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1.
We present the seismic energy, strain energy, frequency–magnitude relation (b-value) and decay rate of aftershocks (p-value) for the aftershock sequences of the Andaman–Sumatra earthquakes of December 26, 2004 (M w 9.3) and March 28, 2005 (M w 8.7). The energy released in aftershocks of 2004 and 2005 earthquake was 0.135 and 0.365% of the energy of the respective mainshocks, while the strain release in aftershocks was 39 and 71% for the two earthquakes, respectively. The b-value and p-value indicate normal value of about 1. All these parameters are in normal range and indicate normal stress patterns and mechanical properties of the medium. Only the strain release in aftershocks was considerable. The fourth largest earthquake in this region since 2004 occurred in September 2007 off the southern coast of Island of Sumatra, generating a relatively minor tsunami as indicated by sea level gauges. The maximum wave amplitude as registered by the Padang, tide gauge, north of the earthquake epicenter was about 60 cm. TUNAMI-N2 model was used to investigate ability of the model to capture the minor tsunami and its effect on the eastern Indian Coast. A close comparison of the observed and simulated tsunami generation, propagation and wave height at tide gauge locations showed that the model was able to capture the minor tsunami phases. The directivity map shows that the maximum tsunami energy was in the southwest direction from the strike of the fault. Since the path of the tsunami for Indian coastlines is oblique, there were no impacts along the Indian coastlines except near the coast of epicentral region.  相似文献   

2.
The b-value of the Gutenberg–Richter’s frequency–magnitude relation and the p-value of the modified Omori law, which describes the decay rate of aftershock activity, were investigated for more than 500 aftershocks in the Aksehir-Afyon graben (AAG) following the 15 December 2000 Sultandagi–Aksehir and the 3 February 2002 Çay–Eber and Çobanlar earthquakes. We used the Kandilli Observatory’s catalog, which contains records of aftershocks with magnitudes ≥2.5. For the Çobanlar earthquake, the estimated b-values for three aftershock sequences are in the range 0.34 ≤  b ≤ 2.85, with the exception of the one that occurred during the first hour (4.77), while the obtained p-values are in the range 0.44 ≤ p ≤ 1.77. The aftershocks of the Sultandagi earthquake have a high p-value, indicating fast decay of the aftershock activity. A regular increase of b can be observed, with b < 1.0 after 0.208 days for the Çay–Eber earthquake. A systematic and similar increase and decrease pattern exists for the b- and p-values of the Çobanlar earthquakes during the first 5 days.  相似文献   

3.
In this paper, we report that the ratio of broadband energy (0.01?C2?Hz) to high-frequency energy (0.3?C2?Hz), E r, estimated from regional seismograms of India, might be a useful parameter in estimating tsunami potential of earthquakes in the Sumatra?CAndaman region. E r is expected to be sensitive to the depth as well as to the source characteristics of an earthquake. Since a shallow and slow earthquake has a greater tsunamigenic potential, E r may be a useful diagnostic parameter. We base our analysis on broadband seismograms of the great earthquakes of Sumatra?CAndaman (2004, M w?~?9.2) and Nias (2005, M w 8.6), 41 of their aftershocks, and the earthquakes of north Sumatra (2010, M w 7.8) and Nicobar (2010, M w 7.4) recorded at VISK, a station located on the east coast of India. In the analysis, we also included the two recent, great strike-slip earthquakes of north Sumatra (2012, M w 8.6, 8.2) recorded at VISK and three south Sumatra earthquakes (2007, M w 8.5; 2007, M w 7.9; 2010, M w 7.8) recorded at PALK, a station in Sri Lanka. We find that E r is a function of depth; shallower earthquakes have higher E r values than the deeper ones. Thus, E r may be indicative of tsunamigenic potential of an earthquake. As M w and E r increase so does the tsunami potential. In addition to the parameter E r, the radiated seismic energy, E s, may be estimated from the regional seismograms in India using empirical Green??s function technique. The technique yields reliable E s for the great Sumatra and Nias earthquakes. E r and E s computed from VISK data, along with M w and focal mechanism, may be useful in estimating tsunami potential along the east coast of India from earthquakes in the Sumatra?CAndaman region in less than ~20?min.  相似文献   

4.
The Killari earthquake of September 29, 1993 (Mw=6.2) in peninsular India triggered several aftershocks that were recorded by a network of 21 stations. We computed the change in regional static stress caused by coseismic slip on the earthquake rupture and correlated it with the aftershocks with a view to constrain some of the rupture parameters of this earthquake. We evaluated the six available estimates of fault plane solutions for this earthquake and concluded that reverse slip on a 42° dipping, N112° trending fault, which extends up to the surface from a depth of 7 km, produces maximum correlation between the increased static stress and aftershock distribution. Our analysis suggests that the majority of coseismic slip occurred on the part of the rupture that lies in the depth range of 3–6.5 km.  相似文献   

5.
We analyzed small repeating earthquakes recorded over a 13-year period and GPS data recorded over an 8-month period to estimate interplate quasi-static slip associated with the 2003 Tokachi-oki earthquake (M8.0) and the 2004 off-Kushiro earthquake (M7.1). The repeating-earthquake analysis revealed that the slip rate near the source region of the Tokachi-oki earthquake was relatively low (< 5 cm/year) prior to the earthquake; however, in the last 3 years leading up to the event, a minor acceleration in slip occurred upon the deeper extension of the coseismic slip area of the earthquake. Repeating-earthquake and GPS data indicate that large amounts of afterslip occurred around the rupture area following the earthquake; the afterslip mainly propagated to the east of the coseismic slip area. We also infer that the occurrence of the 2004 off-Kushiro earthquake, located about 100 km northeast of the epicenter of the Tokachi-oki earthquake, was advanced by the afterslip associated with the Tokachi-oki earthquake.  相似文献   

6.
The role of fluid injection on the occurrence and migration path for the aftershocks of 2004 Sumatra earthquake (Mw 9.3) and January 2005 Andaman earthquake swarm within the aftershock sequence is investigated here from the viewpoint of pore fluid diffusion process. The Sumatra earthquake created a regionally extensive crustal rupture plane exceeding 1,200 km length below the Andaman Sea. The r–t plots (Shapiro et al. 1997) are constructed for these aftershocks in order to examine the role of poroelastic effects as rupturing progressed with time. Their main results are as follows: the r–t plot corresponding to first 3 h of aftershock activity (when only 44 events of mb ≥ 4.5 originated) reveals that 95% of the data points occurred below the modelled parabola with relatively high D value of 20 m2/s, whereas a significantly low D value of 3.5 m2/s characterises the aftershock activity for the first 24 h (when 420 events of mb ≥ 4.0 occurred). Here, the Coulomb stress was transferred from the main shock with a rapid imposition of normal stress, thus inducing the pore-pressure change that started diminishing almost immediately by fluid diffusion, at a rate, defined by the diminishing D value. The modelling results for fault seismicity at far off distances from the main epicentre are interpreted here as potential indicators for large-scale sub-seabed rupturing—consequent to stress changes induced by bending of the Indian Ocean plate. Bathymetric slopes under the Andaman subduction zone are particularly amenable to sub-marine slides where crustal E–W hinge faults inferred seismically cut across the N–S trending regional thrust and strike-slip faults. Seabed rupturing appears to allow deep-slab hydration in these areas, producing pressure gradients along the normal faults. These features are important since they can herald marine geohazards in the Andaman region.  相似文献   

7.
Yaghmaei-Sabegh  Saman  Wang  Hongwei 《Natural Hazards》2022,111(3):2579-2599

The characteristics of aftershocks can be quite different from those of mainshocks. However, most of the works in the past have generally focused on the aftershocks of strong earthquakes, while some moderate earthquakes can cause aftershocks with magnitudes close to the mainshocks. This paper focuses on the aftershocks characteristics of the 2012 Varzaghan–Ahar Doublet event (Mw?=?6.4, 6.2), northwest of Iran. Numerous aftershocks with magnitudes 3.7–5.5 recorded by the local seismic stations provide useful data to study the spectral characteristics at various periods. The predictive capability of the 2008 Next Generation Attenuation along with two regional models has been examined by several aftershock records obtained during these events. This paper is the first attempt made to compare the predictive capability of modern models based on significant aftershock data obtained for these two moderate events. Results confirmed that the ground motion models which have been derived based on larger-magnitude events should not directly be extrapolated to predict aftershock ground motions with magnitude smaller than 5.5, especially when we use relationships that developed without considering aftershock data. The residual analysis showed that the model of Abrahamson and Silva (Earthq Spectra, 24(1): 67–97, 2008) performed the best overall predictions in this study. However, this model performs poorly in the short period of 0.2 s at distances greater than 50 km which underestimates the spectral values for this case.

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8.
In this work, Båth’s Law, the b-value in Gutenberg–Richter Law (G–R Law) in the form of the 1/β relationship, and both the a- and b-values in the G–R Law were introduced in order to estimate maximum aftershock magnitudes of earthquake sequences in the Taiwan region. The averaged difference of magnitude between the mainshock and the maximum aftershock is 1.20, and is consistent with Båth’s Law, however, with a large uncertainty. The large uncertainty implies that the difference may result from a variable controlled by other factors, such as the aftershocks number of an earthquake sequence and magnitude threshold for mainshock. With 1/β, since 86% of the earthquake sequences with a M ⩾ 6.0 mainshock follow this relationship, the upper bound of the maximum magnitude can be estimated for an earthquake sequence with a large mainshock. The a- and b-values in the G–R Law was also considered by evaluating maximum aftershock magnitudes. As there are low residuals between the model and the observations, the results suggest that the G–R Law is a good index for maximum aftershock magnitude determinations. In order to evaluate the temporal decays of maximum aftershock magnitudes, modified Omori’s Law was introduced. Using the approaches mentioned above, the maximum magnitudes and the temporal evolution of an earthquake sequence could be modeled. Among them, the model of the G–R Law has the best fit with observations for most of earthquake sequences. It shows its feasibility. The results of this work may benefit seismic hazards mitigation in the form of rapid re-evaluations for short-term seismic hazards immediately following devastating earthquakes.  相似文献   

9.
During the 1st decade of the 21st century, the study area of Talala, Saurashtra of western India witnessed three damaging earthquakes of moderate magnitude, year 2007 [Mw 5.0; Mw 4.8] and in the year 2011 [Mw 5.1] that generated public panic in the region. The last damaging moderate earthquake of the 20th October 2011 in Talala region (21.09°N;70.45°E), located at about 200 km south to the devastating 2001 Bhuj (23.412°N, 70.232°E) mainshock (Mw 7.6), jolted the entire Saurashtra region of Gujarat. A long series of aftershocks followed hereafter, recorded at nine seismograph/accelerograph stations. Hypocenters of aftershocks were relocated accurately using absolute and relative travel time (double-difference) method. In this study, we, for the first time, determined 3-D tomographic images of the upper crust beneath the 2011 Talala earthquake source zone by inverting about 1135 P and 1125 S wave arrival time data. Estimates of seismic velocities (Vp, Vs) and Poisson’s ratio (σ) structures offer a reliable interpretation of crustal heterogeneities and their bearing on geneses of moderate earthquakes and their aftershock sequences beneath the source zone. It is found that the 2011 Talala mainshock hypocenter depth (6 km) is located near the boundary of the low and high velocity (Vp, Vs) and the source zone is associated with low-σ anomalies guarded by the prominent high-σ anomalies along the active fault zone having strike-slip motion beneath the earthquake source zone. The pattern of distribution of (Vp, Vs, σ) and its association with occurrences of aftershocks provide seismological evidence for the neo-tectonics in the region having left lateral strike-slip motion of the fault.  相似文献   

10.
The most of shallow earthquakes are followed, just after the main shock, by increased residual seismicity known as “aftershocks” or “aftershock sequences”. Because of their disparity in time and space, aftershock sequences are more or less obvious and their productivity is spread out in time. Several studies have been regularly proposed to explain or to understand the mechanisms of the occurrence and the behaviour of these small earthquakes. In a theoretical context, many factors can induce the aftershock triggering: residual friction, subcritical crack growth, pore fluid flow etc. Just after the occurrence of the most destructive main shock of the 21 May 2003 Boumerdes (Algeria) earthquake, a wide sequence of aftershocks was recorded at different geographical locations and with various magnitudes. Based on the fact that the region of Boumerdes (40 km east of the capital Algiers) did not develop major earthquakes in the past, a geostatistical investigation of the data for this aftershock sequence is a valuable input for better seismogeological identification of this area. In the present analysis, after an overview of the geological factors in the likely occurrence of the earthquake, fundamental statistical parameters were chosen: the b value from the Gutenberg–Richter law, the p factor of the extracted respectively from the b value and the fractal variogram defined as a graphic tool to describe the continuity or the roughness of data. Jointly to the geostatistical parameters provided by the variogram like the fractal dimension. The main objective of the calculation and interpretation of these parameters is oriented towards a better understanding of the seismicity of the region of Boumerdes (Algeria) now classified as seismogenic zone.  相似文献   

11.
The recent 10 August 2009 Coco earthquake (Mw 7.5), the largest aftershock of the giant 2004 Sumatra Andaman earthquake, occurred within the subducting India plate under the Burma plate. The Coco earthquake nucleated near the northwestern edge of the 2004 Sumatra-Andaman earthquake rupture under the unruptured updip segment of the plate boundary interface. The earthquake with predominant normal motion on approximately north-south to northeast-southwest oriented plane is very similar to the 27 June 2008 Little Andaman earthquake which occurred in the South Andaman region near the trench. We provide the only available estimate of coseismic offset due to the 2009 Coco earthquake at a survey-mode GPS site in the north Andaman, located about 60 km south of the Coco earthquake epicentre. The not so large coseismic displacement of about 2 cm in the ESE direction is consistent with the earthquake focal mechanism and its magnitude. We suggest that, like the 2008 Little Andaman earthquake, this earthquake too occurred on one of the approximately north-south to northeast-southwest oriented steep planes of the obliquely subducting 90°E ridge which was reactivated in normal motion after subduction, under the favourable influence of coseismic and ongoing postseismic deformation due to the 2004 Sumatra-Andaman earthquake. Another notable feature of this earthquake is its relatively low aftershock productivity. We suggest that the earthquake occurred very close to the aseismic region of the Irrawaddy frontal arc of very low seismicity where pre-existing faults are not so critically stressed and because of which the earthquake could trigger only a few aftershocks in its immediate vicinity.  相似文献   

12.
The Andaman-Sumatra subduction zone is seismically one of the most active and complex subduction zones that produced the 26 December 2004 mega thrust earthquake (Mw 9.3) and large number of aftershocks. About 8,000 earthquakes, including more than 3,000 aftershocks (M ≥ 4.5) of the 2004 earthquake, recorded during the period 1964–2007, are relocated by the EHB method. We have analysed this large data set to map fractal correlation dimension (Dc) and frequency-magnitude relation (b-value) characteristics of the seismogenic structures of this ~3,000-km-long mega thrust subduction zone in south-east Asia. The maps revealed the seismic characteristics of the Andaman-Sumatra-Java trenches, West Andaman fault (WAF), Andaman Sea Ridge (ASR), Sumatra and Java fault systems. Prominent N–S to NW–SE to E–W trending fractal dimension contours all along the subduction zone with Dc between 0.6 and 1.4 indicate that the epicentres mostly follow linear features of the major seismogenic structures. Within these major contours, several pockets of close contours with Dc ~ 0.2 to 0.6 are identified as zones of epicentre clusters and are inferred to the fault intersections as well as asperity zones along the fault systems in the fore arc. A spatial variation in the b-value (1.2–1.5) is also observed along the subduction zone with several pockets of lower b-values (1.2–1.3). The smaller b-value zones are corroborated with lower Dc (0.5–0.9), implying a positive correlation. These zones are identified to be the zones of more stress or asperity where rupture nucleation of intermediate to strong magnitude earthquakes occurred.  相似文献   

13.
Following the December 2004 and March 2005 major shallow foci inter-plate earthquakes in the north Sumatra region, a slab-tear fault located within the subducting Indian plate ruptured across the West Sunda Trench (WST) within the marginal intra-plate region. Trend, length and movement pattern of this New Tear Fault (NTF) segment is almost identical to another such slab-tear fault mapped previously by Hamilton (1979), located around 160 km south of NTF. Seismic activity along the NTF remained quasi-stable till the end of the year 2011, when an earthquake of magnitude 7.2 occurred on 10.01.2012 just at the tip of NTF, only around ～100 km within the intra-plate domain west of WST. The NTF rupture propagated further towards SSW with the generation of two more large earthquakes on 11.04.2012. The foreshock (10.01.12; M7.2) — mainshock (11.04.12; M 8.6) — aftershock (11.04.12; M 8.2) sequence along with numerous smaller magnitude aftershocks unmistakably define the extension of NTF, a slab-tear fault that results tectonic segmentation of the convergent plate margin. Within the intra-plate domain most earthquakes display consistent left-lateral strike slip mechanism along NNE trending fault plane.  相似文献   

14.
The 2001 Bhuj earthquake (Mw 7.6) source zone is examined in the light of crack density (ε), saturation rate (ξ) and porosity parameter (ψ) using new data set derived from a large aftershock sequence recorded by the Gujarat seismic network (GSNet) during November, 2006–December, 2009. Processes of rupture initiations of the mainshock and its aftershock sequence are better understood by synthesizing the dynamic snapshots of the source zone using the new dataset. Pattern of crustal heterogeneities associated with high-ε, high-ξ and high-ψ anomalies at depths varying from 20 km to 25 km is similar to those of earlier study by Mishra and Zhao (2003). The anomalous zone is found extended distinctly by 50–60 km in the lateral direction, indicating the reinforcement of cracks and fractured volume of rock matrix due to long aftershock sequence since 2001 Bhuj earthquake in the source area. It is inferred that the presence of a fluid-filled fractured rock matrix with super saturation may have affected the structural and seismogenic strengths of the source zone and is still contributing significantly to the geneses of earthquakes in and around the source zone. Anomalous pattern of high-ε with wider distribution of high-ξ indicates the existence of micro-cracks in the lower crust, while high-ψ suggests the cementation of cracks through permeation of residual magma/metamorphic fluids into the hypocenter zone. The results suggest that the existence of residual fluids in the fractured rock matrix in the mid to lower crust might have played a key role in triggering the 2001 mainshock and is still responsible for its continued long aftershock sequences.  相似文献   

15.
We observe the spatial distributions of the magnitude of aftershocks following the six earthquakes of focal depth shallower than 20 km with magnitude more than 5.0 from 1983 to 1987 in Japan. The upper limit of the aftershock magnitude is examined as a function of the distance from mainshock hypocentre. The observed spatial distributions of the upper limit are bimodal, with a tendency of the upper limit to decrease as the distance from mainshock hypocentre increases. Moreover, we observe the correlations between the aftershock spatial distribution and earthquake fault length. We focus on the largest aftershocks in each of two aftershock sequences constituting the bimodal distribution. The distances of the two largest aftershocks from the mainshock hypocentre are equal to the fault lengths of shallow earthquakes in Japan and to the maximum earthquake fault lengths.  相似文献   

16.
The 10 June 2012 Mw 6.0 aftershock sequence in southwestern Anatolia is examined. Centroid moment tensors for 23 earthquakes with moment magnitudes (Mw) between 3.7 and 6.0 are determined by applying a waveform inversion method. The mainshock is a shallow focus strike-slip with reverse component event at a depth of 30 km. The seismic moment (Mo) of the mainshock is estimated as 1.28 × 1018 Nm and rupture duration of the Fethiye mainshock is 38 s. The focal mechanisms of the aftershocks are mainly strike-slip faulting with a reverse component. The geometry of the focal mechanisms reveals a strike-slip faulting regime with NE–SW trending direction of T-axis in the entire activated region. A stress tensor inversion of focal mechanism data is performed to obtain a more accurate picture of the Fethiye earthquake stress field. The stress tensor inversion results indicate a predominant strike-slip stress regime with a NW–SE oriented maximum horizontal compressive stress (SH). According to variance of the stress tensor inversion, to first order, the Fethiye earthquake area is characterized by a homogeneous interplate stress field. The Coulomb stress change associated with the mainshock and the largest aftershock are also investigated to evaluate any significant enhancement of stresses along the Gulf of Fethiye and surrounding region. Positive lobes with stress more than 0.4 bars are obtained, indicating that these values are large enough to increase the Coulomb stress failure towards NNW–SSE and E–W directions.  相似文献   

17.
We investigate spatial clustering of 2414 aftershocks along the Izmit Mw = 7.4 August 17, 1999 earthquake rupture zone. 25 days prior to the Düzce earthquake Mw = 7.2 (November 12, 1999), we analyze two spatial clusters, namely Sakarya (SC) and Karadere–Düzce (KDC). We determine the earthquake frequency–magnitude distribution (b-value) for both clusters. We find two high b-value zones in SC and one high b-value zone in KDC which are in agreement with large coseismic surface displacements along the Izmit rupture. The b-values are significantly lower at the eastern end of the Izmit rupture where the Düzce mainshock occurred. These low b-values at depth are correlated with low postseismic slip rate and positive Coloumb stress change along KDC. Since low b-values are hypothesized with high stress levels, we propose that at the depth of the Düzce hypocenter (12.5 km), earthquakes are triggered at higher stresses compared to shallower crustal earthquake. The decrease in b-value from the Karadere segment towards the Düzce Basin supports this low b-value high stress hypothesis at the eastern end of the Izmit rupture. Consequently, we detect three asperity regions which are correlated with high b-value zones along the Izmit rupture. According to aftershock distribution the half of the Düzce fault segment was active before the 12 November 1999 Düzce mainshock. This part is correlated with low b-values which mean high stress concentration in the Düzce Basin. This high density aftershock activity presumably helped to trigger the Düzce event (Mw = 7.2) after the Izmit Mw 7.4 mainshock.  相似文献   

18.
A damaging and widely felt moderate (Mw 5.0) earthquake occurred in the Talala region of Saurashtra, Gujarat (western India) on November 6, 2007. The highly productive sequence comprised about 1300 micro earthquakes (M > 0.5) out of which 325 of M ? 1.5 that occurred during November 6, 2007–January 10, 2008 were precisely located. The spatial aftershock distribution revealed a NE–SW striking fault in accordance with the centroid moment tensor solution, which in turn implies left-lateral motion. The orientation and sense of shear are consistent with similarly orientated geological fault identified in the area from satellite imagery and field investigation.The aftershocks temporal decay, b-value of frequency–magnitude distribution, spatial fractal dimension, D, and slip ratio (ratio of the slip occurred on the primary fault to the total slip) were examined with the purpose to identify the properties of the sequence. The high b-value (1.18 ± 0.01) may be attributed to the paucity of the larger (M ? 4.0) aftershocks and reveals crustal heterogeneity and low stress regime. The high p-value (1.10 ± 0.39), implying fast decay rate of aftershocks, evidences high surface heat flux. A value of the spatial fractal dimension (D) equal to 2.21 ± 0.02 indicates random spatial distribution and source in a two-dimensional plane that is being filled-up by fractures. A slip ratio of 0.42 reveals that more slip occurred on secondary fault systems.The static Coulomb stress changes due to the coseismic slip of the main shock, enhanced off fault aftershock occurrence. The occurrence of a moderate earthquake (Mw 4.3) on October 5, 2008 inside a region of positive Coulomb stress changes supports the postulation on aftershock triggering. When the stress changes were resolved on a cross section including the stronger (M4.8) foreshock plane that is positioned adjacent to the main fault, it became evident that the activity continued there due to stress transfer from the main rupture.  相似文献   

19.
The Van earthquake (M W 7.1, 23 October 2011) in E-Anatolia is typical representative of intraplate earthquakes. Its thrust focal character and aftershock seismicity pattern indicate the most prominent type of compound earthquakes due to its multifractal dynamic complexity and uneven compressional nature, ever seen all over Turkey. Seismicity pattern of aftershocks appears to be invariably complex in its overall characteristics of aligned clustering events. The population and distribution of the aftershock events clearly exhibit spatial variability, clustering-declustering and intermittency, consistent with multifractal scaling. The sequential growth of events during time scale shows multifractal behavior of seismicity in the focal zone. The results indicate that the extensive heterogeneity and time-dependent strength are considered to generate distinct aftershock events. These factors have structural impacts on intraplate seismicity, suggesting multifractal and unstable nature of the Van event. Multifractal seismicity is controlled by complex evolution of crustal-scale faulting, mechanical heterogeneity and seismic deformation anisotropy. Overall seismicity pattern of aftershocks provides the mechanism for strain softening process to explain the principal thrusting event in the Van earthquake. Strain localization with fault weakening controls the seismic characterization of Van earthquake and contributes to explain the anomalous occurrence of aftershocks and intraplate nature of the Van earthquake.  相似文献   

20.
Hydroacoustic data from autonomous arrays and the U.S. Navy's Sound Surveillance System (SOSUS) provide an opportunity to examine the temporal and spatial properties of seismicity along portions of the slow-spreading Mid-Atlantic Ridge (MAR), intermediate-spreading Juan de Fuca Ridge (JdFR) and fast-spreading East Pacific Rise (EPR). Aftershock and foreshock events are selected from the hydroacoustic earthquake catalog using single-link cluster (SLC) analysis, with a combined space–time metric. In the regions examined, hydroacoustic data improve the completeness level of the earthquake catalog by 1.5–2.0 orders of magnitude, allowing the decay constant, p, of the modified Omori law (MOL) to be determined for individual sequences. A non-parametric goodness-of-fit test indicates six of the seven sequences examined are described well by a MOL model. The p-values obtained for individual ridge and transform sequences using hydroacoustic data are larger than that previously estimated from the analysis of a stacked sequence generated from teleseismic data. For three sequences along the Siqueiros, Discovery and western Blanco Transforms, p-values are estimated to be 0.94–1.29. The spatial distribution of aftershocks suggests that the mainshock rupture is constrained by intra-transform spreading centers at these locations. An aftershock sequence following a 7.1Ms thrust event near the northern edge of the Easter Microplate exhibits p=1.02±0.11. Within the sequence, aftershocks are located to the north of a large topographic ridge, which may represent the surface expression of the shallow-dipping fault that ruptured during the mainshock. Two aftershock sequences near 24°25′N and 16°35′N on the MAR exhibit higher p-values, 1.74±0.23 and 2.37±1.65, although the latter estimate is not well constrained because of the small number of aftershocks. Larger p-values along the ridge crest might reflect a hotter thermal regime in this setting. Additional monitoring, however, will be needed to determine if p-value differences between the ridge and transform sequences are robust. A 1999 sequence on the Endeavour segment of the JdFR, which has been correlated with changes in the hydrothermal system, is described poorly by the MOL model. The failure of the MOL model, the anomalously large number of earthquakes within the sequence and absence of a clearly dominant mainshock are inconsistent with aftershock activity and the simple tectonic origin that has been proposed previously for this sequence.  相似文献   

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