Small Magnitude Earthquakes Research Articles

Coda wave attenuation characteristics for Kumaon and Garhwal Himalaya, India

Uttarakhand Himalayas are among one of the most seismically active continental regions of the world. The Himalayan belt in this region is divided into Kumaon and Garhwal Himalaya. The Kumaon Himalayas has not experienced any large earthquake in the recent past; however, this region is frequently visited by small-magnitude earthquakes. The objective of this paper was to determine regional attenuation characteristics from data recorded by local strong motion networks operating in these two regions, using coda waves. Coda wave quality factor estimates for the Kumaon and the Garhwal Himalaya region have been made in high-frequency range using single backscattering model given by Aki and Chouet (J Geophys Res 80:3322–3342, 1975). Coda waves of 30-s window length, filtered at six different frequency bands, centered at 1.5, 3.0, 6.0, 12.0, 20.0 and 28.0 Hz, have been analyzed using the single backscattering method. The frequency-dependent Q c(f) relationship for the Kumaon and Garhwal Himalaya was obtained as Q c = (65 ± 2.4)f (1.07±.04) and Q c = (96 ± 6.9)f (1.06±.04), respectively, which indicate a higher attenuating medium in the Kumaon Himalaya as compared to Garhwal Himalaya. The obtained difference of attenuation characteristics in this region is further compared with the rate of decay of peak ground acceleration with respect to magnitude and distance parameters by using regression model of peak ground acceleration obtained from available strong motion data in this region. Present studies clearly indicate that there is good evidence for high rate of attenuation in the Kumaon Himalaya as compared to the Garhwal Himalaya.

A first look at the Gargano (southern Italy) seismicity as seen by the local scale OTRIONS seismic network

<p>On April 2013, a local scale seismic network, named OTRIONS, composed of twelve short period (1 Hz) three component seismometers, has been located in the northern part of the Apulia (southern Italy). In the first two months of data acquisition, the network recorded about one hundred very small (M<span><sub>L</sub></span><2) magnitude earthquakes. A three-layer 1D V<span><sub>P</sub></span> velocity model was preliminarily computed, using the recordings of earthquakes occurred in the area in the period 2006-2012 and recorded by the national seismic network of INGV (Istituto Nazionale di Geofisica e Vulcanologia). This model was calibrated by means of a multi-scale approach, based on a global search of the minimum misfit between observed and theoretical travel times. At each step of the inversion, a grid-search technique was implemented to infer the elastic properties of the layers, by using HYPO71 to compute the forward models. In a further step, we used P and S travel times of both INGV and OTRIONS events to infer a minimum 1D V<span><sub>P</sub></span> velocity model, using a classical linearized inversion approach. Owing to the relatively small number of data and poor coverage of the area, in the inversion procedure, the V<span><sub>P</sub></span>/V<span><sub>S</sub></span> ratio was fixed to 1.82, as inferred from a modified Wadati diagram. The final 1D velocity model was obtained by averaging the inversion results arising from nine different initial velocity models. The inferred V<span><sub>P</sub></span> velocity model shows a gradual increase of P wave velocity with increasing the depth. The model is well constrained by data until to a depth of about 25-30 km.</p>

NGA-West2 Ground Motion Model for the Average Horizontal Components of PGA, PGV, and 5% Damped Linear Acceleration Response Spectra

We used an expanded PEER NGA-West2 database to develop a new ground motion prediction equation (GMPE) for the average horizontal components of PGA, PGV, and 5% damped linear pseudo-absolute acceleration response spectra at 21 periods ranging from 0.01 s to 10 s. In addition to those terms included in our now superseded 2008 GMPE, we include a more-detailed hanging wall model, scaling with hypocentral depth and fault dip, regionally independent geometric attenuation, regionally dependent anelastic attenuation and site conditions, and magnitude-dependent aleatory variability. The NGA-West2 database provides better constraints on magnitude scaling and attenuation of small-magnitude earthquakes, where our 2008 GMPE was known to be biased. We consider our new GMPE to be valid for estimating horizontal ground motion from shallow crustal continental earthquakes in an active tectonic domain for rupture distances ranging from 0 km to 300 km and magnitudes ranging from 3.3 to 7.5–8.5, depending on source mechanism.

A Paleoseismic Record of Earthquakes for the Dead Sea Transform Fault between the First and Seventh Centuries C.E.: Nonperiodic Behavior of a Plate Boundary Fault

We present new results from a paleoseismic trenching campaign at a site across the Jordan Gorge Fault (JGF), the primary strand of the Dead Sea Transform in northern Israel. In addition to the previously recognized earthquakes of 1202 and 1759 C.E., we observe evidence for eight surface-rupturing earthquakes prior to the second millennium C.E. The past millennium appears deficient in strain release with the occurrence of only two large ruptures, when compared with the preceding 1200 years. Assuming Gutenberg-Richter magnitude-frequency distribution, there is a dis- crepancy between measured rate of small-magnitude earthquakes (M <4) from instrumental records and large earthquake rates from paleoseismic records. The in- terevent time of surface-rupturing earthquakes varies by a factor of two to four during the past 2 ka at our site, and the fault's behavior is not time predictable. The JGF may be capable of rupturing in conjunction with both of its southern and northern neigh- boring segments, and there is tentative evidence that earthquakes nucleating in the Jordan Valley (e.g., the 749 C.E. earthquake) could either rupture through the stepover between the faults or trigger a smaller event on the JGF. We offer a model of earth- quake production for this segment in which the long-term slip rate remains constant while differing earthquake sizes can occur, depending on the segment from which they originated and the time since the last large event. The rate of earthquake occurrence in this model does not produce a time-predictable pattern over a period of 2 ka as a result of the interplay between fault segments to the south and north of the JGF. Online Material: High-resolution photomosaic logs of trench walls probability density function (PDF) and table of unit descriptions for channels 3 and 4.

Source Parameters and Moment Magnitude of 30 January 2012 Earthquake, Northern Red Sea

On 30 January 2012, a small magnitude earthquake of m b 4.6 occurred at 17:04:25.55 near Ras Mohamed area, at the southern end of the Gulf of Suez (European‐Mediterranean Seismological Center [EMSC]). This event is located very close to the active triple junction zone where the northern most part of the Red Sea bifurcates into the Gulf of Suez and Gulf of Aqaba. This zone is characterized by swarm type activities which continue on the order of days or weeks (e.g., Maamoun et al. , 1984; Kulhanek et al. , 1992, Badawy and Abdel‐Fattah, 2006; Hussein et al. , 2006). Earthquakes of this type are generally associated with magmatic, volcanic, and manmade activities like fluid injection into the subsurface. According to EMSC reports, this event was felt by people at distances of 36, 67, and 81 km from the earthquake epicenter. The reporting locations lies approximately 80 km east of El Tur city, 56 km east of Ras Gharib, and 4 km southwest of Hurghada city, respectively (Fig. 1). The observed intensity of V ( MM ) was assigned to the region, which is located about 4 km southwest (SW) of Hurghada city. The felt reports of the earthquake were only received from these few points. A number of persons heard the cracking sound of doors. There was also a sound like a rumbling similar to thunder or a big explosion. Moreover, the evidences show minor damage; books fell off and all pictures were askew. The source parameters of the 30 January 2012 earthquake, as determined by EMSC, are summarized in Table 1 (origin time, location, magnitude, focal depth, and the estimated uncertainty in both epicenter and depth). Furthermore, the seismic waves generated by this event were recorded by a …

Rapid Estimation of Peak Ground Velocity and Earthquake Location Using Small Magnitude Earthquakes in the Tehran Region, Iran

Online Material: Supplemental tables. The Tehran metropolis is located in an extensive area between the Alborz mountain belt to the north and the central Iranian plateau to the south. Tehran, as the political and economic capital of Iran, is considered one of the most populated cities in the world. From the point of view of vulnerability to natural disasters, Tehran is of great concern because of its basic infrastructures and population concentration. The Alborz mountain belt is a part of the Himalayan–Alps orogenic belt with numerous active faults, including the Mosha reverse fault, the North Tehran thrust fault, the Parchin, the Kahrizak, and the South and North Rey faults, with high seismic potential in the study region (Fig. 1). The Mosha fault is one of the fundamental structures of the central Alborz mountains and is situated north of Tehran. This fault is concave to the north and extends from the edge of the mountain range in the west to the eastern Alborz mountains and can be considered an upthrust in some places and an overthrust in others (Moinfar et al. , 1994; Berberian and Yeats, 1999). The North Tehran fault is the most prominent tectonic structure in the immediate vicinity of Tehran for which the trace is east–west to east‐northeast–west‐southwest and slightly concave to the south (Berberian and Yeats, 1999). The Tehran urban district is extended across thick alluvium layers over bedrock with complex geological structure. Based on historic seismic documents, Tehran is surrounded by several specific faults with return periods of about 150 years, but no damaging earthquake has occurred since 1830 (Moinfar et al. , 1994; Berberian and Yeats, 1999), when an estimated 45,000 …

Seismotectonics of the 2008 and 2009 Qaidam Earthquakes and its Implication for Regional Tectonics

Abstract:Three magnitude &gt;6 earthquakes struck Qaidam, Qinghai province, China, in November 10th 2008, August 28th and 31st 2009 respectively. The Zongwulongshan fault has often been designated as the active seismogenic structure, although it is at odd with the data. Our continuous GPS station (CGPS), the Xiao Qaidam station, located in the north of the Qaidam basin, is less than 30 km to the southwest of the 2008 earthquake. This CGPS station recorded the near field co‐seismic deformation. Here we analyzed the co‐seismic dislocation based on the GPS time series and the rupture processes from focal mechanism for the three earthquakes. The aftershocks were relocated to constrain the spatial characteristics of the 2008 and 2009 Qaidam earthquakes. Field geological and geomorphological investigation and interpretation of satellite images show that the Xitieshan fault and Zongwulongshan fault were activated as left lateral thrust during the late Quaternary. Evidence of folding can also be identified. Integrated analyses based on our data and the regional tectonic environment show that the Xitieshan fault is the fault responsible for the 2008 Qaidam earthquake, which is a low dip angle thrust with left lateral strike slip. The Zongwulongshan fault is the seismogenic fault of the 2009 earthquakes, which is a south dipping back thrust of the northern marginal thrust system of the Qaidam basin. Folding takes a significant part of the deformation in the northern marginal thrust system of the Qaidam basin, dominating the contemporary structure style of the northern margin of the Qaidam basin and Qilianshan tectonic system. In this region, this fault and fold system dominates the earthquake activities with frequent small magnitude earthquakes.

Qc, Qβ, Qi and Qs attenuation parameters in the Umbria–Marche (Italy) region

The attenuation of coda and S waves has been inferred for the Umbria–Marche region (central Italy) using seismic waveforms collected during the 1997 seismic crisis. The selected dataset is composed of 343 small magnitude (1.4<ML<4.2) earthquakes recorded at a temporary array composed of 23 seismic stations. The Sato (1977) method, based on the assumption of single isotropic scattering has been used to infer Qc, considering three different lapse times (20, 30 and 40s). The coda normalization method (Aki, 1980) has been used to infer Qβ. Both Qc and Qβ show a clear frequency dependence with a different frequency dependent parameter. The frequency dependence of Qc is comparable with that previously found in the same area and around it. Using the method of Wennerberg (1993), intrinsic and scattering attenuation have been separated. Intrinsic attenuation is found to be close to coda attenuation and dominates over scattering dissipation. Coda Q increases with increasing lapse time but at a rate smaller than that observed in other areas of the Earth. Coda and intrinsic attenuation in the Umbria–Marche region are very high compared to other seismic active regions of the Earth. The retrieved high values of intrinsic attenuation may be explained in terms of the previously hypothesized fluid-pressurized regime of the crust in the central Apennines (Miller et al., 2004).

InSAR Evidence for an active shallow thrust fault beneath the city of Spokane Washington, USA

In 2001, a nearly five month long sequence of shallow, mostly small magnitude earthquakes occurred beneath the city of Spokane, a city with a population of about 200,000, in the state of Washington. During most of the sequence, the earthquakes were not well located because seismic instrumentation was sparse. Despite poor‐quality locations, the earthquake hypocenters were likely very shallow, because residents near the city center both heard and felt many of the earthquakes. The combination of poor earthquake locations and a lack of known surface faults with recent movement make assessing the seismic hazards related to the earthquake swarm difficult. However, the potential for destruction from a shallow moderate‐sized earthquake is high, for example Christchurch New Zealand in 2011, so assessing the hazard potential of a seismic structure involved in the Spokane earthquake sequence is important. Using interferometric synthetic aperture radar (InSAR) data from the European Space Agency ERS2 and ENVISAT satellites and the Canadian Space Agency RADARSAT‐1, satellite we are able to show that slip on a shallow previously unknown thrust fault, which we name the Spokane Fault, is the source of the earthquake sequence. The part of the Spokane Fault that slipped during the 2001 earthquake sequence underlies the north part of the city, and slip on the fault was concentrated between ~0.3 and 2 km depth. Projecting the buried fault plane to the surface gives a possible surface trace for the Spokane Fault that strikes northeast from the city center into north Spokane.

Structural control on along-strike variation in the seismicity of the northwest Himalaya

An overview of seismicity along the northwest Himalaya exhibit along-strike segmentation intricately controlled by the subsurface topographic ridges on the underthrusting Indian Plate as well by rift and nappe structures in the overriding wedge of the Himalaya. The segmentation exists for thrust dominated large magnitude earthquakes (M>6) seated on the active detachment beneath the Outer and Lesser Himalaya. Segmentation also prevails for moderate and small magnitude earthquakes concentrated in a narrow Himalaya Seismic Belt (HSB) straddling northern Lesser Himalaya and southern Higher Himalaya. Numerical calculations of stress distribution favour that the degree of seismicity in the HSB is a good proxy to the presence of mid-crustal ramp connecting the locked section of active detachment to aseismically slipping detachment beneath Higher Himalaya. Further, gap or diffused pattern in the concentrated seismicity in the HSB, in agreement with mapped high electrical conductive structure, suggests ramp structure may be absent where underthrusting Delhi-Hardwar Ridge interacts with the Himalaya arc. In the nappe dominated tectonics, the accommodation of the accumulating strains on the listric thrust faults, produces increased frequency of moderate magnitude earthquakes and thereby possibly influences stress level on the detachment. This may explain relatively less frequent occurrences of large earthquake in the Kangra–Chamba region compared to that in the Garhwal Himalaya. The low level seismicity in the Simla region may also be influenced by the active Kaurik Chango Rift in the Higher Himalaya, as the former casts a stress shadow on the latter.

Earthquake prediction: 20 years of global experiment

Earthquake professionals have for many decades recognized the benefits to society from reliable earthquake predictions, but uncertainties regarding source initiation, rupture phenomena, and accuracy of both the timing and magnitude of the earthquake occurrence have oftentimes seemed either very difficult or impossible to overcome. The problem is that most of these methods cannot be adequately tested and evaluated either because of (a) lack of a precise definition of “prediction” and/or (b) shortage of data for meaningful statistical verification. This is not the case for the pattern recognition algorithm M8 designed in 1984 for prediction of great, Magnitude 8, earthquakes, hence its name. By 1986, the algorithm was rescaled for applications aimed at smaller magnitude earthquakes, down to M5+ range, and since then it has become a useful tool for systematic monitoring of seismic activity in a number of test seismic regions worldwide. After confirmed predictions of both the 1988 Spitak (Armenia) and the 1989 Loma Prieta (California) earthquakes, a “rigid test” to evaluate the efficiency of the intermediate-term middle-range earthquake prediction technique has been designed. Since 1991, each half-year, the algorithm M8 alone and in combination with its refinement MSc has been applied in a real-time prediction mode to seismicity of the entire Earth, and this test outlines, where possible, the areas in the two approximations where magnitude 8.0+ and 7.5+ earthquakes are most likely to occur before the next update. The results of this truly global 20-year-old experiment are indirect confirmations of the existing common features of both the predictability and the diverse behavior of the Earth’s naturally fractal lithosphere. The statistics achieved to date prove (with confidence above 99 %) rather high efficiency of the M8 and M8-MSc predictions limited to intermediate-term middle- and narrow-range accuracy. These statistics support the following general conclusions—(1) precursory seismic patterns do exist; (2) the size of an area where precursory seismic patterns show up is much larger than that of the source zone of the incipient target earthquake; (3) many precursory seismic patterns appear to be similar, even in regions of fundamentally different tectonic environments; and (4) some precursory seismic patterns are analogous to those in advance of extreme catastrophic events in other complex nonlinear systems (e.g., magnetic storms, solar flares, “starquakes”, etc.)—that are of high importance for further searches of the improved earthquake forecast/prediction algorithms and methods.

Seismotectonic state of reservoirs inferred from magnitude distributions of fluid-induced seismicity

Fluid injections in geothermal reservoirs usually induce small magnitude earthquakes (M < 2). Sometimes, however, earthquakes with larger magnitudes (M ~4) occur. Recently, we have shown that under rather general conditions, the probability of an event having a magnitude larger than a given one increases proportionally to the injected fluid mass. The number of earthquakes larger than a given magnitude also depends on the tectonic conditions of an injection site. A convenient parameter for the characterisation of the seismotectonic state of a reservoir location is the seismogenic index Σ. It combines four, generally unknown, site-specific seismotectonic quantities. Using this index, we comparatively analyse the seismotectonic state of several geothermal as well as non-geothermal reservoir locations. The seismogenic indices of the considered locations are in the range of − 10 < Σ < 0.5. Although the number of reservoirs under examination is limited, we see a clear separation between hydrocarbon and geothermal reservoirs with respect to the seismotectonic state. In addition to a higher seismogenic index, geothermal reservoir locations are characterised by a lower b value. It means that fluid injections in geothermal reservoirs have a higher probability to induce an earthquake with a significant magnitude. Our formulation provides a basis for estimating expected magnitudes of induced earthquakes. This can potentially be used to avoid the occurrence of large magnitude earthquakes by correspondingly planning fluid injections.

Geomagnetic variation and its relation to microearthquakes in the seismically inactive Korean Peninsula

Some microearthquakes that occur in the Korean Peninsula show significant geomagnetic variation when subjected to a specialized principal component analysis, wavelet-based semblance filtering and eigenvalue analysis. In this study, the principal component analysis was used to reconstruct and predict the geomagnetic field based on the past observed magnetic fields, and the reconstructed field was compared with the field observed during the earthquake event. The wavelet-based filtering showed improved results in delineating the earthquake event’s geomagnetic variations from their background field. Finally, the change in the tendency of the eigenvalues was detected by the analysis of three components of the geomagnetic fields during the earthquake event. The basement rock of the Korean Peninsula is known to have a highly resistive electrical structure that makes it possible for small-magnitude earthquakes to generate exaggerated geomagnetic variations. This is the first study reporting the relation between these earthquake events and the geomagnetic variations observed in the Korean Peninsula.

Earthquake precursors: activation or quiescence?

SUMMARY We discuss the long-standing question of whether the probability for large earthquake occurrence (magnitudes m > 6.0) is highest during time periods of smaller event activation, or highest during time periods of smaller event quiescence. The physics of the activation model are based on an idea from the theory of nucleation, that a small magnitude earthquake has a finite probability of growing into a large earthquake. The physics of the quiescence model is based on the idea that the occurrence of smaller earthquakes (here considered as magnitudes m > 3.5) may be due to a mechanism such as critical slowing down, in which fluctuations in systems with long-range interactions tend to be suppressed prior to large nucleation events. To illuminate this question, we construct two end-member forecast models illustrating, respectively, activation and quiescence. The activation model assumes only that activation can occur, either via aftershock nucleation or triggering, but expresses no choice as to which mechanism is preferred. Both of these models are in fact a means of filtering the seismicity time-series to compute probabilities. Using 25 yr of data from the California–Nevada catalogue of earthquakes, we show that of the two models, activation and quiescence, the latter appears to be the better model, as judged by backtesting (by a slight but not significant margin). We then examine simulation data from a topologically realistic earthquake model for California seismicity, Virtual California. This model includes not only earthquakes produced from increases in stress on the fault system, but also background and off-fault seismicity produced by a BASS–ETAS driving mechanism. Applying the activation and quiescence forecast models to the simulated data, we come to the opposite conclusion. Here, the activation forecast model is preferred to the quiescence model, presumably due to the fact that the BASS component of the model is essentially a model for activated seismicity. These results lead to the (weak) conclusion that California seismicity may be characterized more by quiescence than by activation, and that BASS–ETAS models may not be robustly applicable to the real data.

지하수위 변화에 따른 지진 유발 산사태의 취약섬 분석

최근 들어 우리나라에서도 지진의 발생빈도가 증가함에 따라 지진관련 재해에 대한 관심이 증가하고 있다. 특히 지진의 발생빈도가 증가함에 따라 지진에 의해 유발되는 산사태의 발생 가능성에 대한 연구의 필요성이 제기되고 있다. 최근 필리핀 레이테섬 산사태의 경우처럼 집중강우에 의해 포화된 사변에서는 소규모의 지진에 의해서도 대규모의 산사태가 유발될 수 있다는 사실이 밝혀짐에 따라 소규모 지진의 발생빈도가 증가하고 있는 우리나라에서도 지진에 의한 산사태의 발생 가능성이 제기되고 있다. 따라서 본 연구에서는 강우에 의해 지하수위가 상승하여 포화된 지반 조건에서 소규모 지진에 의해 유발될 수 있는 산사태의 가능성에 대한 분석을 수행하고자 하였다. 이를 위하여 국내의 지질 및 지형특성을 고려할 수 있는 Newmark displacement model을 해석모델로 선정하고 GIS 분석기법을 활용하여 연구대상지역에 대한 산사태 취약성 분석을 실시하였다. 연구 수행을 위하여 수치지형도와 지질도 등을 이용, 사변의 기하학적 특성과 지질공학적 특성에 대해 커버리지 형태인 10 m <TEX>${\times}$</TEX> 10 m 크기 격자(grid) 형태의 주제도를 작성하였으며 이를 지진특성에 의해 결정되는 Arias intensity와 임계가속도와 결합하여 Newmark 변위를 계산하였다. 본 연구에서는 특히 2007년 l윌 연구지역 주변에서 발생한 규모 4.8의 지진에 대하여 지진에 의해 유발되는 산사태의 취약성을 분석하였으며 지반의 포화정도가 취약성에 미치는 영향을 분석하기 위하여 지하수위를 변동시켜가며 분석을 수행하였다. 또한 지진의 규모와 진앙까지의 거리가 산사태의 취약성 해석결과에 미치는 영향을 파악하기 위하여 3.0 - 4.0 규모의 지진이 연구지역 내의 다양한 위치에서 발생하는 것을 가정하였으며 집중강우에 의해 지반에 포화된 상황을 고려하기 위해 지하수위를 변동시켜가며 산사태의 취약성을 분석하였다. Since the frequency of the earthquake occurrence in Korean peninsular is continuously increasing, the possibility that massive landslides are triggered by earthquake is also growing in Korea. Previously, the landslide is known to be induced by large magnitude earthquake, whose magnitude is larger than 6.0. However, the landslide can be induced by only small magnitude earthquake, especially in the fully saturated soil. Therefore, the susceptibility of landslide caused by small magnitude earthquake in fully saturated soil is analyzed in this study. For that, the topographical and geological characteristics of the site were obtained and managed by GIS software. In the procedure of the study, slope angle, cohesion, friction angle, unit weight of soil were obtained and constructed as a spatial database layer. Combining these data sets in a dynamic model based on Newmark's displacement analysis, the landslide displacements were estimated in each grid cell. In order to check out the possibility of the earthquake induced landslides, the level of the groundwater table is varied from dry to 80% saturated soil. In addition, in order to analyze the effect of the magnitude of earthquake and distance to epicenter, four different earthquakes epicenters were considered in the study area.

Comparison of dike intrusions in an incipient seafloor-spreading segment in Afar, Ethiopia: Seismicity perspectives

[1] Oceanic crust is accreted through the emplacement of dikes at spreading ridges, but the role of dike intrusion in plate boundary deformation during continental rupture remains poorly understood. Between 2005 and 2009 the ∼70 km long Dabbahu-Manda Hararo rift segment in Ethiopia has experienced 14 large volume dike intrusions, 9 of which were recorded on temporary seismic arrays. A detailed comparison of the seismic characteristics of the seismically monitored dikes is presented with implications for dike intrusion processes and magmatic plumbing systems. All of the migrating swarms of earthquakes started from a <5 km radius zone at the middle of the Dabbahu-Manda Hararo segment, and traveled northward and southward along the rift axis. Small magnitude earthquakes associated with the margins of the propagating dike tips are followed by the largest magnitude, primarily low-frequency earthquakes. The seismic moment distributions show >80% of energy is released during the propagation phase, with minimal seismic energy release after the dike propagation ceases. We interpret that faulting and graben formation above the dikes occurs hours after the passage of the dike tip, coincident with the onset of low-frequency earthquakes. Dike lengths show no systematic reduction in length with time, suggesting that topographic loading and stress barriers influence dike length, as well as changes in tectonic stress. The propagation velocities of all the dikes follow a decaying exponential. Northward propagating dikes had faster average velocities than those that propagated southward, suggesting preconditioning by the 2005 megadike, or ongoing heating from a subcrustal magma source north of the midsegment.

Evento principal y réplicas importantes del Terremoto de Limón

Combining local, regional and teleseismic seismological information, we find the principal aftershocks (Mc≥ 4.4) of the Limon earthquake of April 22, l991 (Mw=7.7). They define a rupture area of 85 x 45 km2, it is similar to that found using small magnitude earthquakes (Mc≥ 4.4). Using the arrivals reported by seismological stations located at different epicentral distances, allow us to obtain a focal mechanism solution that corresponds with a thrust oblique faulting event (strike 120°, dip angle 30° , slip 63°). Our interpretation of an acelerogram obtained 70 km southwest of the mainshock epicenter, permit us to identify five subevents during the rupture process, this defines the Limon earthquake as a complex and multiple event. Finally, we find that many aftershocks occurred on thrust and strike slip secondary faults; they are located a different distances and trends from the mainshock epicenter.

Prediction equations for the standardized version of cumulative absolute velocity as adapted for use in the shutdown of U.S. nuclear power plants

The U.S. Nuclear Regulatory Commission (USNRC) uses the standardized version of the cumulative absolute velocity (CAV) together with the response spectra of the recorded ground motion at a site to determine whether a nuclear power plant must be shut down for inspection after an earthquake. In order to better understand the impact of these criteria on potential plant shutdowns, we used several subsets of the PEER-NGA strong motion database to develop empirical prediction equations between a CAV intensity measure that incorporates these criteria and the geometric mean horizontal component of CAV. This particular approach was used because, after applying the USNRC shutdown criteria, we found that there were an insufficient number of records remaining to reliably develop a ground motion prediction equation (GMPE) for this new CAV intensity measure directly from the physical parameters of an earthquake. We use these prediction equations to demonstrate how a lower-bound value of this intensity measure can be used to prevent non-damaging ground motions from both nearby small-magnitude earthquakes and distant large-magnitude earthquakes from contributing to the ground motion hazard computed from a probabilistic seismic hazard analysis (PSHA). We suggest that the use of a CAV criterion, in addition to a criterion based on either peak ground motion parameters or response spectral ordinates, could also be used to prevent the unnecessary shutdown of nuclear power plants outside of the U.S. Furthermore, with some adjustment, a similar approach could be used to rapidly assess the potential damage to conventional structures after an earthquake to aid in emergency response and loss assessment.

Seismic wave propagation analysis in vertical arrays at Kashiwazaki–Kariwa nuclear power plant site by using modified normalized input–output minimization (mod‐NIOM) method

AbstractA new approach on numerical modeling of wave propagation is introduced and is used to analyze the effect of earthquake magnitudes (ground motion amplitudes) on wave propagation. In this method, the sum of the maximum amplitudes of the first output model at time 0 s and rest of the output models at different times are normalized to unity. Considering this as a constraint, the sum of the weighted‐squared Fourier amplitudes is minimized by using the Lagrange multiplier method. The proposed method can reveal the relationship of actual time histories by showing simple clear peaks. This method is used to analyze the time histories of various earthquake events at different vertical array sites of the Kashiwazaki–Kariwa nuclear power plant of Tokyo electric power company (TEPCO). The wave arrival times obtained from this method and down‐hole measurements are compared. The results show increase in the arrival times at surface layer when the magnitude of earthquake is large. The results reveal that the amplitudes of small magnitude earthquakes at depths are small and are largely amplified at surface, whereas in case of large magnitude earthquakes, the amplitudes are large at depths and are deamplified at surface reflecting the effects of the strain‐dependent soil properties that result in non‐linear site response to strong shaking. The results also show that the reflected peak amplitudes are higher for small magnitude earthquakes than for large magnitude earthquakes. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Sea-Bottom Observations from the Western Escarpment of the Sea of Marmara

Abstract The western escarpment of the Sea of Marmara has recently been recognized as the site of intensive gas emissions escaping from the seafloor. Visual observations with the Nautile submersible indicate that gas escapes from elongated tensile cracks oriented to the northwest in the direction of the maximum principal stress. Here, we report results from a 25-day test in 2007 with four ocean-bottom seismometers (OBSs) showing that this area is also characterized by microseismic activity. A cluster of 13 small-magnitude earthquakes aligned northwest occurred in less than 30 hr at shallow crustal depth below the western slope of the Tekirdag basin. The only two focal mechanisms resolvable using land and sea-bottom data reveal normal faulting with strike-slip components, consistent with the stress field expected in this area. It is suggested that tectonic strain below the western slope of the Tekirdag basin contributes to maintaining a high permeability in fault zones and that the fault network provides conduits for deep-seated fluids to rise up to the seafloor.