has shown that the above great earthquakes were preceded as well as followed by long periods (⩾ 19 years) of decreased levels of seismic activity in the epicentral regions. Remarkable decrease in the seismicity following the year 1970 has been observed in the western half of the Central gap as well as in the Assam gap. Local seismic investigation in the Assam gap confirms this feature and the seismicity suggests the existence there of an asperity.The local seismic investigations in Garhwal Himalaya have shown that the small earthquakes are confined to the upper 6–8 km of the crust and may have strike-slip motions. These earthquakes occur in a region where teleseismically recorded events were few.     

Spatial variation of seismicity parameters across India and adjoining areas

An attempt has been made to quantify the variability in the seismic activity rate across the whole of India and adjoining areas (0–45°N and 60–105°E) using earthquake database compiled from various sources. Both historical and instrumental data were compiled and the complete catalog of Indian earthquakes till 2010 has been prepared. Region-specific earthquake magnitude scaling relations correlating different magnitude scales were achieved to develop a homogenous earthquake catalog for the region in unified moment magnitude scale. The dependent events (75.3%) in the raw catalog have been removed and the effect of aftershocks on the variation of b value has been quantified. The study area was divided into 2,025 grid points (1°×1°) and the spatial variation of the seismicity across the region have been analyzed considering all the events within 300 km radius from each grid point. A significant decrease in seismic b value was seen when declustered catalog was used which illustrates that a larger proportion of dependent events in the earthquake catalog are related to lower magnitude events. A list of 203,448 earthquakes (including aftershocks and foreshocks) occurred in the region covering the period from 250 B.C. to 2010 A.D. with all available details is uploaded in the website .     

Seismic behavior in central Taiwan: Response to stress evolution following the 1999 Mw 7.6 Chi-Chi earthquake

Following the 1999 M_w 7.6 Chi-Chi earthquake, a large amount of seismicity occurred in the Nantou region of central Taiwan. Among the seismic activities, eight M_w ⩾ 5.8 earthquakes took place following the Chi-Chi earthquake, whereas only four earthquakes with comparable magnitudes took place from 1900 to 1998. Since the seismicity rate during the Chi-Chi postseismic period has never returned to the background level, such seismicity activation cannot simply be attributed to modified Omori’s Law decay. In this work, we attempted to associate seismic activities with stress evolution. Based on our work, it appears that the spatial distribution of the consequent seismicity can be associated with increasing coseismic stress. On the contrary, the stress changes imparted by the afterslip; lower crust–upper mantle viscoelastic relaxation; and sequent events resulted in a stress drop in most of the study region. Understanding seismogenic mechanisms in terms of stress evolution would be beneficial to seismic hazard mitigation.     

What triggers Koyna region earthquakes? Preliminary results from seismic tomography digital array

The cause for prolific seismicity in the Koyna region is a geological enigma. Attempts have been made to link occurrence of these earthquakes with tectonic strain as well as the nearby reservoirs. With a view to providing reliable seismological database for studying the earth structure and the earthquake process in the Koyna region, a state of the art digital seismic network was deployed for twenty months during 1996–97. We present preliminary results from this experiment covering an area of 60 × 80 km² with twenty seismic stations. Hypocentral locations of more than 400 earthquakes confined to 11×25 km² reveal fragmentation in the seismicity pattern — a NE — SW segment has a dip towards NW at approximately 45°, whilst the other two segments show a near vertical trend. These seismic segments have a close linkage with the Western Ghat escarpment and the Warna fault. Ninety per cent of the seismicity is confined within the depth range of 3–10 km. The depth distribution of earthquakes delimits the seismogenic zone with its base at 10 km indicating a transition from an unstable to stable frictional sliding regime. The lack of shallow seismicity between 0 and 3 km indicates a mature fault system with well-developed gouge zones, which inhibit shallow earthquake nucleation. Local earthquake travel time inversion for P- and S-waves show ≈ 2% higher velocity in the seismogenic crust (0–10 km) beneath the epicentral tract relative to a lower velocity (2–3%) in the adjoining region. The high P- and S-wave velocity in the seismogenic crust argues against the presence of high pressure fluid zones and suggests its possible linkage with denser lithology. The zone of high velocity has been traced to deeper depths (≈ 70 km) through teleseismic tomography. The results reveal segmented and matured seismogenic fault systems in the Koyna region where seismicity is possibly controlled by strain build up due to competent lithology in the seismic zone with a deep crustal root.     

Three-dimensional earthquake locations and upper crustal structure of the Sannio-Matese region (southern Italy)

The Sannio-Matese region is one of the most seismically active regions of Italy and has been struck by large historical earthquakes. At present, the area is characterized by low magnitude background seismicity and small seismic sequences following M4 main events. In this paper, we show Vp and Vp/Vs models and 3D locations for a complete set of earthquakes occurring in the period 1991–2001. We observe a significant crustal heterogeneity, with large scale east-verging high Vp fault-related-folds, stacked by the Pliocene compression. The relocated earthquakes cluster along a 70° east-dipping, NW-striking plane located at the border of the high Vp thrust units. Normal fault earthquakes related to the young and active extension occur within these high Vp zones, interpreted as high strength material. We expect large future earthquakes to occur within these high Vp zones actually characterized by low magnitude seismicity at their borders.     

Cyclicity of late Holocene seismicity in the Alpine-Himalayan belt

It has been shown for particular seismic zones and the Alpine-Himalayan Orogenic Belt as a whole that in addition to Fedotov cycles, the long-period hypercycles of seismicity are distinguished. Long-period variations were revealed in Syria, in southern and central segments of the El-Ghab Fault Zone of the Dead Sea Transform (EG DST), and at the southwestern end of the East Anatolian Fault Zone (EAFZ). The EG DST demonstrates a ～1800-year hypercycle with a maximum in the 3rd-7th and the 19th-20th centuries A.D. To reveal variations in seismicity in the entire central part of the orogenic belt, we have corrected evidence for historical earthquakes, taking into account the probability of missing events and the area of their regular recording domains. As a result, we displayed maximums of seismic energy release from the mid-17th to mid-20th century A.D.; from the mid-4th to the end of the 6th century; and in the 15th-13th centuries B.C. When interpreting hypercycles, it is important to keep in mind that variation of seismicity in EG DST correlates with variation of the rate of elastic deformation accumulation, probably reflecting variability of the stress-and-strain state in the region and of velocity of tectonic movements in active domains. After additional investigations, hypercycles could be taken into account for to refine the seismic hazard estimate.     

Seismotectonics and seismicity of the Silakhor region, Iran

This paper deals with seismotectonic and seismicity of the Silakhor region that shows high seismic activity in western Iran. Silakhor is a vast plain with several villages and cities of Dorud and Borujerd and a small town of Chalanchulan that were destroyed and/or damaged many times by large earthquakes. This paper addresses the historical and instrumental earthquakes and their causative faults, seismotectonic provinces and seismotectonic zones of the region. Available seismic data were normalized by means of time normalization technique that resulted in the magnitude-frequency relation for the Silakhor area and estimation of the return period of earthquakes with different magnitudes. Some active faults in this region include the Dorud fault, the main Zagros thrust, the Galehhatam fault, the Sahneh fault and others. Among them, the Dorud fault is an earthquake fault and is the cause for most of the large and intermediate earthquakes in the region. The return period of large earthquakes with magnitudes greater than 7.0 (Ms) is very low, however, the occurrence of destructive earthquakes is greater in the region than in the neighboring provinces. The study proves the high seismicity of this zone and it is required to develop an accurate national plan for future building and reinforcement of the existing buildings in this region.     

A Pragmatic Approach to Seismic Parameters in a region with Low Seismicity: The Case of Eastern Iberia

In intra-plate regions with low-seismic activity, seismic cycles last between 10³ and 10⁵ years and, consequently, quiescent faults may be capable of producing catastrophic earthquakes. Paleoseismic studies, which are necessary to define capable faults in these regions and to establish their seismic parameters, are scarce and not always possible. In order to overcome the scarcity of paleoseismic data, this paper proposes an approach to evaluate the capability and seismic parameters of faults. This approach takes into account instrumental, historical, paleoseismological and geological data. Assuming that in a given structural region with definite climatic and geological characteristics similar geomorphic features reflect a similar structural-seismic evolution, the capability of faults is established by comparing the fault related geomorphic features of the few proven seismogenic faults with those of the rest of the faults. The seismic parameters were estimated using some mathematical relationships from geological and geomorphological data, where the slip rate of the faults was determined from geological and geomorphological criteria. The case of eastern Iberia is presented to illustrate this approach. This area is characterized by low seismicity, few historical destructive earthquakes, and only one fault with good paleoseismological data. Of the 249 potential faults (longer than 10 km and adequately oriented with respect to the current stress field), 23 were regarded as capable faults. Some of these were located in seismic quiescent areas, and their main seismic parameters were estimated.     

Characterisation of the seismological pattern in a slowly deforming intraplate region: Central and western France

We analyzed the seismicity of central and western France, using historical data, a compilation of all recorded earthquakes from 1962 to 2002 (4574 events, relocated), and all published focal mechanisms (119 focal solutions). The aim is to understand what are the causes of earthquakes and stress accumulation in a slowly deforming intraplate region. The distribution of earthquakes and focal mechanisms is first correlated with recognized faults, geological structures and tomographic images. Then, in order to better understand the distribution of hypocenters and seek deeper crustal sources for stress accumulation, Euler solutions are computed from the available Bouguer anomaly data. The analysis of the obtained pattern for heat flow values, provides a better understanding of the concentration of seismicity in some particular zones.Two different behaviors of this slowly deforming intraplate region are evidenced. One is linked to the presence of a hot spot under the Massif Central, the other to reactivation of the Hercynian structural heritage. Our results highlight that several possible sources for earthquake clustering can be invoked in central–western France.     

Deep roots of seismotectonics of the Far East: The Amur River and Primorye zones

The role of the lateral structure of the lithospheric mantle in the seismotectonics and seismicity of the southern part of the Russian Far East has been investigated. The positions of the epicenters of all the major earthquakes in Sakhalin (M ≥ 6.0), as well as in the Amur region and the Primorye zones (M ≥ 5.0), are defined by the boundaries of the Anyui block of highly ferruginous mantle, which lies at the base of the Sikhote-Alin area. Three cycles of large earthquakes are recognized in the region: the end of the 19th-beginning of the 20th century, the mid-20th century, and end of the 20th-beginning of the 21st century. In the seismic zone of the Amur region (hereafter, the Amur seismic zone), the epicenters of the large earthquakes in each cycle migrate from the SW to NE along the Tan-Lu fault megasystem at a rate of 30–60 km/yr. The specific features of the seismicity of the region are explained by the repeated arrival of strain waves from the west. The waves propagate in the upper part of the mantle and provoke the activation of the deep structure of the region. The detailed analysis of the earthquakes in the Sikhote-Alin area (M ≥ 4.0) in 1973–2009 confirmed the clockwise tectonic rotation of the mantle block. The characteristics of the Primorye zone of deep-focus seismicity at the Russia-China boundary are stated. Since 1973, 13 earthquakes with M ≥ 6.0 have been recorded in the zone at a depth of 300–500 km. This number of earthquakes is at least twice as many as the number of large deep-focus earthquakes elsewhere in the Sea of Japan-Sea of Okhotsk transition zone. The unique genesis of the Primorye seismic zone is related to the additional compression in the seismofocal area due to the creeping of the Anyui mantle block onto the subduction zone during its rotation. The geodynamic implications of the seismotectonic analysis are examined, and the necessity of division of the Amur plate into three geodynamically independent lithospheric blocks is substantiated.     

Earthquake swarms of Mt Cameroon, West Africa

Historical and recent instrumental studies of the central region of the Cameroon Volcanic Line clearly indicate the occurrence of earthquake swarms of volcanic origin. Analyses of more than 3000 micro-earthquakes recorded between 1985 and 1992 show a well-defined seismic pattern characterised by single and swarm events with duration magnitudes between 2 and 3 at depths down to 20 km. On average, the earthquakes here occur at the rate of about 2 events every 3 days with occasional earthquake swarms, which greatly increase this number. The seismic swarms comprising felt earthquakes are shown to be sometimes preceded by, simultaneous with, or followed by swarms from Bimbia and Equatorial Guinea. Mapped epicentres of some of these swarms correspond to regions of volcanic gas emissions and are parallel to the fissures on Mt Cameroon and to the inferred direction of the underlying shear zone.The quiescent periods between swarms are seen to double each year since 1986. This observation was used to predict a major seismic swarm which occurred in 1993. The data coupled with historical data, are used to infer the involvement of a magma chamber in the generation of the earthquake swarms in the region. Since the installation of the network, no eruption has been observed on Mt Cameroon. This warrants more observation in order to study the seismicity that may precede, accompany or follow an eruption of the mountain.     

A reappraisal of the seismicity of the Maghreb countries — Algeria, Morocco, Tunisia

This paper reappraises the seismicity of Algeria and adjacent regions. It presents a general view of the geographical structure, the historical development of the seismological station network, some aspects of the effects of past destructive earthquakes, the state of knowledge of the seismicity and the seismic hazard and risk in the region under survey. Magnitude-frequency relationships for different parts of the region studied are presented as well as magnitude-intensity and intensity-attenuation relationships, semi-empirical formula. It also discusses the macroseismic information, the instrumental data and the social and economic implications of earthquakes in the Maghreb region.     

Deformation and displacement fields of the Himalayan arc,seismicity and large earthquake hazards in its eastern segment and vicinity

The Himalayan arc is a type of plate margin similar to an island arc and is a world-famous region of tectonic and seismic activities, where a series of large earthquakes have occurred in historical time. In this paper, the vertical deformation and horizontal displacement fields of the Himalayan arc are theoretically derived from the viewpoint of the collision between the Indian and Eurasian plates. In the light of the observed data, the seismicity, earthquake focal mechanism, seismotectonic and geomorphological features of the arc and its vicinity are reasonably explained. The characteristics of seismicity and the possibility of earthquakes with magnitude above 8 occurring in this region in the future are studied.     

Estimating magnitudes of prehistoric earthquakes and seismic capability of fault from landslide data in Noor valley (central Alborz,Iran)

Detecting the paleoseismological specifications as well as seismic capability of faults has specific importance in estimating the earthquake hazard in any region. The geomorphic indices are used as indirect procedures in the mountainous area. They are appropriate and applicable methods in recognizing the specifications of active tectonics and evaluating fault seismicity in the mountainous areas. In this regard, giant landslides can be pointed out as proper indices. These landslides are usually related to tectonics and triggered by earthquakes in many cases. In this research, giant landslides existed in Noor valley (central Alborz) have been considered as geomorphological indices for recognizing the seismicity of the region and the seismic capability of its faults. There are four giant landslides in this region (Baladeh, Razan, Vakamar, and Iva) used for the mentioned purpose. No historical earthquake has been reported around Noor valley. However, the existence of giant and old landslides, related to earthquake, indicates the occurrence of numerous prehistoric earthquakes. In this research, three different age classes have been determined (Late Holocene, Early Holocene, and Late Pleistocene) for landslides. By the way, the possibility of identifying multiple earthquakes is provided in this area. The magnitudes of earthquakes are estimated as 7.7 ± 0.49 to 7.9 ± 0.49 based on their relations with maximum volume of displaced material. Regarding the distribution of landslides and other evidences, the eastern segment of Baladeh fault has probably been the main cause of the earthquakes.     

Double-sided subduction with contrasting polarities beneath the Pamir-Hindu Kush: Evidence from focal mechanism solutions and stress field inversion

The Pamir-Hindu Kush region at the western end of the Himalayan-Tibet orogen is one of the most active regions on the globe with strong seismicity and deformation and provides a window to evaluate continental collision linked to two intra-continental subduction zones with different polarities. The seismicity and seismic tomography data show a steep northward subducting slab beneath the Hindu Kush and southward subducting slab under the Pamir. Here, we collect seismic catalogue with 3988 earthquake events to compute seismicity images and waveform data from 926 earthquake events to invert focal mechanism solutions and stress field with a view to characterize the subducting slabs under the Pamir-Hindu Kush region. Our results define two distinct seismic zones: a steep one beneath the Hindu Kush and a broad one beneath the Pamir. Deep and intermediate-depth earthquakes are mainly distributed in the Hindu Kush region which is controlled by thrust faulting, whereas the Pamir is dominated by strike-slip stress regime with shallow and intermediate-depth earthquakes. The area where the maximum principal stress axis is vertical in the southern Pamir corresponds to the location of a high-conductivity low-velocity region that contributes to the seismogenic processes in this region. We interpret the two distinct seismic zones to represent a double-sided subduction system where the Hindu Kush zone represents the northward subduction of the Indian plate, and the Pamir zone shows southward subduction of the Eurasian plate. A transition fault is inferred in the region between the Hindu Kush and the Pamir which regulates the opposing directions of motion of the Indian and Eurasian plates.     

Local seismic array observations at north Evoikos, central Greece, delineate crustal deformation between the North Aegean Trough and Corinthiakos Rift

In order to better constrain and define the microseismic activity at the north Evoikos Gulf and its surrounding area we deployed an onshore/offshore seismic array consisting of 31 three-component seismic digital stations. The array was active from 30 June to 24 October 2003, and covered an area of 2500 km². We located more than 2000 seismic events ranging from 0.7 to 4.5 M_L by using six stations as a minimum in order to define the foci parameters. Recorded seismicity delineated three major zones of deformation: from south to north, the Eretria–Parnis–eastern Corinthiakos zone, the Psachna–Viotia zone, and the Northern Sporades–North Evia–Bralos zone. Alignments of the recorded seismicity follow the tectonic trends and their orientation in the above zones. The whole area accommodates the stress field between the North Aegean Trough and the Corinthiakos Gulf. Rate of deformation intensifies from north to south, as revealed also by historical and instrumental seismicity. The successive change of orientation between the two stress fields fragments the crust in relatively small units and the fault systems developed do not permit the generation of major earthquakes in the north Evoikos area and its immediate vicinity. This is also supported by the instrumental seismicity of the last century. Larger events reported in historical times are probably overestimated.Most seismic activity is crustal. Subcrustal events were recorded mainly below the Lichades area and are interpreted as the consequence of the subduction of the Ionian oceanic lithosphere below the Hellenides. The Lichades volcano is the most northern end of the Hellenic volcanic arc.At present the highest seismic activity is associated with the Psachna region of north Evia that has been continuously active since 2001. Considering, however, the development of the seismic activity during the last decade, there has been a sequence of large events, i.e., Parnis in 1999, Skyros in 2001 and Psachna in 2001–2003. This demonstrates the fact that the tectonic deformation in all this area is intense and important for the accommodation of the stress field of the North Aegean Trough to that of the Corinthiakos Rift.