Swarm Sequence Research Articles

The 2013 and 2017 Ms 5 Seismic Swarms in Jilin, NEChina: Fluid‐Triggered Earthquakes?

AbstractUnderstanding the mechanism of moderate earthquake swarm is important for the time‐dependent hazard assessment. Five Ms ≥ 5.0 earthquakes occurred between October and November 2013 in Qianguo, Jilin, a region with low seismicity in northeast China. After 4 years, two more Ms > 5.0 earthquakes struck Ningjiang, Jilin. To investigate the swarm mechanism and the relationship between these two swarm activities, we determine the full moment tensor solutions for 12 Ms > 3.5 earthquakes, including the seven Ms ≥ 5.0 events. The focal mechanisms show that the 2013 Ms ≥ 5.0 events mostly exhibit thrust compression with significant volume closure components, whereas the 2017 Ms ≥ 5.0 earthquakes illustrate a dominated strike‐slip type. In order to explain such differences, we conduct double‐difference tomography in the study region. Our tomographic results reveal that both swarms are located in the low‐velocity zones and the fault materials exhibit structural variations along the strike, possibly indicating that the structural contrast could be critical for the variations in the focal mechanism and hypocenter migration. The faults provide pathways for the upward intrusion of fluids that control the distribution and migration of the swarm sequences. A long‐term fluid‐rock interaction could lead to the fault materials intensely fractured in the 2013 swarm area but relatively less damaged in the 2017 swarm area. We can conclude that fluids play an important role in triggering the two seismic swarms.

Theoretical analysis of the convergence property of a basic pigeon-inspired optimizer in a continuous search space

The pigeon-inspired optimization (PIO) algorithm is a newly presented swarm intelligence optimization algorithm inspired by the homing behavior of pigeons. AlthoughPIO has demonstrated effectiveness and superiority in numerous fields, particularly in practicalengineering optimization, there have been few results concerning the theoretical foundations of PIO. This paper conductsconvergence analysis of basic PIO in a continuous search space in two aspects. First, we analyze the convergence of each pigeonsexpected position using a difference equation and prove that the average position of each pigeon inthe swarm will converge to the same value. To further study the stochastic globalconvergence property of the pigeon swarm, we apply the martingale theory to investigatethe basic PIO swarm sequence, and achieve a sufficient condition to guarantee global convergenceof the basic PIO. Our theoretical analysis shows that this convergence depends upon the accumulation of the minimum probability with which the pigeon swarm jumps to the global-optimal region at each iteration. The mathematical methods proposed in this study, particularly the martingale technique, also provide a new effective approach for the theoretical analysis of bio-inspired algorithms in continuous optimization.

Heterogeneities in Stress and Strength in Tohoku and Its Relationship with Earthquake Sequences Triggered by the 2011 M9 Tohoku-Oki Earthquake

Inland Tohoku has been recognized as under the WNW-ESE compressional stress state before the 2011 M9 Tohoku-Oki earthquake. Earthquakes that occurred there were characterized by reverse faulting with the compressional axis oriented in almost the WNW-ESE direction. The Tohoku-Oki earthquake reduced this WNW-ESE compressional stress and, therefore, should have suppressed the earthquake occurrence. However, several intensive earthquake sequences were triggered in inland Tohoku. In this study, we investigated the triggering mechanism of these remote earthquake sequences in the stress shadow based on the detailed distribution of stress orientations newly determined from pre-mainshock focal mechanism data. The spatial distribution of stress orientations shows that there exist some regions with anomalous stress fields even before the Tohoku-Oki earthquake on the spatial scale of a few tens of kilometers. This spatial heterogeneity in the stress field suggests that the differential stress magnitude in inland Tohoku is low (a few tens of MPa). Locations of the earthquake clusters tend to correspond to regions where the principal stress axis orientations of the pre-mainshock period are similar to those of the static stress change by the Tohoku-Oki earthquake. This observation suggests that these earthquake sequences were triggered by a local increase in differential stress due to the static stress change. However, a few swarm sequences occurred in central Tohoku with delays ranging from a few days to few weeks after the Tohoku-Oki earthquake despite the reduction in differential stress. These sequences have notable characteristics including upward migration of hypocenters. Such features are similar to the fluid-injection induced seismicity. The source regions of these swarms are located near the ancient caldera structures and major geological boundaries. The swarm activities were probably triggered by the upward fluid movement along such pre-existing structures. These observations demonstrate that information about the temporal evolutions of both stress and frictional strength is necessary to understand the triggering mechanism of earthquakes.

Recurring Slow Slip Events and Earthquake Nucleation in the Source Region of the M 7 Ibaraki‐Oki Earthquakes Revealed by Earthquake Swarm and Foreshock Activity

AbstractSlow slip events (SSEs) on the plate interface are closely related to the occurrence of earthquakes and often trigger earthquake swarms in subduction zones. Moreover, some SSEs, accompanied by intensive foreshocks, precede large interplate earthquakes. Therefore, detecting and monitoring SSEs is important for assessing the potential of future large earthquakes. However, there are many SSEs not followed by large earthquakes, and it is unclear whether these can be distinguished from SSEs preceding large earthquakes. Here we use the epidemic‐type aftershock sequence (ETAS) model and matched‐filter technique to examine the spatial‐temporal distribution of earthquake swarms and foreshocks at Ibaraki‐Oki in the Japan Trench. We found that 19 swarm sequences repeatedly occurred during the period 1982–2008 at the same location as the foreshock sequences of the 1982 and 2008 M 7 Ibaraki‐Oki earthquakes. Both the foreshock and swarm sequences contain repeating earthquakes and have anomalously high seismicity rates inexplicable by the ETAS model, suggesting the recurrence of SSEs. The foreshock sequences in 1982 and 2008 contain more events inexplicable by the ETAS model than the swarm sequences. The fault slip of repeating earthquakes in the 2008 foreshock sequence was also larger than those of the swarm sequences, and the slip rate showed an abrupt increase 12 hr before the 2008 M 7 event. Our results imply that the SSEs that preceded the M 7 events had larger seismic moments than the other SSEs. These large SSEs might be related to the nucleation phase of the M 7 earthquakes.

Aseismic transient during the 2010\u20132014 seismic swarm: evidence for longer recurrence of M\u2009\u2265\u20096.5 earthquakes in the Pollino gap (Southern Italy)?

In actively deforming regions, crustal deformation is accommodated by earthquakes and through a variety of transient aseismic phenomena. Here, we study the 2010–2014 Pollino (Southern Italy) swarm sequence (main shock MW 5.1) located within the Pollino seismic gap, by analysing the surface deformation derived from Global Positioning System and Synthetic Aperture Radar data. Inversions of geodetic time series show that a transient slip, with the same mechanism of the main shock, started about 3–4 months before the main shock and lasted almost one year, evolving through time with acceleration phases that correlate with the rate of seismicity. The moment released by the transient slip is equivalent to MW 5.5, significantly larger than the seismic moment release revealing therefore that a significant fraction of the overall deformation is released aseismically. Our findings suggest that crustal deformation in the Pollino gap is accommodated by infrequent “large” earthquakes (MW ≥ 6.5) and by aseismic episodes releasing a significant fraction of the accrued strain. Lower strain rates, relative to the adjacent Southern Apennines, and a mixed seismic/aseismic strain release are in favour of a longer recurrence for large magnitude earthquakes in the Pollino gap.

Fundamental period elongation of a RC building during the Pollino seismic swarm sequence

A primary school in Rotonda was monitored during an on-going seismic sequence in the Pollino area, Southern Italy. The Reinforced Concrete (RC) building is a typical three story building with a concrete frame, bearing pre-cast slab flooring, concrete block internal walls and pre-cast external infill slabs. The monitoring began in September 2011 with a single station on top of the building, and after the ML=5 mainshock occurred in October 2012 a network was completed with accelerometers on each floor and real-time streaming data was transmitted to the Istituto Nazionale di Oceanografia e Geofisica Sperimentale (Udine-Northern Italy). The school suffered no visible damage during the sequence. The real-time monitoring of the Rotonda school proved to be important for two reasons: (1) the large range of magnitudes and recorded peak accelerations allowed the study of the non-stationary frequency response; (2) the results also show how a simple, real-time monitoring system using cost-effective accelerometers could be used as a tool to provide information on the damage state and usability of the school.

Injection-Driven Swarm Seismicity and Permeability Enhancement: Implications for the Dynamics of Hydrothermal Ore Systems in High Fluid-Flux, Overpressured Faulting Regimes—An Invited Paper

Many types of hydrothermal ore deposits form at overpressured conditions during high fluid flux through fault zones in the continental seismogenic regime. These include many orogenic gold deposits, some types of Fe oxide Cu-Au systems, and a variety of intrusion-related deposits, including some high-sulfidation epithermal systems. The internal structures of these deposits indicate formation while faults were active, in a regime involving episodic slip. Partial to complete sealing of fault zones by precipitation of hydrothermal minerals occurs between slip episodes. Such ore deposits occur typically within low displacement faults, or networks of such faults. Lodes seldom occupy areas much greater than 1 km 2 of fault surfaces, and net slip accumulated during ore formation is typically less than 150 m. Fluid injection experiments in low-permeability rocks, and seismicity styles in hydrothermally active settings, provide new insights about the dynamics of ore formation in high fluid-flux faults. In particular, they indicate that swarm seismicity is the characteristic response to injection of large volumes of overpressured fluids into intrinsically low-permeability rock. Injection-driven swarm seismicity and related permeability enhancement involves repeated sequences of thousands of ruptures, mostly with moment magnitude M w in the range −2 w 2 ; maximum cumulative slip is usually less than a few centimeters. Diffusion-like migration of a seismicity front away from the injection source at rates up to hundreds of meters per day is a key characteristic of injection-driven seismicity, and correlates with migration of a fluid pressure front along activated faults. By analogy with injection experiments and natural injection-driven swarm seismicity, the formation of fault-related ore deposits in overpressured, high fluid-flux regimes is interpreted to involve swarm seismicity, rather than mainshock-aftershock sequences. Recurrence intervals of swarms in contemporary injection-driven swarm sequences indicate that the total slip associated with formation of fault-hosted lodes can accumulate in periods as short as 104 to 105 years, during thousands of swarm episodes, and commonly involve >10 6 ruptures with −2 w 4 to 10 5 m 3 of fluid at injection rates of at least several tens of L.s −1 . Up to tens of kg Au are deposited per swarm. Cascades of thousands of microseismic ruptures and associated permeability enhancement, coseismic dilatancy, and fluid pressure changes during individual injection-driven swarm sequences create very dynamic hydrologic and reaction regimes. Ore formation involving transitory bursts of rapid flow, abrupt fluctuations in flow rates and fluid pressures, and possibly also fluid temperatures, is likely to promote severe chemical disequilibrium. This has implications for controls on ore deposition and grade distribution in many fault-related ore systems.

Interseismic Coupling, Megathrust Earthquakes and Seismic Swarms Along the Chilean Subduction Zone (38°–18°S)

The recent expansion of dense GPS networks over plate boundaries allows for remarkably precise mapping of interseismic coupling along active faults. The interseismic coupling coefficient is related to the ratio between slipping velocity on the fault during the interseismic period and the long-term plates velocity, but the interpretation of coupling in terms of mechanical behavior of the fault is still unclear. Here, we investigate the link between coupling and seismicity over the Chilean subduction zone that ruptured three times in the last 5 years with major earthquakes (Maule Mw 8.8 in 2010, Iquique Mw 8.1 in 2014 and Illapel Mw 8.4 in 2015). We combine recent GPS data acquired over the margin (38°–18° S) with older data to get the first nearly continuous picture of the interseismic coupling variations on the subduction interface. Here, we show that at least six low coupling zones (LCZ), areas where coupling is low relatively to the neighboring highly coupled segments can be identified. We also find that for the three most recent Mw > 8 events, coseismic asperities correlate well with highly coupled segments, while LCZs behaved as barriers and stopped the ruptures. The relation between coupling and background seismicity in the interseismic period before the events is less clear. However, we note that swarm sequences are prone to occur in intermediate coupling areas at the transition between LCZ and neighboring segments, and that the background seismicity tends to concentrate on the downdip part of the seismogenic locked zone. Thus, highly coupled segments usually exhibit low background seismicity. In this overall context, the Metropolitan segment that partly ruptured during the 2015 Illapel earthquake appears as an outlier since both coupling and background seismicity were high before the rupture, raising the issue of the remaining seismic hazard in this very densely populated area.

Rate‐Dependent Incompleteness of Earthquake Catalogs

Important information about the earthquake generation process can be gained from instrumental earthquake catalogs, but this requires complete recordings to avoid biased results. The local completeness magnitude Mc is known to depend on general conditions such as the seismographic network and the environmental noise, which generally limit the possibility of detecting small events. The detectability can be additionally reduced by an earthquake‐induced increase of the noise level leading to short‐term variations of Mc, which cannot be resolved by traditional methods relying on the analysis of the frequency–magnitude distribution. Based on simple assumptions, I propose a new method to estimate such temporal excursions of Mc solely based on the estimation of the earthquake rate resulting in a high temporal resolution of Mc. The approach is shown to be in agreement with the apparent decrease of the estimated Gutenberg–Richter b‐value in high‐activity phases of recorded data sets and the observed incompleteness periods after mainshocks. Furthermore, an algorithm to estimate temporal changes of Mc is introduced and applied to empirical aftershock and swarm sequences from California and central Europe, indicating that observed b‐value fluctuations are often related to rate‐dependent incompleteness of the earthquake catalogs.

Triggering of earthquake swarms following the 2011 Tohoku megathrust earthquake

AbstractEarthquake swarms, often interpreted to result from fluids invading the brittle seismogenic zone, have seismicity patterns that are significantly different from an aftershock sequence. Following the Mw 9.0 Tohoku‐Oki earthquake, an unusual, shallow normal‐faulting swarm sequence occurred near the Pacific coast in the southeast Tohoku district. An integrated approach combining geophysical and geochemical methods was utilized to establish the presence of aqueous fluids around the seismic source region and their derivation. Magnetotelluric inversion defined an anomalous conductor with a width of 20 km and clearly visible to depths of more than 20 km, extending to the base of the crust. Independent geophysical observations, including seismic, strongly support the suggestion that fluid‐filled porous materials and fluids associated with slab dehydration are present in the convergent plate boundary. In order to provide geochemical constraints on the source of the fluids triggering the swarm activity, new helium isotope data were acquired from gas and water samples around the seismic source region. The observed 3He/4He ratios in these samples are significantly lower than the atmospheric value of 1.4 × 10−6, indicating that the mantle helium contribution is less than 10% of the total helium. Assuming the fluid‐triggered swarm activity, the plausible explanations for the generation of fluids are limited to the following: (1) sediment porosity reduction and from smectite‐illite and opal‐quartz reactions in the subducting deep sea sediments, (2) metamorphism of fore‐arc basin sediments, sedimentary, and/or volcanic rocks detached from the plate, or (3) dehydration reactions in the subducted oceanic crust and/or hydrated mantle below the fore‐arc mantle wedge. Geophysical and geochemical findings suggest metamorphic fluids produced by isothermal decompression of altered sediments accompanying uplift and exhumation. Owing to continued fluid production at depths of ~20 km, the fluids migrate into the seismic source region. The swarm sequence would have been triggered by stress changes associated with the Tohoku‐Oki earthquake, enhanced by vertical metamorphic fluid expulsion from the reaction zone.

Earthquake swarms near eastern Himalayan Syntaxis along Jiali Fault in Tibet: A seismotectonic appraisal

The seismotectonic characteristics of ten repeated earthquake swarm sequence within a seismic cluster along Jiali Fault in eastern Himalayan Syntaxis (EHS) have been analysed. The swarms are spatially disposed in and around Yigong Lake (a natural lake formed by blocking of Yigong River by landslide) and are characterized by low magnitude, crustal events with low to moderate b values. Ms : mb discriminant functions though indicate anomalous nature of the earthquakes within swarm but are considered as natural events that occurred under condition of high apparent stress and stress gradients. Composite fault plane solutions of selected swarms indicate strike–slip sense of shear on fault planes; solution parameters show low plunging compression and tensional axes along NW–SE and NE–SW respectively with causative fault plane oriented ENE–WSW, dipping steeply towards south or north. The fault plane is in excellent agreement with the disposition and tectonic movement registered by right lateral Jiali Fault. The process of pore pressure perturbation and resultant ‘r–t plot’ with modelled diffusivity (D = 0.12 m2/s) relates the diffusion of pore pressure to seismic sequence in a fractured poro-elastic fluid saturated medium at average crustal depth of 15–20 km. The low diffusivity depicts a highly fractured interconnected medium that is generated due to high stress activity near the eastern syntaxial bent of Himalaya. It is proposed that hydro fracturing with respect to periodic pore pressure variations is responsible for generation of swarms in the region. The fluid pressure generated due to shearing and infiltrations of surface water within dilated seismogenic fault (Jiali Fault) are causative factors.

The 3D seismotectonic flow of geological masses in the lithosphere of the Baikal Rift Zone

This study of displacements in the lithosphere of the Baikal Rift Zone (BRZ), which was based on data on dynamic earthquake-source parameters for events with energy class KP ≥ 7 for the 1968–1994 period revealed the most conspicuous patterns in the 3D-seismotectonic flow of geological masses in the region. It is shown that the main contribution to the total absolute rock displacement is due to numerous small earthquakes, while the greatest displacements of geological material occur in the zones of long-continued aftershock and swarm sequences. About half of the entire seismotectonic displacements occurred in the northeastern flank of the BRZ, while the total displacements of geological material due to earthquakes that occurred in the southwest and in the middle of the region are approximately equal. When small shocks with energy class KP = 7 are considered, it appears that the total relative displacements of the geological medium are largely due to normal faulting events, while when the KP = 10 earthquakes are in question, the total displacements due to normal mechanisms are considerably below those due to reverse and strike-slip movements. The resulting images of 3D seismotectonic flow of geological masses in the BRZ lithosphere indicate good agreement between negative vertical displacements and rift basins, while the positive movements of geological material are coincident with inter-basin mountain junctions. These local features in the seismotectonic flow of geological material reflect patterns in the 3D inhomogeneous deformation of the BRZ lithosphere and are due to stress rearrangements within the lithosphere of the region among three rifting structures, with compression and tension areas alternating.

Detection of a hidden Boso slow slip event immediately after the 2011 Mw 9.0 Tohoku‐Oki earthquake, Japan

AbstractUtilizing a cross‐correlation detector technique, we discovered an increase in swarm‐like seismicity within the source area of the Boso slow slip events (SSEs) immediately after the 2011 Tohoku‐Oki earthquake. The epicentral distribution of the detected seismicity was similar to that of previously recognized Boso SSEs. In addition, small repeating earthquakes were identified within this seismic swarm sequence. These seismic observations indicate that a hidden SSE occurred along the top surface of the subducting Philippine Sea plate immediately after the Tohoku‐Oki earthquake. We propose that external stress transfer by the coseismic slip of the Tohoku‐Oki earthquake and the following afterslip could have led to the occurrence of the newly detected Boso SSE. In contrast to previous work, we demonstrate that the recurrence interval of the Boso SSEs has not shortened over time but has shown a more complex evolution as a result of external stress perturbations imposed by the Tohoku‐Oki earthquake.

Fault reactivation due to theM7.6 Nicoya earthquake at the Turrialba‐Irazú volcanic complex, Costa Rica: Effects of dynamic stress triggering

AbstractTheM7.6 Nicoya earthquake struck at the interface between the Cocos plate and the Caribbean plate on 5 September 2012 inducing a ground acceleration of 0.5 m s−2at the Irazú‐Turrialba volcanic complex. We use data from six seismic stations deployed around and atop the Irazú‐Turrialba volcanic complex to show the increase of local seismic activity after theM7.6 Nicoya earthquake. The response consists in more than 300 locatable earthquakes occurring in swarm sequences along a fault system that intersects the Irazú‐Turrialba volcanic complex. In addition, we point out that major aftershocks are followed by increases of seismic activity in the same region. The weak static stress variation imposed by the main slip of the Nicoya earthquake at the Irazú‐Turrialba volcanic complex suggests a dynamic triggering mechanism. We expand this concept suggesting that this behavior may be similar to the one observed in the Chilean and Japanese volcanic arcs during theM8.8 2010 Maule, Chile, andM9.0 2011 Tohoku, Japan, earthquakes. Finally, we highlight that the combined action of dynamic stress and short‐lived coseismic relaxation may trigger seismic activity in geological systems in near‐critical conditions.

Seismicity observed under Snæfellsjökull volcano

The Snæfellsnes peninsula in western Iceland is characterized by Pleistocene volcanism dominated by alkalic magmatism across an extinct (>6 Ma) axial rift zone of the Mid-Atlantic Ridge. Although Iceland has an extensive seismic coverage, no systematic seismic monitoring is in place on the Snæfellsnes peninsula. In this paper, we present results of a three months field campaign in the Snæfellsnes peninsula and show for the first time that the region is (micro) seismically active in the depth range of 8–15 km. We identified and located a total of 29 seismic events that occurred in close proximity of the Snæfellsjökull volcano, with most events clustering beneath the volcano in swarm sequences. We propose that seismicity is associated with fluid-induced fracturing related to one or more magmatic reservoir(s), which may be elucidated in future seismic campaigns. Our observations are the first step towards the understanding of the plumbing system of the Snæfellsjökull volcano.

Termination of a 6 year ridge‐spreading event observed using a seafloor seismic network on the Endeavour Segment, Juan de Fuca Ridge

AbstractWe present automatically determined epicenters and magnitudes for 36,523 earthquakes recorded along the Endeavour segment between August 2003 and October 2006 using a local ocean‐bottom seismometer (OBS) network. The catalog is dominated by two swarm sequences in January and February 2005 in the vicinity of the Endeavour overlapping spreading center, which included earthquakes in West Valley, the northern portion of the Endeavour segment, southwest Endeavour Valley, and the Endeavour vent fields. These swarms are attributed to volcanism including a dike intrusion on the northern Endeavour in February 2005 and smaller diking events on the propagating tip of the West Valley segment in both swarms. The dike on the northern Endeavour propagated to the south, which is inconsistent with magma sourced from the axial magma chamber beneath the elevated central portion of the segment. Following the swarms, seismic activity on the Endeavour segment decreased on average to ~15% of pre‐swarm values and almost ceased at the segment ends. We infer that a 6 year non‐eruptive event that started with a swarm in 1999 and finished with the 2005 swarms ruptured the entire segment and relieved plate‐spreading stresses. The inferred coupling between the 1999 and 2005 events, the observation of extensive precursory activity prior to the 2005 swarms, and the interaction between seismically active regions during the swarms is consistent with static triggering with delays influenced by viscoelastic relaxation, hydraulic diffusion, and magma withdrawal and replenishment.

Identification of earthquake aftershock and swarm sequences in the Baikal rift zone

The catalog of earthquakes (КR³6.6) which occurred in the Baikal rift zone (BRZ) was declastered, and the results are presented in the article. Aftershocks of seismic events (КR³12.5) were determined by the software developed by V.B. Smirnov (Lomonosov Moscow State University) with application of the algorithm co-authored by G.M. Molchan and O.E. Dmitrieva. To ensure proper control of the software application, aftershocks were also selected manually. The results of declustering show that aftershocks of the earthquakes (КR³12.5) account for about 25 per cent of all seismic events in the regional catalog. Aftershocks accompanied 90 per cent of all the earthquakes considered as main shocks. Besides, earthquake swarms, including events with КR³11, were identified. The results of this study show that, in the BRZ, the swarms and strong events with aftershocks are not spatially separated, and this conclusion differs from the views of the previous studies that reviewed data from a shorter observation period. Moreover, it is noted that the swarms may consist of several main shocks accompanied by aftershocks. The data accumulated over the last fifty years of instrumental observations support the conclusion made earlier that the swarms in BRZ occur mainly in the north-eastward direction from Lake Baikal and also confirm the trend of a small number of aftershocks accompanying earthquakes in the south-western part of the Baikal rift zone.

Detecting Earthquakes around Salton Sea Following the 2010 Mw7.2 El Mayor-Cucapah Earthquake Using GPU Parallel Computing

The recently developed matched filter technique is effective in detecting earthquakes during intensive aftershock or swarm sequences. However, currently our detection code can only process on single-CPU computers, which takes a long time to perform the cross-correlation between continuous seismic data and template events. In this paper, we present a GPU-based computation method to significantly accelerate the detection algorithm. By dividing the procedure into several routines and processing them in parallel, we achieve ∼40 times speedup for one Nvidia GPU card compared to sequential CPU code. We apply the paralleled code to search around the Salton Sea geothermal field for missing earthquakes in a 90-day time window around the occurrence time of the 2010 Mw7.2 El Mayor-Cucapah earthquake. We obtain ∼70 times more earthquakes than listed in the official Southern California Seismic Network catalog. These newly detected events could be used to help to better understand how earthquakes are triggered in the immediate vicinity of a mainshock rupture.

Detection of the seismic quiescence along the seismic active faults in Kalabsha area, west of Lake Nasser, Aswan, Egypt

The spatial and temporal distribution of seismicity around the northern part of Lake Nasser in Aswan has been continuously monitored by a short-period seismograph network since 1982. Data of this network, which consists of 13 field stations distributed around the Kalabsha fault, has demonstrated occurrences of three swarm sequences during the past 26 years. The first swarm occurred in June 1987 on a fault segment about 8 km in length beneath the lake water. The largest magnitude of this swarm was 3.7. Its hypocenters were located within the shallow depth’s interval (i.e., 2 to 5 km from the ground surface). The estimated b-value for this swarm was 0.63. The second swarm sequence took place at the intersection of the Seiyal fault with Kurkur fault about 10 km to the north of the June 1987 sequence. This swarm occurred during the period from August to December 2004 with largest magnitude of 4.1. The focal depths of this swarm also ranged from 2 to 5 km. The b-value of this swarm was found to be 0.69. The third swarm occurred recently in April 2007 with largest magnitude of 4.2 at the same location of 1987 sequence but its focal depths ranged from 6 to 8 km and has the b-value of 0.58. The composite focal mechanism study of these three swarm sequences revealed right-lateral strike slip faults with horizontal stress axes pattern. The relationship between the lake water level and the swarm sequences showed that both 1987 swarm and 2007 swarm occurred during the decreasing of water level, whereas the 2004 swarm occurred during the increasing of the water level. The study of these three swarm sequences illustrates an interesting phenomena, that there is a seismic quiescence preceding each swarm sequence. The duration period of the seismic quiescence ranges from 3 to 5 months.

Earthquake Monitoring in Southern California for Seventy-Seven Years (1932-2008)

The Southern California Seismic Network (SCSN) has produced the SCSN earthquake catalog from 1932 to the present, a period of more than 77 yrs. This catalog consists of phase picks, hypocenters, and magnitudes. We present the history of the SCSN and the evolution of the catalog, to facilitate user understanding of its limitations and strengths. Hypocenters and magnitudes have improved in quality with time, as the number of stations has increased gradually from 7 to ~400 and the data acquisition and measuring procedures have become more sophisticated. The magnitude of completeness (M_c) of the network has improved from M_c ~3.25 in the early years to M_c ~1.8 at present, or better in the most densely instrumented areas. Mainshock–aftershock and swarm sequences and scattered individual background earthquakes characterize the seismicity of more than 470,000 events. The earthquake frequency-size distribution has an average b-value of ~1.0, with M≥6.0 events occurring approximately every 3 yrs. The three largest earthquakes recorded were 1952 M_w 7.5 Kern County, 1992 M_w 7.3 Landers, and 1999 M_w 7.1 Hector Mine sequences, and the three most damaging earthquakes were the 1933 M_w 6.4 Long Beach, 1971 M_w 6.7 San Fernando, and 1994 M_w 6.7 Northridge earthquakes. All of these events ruptured slow-slipping faults, located away from the main plate boundary fault, the San Andreas fault. Their aftershock sequences constitute about a third of the events in the catalog. The fast slipping southern San Andreas fault is relatively quiet at the microseismic level and has not had an M>6 earthquake since 1932. In contrast, the slower San Jacinto fault has the highest level of seismicity, including several M>6 events. Thus, the spatial and temporal seismicity patterns exhibit a complex relationship with the plate tectonic crustal deformation.