Showing papers on "Hypocenter published in 2012"

Crustal structure and rheology of the Longmenshan and Wenchuan Mw 7.9 earthquake epicentral area from magnetotelluric data

Abstract: The Longmenshan forms the eastern margin of the Tibetan Plateau adjacent to the Sichuan Basin. This range is anomalous because it formed despite low convergence and slip rates and without the development of a foreland basin. The devastating A.D. 2008 Wenchuan earthquake (Mw = 7.9) has renewed debate about the tectonics of the Longmenshan. A magnetotelluric (MT) study was undertaken subsequent to the earthquake to investigate the crustal structure of the Longmenshan, and inversion of the data reveals a low-resistivity (high-conductivity) layer at a depth of ∼20 km beneath the eastern Tibetan Plateau that terminates ∼25 km west of the Wenchuan-Maoxian fault. Its electrical properties are consistent with it being fluid-rich and mechanically weak. Beneath the Longmenshan and Sichuan Basin, a high-resistivity zone extends through the entire crust, but with a zone of low resistivity in the vicinity of the Wenchuan hypocenter. We show that the MT data, combined with other geological and geophysical observations, support geodynamic models for the uplift of eastern Tibet being caused by southeast-directed crustal flow that is blocked by stable lithosphere beneath the Sichuan Basin and Longmenshan, leading to inflation of the Songpan-Ganzi terrane. This rigid high-resistivity backstop not only provided a block to flow, but also may have accumulated stress prior to the earthquake. The MT observations provide new insights into the generation of the Wenchuan earthquake, which occurred in a region with low convergence rates prior to the earthquake.

Rupture directivity of the 2011, Mw 5.2 Lorca earthquake (Spain)

Abstract: [1] On May 11th 2011, a rather small earthquake caused nine fatalities in the city of Lorca, SE-Spain. We analyze seismograms from a dense network to characterize the source of this earthquake. We estimate an oblique reverse faulting mechanism, moment magnitude of 5.2 and a shallow hypocenter (4.6 km), at only 5.5 km epicentral distance from the city center. Double difference relocations yield a ∼5 km long, NE-SW trending distribution of aftershocks SW of the mainshock, suggesting a SW propagating rupture along the Alhama de Murcia fault. We use the Mw 4.6 foreshock and an Mw 3.9 aftershock as empirical Greens functions to estimate apparent source time functions, observing a clear directivity effect. We model apparent durations with a unilateral and asymmetric bilateral rupture, in both cases obtaining rupture directivity of ∼N220°E, towards Lorca. In addition to the near epicenter and shallow depth, directivity may have contributed to the significant impact.

Microseismicity and stress in the vicinity of the Alpine Fault, central Southern Alps, New Zealand

Abstract: [1] We investigate present-day microseismicity associated with the central Alpine Fault and the zone of active deformation and uplift in the central Southern Alps. Using 14 months of data, robust hypocenter locations have been obtained for ∼1800 earthquakes of magnitudes between −0.3 and 4.2. We derived a magnitude scale with a frequency-dependent attenuation factor, γ(f) = γ0f, where γ0 = 1.89 ± 0.02 × 10−3 s/km, that enables magnitudes to be calculated consistently for earthquakes of different sizes and frequency contents. The maximum depth of the seismicity varies systematically with distance from the Alpine Fault, from 10 ± 2 km near the fault to 8 ± 2 km within 20 km and 15 ± 2 km further southeast. This distribution correlates with lateral variations in crustal resistivity: earthquake hypocenters are concentrated in areas of strong resistivity gradients and restricted to depths of resistivities >100 Ωm. Rocks at greater depth are too hot, too fluid-saturated, or too weak to produce detectable earthquakes. Focal mechanism solutions computed for 211 earthquakes (ML > 0.44) exhibit predominantly strike-slip mechanisms. We obtain a maximum horizontal compressive stress direction of 115 ± 10° from focal mechanism inversion. This azimuth is consistent with findings from elsewhere in the central and northern South Island, and indicates a uniform crustal stress field despite pronounced variations in crustal structure and topographic relief. Our stress estimates suggest that the Alpine Fault (with a mean strike of 055°) is poorly oriented in an Andersonian sense but that individual thrust and strike-slip segments of the fault's surface trace have close to optimal orientations.

Source model for strong ground motion generation in the frequency range 0.1–10 Hz during the 2011 Tohoku earthquake

Abstract: The source model of the 2011 Tohoku earthquake, which is composed of four strong motion generation areas (SMGAs), is estimated based on the broadband strong ground motion simulations in the frequency range 0.1–10 Hz using the empirical Green’s function method. Two strong motion generation areas are identified in the Miyagi-oki region west of the hypocenter. Another two strong motion generation areas are located in the Fukushima-oki region southwest of the hypocenter. The strong ground motions in the frequency range 0.1–10 Hz along the Pacific coast are mainly controlled by these SMGAs. All the strong motion generation areas exist in the deeper portion of the source fault plane. The stress drops of the four SMGAs range from 6.6 to 27.8 MPa, which are similar to estimations for past M 7-class events occurring in this region. Compared with the slip models and aftershock distributions of past interplate earthquakes in the Miyagi-oki and Fukushima-oki regions since the 1930s, the SMGAs of the 2011 Tohoku earthquake spatially correspond to the asperities of M 7-class events in 1930s. In terms of broadband strong ground motions, the 2011 Tohoku earthquake is not only a tsunamigenic event with a huge coseismic slip near the trench but is also a complex event simultaneously rupturing pre-existing asperities.

Dynamic rupture of the 2011 Mw 9.0 Tohoku‐Oki earthquake: Roles of a possible subducting seamount

Abstract: [1] Using a hybrid MPI/OpenMP parallel finite element method for spontaneous rupture and seismic wave propagation simulations, we investigate features in rupture propagation, slip distribution, seismic radiation, and seafloor deformation of the 2011 Mw 9.0 Tohoku-Oki earthquake to gain physical insights into the event. With simplified shallow dipping (10°) planar fault geometry, 1D velocity structure, and a slip-weakening friction law, we primarily investigate initial stress and strength conditions that can produce rupture and seismic radiation characteristics of the event revealed by kinematic inversions, and seafloor displacements observed near the epicenter. By a large suite of numerical experiments aided by parallel computing on modern supercomputers, we find that a seamount of a dimension of ∼70 km by 23 km just updip of the hypocenter on the subducting plane, parameterized by higher static friction, lower pore fluid pressure, and higher initial stress than surrounding regions, may play a dominant role in the 2011 event. Its high strength stalls updip rupture for tens of seconds, and its high stress drop generates large slip. Its failure drives the rupture to propagate into the shallow portion that is likely velocity-strengthening, resulting in significant slip near the trench within a limited area. However, the preferred model suggests that the largest slip in the event occurs near the hypocenter. High-strength patches along the downdip portion of the subducting plane are most effective among several possible factors in generating high-frequency seismic radiations, suggesting the initial strength distribution there is very heterogeneous.

Control of pore fluid pressure diffusion on fault failure mode: Insights from the 2009 L'Aquila seismic sequence

Abstract: [1] The MW 6.13 L'Aquila earthquake ruptured the Paganica fault on 2009/04/06 at 01:32 UTC, and started a strong sequence of aftershocks. For the first four days, the region north of the hypocenter of the main quake was shaken by three large events (MW ∼ 5.0) that ruptured different patches of the Monti della Laga fault (hereafter “Campotosto”). In our hypothesis, these aftershocks were induced by a dramatic reduction in the fault's shear strength due to a pulse of pore fluid pressure released after the L'Aquila main earthquake. Here we model the time evolution of the pore fluid pressure northward from the main hypocenter. We show that, during the sequence, the Campotosto fault failed in multiple episodes, when the specific patches/asperities underwent fluid pressure-related strength reductions of 7–10 MPa. Although such drops in strength are very large in amplitude, the contribution of other weakening mechanisms (perturbations of the Coulomb shear stress, and/or dynamic stresses induced by passing seismic waves) cannot be ruled out by our observations. However, the Coulomb shear stress variations either had negative amplitudes down to −0.2 MPa (i.e., tended to inhibit further seismic activity), or had very small positive amplitudes (<0.05 MPa). Paleoseismological evidence supports the hypothesis that larger events (MW 6.5–7) have occurred on the Paganica fault [EMERGEO Working Group, 2009], whereas Lucente et al. [2010] concluded that an important migration of pore fluids characterized the preparatory phase of the L'Aquila main shock. Consequently, the MW 6.13 L'Aquila earthquake may be analogous, at a larger scale, to one of the three Campotosto largest aftershocks. The complex behavior observed for the L'Aquila-Campotosto fault system seems to be common to other seismogenic structures in the Central Apennines (e.g., the Umbria-Marche fault system), and need to be taken into consideration for the assessment of seismic hazard.

The East Anatolian Fault Zone: Seismotectonic setting and spatiotemporal characteristics of seismicity based on precise earthquake locations

Abstract: [1] The East Anatolian Fault Zone (EAFZ) represents a plate boundary extending over ∼500 km between the Arabian and Anatolian plates. Relative plate motion occurs with slip rates ranging from 6 to 10 mm/yr and has resulted in destructive earthquakes in eastern Turkey as documented by historical records. In this study, we investigate the seismic activity along the EAFZ and fault kinematics based on recordings from a densified regional seismic network providing the best possible azimuthal coverage for the target region. We optimize a reference 1-D velocity model using a grid-search approach and re-locate hypocenters using the Double-Difference earthquake relocation technique. The refined hypocenter catalog provides insights into the kinematics and internal deformation of the fault zone down to a resolution ranging typically between 100 and 200 m. The distribution of hypocenters suggests that the EAFZ is characterized by NE-SW and E-W oriented sub-segments that are sub-parallel to the overall trend of the fault zone. Faulting mechanisms are predominantly left-lateral strike-slip and thus in good correlation with the deformation pattern derived from regional GPS data. However, we also observe local clusters of thrust and normal faulting events, respectively. While normal faulting events typically occur on NS-trending subsidiary faults, thrust faulting is restricted to EW-trending structures. This observation is in good accordance with kinematic models proposed for evolving shear zones. The observed spatiotemporal evolution of hypocenters indicates a systematic migration of micro- and moderate-sized earthquakes from the main fault into adjacent fault segments within several days documenting progressive interaction between the major branch of the EAFZ and its secondary structures. Analyzing the pre versus post-seismic phase for M > 5 events we find that aftershock activities are initially spread to the entire source region for several months but start to cluster at the central part of the main shock rupture thereafter.

Space-time distribution of afterslip following the 2009 L'Aquila earthquake

Abstract: [1] The inversion of multitemporal DInSAR and GPS measurements unravels the coseismic and postseismic (afterslip) slip distributions associated with the 2009 MW 6.3 L'Aquila earthquake and provides insights into the rheological properties and long-term behavior of the responsible structure, the Paganica fault. Well-resolved patches of high postseismic slip (10–20 cm) appear to surround the main coseismic patch (maximum slip ≈1 m) through the entire seismogenic layer above the hypocenter without any obvious depth-dependent control. Time series of postseismic displacement are well reproduced by an exponential function with best-fit decay constants in the range of 20–40 days. A sudden discontinuity in the evolution of released postseismic moment at ≈130 days after the main shock does not correlate with independent seismological and geodetic data and is attributed to residual noise in the InSAR time series. The data are unable to resolve migration of afterslip along the fault probably because of the time interval (six days) between the main shock and the first radar acquisition. Surface fractures observed along the Paganica fault follow the steepest gradients of postseismic line-of-sight satellite displacements and are consistent with a sudden and delayed failure of the shallow layer in response to upward tapering of slip. The occurrence of afterslip at various levels through the entire seismogenic layer argues against exclusive depth-dependent variations of frictional properties on the fault, supporting the hypothesis of significant horizontal frictional heterogeneities and/or geometrical complexities. We support the hypothesis that such heterogeneities and complexities may be at the origin of the long-term variable behavior suggested by the paleoseismological studies. Rupture of fault patches with dimensions similar to that activated in 2009 appears to have a ≈500 year recurrence time interval documented by paleoseismic and historical studies. In addition to that, paleoseismological evidence of large (>0.5 m) coseismic offsets seems to require seismic events, recurring every 1000–2000 years, characterized by (1) multisegment linkage, (2) surface ruptures larger than in 2009, and (3) complete failure of the 2009 coseismic and postseismic patches.

Geodetic constraints on afterslip characteristics following the March 9, 2011, Sanriku‐oki earthquake, Japan

Abstract: [1] A magnitude 7.3 foreshock occurred at the subducting Pacific plate interface on March 9, 2011, 51 h before the magnitude 9.0 Tohoku earthquake off the Pacific coast of Japan. We propose a coseismic and postseismic afterslip model of the magnitude 7.3 event based on a global positioning system network and ocean bottom pressure gauge sites. The estimated coseismic slip and afterslip areas show complementary spatial distributions; the afterslip distribution is located up-dip of the coseismic slip for the foreshock and northward of hypocenter of the Tohoku earthquake. The slip amount for the afterslip is roughly consistent with that determined by repeating earthquake analysis carried out in a previous study. The estimated moment release for the afterslip reached magnitude 6.8, even within a short time period of 51h. A volumetric strainmeter time series also suggests that this event advanced with a rapid decay time constant compared with other typical large earthquakes.

A dynamic model of the frequency-dependent rupture process of the 2011 Tohoku-Oki earthquake

Abstract: We present a 2D dynamic rupture model that provides a physical interpretation of the key features of the 2011 Tohoku-Oki earthquake rupture. This minimalistic model assumes linear slip-weakening friction, the presence of deep asperities and depth-dependent initial stresses. It reproduces the first-order observations of the along-dip rupture process during its initial 100 s, such as large static slip and low-frequency radiation up-dip from the hypocenter, and slow rupture punctuated by high-frequency radiation in deeper regions. We also derive quantitative constraints on the ratio of shallow versus deep radiation from teleseismic back-projection source imaging. This ratio is explained in our model by the rupture of deep asperities surrounded by low stress drop regions, and by the decrease of initial stresses towards the trench.

Source mechanism of the 23 October, 2011, Van (Turkey) earthquake ( M w = 7.1) and aftershocks with its tectonic implications

Abstract: This study has investigated the rupture process of the 23 October, 2011, Van (Turkey) earthquake (Mw = 7.1) by using inversion of teleseismic waveform analysis and its tectonic implications. Focal parameters of the main shock and 21 aftershocks were obtained by using the first motion polarities of regional P-waves. The first results for the source rupture process were derived from broadband teleseismic P-waves. The main outcomes of the analysis are: (a) the main rupture is located around the initial break point, and the maximum slip amount was 3.6 m; (b) the size of the main fault plane area was about 40 km in length and 20 km in width, the duration of rupture was approximately 19 seconds and the seismic moment of the earthquake was estimated to be 5.53 × 1019 N m (Mw = 7.1); (c) the rupture gradually expanded near the hypocenter and propagated both northeast and southwest, but mainly to the southwest. Tectonic implications of the earthquake were defined by field observations. The 23 October, 2011, Van earthquake occurred on a main thrust fault plane trending NE-SW between Lake Van and Lake Ercek located in the East Anatolian compressional province. This main fault plane and the secondary structural elements were generated by a continental-continental collision taking place in a region located 200 km north of the the Bitlis-Zagros Suture Zone.

Detailed geometry of the subducting Indian Plate beneath the Burma Plate and subcrustal seismicity in the Burma Plate derived from joint hypocenter relocation

Abstract: With the aim of delineating the subducting Indian Plate beneath the Burma Plate, we have relocated earthquakes by employing teleseismic P-wave arrival times. We were able to obtain the detailed geometry of the subducting Indian Plate by constructing iso-depth contours for the subduction earthquakes at depths of 30–140 km. The strikes of the contours are oriented approximately N-S, and show an “S” shape in map view. The strike of the slab is N20°Eat25°N, but moving southward, the strike rotates counterclockwise to N20°Wat20°N, followed by a clockwise rotation to a strike of N10°E at 17.5°N, where slab earthquakes no longer occur. The plate boundary north of 20°N might exist near, or west, of the coast line of Myanmar. The mechanisms of subduction earthquakes are down-dip extension, and T axes are oriented parallel to the local dip of the slab. Subcrustal seismicity occurs at depths of 20–50 km in the Burma Plate. This activity starts near the 60-km-depth contour of the subduction earthquakes and becomes shallower toward the Sagaing Fault, indicating that this fault is located where the cut-off depth of the seismicity becomes shallower.

Induced seismicity during the construction of the Gotthard Base Tunnel, Switzerland: hypocenter locations and source dimensions

Abstract: A series of 112 earthquakes was recorded between October 2005 and August 2007 during the excavation of the MFS Faido, the southernmost access point of the new Gotthard Base Tunnel. Earthquakes were recorded at a dense network of 11 stations, including 2 stations in the tunnel. Local magnitudes computed from Wood–Anderson-filtered horizontal component seismograms ranged from −1.0 to 2.4; the largest earthquake was strongly felt at the surface and caused considerable damage in the tunnel. Hypocenter locations obtained routinely using a regional 3-D P-wave velocity model and a constant Vp/Vs ratio 1.71 were about 2 km below the tunnel. The use of seismic velocities calibrated from a shot in the tunnel revealed that routinely obtained hypocenter locations were systematically biased to greater depth and are now relocated to be on the tunnel level. Relocation of the shot using these calibrated velocities yields a location accuracy of 25 m in longitude, 70 m in latitude, and 250 m in focal depth. Double-difference relative relocations of two clusters with highly similar waveforms showed a NW–SE striking trend that is consistent with the strike of mapped faults in the MFS Faido. Source dimensions computed using the quasidynamic model of Madariaga (Bull Seismo Soc Am 66(3):639–666, 1976) range from 50 to 170 m. Overlapping source dimensions for earthquakes within the two main clusters suggests that the same fault patch was ruptured repeatedly. The observed seismicity was likely caused by stress redistribution due to the excavation work in the MFS Faido.

Regional Centroid-Moment-Tensor Analysis for Earthquakes in Canada and Adjacent Regions: An Update

Abstract: Reliable determination of earthquake source parameters is a subject of fundamental importance for seismological research. It also provides critical observational constraints to the study of deformation and stress within the lithosphere. A comprehensive catalog of earthquake source parameters is not only essential to the understanding of global and regional tectonics, but also offers critical information on geological structures that may have engineering, economic, and hazard implications. In addition to the origin time, hypocenter, and magnitude, an earthquake’s source parameters include its focal mechanism, source time function, and the spatiotemporal distribution of slip. However, unless the source dimension is considerably larger than the wavelengths of propagating seismic waves ( e.g., P or S ), the source time function and slip distribution of a seismic event are difficult to resolve, meaning that it is effectively a point-source. The focal mechanism, on the other hand, is directly related to fault movement in the source region and can be determined by several different methods even if the event is an effective point-source. The simplest way to determine an earthquake’s focal mechanism is the “first-motion” method that derives the orientation of the fault plane (strike and dip) and the relative movement between the hanging wall and the foot wall (rake) from the polarity and distribution of first arrivals on the focal sphere (Aki and Richards 1980). A big drawback of this method is that it needs a large number of well-distributed stations with good data quality to sufficiently cover the focal sphere. The process of picking first arrivals and determining their polarity of motion can also be time-consuming, labor-intensive, and often ambiguous, even for well-experienced data analysts. One breakthrough in seismology during the past several decades was the realization that a seismic source can be mathematically represented by a moment tensor consisting of six independent elements (Aki …

A Rayleigh wave back‐projection method applied to the 2011 Tohoku earthquake

Abstract: [1] We study the rupture process of the 2011 Tohoku megathrust by analyzing 384 regional strong-motion records using a novel back-projection method for Rayleigh waves with periods between 13 and 100 s. The proposed approach is based on isolating the signal at the selected period with a continuous wavelet transform, and generating the stack using arrival times predicted from detailed fundamental mode Rayleigh wave group velocity maps. We verify the method by back-projecting synthetic time series representing a point source off the coast of Tohoku, which we generate with a 3D finite difference method and a mesh based on the Japan Integrated Velocity Structure Model. Application of the method to K-NET/KiK-net records of theMw9.1 Tohoku earthquake reveals several Rayleigh wave emitters, which we attribute to different stages of rupture. Stage 1 is characterized by slow rupture down-dip from the hypocenter. The onset of stage 2 is marked by energetic Rayleigh waves emitted from the region between the JMA hypocenter and the trench within 60 s after hypocentral time. During stage 3 the rupture propagates bilaterally towards the north and south at rupture velocities between 3 and 3.5 km · s−1, reaching Iwate-oki 65 s and Ibaraki-oki 105 s after nucleation. In contrast to short-period back-projections from teleseismic P-waves, which place radiation sources below the Honshu coastline, Rayleigh wave emitters identified from our long-period back-projection are located 50–100 km west of the trench. This result supports the interpretation of frequency-dependent seismic wave radiation as suggested in previous studies.

Classification of road damage due to earthquakes

Abstract: Earthquakes cause massive road damage which in turn causes adverse effects on the society. Previous studies have quantified the damage caused to residential and commercial buildings; however, not many studies have been conducted to quantify road damage caused by earthquakes. In this study, an attempt has been made to propose a new scale to classify and quantify the road damage due to earthquakes based on the data collected from major earthquakes in the past. The proposed classification for road damage due to earthquake is called as road damage scale (RDS). Earthquake details such as magnitude, distance of road damage from the epicenter, focal depth, and photographs of damaged roads have been collected from various sources with reported modified Mercalli intensity (MMI). The widely used MMI scale is found to be inadequate to clearly define the road damage. The proposed RDS is applied to various reported road damage and reclassified as per RDS. The correlation between RDS and earthquake parameters of magnitude, epicenter distance, hypocenter distance, and combination of magnitude with epicenter and hypocenter distance has been studied using available data. It is observed that the proposed RDS correlates well with the available earthquake data when compared with the MMI scale. Among several correlations, correlation between RDS and combination of magnitude and epicenter distance is appropriate. Summary of these correlations, their limitations, and the applicability of the proposed scale to forecast road damages and to carry out vulnerability analysis in urban areas is presented in the paper.

Thermal structure and intermediate-depth seismicity in the Tohoku-Hokkaido subduction zones

Abstract: . The cause of intermediate-depth (>40 km) seismicity in subduction zones is not well understood. The viability of proposed mechanisms, which include dehydration embrittlement, shear instabilities and the presence of fluids in general, depends significantly on local conditions, including pressure, temperature and composition. The well-instrumented and well-studied subduction zone below Northern Japan (Tohoku and Hokkaido) provides an excellent testing ground to study the conditions under which intermediate-depth seismicity occurs. This study combines new finite element models that predict the dynamics and thermal structure of the Japan subduction system with a high-precision hypocenter data base. The upper plane of seismicity is principally contained in the crustal portion of the subducting slab and appears to thin and deepen within the crust at depths >80 km. The disappearance of seismicity overlaps in most of the region with the predicted phase change of blueschist to hydrous eclogite, which forms a major dehydration front in the crust. The correlation between the thermally predicted blueschist-out boundary and the disappearance of seismicity breaks down in the transition from the northern Japan to Kurile arc below western Hokkaido. Adjusted models that take into account the seismically imaged modified upper mantle structure in this region fail to adequately recover the correlation that is seen below Tohoku and eastern Hokkaido. We conclude that the thermal structure below Western Hokkaido is significantly affected by time-dependent, 3-D dynamics of the slab. This study generally supports the role of fluids in the generation of intermediate-depth seismicity.

The Sarez-Pamir Earthquake and Landslide of 18 February 1911

Abstract: A hundred years ago 2.4 cubic km of rock fell from a Pamir mountainside >700 m to the valley floor, releasing potential energy equivalent to an Mw 7.8 ± 0.1 earthquake. Its fall created the world’s highest dam, impounding a 17-km3 lake that remains to this day. Seismograms recorded in Europe and Asia registered an earthquake at the approximate time of the fall, and soon after the details of the landslide had been evaluated a controversy arose concerning whether these seismograms had recorded an earthquake that had triggered the landslide, or whether the seismograms had merely registered waves generated by the potential energy release of the landslide’s impact. Boris Galitzin (1915) reasoned that the radiated energy almost exactly equaled the potential energy released by the fall, and hence represented the unique case of the hypocenter and the epicenter being identical. Otto Klotz (1916) translated Galitzin’s article with a preface underlining its importance, and Harold Jeffreys, in a 1923 article, despite revealing flaws in Galitzin’s calculations, confirmed both the approximate coincidence in location of the two events and the equality of energy release computed by Galitzin. However, that same year Richard Oldham dismissed the implications of Jeffreys’s calculations, noting that the maximum epicentral damage was offset from the landslide and that the area of felt shaking was typical of a deep earthquake. Though Oldham’s arguments were eventually to win, it would take another decade before it was realized that it was the long duration of energy release in the landslide that accounted for its apparent absence in distant seismograms. The details of the causal earthquake, and the curious equality in landslide and earthquake energy, have never been fully resolved. We attempt to do so in this article. We quantify what is known of the earthquake and trace the history of …

Liquefaction phenomena along the paleo-Reno River caused by the May 20, 2012, Emilia (northern Italy) earthquake

Abstract: On the May 20, 2012 (04:03:52 local time; 02:03:52 UTC), a moderate earthquake (Ml 5.9) [Scognamiglio et al. 2012, this volume] with a focal mechanism showing E-W-trending, S-dipping, reverse-faulting occurred in the eastern sector of the alluvial plain of the Po River, close to the border between the Regions of Emilia-Romagna and Lombardia (northern Italy). The tectonic structure is completely blind, but it was well known from a dense grid of seismic profiles for hydrocarbon explorations [e.g., Pieri and Groppi 1981, Toscani et al. 2009]. The earthquake triggered extensive liquefaction-induced ground effects at the surface, and caused severe structural damage to nonreinforced masonry and precast industrial buildings within the broader epicentral area. The hypocenter was at 44.89 ˚N, 11.23 ˚E, at a depth of 6.3 km [Scognamiglio et al. 2012], while the maximum acceleration was recorded in Mirandola, with peak ground acceleration 310 cm/s2 and 264 cm/s2 along the vertical and horizontal components, respectively [Bozzoni et al. 2012, this volume]. In this report, we focus on a zone including the Sant'A-gostino, San Carlo and Mirabello villages (west Ferrara Province), which were built along an abandoned reach of the Reno River and where liquefaction phenomena were particularly diffuse, with very intense local effects. […]

Hypocenter distribution and heterogeneous seismic velocity structure in and around the focal area of the 2008 Iwate-Miyagi Nairiku Earthquake, NE Japan - Possible seismological evidence for a fluid driven compressional inversion earthquake

Abstract: We have done seismic tomography in and around the focal area of the 2008 Iwate-Miyagi Nairiku Earthquake (M 7.2) occurred on June 14, 2008 in NE Japan. We used data from temporary aftershock observation network deployed just after the occurrence of the present earthquake. Based on the distribution of aftershocks, the fault plane of the mainshock is inferred to dip to the west. Small immediate foreshocks and preceding seismic activity in 1999–2000 on the fault plane in the vicinity of the hypocenter of the mainshock of this earthquake were observed. Lower-seismic-velocity hanging wall can be imaged in the central and the northern part of the focal area. This possibly suggests the present earthquake is a compressional inversion earthquake. The low-velocity zone in the lower crust extends upward to the upper crust, branches into three portions and reaches each active volcano. This low-velocity region can be seen just beneath the mainshock hypocenter and the whole focal area, suggesting that crustal fluid possibly promote the occurrence of the 2008 earthquake.

Seismicity near the hypocenter of the 2011 off the Pacific coast of Tohoku earthquake deduced by using ocean bottom seismographic data

Abstract: We relocated hypocenters of the foreshock, mainshock, and aftershocks of the 2011 off the Pacific coast of Tohoku earthquake (M 9.0) in the middle part of the Japan Trench where the earthquake rupture initiated. Ocean Bottom Seismographs (OBSs), deployed in the area, recorded the earthquakes and these data provide improved images of the hypocenter distribution. The mainshock hypocenter was relocated slightly westward from that reported by Japan Meteorological Agency (JMA), placing it near the intersection between the plate boundary and the Moho of the overriding plate. The foreshock seismicity mainly occurred on the trenchward side of the mainshock hypocenter, where the Pacific slab contacts the island arc crust. The foreshocks were initially activated at the up-dip limit of the seismogenic zone ~30 km trenchward of the largest foreshock (M 7.3, two days before the mainshock). After the M-7.3 earthquake, intense interplate seismicity, accompanied by epicenters migrating toward the mainshock hypocenter, was observed. The focal depth distribution changed significantly in response to the M-9 mainshock. Earthquakes along the plate boundary were almost non-existent in the area of huge coseismic slip, whereas earthquakes off the boundary increased in numbers in both the upper and the lower plates.

Role of fluids in the earthquake occurrence around Aswan reservoir, Egypt

Abstract: [1] Continuing seismicity for about 30 years near a large western embayment of the Lake Nasser, about 50 km from the Aswan High Dam in Egypt, has led to a debate about the possibility of its relation with the reservoir impoundment. The largest event in the region occurred on 14 November 1981 (M 5.3), 20 km beneath the Wadi Kalabsha embayment, a westward extension of the Lake Aswan. Since then, continuous monitoring of seismic activity has given an excellent opportunity to study the spatiotemporal distribution of seismicity in the area. Most of the immediate aftershocks of the 1981 main shock were located in the Gebel Marawa area at depths between 15 and 30 km. Depths of almost all earthquakes away from this zone were shallower than 12 km. To quantify the effect of the reservoir impoundment on the seismicity of the Aswan area, we calculated changes in stress and pore pressure due to the reservoir impoundment using Green's function approach. The change in Coulomb stress (ΔS) is calculated on the fault planes responsible for majority of the seismicity of the region. We found that for all the seismogenic faults, ΔS is negative, i.e., stabilizing, when we consider the effect of the reservoir load only, whereas it is positive, i.e., destabilizing, when we include pore pressure. For example, at the hypocenter of the main earthquake, shear stress, normal stress, and pore pressure due to reservoir operation are estimated as 5.5, 13.2, and 13.5 kPa, respectively, which suggest that ΔS is −3.1 kPa when we do not consider the effect of pore pressure and 5.7 kPa when contribution from pore pressure is considered. Hence, the seismicity in the Aswan lake region is driven by the pore pressure due to reservoir impoundment.

Rupture Process of the 2011 Tohoku-Oki Earthquake Based upon Joint Source Inversion of Teleseismic and GPS Data

Abstract: This study investigated 18 broadband teleseismic records and 451 near field GPS coseismic deformation data to determine the spatial and temporal slip distribution of the 2011 Tohoku-Oki earthquake (M 9.0). The results show a large triangular shaped slip zone with several asperities. The largest asperity centered above the hypocenter at about 5 - 30 km depth. A secondary large asperity was found in the deeper subduction zone beneath the hypocenter. The average slip on the fault is ~15 m and the maximum displacement on the biggest asperity is ＞ 30 m. The temporal rupture process shows that the slip nucleated near the hypocenter at the beginning, and then ruptured to the shallow fault plane forming the largest asperity. The slip developed in the deeper subduction zone in the second stage. Finally, the rupture propagated toward the north and south of the fault along the Japan Trench. The source time function shows three segments of energy releases with two large peaks related to the development of the asperities. The overall rupture process is ~180 seconds. This source model coincides well with the aftershock distribution and provides a first-order information on the source complexity of the earthquake which is crucial for further studies.

Tunnel heading-along earthquake advanced detection device taking heading machine as earthquake focus and method thereof

Abstract: The invention discloses a tunnel heading-along earthquake advanced detection device taking a heading machine as an earthquake focus and a method thereof. The device comprises an earthquake detector, an earthquake focus signal receiving sensor and an earthquake wave receiving detector array, wherein the earthquake focus signal receiving sensor is arranged on the arm of the heading machine; and the earthquake wave receiving detector array is arranged on a tunnel wall. The method comprises the following steps of: continuously acquiring earthquake wave signals through a three-component sensor which is coupled on the arm of the heading machine and a three-component sensor array which is arranged on tunnel surrounding rock by taking vibration generated by cutting rock with a cutting head during working of the heading machine as a hypocenter; and recording the acquired signals for processing, and forecasting the position and structural detail of a geological anomalous body in front of a tunnel by analyzing received wave field imaging geometrical characteristics. Due to the adoption of the device and the method, the problem of oneness of an earthquake wave field of a pure explosive source in near flat ground layer tunnel advanced detection can be solved without influencing the tunnel heading work; the device has high working efficiency and high detection accuracy, and is free from safety risks; and accurate positioning of a geometrical anomalous body can be realized.