Statistical search for non-random features of the seismicity of strong earthquakes

The space-time-magnitude relationship among worldwide earthquakes of magnitude M ? 7 has been investigated with a view toward discerning the statistical reliability of possibilities of epicenter migration, gaps in seismic activity, and techniques of identifying aftershocks and foreshocks. The statistical procedure involves the calculation of the second-order moment of the sequences. The statistical validity of the results was obtained by modeling the interaction of discrete seismic events by a multidimensional branching stochastic point process.The results are that epicenters of large earthquakes migrate with velocities between 300 and 2,000 km/year with a significance level greater than 99.5%; the maximum likelihood estimate of the velocity of migration of large aftershocks of large earthquakes is about 1,400 km/year; foreshocks of large earthquakes have a maximum likelihood estimate of velocity of migration of about 2,800 km/year. A gap in seismic activity occurs before large earthquakes; with somewhat less confidence, a gap also occurs after large earthquakes, after the aftershock sequence has ended. The number of immediate foreshocks is 35–40% of the number of aftershocks. Aftershocks are comparatively weaker than foreshocks when compared with the main shock. There are hints, that are not fully statistically confirmed, of interaction among large earthquakes in different depth ranges.     

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.     

Source parameters for the M<Subscript>w</Subscript> = 6.6, 03 February 2002, Çay Earthquake (Turkey) and aftershocks from GPS,Southwestern Turkey

The 03 February 2002 Çay Earthquake (M_w ～6.7) occurred on the fault segment between Eber and Ak?ehir Lakes followed by a large aftershock (M_w ～5.6) near the western end of the fault and two sequential aftershocks. We computed the coseismic surface displacements from static GPS measurements to determine the fault geometry parameters and uniform slip components. The coseismic displacements were obtained through combining the regional pre-earthquake and post-earthquake GPS data. Fault geometry and slips were acquired through the inversion of GPS data modeling the events as elastic dislocations in a half-space and assuming all four events took place on the same fault plane. Results suggest that one-segment fault of ～33 km length and dipping ～43° northward suffices to model the dislocation, assuming uniform slip distribution with 0.51 m dip slip, 0.26 m left-lateral slip extending to a depth down to ～11.5 km which is consistent with seismological evidence. The results also verify the normal faulting in the eastern flank of Isparta Angle which has long been assumed as a thrusting structure. While the available data cannot identify the four individual events on the same day, an attempted distributed slip model differentiates dip slip and left-lateral slips near the hypocenter with maximum values of ～1 and 0.6 m, respectively.     

Long aftershock sequences in North China and Central US: implications for hazard assessment in mid-continents

Because seismic activity within mid-continents is usually much lower than that along plate boundary zones, even small earthquakes can cause widespread concerns, especially when these events occur in the source regions of previous large earthquakes. However, these small earthquakes may be just aftershocks that continue for decades or even longer. The recent seismicity in the Tangshan region in North China is likely aftershocks of the 1976 Great Tangshan earthquake. The current earthquake sequence in the New Madrid seismic zone in central United States, which includes a cluster of M ~ 7.0 events in 1811–1812 and a number of similar events in the past millennium, is believed to result from recent fault reactivation that releases pre-stored strain energy in the crust. If so, this earthquake sequence is similar to aftershocks in that the rates of energy release should decay with time and the sequence of earthquakes will eventually end. We use simple physical analysis and numerical simulations to show that the current sequence of large earthquakes in the New Madrid fault zone is likely ending or has ended. Recognizing that mid-continental earthquakes have long aftershock sequences and complex spatiotemporal occurrences are critical to improve hazard assessments.     

Time Distribution of Immediate Foreshocks Obtained by a Stacking Method

—We apply a stacking method to investigate the time distribution of foreshock activity immediately before a mainshock. The foreshocks are searched for events with M≥ 3.0 within a distance of 50 km and two days from each mainshock with M≥ 5.0, in the JMA catalog from 1977 through 1997/9/30. About 33% of M≥ 5.0 earthquakes are preceded by foreshocks, and 50–70% in some areas. The relative location and time of three types of representative foreshocks, that is, the largest one, the nearest one to the mainshock in distance, and the nearest one in time, are stacked in reference to each mainshock. The statistical test for stacked time distribution of foreshocks within 30km from and two days before mainshocks shows that the inverse power-law type of a probability density time function is a significantly better fit than the exponential one for all three types of representative foreshocks. Two explanations possibly interpret the results. One is that foreshocks occur as a result of a stress change in the region, and the other one is that a foreshock is the cause of a stress change in the region and it triggers a mainshock. The second explanation is compatible with the relationship between a mainshock and aftershocks, when an aftershock happens to become larger than the mainshock. However the values of exponent of the power law obtained for stacked foreshocks are significantly smaller than those for similarly stacked aftershocks. Therefore the foreshock–mainshock relation should not be explained as a normal aftershock activity. Probably an increase of stress during foreshock activity results in apparently smaller values of the exponent, if the second explanation is the case.     

强余震的断层面解特征

本文分析了我国及邻区16次大地震的前震、主震和强余震的断层面解.其结果表明:前震和主震具有很好的一致性;一部分强余震同主震相比具有明显的差异, 具有明显差异的强余震发生的时间, 一般都在主震之后的一、二天内.这些特点有助于对地震趋势的估计.断层面解的差异性可能由于局部应力场引起, 也可能由于震源区断层面错动性质的多样性所致.      

The Lemnos 8 January 2013 (M w = 5.7) earthquake: fault slip,aftershock properties and static stress transfer modeling in the north Aegean Sea

We investigate mainshock slip distribution and aftershock activity of the 8 January 2013 M _w?=?5.7 Lemnos earthquake, north Aegean Sea. We analyse the seismic waveforms to better understand the spatio-temporal characteristics of earthquake rupture within the seismogenic layer of the crust. Peak slip values range from 50 to 64 cm and mean slip values range from 10 to 12 cm. The slip patches of the event extend over an area of dimensions 16?×?16 km². We also relocate aftershock catalog locations to image seismic fault dimensions and test earthquake transfer models. The relocated events allowed us to identify the active faults in this area of the north Aegean Sea by locating two, NE–SW linear patterns of aftershocks. The aftershock distribution of the mainshock event clearly reveals a NE–SW striking fault about 40 km offshore Lemnos Island that extends from 2 km up to a depth of 14 km. After the mainshock most of the seismic activity migrated to the east and to the north of the hypocenter due to (a) rupture directivity towards the NE and (b) Coulomb stress transfer. A stress inversion analysis based on 14 focal mechanisms of aftershocks showed that the maximum horizontal stress is compressional at N84°E. The static stress transfer analysis for all post-1943 major events in the North Aegean shows no evidence for triggering of the 2013 event. We suggest that the 2013 event occurred due to tectonic loading of the North Aegean crust.     

Paleoseismic study of the Kamishiro Fault on the northern segment of the Itoigawa–Shizuoka Tectonic Line,Japan

The M_w 6.2 (Mj 6.8) Nagano (Japan) earthquake of 22 November 2014 produced a 9.3-km long surface rupture zone with a thrust-dominated displacement of up to 1.5 m, which duplicated the pre-existing Kamishiro Fault along the Itoigawa–Shizuoka Tectonic Line (ISTL), the plate-boundary between the Eurasian and North American plates, northern Nagano Prefecture, central Japan. To characterize the activity of the seismogenic fault zone, we conducted a paleoseismic study of the Kamishiro Fault. Field investigations and trench excavations revealed that seven morphogenic paleohistorical earthquakes (E2–E8) prior to the 2014 M_w 6.2 Nagano earthquake (E1) have occurred on the Kamishiro Fault during the last ca. 6000 years. Three of these events (E2–E4) are well constrained and correspond to historical earthquakes occurring in the last ca. 1200 years. This suggests an average recurrence interval of ca. 300–400 years on the seismogenic fault of the 2014 Kamishiro earthquake in the past 1200 years. The most recent event prior to the 2014 earthquakes (E1) is E2 and the penultimate and antepenultimate faulting events are E3 and E4, respectively. The penultimate faulting event (E3) occurred during the period of AD 1800–1400 and is associated with the 1791 M_w 6.8 earthquake. The antepenultimate faulting event (E4) is inferred to have occurred during the period of ca. AD 1000–700, likely corresponding to the AD 841 M_w 6.5 earthquake. The oldest faulting event (E8) in the study area is thought to have occurred during the period of ca. 5600–6000 years. The throw rate during the early Holocene is estimated to be 1.2–3.3 mm/a (average, 2.2 mm/a) with an average amount of characteristic offset of 0.7–1.1 m produced by individual event. When compared with active intraplate faults on Honshu Island, Japan, these slip rates and recurrence interval estimated for morphogenic earthquakes on the Kamishiro Fault along the ISTL appear high and short, respectively. This indicates that present activity on this fault is closely related to seismic faulting along the plate boundary between the Eurasian and North American plates.     

Source mechanism of strong aftershocks (<Emphasis Type="Italic">M</Emphasis><Subscript>s</Subscript>⩾5.6) of the 2008/05/12 Wenchuan earthquake and the implication for seismotectonics

Dozens of >M5, hundreds of >M4, and much more >M3 aftershocks occurred after the 2008/05/12 Wenchuan earthquake, which were well recorded by permanent and portable seismic stations. After relocated with P arrival, the >M3 aftershocks show two trends of distribution, with most of the aftershocks located along the north-east strike consistent with Longmenshan fault system, yet there is a north-west trend around the epicenter. It seems that substantially more aftershocks occur in regions with crystalline bedrocks. Then we collected waveform data from National Digital Seismograph Network and regional seismograph network of China, and employed “Cut and Paste” method to obtain focal mechanisms and depths of the big aftershocks (M⩾5.6). While most of those aftershocks show thrust mechanism, there are some strike slip earthquakes in the northern-most end of the rupture. Focal mechanisms show that the events located on the southern part of central Beichuan-Yingxiu Fault (BY) are mainly thrust earthquakes, which is consistent with initial mechanism of the main shock rupture. In the north part the aftershocks along the BY are also dominated by thrust slip, which is quite different from the right slip rupture of the main shock. Around Qingchuan-Pingwu Fault, the focal mechanisms are dominated by right-slip rupture with large depths (∼18 km). So we suspected that in the north part the main shock might rupture on two faults: Beichuan Fault and Qingchuan-Pingwu Fault. The complex pattern of aftershock mechanisms argues for presence of a complicated fault system in the Longmenshan area. Supported by Knowledge Innovation Project of Chinese Academy of Sciences (Grant Nos. KZCX3-SW-153, KZCX2-YW-116-1), National Natural Science Foundation of China (Grant No. 40604004), and National Basic Technology R & D Program (Grant No. 2006BAC01B02-01-02).     

Scaling Transition in Earthquake Sources: A Possible Link Between Seismic and Laboratory Measurements

We estimate the corner frequencies of 20 crustal seismic events from mainshock–aftershock sequences in different tectonic environments (mainshocks 5.7 < M _W < 7.6) using the well-established seismic coda ratio technique (Mayeda et al. in Geophys Res Lett 34:L11303, 2007; Mayeda and Malagnini in Geophys Res Lett, 2010), which provides optimal stability and does not require path or site corrections. For each sequence, we assumed the Brune source model and estimated all the events’ corner frequencies and associated apparent stresses following the MDAC spectral formulation of Walter and Taylor (A revised magnitude and distance amplitude correction (MDAC2) procedure for regional seismic discriminants, 2001), which allows for the possibility of non-self-similar source scaling. Within each sequence, we observe a systematic deviation from the self-similar \( M_{0} \propto \mathop f\nolimits_{\text{c}}^{ - 3} \) line, all data being rather compatible with \( M_{0} \propto \mathop f\nolimits_{\text{c}}^{ - (3 + \varepsilon )} \) , where ε > 0 (Kanamori and Rivera in Bull Seismol Soc Am 94:314–319, 2004). The deviation from a strict self-similar behavior within each earthquake sequence of our collection is indicated by a systematic increase in the estimated average static stress drop and apparent stress with increasing seismic moment (moment magnitude). Our favored physical interpretation for the increased apparent stress with earthquake size is a progressive frictional weakening for increasing seismic slip, in agreement with recent results obtained in laboratory experiments performed on state-of-the-art apparatuses at slip rates of the order of 1 m/s or larger. At smaller magnitudes (M _W < 5.5), the overall data set is characterized by a variability in apparent stress of almost three orders of magnitude, mostly from the scatter observed in strike-slip sequences. Larger events (M _W > 5.5) show much less variability: about one order of magnitude. It appears that the apparent stress (and static stress drop) does not grow indefinitely at larger magnitudes: for example, in the case of the Chi–Chi sequence (the best sampled sequence between M _W 5 and 6.5), some roughly constant stress parameters characterize earthquakes larger than M _W ~ 5.5. A representative fault slip for M _W 5.5 is a few tens of centimeters (e.g., Ide and Takeo in J Geophys Res 102:27379–27391, 1997), which corresponds to the slip amount at which effective lubrication is observed, according to recent laboratory friction experiments performed at seismic slip velocities (V ~ 1 m/s) and normal stresses representative of crustal depths (Di Toro et al. in Nature in press, 2011, and references therein). If the observed deviation from self-similar scaling is explained in terms of an asymptotic increase in apparent stress (Malagnini et al. in Pure Appl Geophys, 2014, this volume), which is directly related to dynamic stress drop on the fault, one interpretation is that for a seismic slip of a few tens of centimeters (M _W ~ 5.5) or larger, a fully lubricated frictional state may be asymptotically approached.     

1975年2月4日辽宁省海城地震的震源机制

由地震纵波初动符号的资料,求得了海城地震系列中Ms≥4.0的24个地震的断层面解。主震发生于1975年2月4日,它的一个节面走向N70°W,倾向NE,倾角81°;另一个节面走向N23°E,倾向SE,倾角75°。根据余震的空间分布以及地面形变资料选取N70°W的节面为断层面,主震是发生在这个近乎直立的断层面上的左旋走向滑动,略具正的倾向滑动分量。前震及大多数余震的震源机制和主震的相似,有四个Ms≥4.0的余震的震源机制和主震的迥然不同,表现出滑动向量和主震的滑动向量相反的断层错动方式。这种情况的一种可能的解释是主震时在断层的一些地段发生错动过头。 由野外资料及余震的空间分布资料计算了主震的震源参数。主震断层长70公里,宽20公里,平均错距45厘米,地震矩2.1×10²⁶达因·厘米,应力降4.8巴,应变降7.3×10^-6。它是发生在不能积累起较高应力的薄弱地带的一次低应力降的地震。 由地震纵波初动的半周期和振幅的资料计算了81个前震和余震的震源尺度、地震矩、应力降和平均错距。结果表明前震和余震的应力降都比较低,一般在0.1-1巴之间。余震区中有两个应力降相对说来比较高（高于0.8巴）的地区,它们恰好对应于主破裂错动过头的部位。这些结果意味着震前高应力、错动过头、相对高应力降和震源机制反向四者之间     

Earthquake swarms and volcanism in New Zealand

The term « swarm » is used to describe a group of related earthquakes, concentrated in space and time, without an obvious principal event. Large shallow earthquakes are often followed by aftershocks, but the pattern in which aftershocks occur differs in detail from that of a swarm. Sequences of New Zealand earthquakes that have been called swarms differ markedly from one another. The most vigorous of them, near Taupo in 1922, appears to have been an ordinary tectonic earthquake accompanied by foreshocks and aftershocks, and by surface faulting. No fault movements accompanied the 1964 swarm in the same area. Other localities that have experienced swarms include Great Barrier Island, Matamata, Kawerau, and Opunake. Swarms are considered by some writers to be characteristic of volcanic regions. Although all New Zealand swarms have occurred in areas of Quaternary volcanism, there are still no observations showing what part, if any, volcanism plays in the generation of earthquake swarms.     

Tectonic setting of the recent damaging seismic series in the Southeastern Betic Cordillera,Spain

In this work we analyze the tectonic setting of the recent damaging seismic series occurred in the Internal Zones of the eastern Betic Cordillera (SE Spain) and surrounding areas, the tectonic region where took place the 11th May 2011 Mw 5.2 Lorca earthquake. We revisit and make a synthesis of the seven largest and damaging seismic series occurred from 1984 to 2011. We analyze their seismotectonic setting, and their geological sources under the light of recent advances in the knowledge on active faults, neotectonics, seismotectonics and stress regime, with special attention focused on the Lorca Earthquake. These seismic series are characterized by two types of focal mechanisms, produced mainly by two sets of active faults, NNW–SSE to NNE–SSW small (no larger than 20–30 km) extensional faults with some strike slip component, and E–W to NE–SW large strike slip faults (more than 50 km long) with some compressional component (oblique slip faults). The normal fault earthquakes related to the smaller faults are dominant in the interior of large crustal tectonic blocks that are bounded by the large E–W to NE–SW strike-slip faults. The strike slip earthquakes are associated to the reactivation of segments or intersegment regions of the large E–W to NE–SW faults bounding those crustal tectonic blocks. Most of the seismic series studied in this work can be interpreted as part of the background seismicity that occurs within the crustal blocks that are strained under a transpressional regime driven by the major strike slip shear corridors bounding the blocks. The seismotectonic analysis and the phenomenology of the studied series indicate that it is usual the occurrence of damaging compound earthquakes of M  \(\sim \)  5.0 associated with triggering processes driven by coseismic stress transfer. These processes mainly occur in the seismic series generated by NNW–SSE to NNE–SSW faults. These mechanical interaction processes may induce a higher frequency of occurrence of this kind of earthquakes than considered in traditional probabilistic seismic hazard assessments and it should be taken into account in future seismic hazard assessments.     

活动断层周期性粘滑变形的宏观和微观证据

收集和归纳了活动断层周期性破裂(高速)变形的宏观和微观证据。这些对于我们识别断层上的过去发生的粘滑(古地震)事件遗迹,研究断层活动习性,以更有效地进行地震预测预报是有益的。     

Use of Potential Foreshocks to Estimate the Short-term Probability of Large Earthquakes, Tohoku, Japan

This study seeks to construct a hazard function for earthquake probabilities based on potential foreshocks. Earthquakes of magnitude 6.5 and larger that occurred between 1976 and 2000 in an offshore area of the Tohoku region of northeast Japan were selected as events for estimating probabilities. Later occurrences of multiple events and aftershocks were omitted from targets. As a result, a total of 14 earthquakes were employed in the assessment of models. The study volume spans 300 km (East-West) × 660 km (North-South) × 60 km in depth. The probability of a target earthquake occurring at a certain point in time-space depends on the number of small earthquakes that occurred per unit volume in that vicinity. In this study, we assume that the hazard function increases geometrically with the number of potential foreshocks within a constrained space-time window. The parameters for defining potential foreshocks are magnitude, spatial extent and lead time to the point of assessment. The time parameter is studied in ranges of 1 to 5 days (1-day steps), and spatial parameters in 20 to 100 km (20-km steps). The model parameters of the hazard function are determined by the maximum likelihood method. The most effective hazard function examined was the following case: When an earthquake of magnitude 4.5 to 6.5 occurs, the hazard for a large event is increased significantly for one day within a 20 km radius surrounding the earthquake. If two or more such earthquakes are observed, the model expects a 20,000 times greater probability of an earthquake of magnitude 6.5 or greater than in the absence of such events.     

利用滑动速率、古地震资料估算走滑断裂的粘滑及蠕滑量的方法

断层走滑包括粘滑和蠕滑,前者是伴有强震发生的快速运动,后者则是一种缓慢的无震稳滑运动。这２ 种运动常随时间交替出现,共同构成了断层的基本运动方式。在利用断层滑动速率讨论大震重复率的问题中,人们最关心的问题则是如何从总滑动量中分辨和划分出其中所包含的粘滑和蠕滑量,特别是对蠕滑量的划分,因为它直接影响着大震重复率的正确性。笔者在野外考察的基础上,对昌马活动断裂的位移量进行了分级,确定出昌马断裂带全新世以来的水平滑动量大致可划分为５个级别：３１～４１ ｍ ;２５～３１ ｍ ; １５～２２ ｍ ; ８～１３ ｍ ;１～５．５ ｍ ,同时依据古地震学方法并结合１４Ｃ断代法及断层崖形成年代的数学模拟计算,求得全新世以来在昌马断裂带上共发生５ 次古地震事件。在上述２ 项资料确定的基础上,进一步对昌马断裂带的粘滑及蠕滑量进行了划分,并给出了它们随时间的变化情况     

Large Earthquake Hazard of the San Jacinto Fault Zone,CA, from Long Record of Simulated Seismicity Assimilating the Available Instrumental and Paleoseismic Data

We investigate spatio-temporal properties of earthquake patterns in the San Jacinto fault zone (SJFZ), California, between Cajon Pass and the Superstition Hill Fault, using a long record of simulated seismicity constrained by available seismological and geological data. The model provides an effective realization of a large segmented strike-slip fault zone in a 3D elastic half-space, with heterogeneous distribution of static friction chosen to represent several clear step-overs at the surface. The simulated synthetic catalog reproduces well the basic statistical features of the instrumental seismicity recorded at the SJFZ area since 1981. The model also produces events larger than those included in the short instrumental record, consistent with paleo-earthquakes documented at sites along the SJFZ for the last 1,400 years. The general agreement between the synthetic and observed data allows us to address with the long-simulated seismicity questions related to large earthquakes and expected seismic hazard. The interaction between m ≥ 7 events on different sections of the SJFZ is found to be close to random. The hazard associated with m ≥ 7 events on the SJFZ increases significantly if the long record of simulated seismicity is taken into account. The model simulations indicate that the recent increased number of observed intermediate SJFZ earthquakes is a robust statistical feature heralding the occurrence of m ≥ 7 earthquakes. The hypocenters of the m ≥ 5 events in the simulation results move progressively towards the hypocenter of the upcoming m ≥ 7 earthquake.     

Simulation of Unstable Fault Slip in Granite Using a Bonded-particle Model

—?A bonded-particle model is used to simulate shear-type microseismic events induced by tunnel excavation in granite. The model represents a volume of granite by an assembly of 50,000 individual particles bonded together at points of contact. A plane of weakness is included in the model and this plane is subjected to increasing shear load while the normal load across the plane is held constant. As shear stress in the model increases, bonds begin to break and small acoustic emissions (AE) result. After enough bonds have broken, macro-slip occurs across the large portions of the fault in an unstable manner. Since the model is run dynamically, seismic source information can be calculated for the simulated AE and macro-slip events. This information is compared with actual results obtained from seismic monitoring around an underground excavation. Although the modelled events exhibit larger magnitudes than the actual recorded events, there are many similarities between the model and the actual results, namely the presence of foreshocks before the macro-slip events and the patterns of energy release during loading. In particular, the model provides the ability to examine the complexity of the slip events in detail.     

Tsunami Source of the 2010 Mentawai, Indonesia Earthquake Inferred from Tsunami Field Survey and Waveform Modeling

The 2010 Mentawai earthquake (magnitude 7.7) generated a destructive tsunami that caused more than 500 casualties in the Mentawai Islands, west of Sumatra, Indonesia. Seismological analyses indicate that this earthquake was an unusual “tsunami earthquake,” which produces much larger tsunamis than expected from the seismic magnitude. We carried out a field survey to measure tsunami heights and inundation distances, an inversion of tsunami waveforms to estimate the slip distribution on the fault, and inundation modeling to compare the measured and simulated tsunami heights. The measured tsunami heights at eight locations on the west coasts of North and South Pagai Island ranged from 2.5 to 9.3 m, but were mostly in the 4–7 m range. At three villages, the tsunami inundation extended more than 300 m. Interviews of local residents indicated that the earthquake ground shaking was less intense than during previous large earthquakes and did not cause any damage. Inversion of tsunami waveforms recorded at nine coastal tide gauges, a nearby GPS buoy, and a DART station indicated a large slip (maximum 6.1 m) on a shallower part of the fault near the trench axis, a distribution similar to other tsunami earthquakes. The total seismic moment estimated from tsunami waveform inversion was 1.0 × 10²¹ Nm, which corresponded to M_w 7.9. Computed coastal tsunami heights from this tsunami source model using linear equations are similar to the measured tsunami heights. The inundation heights computed by using detailed bathymetry and topography data and nonlinear equations including inundation were smaller than the measured ones. This may have been partly due to the limited resolution and accuracy of publically available bathymetry and topography data. One-dimensional run-up computations using our surveyed topography profiles showed that the computed heights were roughly similar to the measured ones.     

Evolution of the vigorous 2006 swarm in Zakynthos (Greece) and probabilities for strong aftershocks occurrence

A multiplet of moderate-magnitude earthquakes (5.1?≤?M?≤?5.6) took place in Zakynthos Island and offshore area (central Ionian Islands, Greece) in April 2006. The activity in the first month occupied an area of almost 35 km long, striking roughly NNW–SSE, whereas aftershocks continued for several months, decaying with time but persisting at the same place. The properties of the activated structure were investigated with accurate relocated data and the available fault plane solutions of some of the stronger events. Both the distribution of seismicity and fault plane solutions show that thrusting with strike-slip motions are both present in high-angle fault segments. The segmentation of the activated structure could be attributed to the faulting complexity inherited from the regional compressive tectonics. Investigation of the spatial and temporal behavior of seismicity revealed possible triggering of adjacent fault segments that may fail individually, thus preventing coalescence in a large main rupture. In an attempt to forecast occurrence probabilities of six of the strong events (M _w?≥?5.0), estimations were performed following the restricted epidemic-type aftershock sequence model, applied to data samples before each one of these strong events. Stochastic modeling was also used to identify “quiescence” periods before the examined aftershocks. In two out of the six cases, real aftershock rate did decrease before the next strong shock compared to the modeled one. The latter results reveal that rate decrease is not a clear precursor of strong shocks in the swarm and no quantitative information, suitable to supply probability gain, could be extracted from the data.