Space-time seismicity and earthquake swarms: Certain observations along the slow-spreading mid-Indian Ocean ridges

The slow spreading mid-Indian Ocean ridge system containing the Carlsberg, Central and Southwest Indian ridges is seismically very active. In the present study, a detailed analysis has been carried out of the data of earthquake sources along different ridge segments in order to investigate the spatial and temporal clustering patterns and to evaluate crustal processes related to the swarm occurrences along these ridges. The spatial and temporal clustering pattern of the recent earthquakes (1980–1990) pertaining to nine major spreading segments and eight fracture zones suggests that the events cluster in greater proportion along the spreading segments than along the fracture zones. We performed a systematic search of earthquake catalogue during the period 1964–1990 by examining the spatio-temporal hypocentral clusters in order to identify the swarm occurrences along these ridges. The search included eighteen prominent sequences, of which, thirteen were earthquake swarms. Except two, all other swarms were found to be occurring mainly on the spreading segments. The maximum magnitude observed in these swarms is m_b = 5.4 and have many events predominantly showing normal faulting mechanisms. The spatial disposition and temporal activity of the events in swarms is much similar to the foreshock-mainshock-aftershock sequences observed along the spreading rift valley zones. These characteristics help us to support that swarms along the slow spreading mid-Indian Ocean ridges are the result of extensional tectonic activity, leading to the development of the median valley topography, a mechanism similar to that proposed by Bergman and Solomon (1990) for the Mid-Atlantic Ridge.     

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.     

Meteorological triggering of earthquake swarms at Mt. Hochstaufen, SE-Germany

A growing body of evidence suggests that fluids are intimately linked to a variety of faulting processes. Yet, the particular mechanisms through which fluids and associated parameters influence the stress regime and thus the seismicity of a particular area are not well understood.We carry out a study of the spatio-temporal behavior of earthquakes, fluid-related parameters (groundwater levels) and meteorological observables (precipitation) in the swarm earthquake area of Bad Reichenhall, southeastern Germany. The small volume in which the earthquakes take place, almost yearly occurring earthquake swarms and a permanent, seismo-meteorological monitoring network, provide nearly controlled experimental conditions to study the physics of earthquake swarms and to infer characteristic properties of the seismogenic crust.In this paper we (1) describe this fairly unique study area in terms of geology, seismicity and atmospheric conditions; (2) present two cases of earthquake swarms that seem to follow above-average rainfall events; and (3) examine the observed migration of hypocenters with a simple pore pressure diffusion model.We find significant correlation of seismicity with rainfall and groundwater level increase, and estimate an average hydraulic diffusivity of D = 0.75 ± 0.35 m²/s for Mt. Hochstaufen in 2002.     

Precursory swarm and its application for long range earthquake forecasting in Taiwan region

At trenches a few earthquake swarms of low magnitude have been observed before the medium size earthquake swarms. The first swarm was designated as precursory swarm and the second as mainshocks. Seismicity fluctuations before six such mainshocks events of medium size earthquakes of magnitudes ranging from 5.3 to 6.1 occurring in the east belt of Taiwan region have been discussed. A precursory gap between the precursory swarm and mainshock events has been observed. The duration of the gaps increases with magnitudes of the mainshocks suggesting a causal relationship between the two. Regression equations between the largest magnitude in the precursory swarms, the largest mainshock magnitude and the precusory gaps have been given.     

Weak tidal correlation of NW-Bohemia/Vogtland earthquake swarms

We analyze the possible effect of solid Earth tidal stresses upon a vertical strike-slip fault in NW-Bohemia/Vogtland, central Europe, typical by occurrence of swarm earthquakes. The horizontal components of solid Earth tidal stresses were found strongly to prevail and to reach the level of 2 kPa. We examined tidal triggering as influence of tidal stresses to launching the swarm activity in relative absence of other stress disturbances. The onset times of 46 swarms of mostly M_L < 3 earthquakes that occurred in the period 1991–2005 displayed an increased occurrence near the fortnightly maximum of tidal extensive normal stress. The statistical test however did not prove a statistically significant correlation indicating a triggering effect of fault extension due to tidal loading. We also examined tidal effects to the already running seismic activity of the prominent 2000 swarm by comparing the tidal stress distribution in the investigated period with the distribution of tidal stresses in the occurrence times of each earthquake. The results show that these distributions are almost similar, which indicates that individual earthquakes occur independent of tidal stresses. The unclear tidal correlation of the swarm seismicity may be interpreted by small amplitudes and rates of tidal stress changes compared to the amplitudes and rates of coseismic stress perturbations and of pressure bursts of deep generated fluids.     

Earthquake swarms in continental rifts — A comparison of selected cases in America, Africa and Europe

The occurrence of earthquake swarms is typically related to magmatic activity in volcanoes, yet swarms are also common in other intracontinental regions such as continental rifts. We present here a summary of geophysical observations that have been made in earthquake swarm areas of the Rio Grande, Kenya, and Eger rifts, focusing on characteristic parameters for the origin and generation of the swarm earthquakes.Our compilation of seismological parameters such as spatial distribution and focal parameters of hypocenters, magnitude statistics, and the location of the swarm centres in the rift environments reveals major similarities. The earthquake swarms take place at shallow depth between 0 and 10 km. The maximum magnitudes are mostly less than 4.5. The b-values, indicating the magnitude frequency relation of the seismicity, are about 0.8. They are hence not deviating from a normal non-volcanic intraplate environment, but are considerably lower than those of volcanic earthquake swarms. Focal mechanism studies give uniform pictures of stress field orientation and faulting style for the swarm areas. In all three rifts, the centres of swarm activity seem to be restricted to rift valley sections that may be influenced by large-scale fracture or shear zones that intersect the rifts. We conclude that these deep-reaching zones of weakness allow intrusions of upper mantle material into crustal layers, where magma-related fluids or fluctuations of the magma bodies themselves cause the generation of earthquake swarms.     

Usage of an early warning and information system Web-site for real-time seismicity in Iceland

Iceland has been subjected to destructive earthquakes and volcanic eruptions throughout history. Such events are often preceded by changes in earthquake activity over varying timescales. Although most seismicity is confined to micro-earthquakes, large earthquakes have occurred within populated regions. Following the most recent hazardous earthquakes in 2000, the Icelandic Meteorological Office (IMO) developed an early warning and information system (EWIS) Web-site for viewing near-real-time seismicity in Iceland. Here we assess Web-site usage data in relation to earthquake activity, as recorded by the South Iceland Lowland (SIL) seismic network. Between March 2005 and May 2006 the SIL seismic network recorded 12,583 earthquakes. During this period, the EWIS Web-site logged a daily median of 91 visits. The largest onshore event (M _L 4.2) struck 20 km from Reykjavík on 06 March 2006 and was followed by an immediate, upsurge in usage resulting in a total of 1,173 unique visits to the Web-site. The greatest cluster of large (≥M _L 3) events occurred 300 km offshore from Reykjavík in May 2005. Within this swarm, 9 earthquakes ≥M _L 3 were detected on 11 May 2005, resulting in the release of a media bulletin by IMO. During the swarm, and following the media bulletin, the EWIS Web-site logged 1,234 unique visits gradually throughout the day. In summary, the data reveal a spatial and temporal relationship between Web-site usage and earthquake activity. The EWIS Web-site is accessed immediately after the occurrence of a local earthquake, whereas distant, unfelt earthquakes generate gradual interest prompted by media bulletins and, possibly, other contributing factors. We conclude that the Internet is a useful tool for displaying seismic information in near-real-time, which has the capacity to help increase public awareness of natural hazards.     

The foreshock sequence of haicheng earthquake and earthquake swarm—the use of foreshock sequences in earthquake prediction

We have compared the Haicheng foreshock sequence with several earthquake swarms which occurred in its neighborhood. The spatial distribution of the earthquakes is relatively concentrated. For the most part, the events occurred within a few kilometers of each other. The focal mechanisms are comparatively stable. However, there are several swarms in which the variations of focal mechanisms are quite obvious after the occurrence of the largest event of the sequence, which would allow it to be recognized as a swarm. However, there are also swarms whose focal mechanisms are no less stable throughout the sequence compared to the Haicheng foreshock sequence. This feature could thus not be used to identify a foreshock sequence. The temporal distributions of foreshocks and swarms are quite similar in some cases. This is again not a definite criterion for identifying foreshocks, but is worthy of further study. Thus, no definite criterion for identifying foreshock sequences has been found. However, some earthquake swarms may be recognized in their later stage.Finally, we introduced a magnitude sequence with gaps which can be used to see whether a large event is still forthcoming. This method (in conjunction with other methods) could be used in areas prone to large earthquakes, immediately before a large event, to improve the probability of predicting the occurrence of a large event. We also report that the temporal distribution of all the sequences showed a 12-hour recurrence pattern that corresponded with the earth tides, indicating that tidal forces might be influencing foreshocks and earthquake swarm occurrence.     

200-m-deep earthquake swarm in Tricastin (lower Rhône Valley, France) accounts for noisy seismicity over past centuries

In the lower Rhône Valley (France), the Tricastin area was struck in 2002–2003 by an earthquake swarm with a maximum M _L-magnitude of 1.7. These shocks would have gone unnoticed if they had not occurred beneath habitations and close to the surface, some events being only 200-m deep. A several months' monitoring of the seismic activity by a 16-station mobile network showed that earthquakes clustered along a N–S-trending, at least 5-km long, shallow rupture zone, with no corresponding fault mapped in the surface. Half of the seismic events occurred in a massive, c.  250-m-thick, Lower Cretaceous limestone slab that outcrops near by. Since the late eighteenth century, several much more severe earthquake swarms have struck Tricastin. The 1772–1773 and 1933–1936 swarms were prolific and protracted, with reports of numerous detonations and even damage. Obviously, the abnormal noises that caused panic in the past centuries can be explained by the shallowness of the phenomena, a 200-m focal depth being perhaps a record value for tectonic earthquakes.     

Seismic activity in the McNaughton Lake area, Canada

Filling of McNaughton Lake, with a capacity at full load of25 · 10⁹m³ and maximum depth 191 m, was initiated on March 29, 1973. An earthquake swarm of 747 events (M_L > 0) with largest eventM_L = 4.7 occurred within 17 km of the reservoir just prior to loading. Subsequent to this, three swarms of 194, 292 and 22 events with maximumM_L = 4.1 occurred in the same region; however, no earthquakes have occurred between the reservoir and the swarm area. The level of regional seismic activity is similar to that observed prior to loading. The distribution of this activity, excluding swarm events, exhibits a spatial pattern similar to that recorded earlier by regional seismic stations, except that several events appear to be associated with the fault underlying the Rocky Mountain Trench in which the lake is formed. During a loading-unloading cycle in which the maximum water depth varied from 98 m to 171 m to 131 m, the change ofv_p was less than 2%. This indicates that no significant change in dilatancy or degree of water saturation occurred in the upper crustal layer during this cycle.     

Modelling fundamental waiting time distributions for earthquake sequences

The distribution of waiting times between time-neighbouring events for a time series obeying the Omori law is examined theoretically and numerically with the aim of understanding the characteristics of these distributions, how these characteristics change (e.g. scale) with the parameters of the Omori series, and thus how empirical waiting time data may be correctly interpreted. It is found that the waiting time distribution, for a single Omori aftershock sequence, consists in general of two power law segments followed by a rapid decay at larger waiting times. The analyses are illustrated using real data from the SIL network on Iceland. This data often shows characteristics predominantly consistent with the Omori law, but there are significant exceptions. We conclude that waiting time distributions and related statistical analysis has meaningful potential for the analysis of earthquake data sets, as a step towards developing physical models of the earthquake process.     

Seismic swarms,fluid flow and hydraulic conductivity in the forearc offshore North Costa Rica and Nicaragua

At the continental margin of north Costa Rica and Nicaragua, the strongly hydrated Cocos Plate subducts beneath the Caribbean Plate. From the downgoing Cocos plate fluids are released through extensional fractures in the overriding plate. At the seafloor, they form fluid seeps, mounds and other types of fluid expulsion. Using an offshore temporary seismic network, we investigated seismicity possibly related to these processes and observed several swarms of earthquakes located on the continental slope trenchward of the seismogenic zone of S Nicaragua. The seismicity occurred within the downgoing plate, near the plate interface and in the overriding plate. We interpret these swarm events as an expression of pore pressure propagation under critical stress conditions driven by fluid release from the downgoing plate. In order to estimate hydraulic diffusivity and permeability values, we applied a theory developed for injection test interpretation to the spatio-temporal development of the swarms. The resulting diffusivity and permeability values are in the ranges of 28–305 m²/s and 3.2 × 10^?14 m²–35.1 × 10^?14 m², respectively, applying to the continental and oceanic crust near the plate interface. These values are somewhat larger than observed in drill logs on the margin wedge off north Costa Rica, but of comparable magnitude to values estimated for the Antofagasta 1995 earthquake aftershock sequence.     

Dike swarms and related igneous complexes in the Urals

The dike swarms of the entire Urals are classified for the first time; the related igneous complexes associated with them in space and time are named. The following types and chronological levels of the Uralian dikes are distinguished (proper names are given after type localities). The epicontinental type comprises the Middle Riphean Mashak, Late Riphean Arsha-Serebryanka, Late Cambrian-Early Ordovician Kidryasovo-Lemva, Ordovician-Silurian Ushat, Devoninan Inzer-Timaiz (the most extended of all), Early Carboniferous Magnitogorsk-Mugodzhary, and Triassic Borisovo dike swarms. Many of them are probably related to plume events. The existence of the Early Riphean dike complex remains unclear. Oceanic (spreading or suprasubduction) dike-in-dike type: Ordovician Man’ya oceanic type, Devonian Aktogai backarc and Khabarny suprasubduction types. The igneous complexes associated with dike swarms are rather diverse. In addition to rhyolite dikes, in many cases determining the contrasting character of magmatism, large comagmatic gabbro and gabbro-granite intrusions are noted, as well as minor intrusions of subalkali granitoids, syenites, and, apparently, carbonatites and kimberlites. Flood basalt fields are noted at the periphery of the Urals, implying the occurrence of a feeding dike swarm beneath them.     

Use of tsunami waveforms for earthquake source study

Tsunami waveforms recorded on tide gauges, like seismic waves recorded on seismograms, can be used to study earthquake source processes. The tsunami propagation can be accurately evaluated, since bathymetry is much better known than seismic velocity structure in the Earth. Using waveform inversion techniques, we can estimate the spatial distribution of coseismic slip on the fault plane from tsunami waveforms. This method has been applied to several earthquakes around Japan. Two recent earthquakes, the 1968 Tokachi-oki and 1983 Japan Sea earthquakes, are examined for calibration purposes. Both events show nonuniform slip distributions very similar to those obtained from seismic wave analyses. The use of tsunami waveforms is more useful for the study of unusual or old earthquakes. The 1984 Torishima earthquake caused unusually large tsunamis for its earthquake size. Waveform modeling of this event shows that part of the abnormal size of this tsunami is due to the propagation effect along the shallow ridge system. For old earthquakes, many tide gauge records exist with quality comparable to modern records, while there are only a few good quality seismic records. The 1944 Tonankai and 1946 Nankaido earthquakes are examined as examples of old events, and slip distributions are obtained. Such estimates are possible only using tsunami records. Since tide-gauge records are available as far back as the 1850s, use of them will provide unique and important information on long-term global seismicity.     

http://www.sciencedirect.com/science/article/pii/S1674987111001022

This study proposes three models to explain the mechanism of the three major types of mafic dyke swarms.Parallel dyke swarms form in response to a regional stress field,e.g.the mafic dyke swarms in the...     

Geochemical characteristics and petrogenesis of four Palaeoproterozoic mafic dike swarms and associated large igneous provinces from the eastern Dharwar craton,India

Palaeoproterozoic mafic dike swarms of different ages are well exposed in the eastern Dharwar craton of India. Available U-Pb mineral ages on these dikes indicate four discrete episodes, viz. (1) ~2.37 Ga Bangalore swarm, (2) ~2.21 Ga Kunigal swarm, (3) ~2.18 Ga Mahbubnagar swarm, and (4) ~1.89 Ga Bastar-Dharwar swarm. These are mostly sub-alkaline tholeiitic suites, with ~1.89 Ga samples having a slightly higher concentration of high-field strength elements than other swarms with a similar MgO contents. Mg number (Mg#) in the four swarms suggest that the two older swarms were derived from primary mantle melts, whereas the two younger swarms were derived from slightly evolved mantle melt. Trace element petrogenetic models suggest that magmas of the ~2.37 Ga swarm were generated within the spinel stability field by ~15–20% melting of a depleted mantle source, whereas magmas of the other three swarms may have been generated within the garnet stability field with percentage of melting lowering from the ~2.21 Ga swarm (~25%), ~2.18 Ga swarm (~15–20%), to ~1.89 Ga swarm (~10–12%). These observations indicate that the melting depth increased with time for mafic dike magmas. Large igneous province (LIP) records of the eastern Dharwar craton are compared to those of similar mafic events observed from other shield areas. The Dharwar and the North Atlantic cratons were probably together at ~2.37 Ga, although such an episode is not found in any other craton. The ~2.21 Ga mafic magmatic event is reported from the Dharwar, Superior, North Atlantic, and Slave cratons, suggesting the presence of a supercontinent, ‘Superia’. It is difficult to find any match for the ~2.18 Ga mafic dikes of the eastern Dharwar craton, except in the Superior Province. The ~1.88–1.90 Ga mafic magmatic event is reported from many different blocks, and therefore may not be very useful for supercontinent reconstructions.     

From surface fault traces to a fault growth model: The Vogar Fissure Swarm of the Reykjanes Peninsula,Southwest Iceland

The Vogar Fissure Swarm is one of four en-echelon fracture swarms that connect the Reykjanes Ridge to the South Iceland Seismic Zone and the Western Volcanic Zone. Occurring in an area of flat topography, this fissure swarm is clearly visible at the surface, where it can be seen to affect recent postglacial lavas. Using remote sensing methods to identify and measure all the faults and fractures in the swarm, combined with additional field observations and measurements, we measured 478 individual fractures, 33% of them being faults and 67% being fissures. The fracture lengths show roughly log-normal distributions. Most of the individual fractures belong to 68 main composite fractures, seven of which are longer than 2500 m and correspond to the main fault scarps of the fissure swarm. We showed that these main faults are distributed along five, equally spaced zones, ∼500 m apart and a few kilometers long. We drawn 71 across-strike profiles to characterize the shape of the fault scarps, and 5 along-strike profiles to characterize the evolution of vertical throw along the main faults. Each fault consists of a coalescence of individual segments of approximately equal length. Fault throws are never larger than 10 m and are smallest at the junctions between individual segments. Analyses of along-strike throw profiles allowed us to determine the early stages of growth after coalescence. The earliest stage is characterized by an increase in the throw of the central parts of segments. This is followed by a second stage during which the throw increases at the junctions between segments, progressively erasing these small-throw zones.     

Seismicity of the Baikal rift system from regional network observations

In the paper we report the state-of-the-art of seismicity study in the Baikal rift system and the general results obtained. At present, the regional earthquake catalog for fifty years of the permanent instrumental observations consists of over 185,000 events. The spatial distribution of the epicenters, which either gather along well-delineated belts or in discrete swarms is considered in detail for different areas of the rift system. At the same time, the hypocenters are poorly constrained making it difficult to identify the fault geometry. Clustered events like aftershock sequences or earthquake swarms are typical patterns in the region; moreover, aftershocks of M ⩾ 4.7 earthquakes make up a quarter of the whole catalog. The maximum magnitude of earthquakes recorded instrumentally is M_LH7.6 for a strike-slip event in the NE part of the Baikal rift system and M_LH6.8 for a normal fault earthquake in the central part of the rift system (Lake Baikal basin). Predominant movement type is normal faulting on NE striking faults with a left lateral strike-slip component on W–E planes. In conclusion, some shortcomings of the seismic network and data processing are pointed out.     

U–Pb baddeleyite ages linking major Archean dyke swarms to volcanic-rift forming events in the Kaapvaal craton (South Africa), and a precise age for the Bushveld Complex

The Archean basement in the northeastern part of the Kaapvaal craton is intruded by a large number of mafic dykes, defining three major dyke swarms, which collectively appear to fan out from the Bushveld Complex. Herein we present U–Pb baddeleyite ages for two of these dyke swarms, the northwest trending Badplaas Dyke Swarm and the east-west trending Rykoppies Dyke Swarm, and infer their correlation with tectonic events in the Kaapvaal craton. We also present a U–Pb baddeleyite age for a noritic phase of the Marginal Zone of the Rustenburg Layered Suite (Bushveld Complex).     

Shear‐wave splitting and earthquake forecasting

Seismic shear‐wave splitting (SWS) monitors the low‐level deformation of fluid‐saturated microcracked rock. We report evidence of systematic SWS changes, recorded above small earthquakes, monitoring the accumulation of stress before earthquakes that allows the time and magnitude of impending large earthquakes to be stress‐forecast. The effects have been seen with hindsight before some 15 earthquakes ranging in magnitude from an M1.7 seismic swarm event in Iceland to the Ms7.7 Chi‐Chi Earthquake in Taiwan, including a successfully stress‐forecast of a M5.0 earthquake in SW Iceland. Characteristic increases in SWS time‐delays are observed before large earthquakes, which abruptly change to deceases shortly before the earthquake occurs. There is a linear relationship between magnitudes and logarithms of durations of both increases and decreases in SWS time‐delays before large impending earthquakes. However, suitably persistent swarms of small earthquakes are too scarce for routine stress‐forecasting. Reliable forecasting requires controlled‐source cross‐hole seismics between neighbouring boreholes in stress‐monitoring sites (SMS). It would be possible to stress‐forecast damaging earthquakes worldwide by a global network of SMS in real time.