Complex Rupture Process Research Articles

Slip history of the 2016 Mw 7.0 Kumamoto earthquake: Intraplate rupture in complex tectonic environment

AbstractRupture history of the 2016 Mw 7.0 Kumamoto earthquake is constrained by using the waveforms of strong motion observations, teleseismic broadband body waves, and long‐period surface waves. Its fault geometry is modeled with Hinagu (orienting 205° and dipping 73°) and Futagawa (orienting 235° and dipping 60°), two segments. The result reconciles the difference between moment tensor solutions and the surface fault trace. It reveals a complex rupture process that initiated on the Hinagu segment in dextral motion, propagated northeastward unilaterally, and after 15 s ceased near Aso volcano with normal fault motion. The average slip, rise time, and slip rate are 1.8 m, 2.0 s, and 1.2 m/s, respectively. The rupture broke through an ~30° fault intersection without notable delay, which can be a result of dynamic “unclamping.” The northeast boundary of the largest asperity might mark the bottom of the seismogenic zone, which becomes shallower gradually near Aso volcano.

Diversity of the 2014 Iquique’s foreshocks and aftershocks: clues about the complex rupture process of a Mw 8.1 earthquake

We study the foreshocks and aftershocks of the 1 April 2014 Iquique earthquake of Mw 8.1. Most of these events were recorded by a large digital seismic network that included the Northern Chile permanent network and up to 26 temporary broadband digital stations. We relocated and computed moment tensors for 151 events of magnitude Mw ≥ 4.5. Most of the foreshocks and aftershocks of the Iquique earthquake are distributed to the southwest of the rupture zone. These events are located in a band of about 50 km from the trench, an area where few earthquakes occur elsewhere in Chile. Another important group of aftershocks is located above the plate interface, similar to those observed during the foreshock sequence. The depths of these events were constrained by regional moment tensor (RMT) solutions obtained using the records of the dense broad band network. The majority of the foreshocks and aftershocks were associated to the interplate contact, with dip and strike angles in good agreement with the characteristics of horst and graben structures (>2000 m offset) typical of the oceanic Nazca Plate at the trench and in the outer rise region. We propose that the spatial distribution of foreshocks and aftershocks, and its seismological characteristics were strongly controlled by the rheological and tectonics conditions of the extreme erosive margin of Northern Chile.

Complex Rupture Process of the 19 March 2013, Rudna Mine (Poland) Induced Seismic Event and Collapse in the Light of Local and Regional Moment Tensor Inversion

On 19 March 2013, a strong, shallow, induced seismic event struck a mining panel in the room‐and‐pillar Rudna copper mine in southeastern Poland. The event caused important damage at the mining tunnel and trapped 19 miners, who were safely rescued a few hours later. Although mining‐induced seismicity is frequent at this mine, the 19 March event was unusual because of its larger magnitude, its occurrence far from the mining stopes, and because it was accompanied by a strong hazardous rockburst. The mining inspections following the event verified the occurrence of a rockfall with tunnel floor uplift but also recognized the presence of a faulting structure at the hypocentral location. The availability of three monitoring networks (including local and regional data, short‐period and broadband seismometers, and surface and in‐mine installation) presented an optimal setup to determine rupture parameters and to compare the performance and results from different installations. We performed waveform and spectral‐based analysis to infer source properties, with a particular interest to the determination of the rupture processes, using different moment tensor (MT) inversion techniques. Our results are surprisingly different, ranging from a dominant thrust mechanism, resolved at closest distances, to a collapse‐type rupture, resolved at regional distances. We demonstrate that a complex rupture model is needed to explain all observations and to justify these discrepancies. The final scenario indicates that the rupture nucleated as a weaker thrust mechanism along a pre‐existing weakened surface and then continued in a more energetic collapse event. The local LUMINEOS surface network has the potential to resolve both subevents but not using a standard MT decomposition. Here, we propose a new MT decomposition and an alternative MT fitting procedure that can be used to analyze the MT of collapse sources.

Rupture process for micro-earthquakes inferred from borehole seismic recordings

We investigate the spatial extent of rupture and variability in fault slip for micro-earthquakes by inverting seismic moment rate functions derived from empirical Green’s function deconvolution. By using waveforms from an array of borehole seismometers, we determine the spatial distributions of fault slip for M 3+ earthquakes that occurred along the Hayward fault in central California and identify a variety of slip behaviors including subevents, directivity, and high stress drop. The 2013 M w 3.2 Orinda earthquake exhibits a complex rupture process involving two subevents with northwest and up-dip directivity. The two subevents release 43 and 18 % of the total seismic moment (6.7 × 1013 N m), and their inferred peak stress drops are 18 and 8 MPa. The 2011 M w 4.0 Berkeley and 2012 M w 4.0 El Cerrito earthquakes are marked by high stress drop. The inferred peak and mean stress drops are about 100–130 and 40 MPa, respectively, which suggests that there are locally high levels of fault strength on the Hayward fault. Our finite-source modeling suggests that the radiation efficiency determined for these two earthquakes is very low (<0.1) and implies that most energy is dissipated during the earthquake rupture process.

Modeling Earthquakes Using Fractal Circular Patch Models with Lessons from the 2011 Tohoku-Oki Earthquake

Earthquakes occur in a complex hierarchical fault system, meaning that a realistic mechanically-consistent model is required to describe heterogeneity simply and over a wide scale. We developed a simple conceptual mechanical model using fractal circular patches associated with fracture energy on a fault plane. This model explains the complexity and scaling relation in the dynamic rupture process. We also show that such a fractal patch model is useful in simulating longterm seismicity in a hierarchal fault system by using external loading. In these studies, an earthquake of any magnitude appears as a completely random cascade growing from a small patch to larger patches. This model is thus potentially useful as a benchmarking scenario for evaluating probabilistic gain in probabilistic earthquake forecasts. The model is applied to the real case of the 2011 Tohoku-Oki earthquake based on prior information from a seismicity catalog to reproduce the complex rupture process of this very large earthquake and its resulting ground motion. Provided that a high-quality seismicity catalog is available for other regions, similar approach using this conceptual model may provide scenarios for other potential large earthquakes.

2011년 토호쿠 대지진의 수중음향 관측

2011년 3월 11일에 일본 토호쿠 앞바다에서 규모 9.0의 대지진이 발생하였고 이는 전 세계 지진관측망뿐 아니라 IMS(International Monitoring System)에서 운영하는 하와이 수중음향 관측망에도 기록되었다. 우리는 이 대지진을 비롯한 두개의 규모 7 이상의 여진의 수중음향 자료에 대하여 방위각 및 스펙트럼 분석을 통해 복잡한 지진원에서 발생한 T 파의 특성을 파악하고자 연구를 수행하였다. 이를 통해 복잡한 단층 파열 현상이 직접적으로 T 파의 형태에 영향을 미치는 것을 관측하였고, 빠른 단층 파열 과정에 의하여 발생한 T 파가 중첩되는 구간에서 큰 에너지가 기록된 것을 확인할 수 있었다. 두 여진에서 발생한 T 파를 비교한 결과 단층 종류와 깊이에 의해 T 파의 형태와 주파수 특성이 변하는 것을 확인하였다. The <TEX>$M_W$</TEX> 9.0 thrust-fault earthquake has occurred in the Pacific coast of Tohoku, Japan, on March 11, 2011. We present the detection of the great earthquake and analyze T-waves associated with the main event and two other big aftershocks (<TEX>$M_W$</TEX> > 7) recorded in a hydroacoustic array (H11N) in the Pacific Ocean by performing array and spectral analysis to examine characteristics of T-waves generated from the big events. The complex rupture process of the main event directly influences on the shape of the T-waves, and the peak locates on where T-waves excited from fast rupturing process arrive. We compare the two aftershocks with different fault type and show that the fault type and the source depth change shape and spectral contents of T-waves.

Nonplanar Fault Source Modeling of the 2008Mw 6.9 Iwate–Miyagi Inland Earthquake in Northeast Japan

We use crustal deformation data sets acquired by the Phased Array‐Type L ‐Band Synthetic Aperture Radar (PALSAR) and the Japanese nationwide Global Positioning System (GPS) Earth Observation Network (GEONET) to develop a fault source model for the 2008 Iwate–Miyagi inland earthquake ( M w 6.9) that occurred on 14 June 2008 in northeastern Honshu, Japan. The PALSAR data include Interferometric Synthetic Aperture Radar (InSAR), range offset, and azimuth offset acquired from both ascending and descending paths. The range‐ and azimuth‐offset data are the only quantitative deformation signals around the epicenter, and exhibit complicated signals that were not detected by the GPS data. We refine our preliminary model by using Green’s functions from triangular dislocation elements to represent nonplanar fault geometries. Although a single segment model can well explain the GPS data, it generates significant misfit residuals in the offset data; thus, both west‐dipping and east‐dipping segments are necessary, as also suggested from aftershock distribution data. The moment magnitudes of the west‐dipping and east‐dipping faults are 6.53 and 6.76, respectively, and the total sum, 6.87, is almost identical to the seismological moment magnitude. This result suggests that both segments contributed equally to the total seismic moment, demonstrating a complex rupture process of the inland earthquake.

Dynamic rupture simulation of the 2011 off the Pacific coast of Tohoku Earthquake: Multi-event generation within dozens of seconds

We focus on a complex rupture process, namely, multi-event generation within about 50 seconds during the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0). We perform a 2D dynamic rupture simulation in order to explain physically the rupture process along the cross-section passing through the hypocenter. Realistic velocity structures are introduced into the simulation model. The dynamic parameters are selected by referring to kinematic source inversion results. The first significant event is generated on the deeper side of the fault. The scattered waves, mainly from the free surface, generate a stick-slip around the hypocenter, and then a second significant event is triggered. The synthetic waveforms consist of two major wave groups that are consistent with the observed ground motions.

Source complexity of the 4 March 2010 Jiashian, Taiwan, Earthquake determined by joint inversion of teleseismic and near field data

The Jiashian earthquake (ML 6.4) occurred on 4 March 2010. It was the largest inland event in southern Taiwan of 2010. The mainshock location was unexpected since it occurred in an area with relatively low background seismicity. In addition, reports of earthquake focal mechanisms do not fit with any known active fault geometry. In order to understand the origin of this earthquake, especially its rupture process, we perform a joint source inversion by using teleseismic body wave, GPS coseismic displacements and near field ground motion data. In this study, we considered a northwest–southeast trending fault with a northeast dip retrieved from GPS coseismic data and aftershocks distribution. To analyze the detailed slip distribution in space and time, we used near field 3D Green’s functions provided by spectral-element method and a full time–space inversion technique. We find a complex rupture process with several slip patches distributed inside two main asperities. The slip map reveals a mean slip of 12.9 cm for a maximum slip of 27.3 cm leading to a Mw 6.47 for this event. The rupture initiates in the deepest portion of the fault at 20 km depth, and propagated upward up to 2 km depth to form the two asperities. The source time function of this event revealed two pulses corresponding to the two asperities, for a total duration time of about 16 s. Most aftershocks occurred near the upper boundary of the deepest asperity while no aftershocks are located close to the shallowest one. We infer that the locations of these slip patches are related to the surrounding fault systems that may have restricted the rupture propagation during the earthquake.

Historical seismicity and dynamic rupture process of the 2011 Tohoku-Oki earthquake

A number of studies have documented the complex rupture process of the M w 9.0 March 11, 2011 Tohoku-Oki earthquake, which is characterized by different stages of rupture, including a large delayed slip near the trench and multiple high-frequency ruptures along the down-dip edge of the seismogenic zone. In this study, a multiscale circular patch model was used to represent the spatial heterogeneity of fracture energy inferred on the plate interface, with spontaneous dynamic rupture simulated by solving the elasto-dynamic equation with a slip-weakening friction law. Historical seismicity, recorded over the past 100 years, was used to determine the spatial heterogeneity of seismic events, with an additional large patch used to model large slip movements near the trench. Dynamic rupture processes qualitatively consistent with observations were successfully modeled during this study, although slight adjustments, including the introduction of foreshock-related stress concentrations, were needed to accomplish this. The delayed failure of the shallow largest patch, representing the main rupture stage, was only achieved by a dynamic nucleation process comprising cascading ruptures within small and moderately sized patches near the down-dip edge of the seismogenic zone; this failure was followed by successive ruptures along the down-dip edge of the zone. This study confirms that a free surface significantly increases fault slip and released seismic moment during an earthquake. In addition, we found that the largest patch occupies the area where the b-value of the Gutenberg–Richter magnitude–frequency relationship is much smaller than 1, while the hypocenter corresponds to a local maximum b-value of about 1.5. This research also suggests that seismicity catalogs can be used to constrain the spatial segmentation of plate boundaries and to propose possible scenarios for the dynamic rupture processes of future earthquakes. ► Historical seismicity data are used to model plate interface heterogeneity. ► Simple model explains complex rupture processes during the Tohoku-Oki earthquake. ► Cascading ruptures of smaller patches work as a dynamic nucleation process.

Complexity of the rupture process during the 2009 L’Aquila, Italy, earthquake

In this study, we investigate the rupture history of the 2009 April 6 (Mw 6.1) L'Aquila normal faulting earthquake by using a non-linear inversion of strong motion, GPS and DInSAR data. Both the separate and joint inversion solutions reveal a complex rupture process and a heterogeneous slip distribution. Slip is concentrated in two main asperities: a smaller shallow patch of slip located updip from the hypocentre and a second deeper and larger asperity located southeastwards along-strike direction. The key feature of the source process emerging from our inverted models concerns the rupture history, which is characterized by two distinct stages. The first stage begins with rupture nucleation and with updip propagation at relatively high (∽4.0 km s−1), but still subshear, rupture velocity. The second stage starts nearly 2.0–2.5 s after nucleation and it is characterized by the along-strike rupture propagation. The largest and deeper asperity fails during this stage of the rupture process. The rupture velocity is larger in the updip than in the along-strike direction. The updip and along-strike rupture propagation are separated in time and associated with a Mode II and a Mode III crack, respectively. The comparison between the source models inferred in this study with the Poisson ratio anomalies in the crustal volume containing the fault plane allows the interpretation of the delay in along-strike rupture propagation in terms of a structural control of the rupture history. Our results show that the L'Aquila earthquake featured a very complex rupture, with strong spatial and temporal heterogeneities suggesting a strong frictional and/or structural control of the rupture process.

Seismicity, active faults, stress patterns, and rupture processes in the Hualien region, Taiwan, investigated using the 1990 Hualien earthquake sequence

The Hualien mainshock–aftershock sequence from December 1990 to January 1991 was used to investigate seismicity, active faults, stress patterns, and complex rupture processes in the Hualien region of eastern Taiwan. A combined data set, compiled from observations by the regional seismic network (Taiwan Telemetered Seismograph Network, TTSN) and a dense aftershock monitoring array (Hualien Temporary Seismic Network, HTSN), provided high-quality seismic data. Using these data, earthquake source parameters could be well constrained by the joint hypocenter determination (JHD) and double-difference relocation methods. These two relocation methods presented similar results and produced improved estimations of earthquake location. Station corrections determined by JHD showed significant lateral velocity variations across the Yuehmei fault. The relocated seismicity indicated that a previously known active fault, the Chimei fault, extends to offshore. Earthquake relocations also revealed a northwest dipping seismicity probably related to the Chimei fault. In addition, three sets of focal mechanism solutions are presented and their discrepancies are attributed to the complex rupture process. At first, earthquakes were initiated by thrust motions. Later, the rupture pattern changed to strike–slip motions. Variations in slip may reflect the fault geometry or rupture boundary conditions in the region. The observed stress patterns are considered to relate not only to the regional collision and mountain-building processes, but also to active faults in local structures.

Estimation of Earthquake Source Parameters in the Kachchh Seismic Zone, Gujarat, India, from Strong-Motion Network Data Using a Generalized Inversion Technique

The generalized inversion of S -wave spectral amplitude data from 18 strong-motion sites in the Kachchh seismic zone, Gujarat, India, has been carried out to estimate source parameters of 38 aftershocks ( M w 2.93–5.32) from the 2001 Bhuj earthquake ( M w 7.7). The result shows that the seismic moment M and source radius r of aftershocks are between 3.1×1013 and ![Graphic][1] and 226–889 m, respectively, while stress drops ( Δσ ) vary from 0.11 to 7.44 MPa. The regression analysis between M and r shows a break in linear scaling at ![Graphic][2] . The estimated stress-drop values scatter significantly with M (![Graphic][3] ) for larger aftershocks (![Graphic][4] ) but vary in proportion with M (![Graphic][5] ) in case of smaller magnitude events. This can be due to the complex rupture process associated with relatively larger aftershocks in comparison to circular rupture for smaller events. The high stress-drop values are observed for events located on the north Wagad fault (at 15–30 km depths) and Gedi fault (at 0–5 km depths). This could be attributed to the large stress develop at the hypocentral depth as a result of high pore-fluid pressure beneath these two fault zones. The regression between M and corner frequency f c shows M is proportional to ![Graphic][6] , which is consistent for earthquakes with short rupture lengths and high stress-drop values. High stress-drop values commonly associated with the events from Kachchh and adjacent Narmada-Son lineament rift zones indicate strong potential for generating high peak ground accelerations and hence high seismic hazard. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif [4]: /embed/inline-graphic-4.gif [5]: /embed/inline-graphic-5.gif [6]: /embed/inline-graphic-6.gif

Focal mechanism and slip history of the 2011 M w 9.1 off the Pacific coast of Tohoku Earthquake, constrained with teleseismic body and surface waves

Focal mechanism and slip history of the 2011 Mw 9.1 off the Pacific coast of Tohoku, Japan earthquake were derived rapidly from teleseismic body and surface waves. Multiple double couples (MDC) analysis was first conducted using 1-hour long period seismic waves, yielding a single double couple with a seismic moment of 5.06×1022 N m, whose low angle nodal plane orients 199° and dips 10° west. Spatiotemporal rupture history was then constrained using both broadband body waves and long period seismic waves. The solution was updated twice in three days. Our preferred model, which based on the MDC fault plane, local JMA hypocenter and calibrated alignments of body and surface waves, revealed a complex rupture process. The rupture initiated slowly at a depth of 23 km. It first propagated in downdip and bilaterally directions along the subduction interface for 45 s and then broke a 80 km by 250 km near trench asperity in the up-dip direction and produced up to 60 m slip. The fault plane below the hypocenter had two more subevents, occurring in 70–95 s and 110–160 s, respectively. The total seismic moment was 5.8 × 1022 N m. The abnormal high slip near the trench suggests a recurrence interval larger than 500 yr.

Testing Earthquake Source Inversion Methodologies

Source Inversion Validation Workshop; Palm Springs, California, 11–12 September 2010; Nowadays earthquake source inversions are routinely performed after large earthquakes and represent a key connection between recorded seismic and geodetic data and the complex rupture process at depth. The resulting earthquake source models quantify the spatiotemporal evolution of ruptures. They are also used to provide a rapid assessment of the severity of an earthquake and to estimate losses. However, because of uncertainties in the data, assumed fault geometry and velocity structure, and chosen rupture parameterization, it is not clear which features of these source models are robust. Improved understanding of the uncertainty and reliability of earthquake source inversions will allow the scientific community to use the robust features of kinematic inversions to more thoroughly investigate the complexity of the rupture process and to better constrain other earthquake‐related computations, such as ground motion simulations and static stress change calculations.

Large surface wave of the 2004 Sumatra-Andaman earthquake captured by the very long baseline kinematic analysis of 1-Hz GPS data

Abstract The 26 December 2004 Sumatra-Andaman great earthquake had a −1500 km long rupture of more than 600 seconds duration, and may have involved a complex rupture process including slow slip. We processed International GNSS Service (IGS) 1-Hz Global Positioning System (GPS) data using kinematic analysis to investigate ground motion caused by this large earthquake. Since there are few 1-Hz stations, we had to process long baseline up to several thousand kilometers long. Long baselines degrade the GPS carrier phase ambiguity resolution. Nevertheless, clear seismic surface waves of the earthquake are recorded in our longdistance kinematic GPS solutions, which are in good agreement with response-corrected broadband seismic record. Our long baseline kinematic GPS solutions clearly indicated directivity of the seismic wave associated by rupture process of this earthquake. Also at the GPS stations that are 2,000 km away from the epicenter, dynamic displacements exceeding 5–10 cm were detected. In contrast, short baseline kinematic analysis shows large strain change caused by passage of surface wave, which reaches 6 × 10-6. Based on the comparison with seismometer and spectrum analysis of GPS results, it is difficult to discuss for very long time period displacement such as with a period more than 600 seconds in this study.

Subevent analysis for the Tabas earthquake of September 16, 1978, using near field accelerograms

We analyze the strong motion accelerograms of the large magnitude ( m b = 6.4, M S = 7.7, M w = 7.4, M 0 = 1.32 × 10 20) Tabas earthquake of September 16, 1978. The earthquake occurred due to a unilaterally propagating complex rupture process on a thrust fault system. The imprints of rupture of four (and possibly five) asperities (sub-faults) have been identified on the accelerograms. Using derived ‘SH-wave’ records, the locations of these asperities, which lie on the main causative fault, and the relative times of their occurrence have been determined. A foreshock that occurred about 6 s prior to the main shock has also been identified and located. Further, using ‘SH-wave’ spectral data, we provide near field estimates of the strike, dip and slip of these asperities as well as their seismic moment, moment magnitude, fault area, released shear wave energy and average stress drop. The spectra of the sub-events conform to the ω −2 Brune spectra. The total seismic energy of the sub-events is consistent with the radiated energy estimated from teleseismic observations. The fault plane solutions show generally WNW–ESE strike, which is sub-parallel to the regional strike of the geological fault system existing in the area. The stress drops are in the range of 7–30 bar, with the exception of the main sub-event that showed a stress drop of 100 bar. Overall the sub-events show self-similarity. The average shear wave quality factor Q S, which in the near field is quite sensitive to the local site geology, has been found to lie between 600 and 1200. The decay of PGA with distance is found to be compatible with Niazi and Bozorgnia's [Niazi, M., Bozorgnia, Y., 1992. The 1990 Manjil, Iran earthquake: geology and seismological overview, PGA attenuation and observed damage. Bull. Seism. Soc. Am. 82, 774–799] attenuation relation for the region.

The 2002 Denali Fault and 2001 Kunlun Fault Earthquakes: Complex Rupture Processes of Two Large Strike-Slip Events

We studied the source processes of two large continental earthquakes, the 3 November 2002 Denali fault earthquake and the 14 November 2001 Kunlun fault earthquake, associated with strike-slip faulting along ancient sutures. We inverted teleseismic P waveforms using a pulse-stripping method for multiple time windows with different focal mechanisms and derived composite source models. According to our results, the 2002 Denali fault earthquake began with initial thrusting ( M w 7.3) along a 40-km-long segment of the north-northwest-dipping Susitna Glacier fault and later ruptured a 300-km-long segment along the Denali and Totschunda faults with a right-lateral strike-slip mechanism ( M w 7.7). In contrast, the 2001 Kunlun fault earthquake nucleated near an extensional step-over with a subevent pair consisting of 30-km-long strike-slip ( M w 6.9) event and 40-km-long normal ( M w 6.8) faulting event and later ruptured a 350-km-long segment along the Kunlun fault with a left-lateral strike-slip mechanism ( M w 7.7). Both earthquakes propagated primarily unilaterally to the east and released most of their energy along slip patches (asperities) far from the hypocenter locations. We find that both the Denali fault and Kunlun fault earthquakes had high-average rupture velocities of 3.2 km/sec and 3.4 km/sec, respectively. We also compared the source properties of these two earthquakes with other strike-slip earthquakes. For scaling purposes, large strike-slip earthquakes were classified as interplate, oceanic intraplate, or continental intraplate events. By using this classification the Denali fault and Kunlun fault earthquakes have an interplate signature that suggests overall weak faulting.

Active faulting and seismicity of the Dasht-e-Bayaz region, eastern Iran

The Dasht-e-Bayaz left-lateral strike-slip fault system in NE Iran ruptured over a length of ca 120 km during two earthquakes in 1968 and 1979. We constrain source parameters of the 1968 August 31 (Dasht-e-Bayaz) and 1979 November 27 (Khuli-Buniabad) earthquakes by analysing long-period body wave seismograms. Both earthquakes involved complex rupture processes, with at least two subevents in each case. Coseismic surface ruptures and cumulative scarps in alluvium indicate fault segmentation on numerous short (ca 20 km long) strands with small pull-aparts between them. The earthquake subevents seen in the seismograms probably relate to this segmentation. The total, cumulative, offset on the fault system is estimated at ca 4–5 km. This is small compared to the total amount of Late Tertiary deformation expected in this part of Iran, and indicates that the Dasht-e-Bayaz fault may be relatively young. Distributed strike-slip faulting is widespread in the region and there are indications that the Dasht-e-Bayaz fault is evolving from several short faults that are coalescing. These results are important not only for understanding of the regional tectonics but for the development and evolution of strike-slip faults in general.

Analysis of the southeast Beni-Suef, northern Egypt earthquake sequence

On 11 October 1999, a moderate earthquake of local magnitude ML 4.9 hit northern Egypt. This event shook the greater Cairo region and northern part of the Nile valley and was felt in much of northern Egypt with epicentral intensity of V (EMS). It was followed by eight aftershocks with local magnitude ranging from 1.6 to 3.2. A detailed analysis of this sequence, cross-correlation analysis, fault plane solutions and source parameter estimations is performed in the present article. A cross-correlation analysis allowed us to recognize a number of correlated events, which were used for earthquake ancestry using a master event technique. The focal mechanism solutions for the main-shock and the largest aftershock are moderately controlled and indicate strike-slip faulting with a dip-slip normal component. Rupture directivity, aftershock distributions and isoseismal lines were combined in an attempt to determine which of the two nodal planes is the fault plane. The dynamic source parameters of the southeast Beni-Suef earthquake sequence have been estimated from the P-wave spectra of the Egyptian National Seismograph Network (ENSN). The spectral amplitude of the main shock records displayed two distinct corner frequencies that reflect a complex rupture process. The averaging of the obtained values at different stations shows that the main shock occurred on a fault length of 1.5 km with relative displacement 9 cm, stress drop of 49 bar and seismic moment of 5×1022 dyn/cm. The aftershocks show a relatively similar source parameters fault length of 0.3⩽L⩽0.6 km, relative displacement of 0.05⩽u⩽0.2 cm, seismic moment of 2.3×1019⩽M0⩽1.1×1020 dyn.cm and stress drop of 0.7⩽Δσ⩽4 bar.