Assessment of expected damage on buildings subjected to Lorca earthquake through an energy-based seismic index method and nonlinear dynamic response analyses

The city of Lorca (Spain) was hit on May 11th, 2011, by two consecutive earthquakes of magnitudes 4.6 and 5.2 M \(_\mathrm{w}\) , causing casualties and important damage in buildings. Many of the damaged structures were reinforced concrete frames with wide beams. This study quantifies the expected level of damage on this structural type in the case of the Lorca earthquake by means of a seismic index \(I_{v}\) that compares the energy input by the earthquake with the energy absorption/dissipation capacity of the structure. The prototype frames investigated represent structures designed in two time periods (1994–2002 and 2003–2008), in which the applicable codes were different. The influence of the masonry infill walls and the proneness of the frames to concentrate damage in a given story were further investigated through nonlinear dynamic response analyses. It is found that (1) the seismic index method predicts levels of damage that range from moderate/severe to complete collapse; this prediction is consistent with the observed damage; (2) the presence of masonry infill walls makes the structure very prone to damage concentration and reduces the overall seismic capacity of the building; and (3) a proper hierarchy of strength between beams and columns that guarantees the formation of a strong column-weak beam mechanism (as prescribed by seismic codes), as well as the adoption of counter-measures to avoid the negative interaction between non-structural infill walls and the main frame, would have reduced the level of damage from \(I_{v}\) \(\,=\,\) 1 (collapse) to about \(I_{v}\) \(\,=\,\) 0.5 (moderate/severe damage).     

The structural role played by masonry infills on RC building performances after the 2011 Lorca,Spain, earthquake

On May 11, 2011 an earthquake of magnitude 5.1 ( \(M_{w}\) ) struck Murcia region causing nine casualties and damage to buildings and infrastructures. Even if the main characteristics of the event would classify it as a moderate earthquake, the maximum Peak Ground Acceleration (PGA) registered (equal to 0.37 g) exceeded significantly local code provisions in terms of hazard at the site. This high PGA was a result of directivity effects in the near source region. An overview of earthquake characteristics and damage observed is provided. Notwithstanding the lack of proper structural design characterizing building stock in the area, most of the losses were caused by non-structural damage. According to in field observations, it emerges that masonry infills provided additional, “not designed”, strength to reinforced concrete (RC) buildings. Observed damage data, collected after the earthquake, are shown and compared to the results of a simplified approach for nonstructural damage assessment of RC infilled structures (FAST vulnerability approach). The latter comparison provided a fair accordance between observed data and analytical results.     

Field evidence and numerical investigation of the \text{ M}_\mathrm{w}= 7.1 October 23 Van, Tabanlı and the \text{ M}_\mathrm{W}> 5.7 November earthquakes of 2011

On Sunday, October 23rd, 2011, the Van province, in the Eastern Turkey, was stricken by a magnitude $\text{ M}_{\mathrm{w}} \!=\! 7.1$ earthquake. The maximum horizontal peak ground acceleration, i.e. 0.182 g, was measured from the seismic station in Muradiye, at about 40 km from the epicenter. Several $\text{ M}_\mathrm{w} > 5.7$ strong motion aftershocks were recorded in November 2011. The exceptionally rich sequence of ground motions was due to the dense seismotectonic activity of the Eastern Turkey, where many active historical faults exist and newly generated can also be found because of the ongoing continental collision between the Arabian and the Eurasian Plates. The 2011 Van earthquake sequence caused 644 casualties, 1966 injuries with 252 rescues; the total economic losses are estimated at around 1 billion US dollars. The present paper deals with the seismological and structural damage assessment of two major seismic events and aftershock sequences in Van region; special emphasis is on the findings of the site investigations performed in the aftermath of the major seismic event. The performed investigation has shown that there is substantial field evidence demonstrating that the losses generated to the local social communities were caused by typical structural and non-structural deficiencies that have been surveyed in the past in several moderate-to-major earthquakes worldwide, especially in poor countries. Comprehensive numerical simulations were also carried out to assess the characteristics of the strong motion records and their effects on existing representative building type of structures in the earthquake-affected region. It was found that the local building stock is highly vulnerable and requires urgent major structural interventions for seismic strengthening. A cost-efficient retrofitting scheme is however not straightforward. It should be a trade-off between two competing aspects: the use of innovative materials and technologies on one hand, and the low-quality of the workmanships and lack of adequate quality control during construction phase, which are available in the Van province, on the other hand.     

Using the damage from 2010 Haiti earthquake for calibrating vulnerability models of typical structures in Port-au-Prince (Haiti)

After the 2010 Haiti earthquake, that hits the city of Port-au-Prince, capital city of Haiti, a multidisciplinary working group of specialists (seismologist, geologists, engineers and architects) from different Spanish Universities and also from Haiti, joined effort under the SISMO-HAITI project (financed by the Universidad Politecnica de Madrid), with an objective: Evaluation of seismic hazard and risk in Haiti and its application to the seismic design, urban planning, emergency and resource management. In this paper, as a first step for a structural damage estimation of future earthquakes in the country, a calibration of damage functions has been carried out by means of a two-stage procedure. After compiling a database with observed damage in the city after the earthquake, the exposure model (building stock) has been classified and through an iteratively two-step calibration process, a specific set of damage functions for the country has been proposed. Additionally, Next Generation Attenuation Models (NGA) and \(\hbox {Vs}^{30}\) models have been analysed to choose the most appropriate for the seismic risk estimation in the city. Finally in a next paper, these functions will be used to estimate a seismic risk scenario for a future earthquake.     

The role of site effects in the comparison between code provisions and the near field strong motion of the Emilia 2012 earthquakes

A damaging seismic sequence hit a wide area mainly located in the Emilia-Romagna region (Northern Italy) during 2012 with several events of local magnitude \(\hbox {M}_\mathrm{l} \ge 5\) , among which the \(\hbox {M}_\mathrm{l}\) 5.9 May 20 and the \(\hbox {M}_\mathrm{l}\) 5.8 May 29 were the main events. Thanks to the presence of a permanent accelerometric station very close to the epicentre and to the temporary installations performed in the aftermath of the first shock, a large number of strong motion recordings are available, on the basis of which, we compared the recorded signals with the values provided by the current Italian seismic regulations, and we observed several differences with respect to horizontal components when the simplified approach for site conditions (based on Vs30 classes) is used. On the contrary, when using the more accurate approach based on the local seismic response, we generally obtain a much better agreement, at least in the frequency range corresponding to a quarter wavelength comparable with the depth of the available subsoil data. Some unresolved questions still remain, such as the low frequency behaviour ( \(<\) 1 Hz) that could be due either to complex propagation at depth larger than the one presently investigated or to near source effects, and the behaviour of vertical spectra whose recorded/code difference is too large to be explained with the information currently available.     

Slope instabilities triggered by the 11th May 2011 Lorca earthquake (Murcia,Spain): comparison to previous hazard assessments and proposition of a new hazard map and probability of failure equation

The Lorca Basin has been the object of recent research aimed at studying the phenomena of earthquake-induced landslides and its assessment in the frame of different seismic scenarios. However, it has not been until the 11th May 2011 Lorca earthquakes when it has been possible to conduct a systematic approach to the problem. In this paper we present an inventory of slope instabilities triggered by the Lorca earthquakes which comprises more than 100 cases, mainly rock and soil falls of small size (1–100  \(\hbox {m}^{3}\) ). The distribution of these instabilities is here compared to two different earthquake-triggered landslide hazard maps: one considering the occurrence of the most probable earthquake for a 475-years return period in the Lorca Basin \((\hbox {M}_{\mathrm{w}}=5.0)\) based on both low- and high-resolution digital elevation model (DEM); and a second one matching the occurrence of the \(\hbox {M}_{\mathrm{w}}=5.2\) 2011 Lorca earthquake, which was performed using the higher resolution DEM. The most frequent Newmark displacements related to the slope failures triggered by the 2011 Lorca earthquakes are lower than 2 cm in both the hazard scenarios considered. Additionally, the predicted Newmark displacements were correlated to the inventory of slope instabilities to develop a probability of failure equation. The fit seems to be very good since most of the mapped slope failures are located on the higher probability areas. The probability of slope failure in the Lorca Basin for a seismic event similar to the \(\hbox {M}_{\mathrm{w}}\) 5.2 2011 Lorca earthquake can be considered as very low (0–4 %).     

Strong motion recorded during the Emilia 2012 thrust earthquakes (Northern Italy): a comprehensive analysis

A complex seismic sequence characterised by two thrust earthquakes of magnitudes M \(_\mathrm{L}\) 5.9 and M \(_\mathrm{L}\) 5.8 occurred on May 20 and 29, 2012, respectively, and activated the central portion of the Ferrara Arc structure beneath the Po Plain in northern Italy. The sequence, referred to as Emilia 2012, was recorded by the Italian Strong Motion Network, the Irpinia Network, the Friuli Venezia Giulia Network and 15 temporary stations installed by the Civil Protection Department. In this study, we compile and analyse a large dataset that contains 3,273 waveforms from 37 \(M_\mathrm{L} \ge 4.0\) seismic events. The main aim of this paper is to characterise the ground motion induced by the Emilia 2012 seismic sequence and compare it with other data in the Italian strong motion database and to the recent Ground Motion Prediction Equations (GMPEs) developed for northern Italy, all of Italy and Europe. This is achieved by (1) the computation and analysis of the strong motion parameters of the entire Emilia Strong Motion Dataset (ESMD) and (2) a comprehensive investigation of the May 29 event recordings in terms of time–frequency analysis, the ground motion parameters and the response spectra. This detailed analysis was made possible by the temporary Civil Protection Department stations that were installed soon after the May 20 event at several municipalities in the epicentral area. Most of the recordings are characterised by low-frequency content and long durations, which is a result of the thick sedimentary cover that is typical of the Po Plain. The distributions of the observed horizontal peak ground accelerations and velocities (PGAs and PGVs) with distance are generally consistent with the GMPEs. This is particularly true for the data from M \(_\mathrm{L} \ge \) 5.0 (M \(_\mathrm{W}\ge \) 5.0) events, though the data are scattered at distances beyond approximately 60–70 km and show faster attenuation than the European GMPEs. The horizontal components for the May 29 event at two near-fault stations (Mirandola and San Felice sul Panaro) are overestimated by all of the analysed GMPEs. In contrast, the vertical components, which played an important role in the shaking near the source, are underestimated. The May 29 event produced intense velocity pulses on the horizontal components and the highest peak ground acceleration ever recorded in Italy on the vertical component of the Mirandola near-fault station. The ground motion recordings contained in the ESMD significantly enrich the Italian strong motion database. They contribute new information about (1) the possibility of exceeding the largest recorded PGA in Italy, (2) the development of a spectral design that takes into account the role of the vertical component and the extreme variability of the near-fault ground shaking, and (3) the characterisation of the ground motions in deep sedimentary basins.     

Performance of masonry buildings during the Emilia 2012 earthquake

The earthquake sequence started on May \(20\) th 2012 in Emilia (Italy) affected a region where masonry constructions represent a large part of the existing building stock and the construction of new modern masonry buildings is a common practice. The paper is focused on the performance of common architectural configurations, typical for residential or business use. The large majority of old masonry buildings is made of fired clay bricks. The seismic performance of these buildings is particularly interesting since major past earthquakes in Italy affected areas with mainly stone masonry structures. Apart from examples showing systematic or peculiar structural deficiencies governing the vulnerability of several buildings, the overall seismic performance of these structures to repeated shaking, with PGA as large as 0.25–0.3 g was rather good, despite the major part of them were only conceived for carrying vertical loads. In fact, seismic design is mandatory in the area only since 2003. Modern low-rise masonry buildings erected after this date and incorporating seismic design and proper detailing resulted in most cases practically undamaged. The examples reported in the paper allow an evaluation of the superior performance of seismically designed modern masonry buildings in comparison to older ones.     

Identification of the structural model and analysis of the global seismic behaviour of a RC damaged building

The study of the structural behaviour of damaged RC buildings during ground motion is a fundamental topic in the modern earthquake engineering. Many studies have been carried out in order to better understand the real evolution of the damage in RC buildings during a seismic event. In this work, a damaged RC building has been intensively investigated in terms of materials property and stiffness evolution in order to interpreting the structural and nonstructural surveyed damage. The peculiarity of this building is its damage sequence during the 2002 Molise earthquake. In fact, the town of Bonefro suffered moderate damage (MCS intensity VII), with the exception of the investigated reinforced concrete building. The October 31, 2002 event (M=5.4) caused some structural damage to this building. The second event (M=5.3), on November 1, 2002, increased substantially the damage level (grade 4 according to the 1998 European Macroseismic Scale). It occurred just while, due to fortuitous circumstances, a 5 min. seismic velocimetric recording was being taken. The working group has performed some frequency analyses based on the recording. Several non linear models have been defined to understand the damage evolution of the building and the local and global damage patterns through for static analyses. Finally, linear and non linear models have been developed with the main goal of identifying the characteristics of a reliable undamaged structural model.     

The 2011 Lorca seismic series: Temporal evolution,faulting parameters and hypocentral relocation

The Lorca 2011 seismic series was recorded by an unprecedented set of high quality on scale broadband seismograms and strong motion accelerograms. The waveforms from permanent and temporary broadband seismic networks deployed in the region by different institutions allowed to invert regional moment tensor for the fore, main and largest aftershock of the complete seismic sequence. Using double-difference algorithm we have performed a precise relocation of the seismic series, where body wave travel times from strong ground motion accelerograms were included in the data set. Regional moment tensor inversion for the three main events show similar oblique-reverse faulting regime with a northeast-southwest fault orientation. The scalar seismic moment, moment magnitude and focal depth retrieved from the inversion yield the following values for each event: \(\hbox {Mo}=6.5\times 10^{16}\) Nm (Mw = 5.2) for the mainshock, \(\hbox {Mo}= 9.6 \times 10^{15}\) Nm (Mw = 4.6) for the foreshock and \(\hbox {Mo}=7.3\times 10^{14}\) Nm (Mw = 3.9) for the large aftershock. The centroid depths range between 4 and 6 km. The double-difference relocation of the seismic series shows significant epicentral differences with the preliminary routine location. The epicentral solutions given by this relocation show a seismic sequence distributed following a NE–SW strike, subparallel to the Alhama de Murcia fault and compatible with the faulting parameters inverted from the moment tensor analysis. The hypocenters of the series generate a subvertical trend in depth distribution, being concentrated between 2 and 6 km. The depth distribution of the main events, which range from 4.6 to 5.5 km, is in good relationship with the faulting and depth parameters deduced from the moment tensor inversion technique. The regional moment tensor solutions for the three largest earthquakes, the epicentral distribution and the focal depths show good relationship with the surface geometry and tectonic regime of the Alhama de Murcia fault. The stress drop deduced for the mainshock gives a value ranging between 58 and 85 bars, which does not support the idea of a high stress drop release as a main factor contributing to the high ground acceleration recorded at Lorca. The PGA values observed at Lorca, which contributed to the high damage independently of structural deficiencies, could be generated mainly by shallowness and proximity to the seismic source together with a directivity effect in the seismic radiation.     

Consideration of aging and SSI effects on seismic vulnerability assessment of RC buildings

At present, the seismic vulnerability assessment of reinforced concrete (RC) buildings is made considering fixed base conditions; moreover, the mechanical properties of the building remain intact in time. In this study we investigate whether these two fundamental hypotheses are sound as aging and soil-structure interaction (SSI) effects might play a crucial role in the seismic fragility analysis of RC structures. Among the various aging processes, we consider the chloride-induced corrosion based on probabilistic modeling of corrosion initiation time and corrosion rate. Different corrosion aspects are considered in the analysis including the loss of reinforcement cross-sectional area, the degradation of concrete cover and the reduction of steel ultimate deformation. SSI is modeled by applying the direct one-step approach, which accounts simultaneously for inertial and kinematic interactions. Two-dimensional incremental dynamic analysis is performed to assess the seismic performance of the initial uncorroded ( \(\hbox {t}=0\) years) and corroded ( \(\hbox {t}=50\) years) RC moment resisting frame structures, having been designed with different seismic code levels. The time-dependent fragility functions are derived in terms of the spectral acceleration at the fundamental mode of the structure \(\hbox {S}_{\mathrm{a}}(\hbox {T}_{1}\) , 5 %) and the outcropping peak ground acceleration for the immediate occupancy and collapse prevention limit states. Results show an overall increase in seismic vulnerability over time due to corrosion highlighting the important influence of deterioration due to aging effects on the structural behavior. Moreover, the consideration of SSI and site effects may significantly alter the expected structural performance leading to higher vulnerability values.     

Changes in dynamic characteristics of Lorca RC buildings from pre- and post-earthquake ambient vibration data

This paper estimates fundamental translational period and damping ratio parameters and examines the changes in dynamic characteristics of a set of low-to-medium rise buildings in Lorca town (SE of Spain) affected by the May 11th, 2011 earthquake. These building parameters have been calculated analysing structural dynamic response from ambient vibration measurements recorded at top RC buildings pre- and post earthquake, using the Fast Fourier Transform and the Randomdec technique. The empirical expression relating period \((T)\) and number of floor \((N)\) here obtained analysing ambient noise recorded on 59 healthy buildings before the earthquake is \(T= (0.054\pm 0.002)\, N\) , very similar to others empirical period–height relationships obtained for RC structures in the European built environment but quite different from code provisions. Measurements performed in 34 damaged buildings show a period elongation after the quake according to \(T^{*} =(0.075\pm 0.002)\,N\) expression. Moreover, we found a rise of the fundamental period with the EMS’s grade of damage of buildings. In contrast to natural frequency, damping ratio \((\xi )\) do not shows a significant variation with earthquake damage degree and the product \(\xi \, T\) remains near constant.     

Characterizing and Locating Very Weak (−2.2 ≥ M L ≥ −3.4) Induced Seismicity in Unstable Sandstone Cliffs by Nanoseismic Monitoring

Unloaded natural rock masses are known to generate seismic signals (Green et al., 2006; Hainzl et al., 2006; Husen et al., 2007; Kraft et al., 2006). Following a 1,000 m³ mass failure into the Mediterranean Sea, centimeter-wide tensile cracks were observed to have developed on top of an unstable segment of the coastal cliff. Nanoseismic monitoring techniques (Wust-Bloch and Joswig, 2006; Joswig, 2008), which function as a seismic microscope for extremely weak seismic events, were applied to verify whether brittle failure is still generated within this unconsolidated sandstone mass and to determine whether it can be detected. Sixteen days after the initial mass failure, three small-aperture sparse arrays (Seismic Navigation Systems-SNS) were deployed on top of this 40-m high shoreline cliff. This paper analyzes dozens of spiky nanoseismic (?2.2 ≥ M _L ≥ ?3.4) signals recorded over one night in continuous mode (at 200 Hz) at very short slant distances (3–67 m). Waveform characterization by sonogram analysis (Joswig, 2008) shows that these spiky signals are all short in duration (>0.5 s). Most of their signal energy is concentrated in the 10–75 Hz frequency range and the waveforms display high signal similarity. The detection threshold of the data set reaches M _L ?3.4 at 15 m and M _L ?2.7 at 67 m. The spatial distribution of source signals shows 3-D clustering within 10 m from the cliff edge. The time distribution of M _L magnitude does not display any decay pattern of M _L over time. This corroborates an unusual event decay over time (modified Omori’s law), whereby an initial quiet period is followed by regained activity, which then fades again. The polarization of maximal waveform amplitude was used to estimate spatial stress distribution. The orientation of ellipses displaying maximal signal energy is consistent with that of tensile cracks observed in the field and agrees with rock mechanics predictions. The M _L– surface rupture length relationship displayed by our data fits a constant-slope extrapolation of empirical data collected by Wells and Coppersmith (1994) for normal fault features at much larger scale. Signal characterization and location as well as the absence of direct anthropogenic noise sources near the monitoring site, all indicate that these nanoseismic signals are generated by brittle failure within the top section of the cliff. The atypical event decay over time that was observed suggests that the cliff material is undergoing post-collapse bulk strain accommodation. This feasibility study demonstrates the potential of nanoseismic monitoring in rapidly detecting, locating and analyzing brittle failure generated within unconsolidated material before total collapse occurs.     

On the ability of Moho reflections to affect the ground motion in northeastern Italy: a case study of the 2012 Emilia seismic sequence

It has been observed that post-critically reflected S-waves and multiples from the Moho discontinuity could play a relevant role on the ground motion due to medium to strong size earthquakes away from the source. Although some studies investigated the correlation between the Moho reflections amplitudes and the damage in the far field, little attention was given to the frequency content of these specific phases and their scaling with magnitude. The 2012 Emilia seismic sequence in northern Italy, recorded by velocimetric and accelerometric networks, is here exploited to investigate Moho reflections and multiples (SmSM). A single station method for group velocity-period estimation, based on the multiple filter technique, is applied to strong motion data to detect SmSM. Amplitude and frequency scaling with magnitude is defined for earthquakes from \(\hbox {Mw}=3.9\) to \(\hbox {Mw}=5.9\) . Finally, the ability of SmSM to affect the ground motion for a maximum credible earthquake within the Po plain is investigated by extrapolating observed engineering parameters. Data analysis shows that high amplitude SmSM can be recognized within the Po plain, and at the boundaries between the Po plain and the Alpine chain, at epicentral distances larger than 80 km, in the period range from 0.25 to 3 s and in the group velocity window from about 2.6 to 3.2 km/s. 5 % damped pseudo-spectral accelerations at different periods (0.3, 1.0 and 2.0 s), and Housner intensities, are obtained from data characterized by large amplitude SmSM. A scaling relationship for both pseudo-spectral accelerations and Housner intensities is found for the earthquakes of the 2012 Emilia seismic sequence. \(\hbox {I}_{\mathrm{MCS}}\) from VII to VIII is estimated, as a result of SmSM amplitude enhancement, at about 100 km for a maximum credible earthquake ( \(\hbox {Mw}=6.7\) ) in the Po plain, showing that moderate to high damage cloud be caused by these specific phases.     

Shakemaps uncertainties and their effects in the post-seismic actions for the 2012 Emilia (Italy) earthquakes

The May 20, 2012, Emilia Ml 5.9 earthquake was followed by some major aftershocks, well recorded by a large number of temporary stations that were installed to monitor the sequence. These additional recordings allowed us a thorough testing of the performance of the ShakeMap—a procedure designed to provide rapid information on the experienced ground motion. We found that the shakemaps for the May 29, 2012, Ml 5.8 earthquake, obtained using the permanent stations only, underestimate significantly the ground motion computed with the highest station density, especially for PSA at long periods (T \(=\) 3.0 s). This low-frequency motion is controlled primarily by the surface waves recorded in the Po plain: the observed site effects are likely not accounted properly by the site correction coefficient based on Vs30 as implemented in the ShakeMap procedure. The shakemaps determined during the seismic sequence have been included in an Italian national law that was passed after the 2012 earthquake. According to this law, the factories safety verifications were bound to the comparison between the shakemaps and the design acceleration required by the current national seismic code. We then decide to appraise the impact of the shakemap accuracy on the law provisions. Following the law recommendations, we have estimated the possible errors resulting from the incomplete evaluation of the ground shaking: our results show that, if the complete dataset were available at the time of the law approval, the number of buildings for which the safety check was required would have been significantly smaller.     

Performance-based seismic design of nonstructural building components:The next frontier of earthquake engineering

With the development and implementation of performance-based earthquake engineering,harmonization of performance levels between structural and nonstructural components becomes vital. Even if the structural components of a building achieve a continuous or immediate occupancy performance level after a seismic event,failure of architectural,mechanical or electrical components can lower the performance level of the entire building system. This reduction in performance caused by the vulnerability of nonstructural components has been observed during recent earthquakes worldwide. Moreover,nonstructural damage has limited the functionality of critical facilities,such as hospitals,following major seismic events. The investment in nonstructural components and building contents is far greater than that of structural components and framing. Therefore,it is not surprising that in many past earthquakes,losses from damage to nonstructural components have exceeded losses from structural damage. Furthermore,the failure of nonstructural components can become a safety hazard or can hamper the safe movement of occupants evacuating buildings,or of rescue workers entering buildings. In comparison to structural components and systems,there is relatively limited information on the seismic design of nonstructural components. Basic research work in this area has been sparse,and the available codes and guidelines are usually,for the most part,based on past experiences,engineering judgment and intuition,rather than on objective experimental and analytical results. Often,design engineers are forced to start almost from square one after each earthquake event: to observe what went wrong and to try to prevent repetitions. This is a consequence of the empirical nature of current seismic regulations and guidelines for nonstructural components. This review paper summarizes current knowledge on the seismic design and analysis of nonstructural building components,identifying major knowledge gaps that will need to be filled by future research. Furthermore,considering recent trends in earthquake engineering,the paper explores how performance-based seismic design might be conceived for nonstructural components,drawing on recent developments made in the field of seismic design and hinting at the specific considerations required for nonstructural components.     

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.     

Seismic Source Characteristics of Nuclear and Chemical Explosions in Granite from Hydrodynamic Simulations

Seismic source characteristics of low-yield (0.5–5 kt) underground explosions are inferred from hydrodynamic simulations using a granite material model on high-performance (parallel) computers. We use a non-linear rheological model for granite calibrated to historical near-field nuclear test data. Equivalent elastic P-wave source spectra are derived from the simulated hydrodynamic response using reduced velocity potentials. Source spectra and parameters are compared with the models of Mueller and Murphy (Bull Seism Soc Am 61:1675–1692, 1971, hereafter MM71) and Denny and Johnson (Explosion source phenomenology, pp 1–24, 1991, hereafter DJ91). The source spectra inferred from the simulations of different yields at normal scaled depth-of-burial (SDOB) match the MM71 spectra reasonably well. For normally buried nuclear explosions, seismic moments are larger for the hydrodynamic simulations than MM71 (by 25 %) and for DJ91 (by over a factor of 2), however, the scaling of moment with yield across this low-yield range is consistent for our calculations and the two models. Spectra from our simulations show higher corner frequencies at the lower end of the 0.5–5.0 kt yield range and stronger variation with yield than the MM71 and DJ91 models predict. The spectra from our simulations have additional energy above the corner frequency, probably related to non-linear near-source effects, but at high frequencies the spectral slopes agree with the f ^?2 predictions of MM71. Simulations of nuclear explosions for a range of SDOB from 0.5 to 3.9 show stronger variations in the seismic moment than predicted by the MM71 and DJ91 models. Chemical explosions are found to generate higher moments by a factor of about two compared to nuclear explosions of the same yield in granite and at normal depth-of-burial, broadly consistent with comparisons of nuclear and chemical shots at the US Nevada Test Site (Denny, Proceeding of symposium on the non-proliferation experiment, Rockville, Maryland, 1994). For all buried explosions, the region of permanent deformation and material damage is not spherical but extends along the free surface above and away from the source. The effect of damage induced by a normally buried nuclear explosion on seismic radiation is explored by comparing the motions from hydrodynamic simulations with those for point-source elastic Green’s functions. Results show that radiation emerging at downward takeoff angles appears to be dominated by the expected isotropic source contribution, while at shallower angles the motions are complicated by near-surface damage and cannot be represented with the addition of a simple secondary compensated linear vector dipole point source above the shot point. The agreement and differences of simulated source spectra with the MM71 and DJ91 models motivates the use of numerical simulations to understand observed motions and investigate seismic source features for underground explosions in various emplacement media and conditions, including non-linear rheological effects such as material strength and porosity.