Small Local Earthquakes Research Articles

Shaken and Stirred: A Comparative Study of Earthquake‐Triggered Soft‐Sediment Deformation Structures in Lake Sediments

AbstractSubaqueous paleoseismic studies used soft sediment deformation structures (SSDS) to discern the shaking strength of past earthquakes, as the deformation degree of SSDS related to Kelvin Helmholtz Instability evolves from disturbed lamination and folds to intraclast breccia with higher peak ground accelerations (PGA). We lack comparative studies of different sediment types with SSDS related to earthquakes from different seismogenic sources to comprehend how these factors modulate earthquake‐induced deformation. Here, we compile sediment records with seven earthquake‐triggered SSDS from 10 lakes with organic‐, carbonate‐, siliciclastic‐, and diatom‐rich sediment from three subduction zones and one collisional setting. We target basin sequences with slope angles <0.65° to reduce the influence of gravitational downslope stress. We find that even minimal increases in slope angle, maximal 1°, lead to higher deformation degrees and, for some earthquakes, SSDS are only present at >0.65°. Fine‐grained clastics enhance sediment susceptibility to deformation, whereas abundant diatoms reduce it, demonstrating the influence of composition. Deformation correlates best with PGA and the vicinity of the earthquakes, suggesting that high frequency shaking promotes deformation. In addition, deformation only occurs above a minimum magnitude dependent on sediment composition, and higher deformation degrees in our studied basin sedimentary sequences only above Mw 4.9 for all sediment types, suggesting that sufficient duration of shaking—magnitude correlates with duration—is essential for SSDS development. We advise taking multiple cores on gentle slopes to study SSDS—additional to basin cores—to resolve small magnitude local earthquakes and relative differences in frequency content of past events.

Potential of Earthquake Strong Motion Observation Utilizing a Linear Estimation Method for Phase Cycle Skipping in Distributed Acoustic Sensing

AbstractDistributed acoustic sensing (DAS) using existing optical fiber cables facilitates high‐density seismic observation. However, few studies have examined the reliability of the seismic waveform amplitude recorded by DAS. In this study, a DAS network was connected to optical fiber cables installed over a distance of 75 km along a high‐speed train (Shinkansen) railway in the Kumamoto prefecture, Japan. We successfully observed strong motions of the Mj6.6 earthquake (approximately 150 km from the fiber) on 22 January 2022, in Hyuga‐nada, in addition to several small local earthquakes. The observed strong motions from the Mj6.6 earthquake, using DAS, exhibited cycle skipping (clipping) issues due to dynamic range limitations at numerous channels. To address this, we estimated the shaking map, representing maximum strain distributions for Mj6.6, by replacing the clipped data with information from nearby unclipped channels and scaling their RMS amplitudes based on S‐coda (unclipped). Furthermore, we verified the reliability of the amplitude information obtained from DAS by estimating the distance attenuation of seismic waves while correcting for the differences in the structure type and coupling as much as possible. The distance attenuation property of local earthquakes was consistent with that of the peak ground velocities obtained from seismometers, indicating that DAS data acquired using fibers installed on infrastructure (various structures) can also be utilized to assess the spatial distribution of the relative amplitude values along the fiber. Obtaining high‐density seismic motion distributions is important for earthquake early warning and accurate damage estimation of strong motions.

Seismic anisotropy in the upper crust around the north segment of Xiaojiang faults in the SE margin of Tibetan Plateau

The Xiaojiang faults located in the SE margin of the Tibetan Plateau is a fault system of left-lateral strike-slip, striking NS, between the 2nd-order Sichuan-Yunnan block and the 1st-order South China block. The Xiaojiang faults and the surrounding areas are characterized by strong tectonic movements and intense seismic activities. Using seismic data from January 2013 to November 2020 recorded at the stations of the temporary QiaoJia seismic Array (QJ Array), deployed by the Institute of Geophysics, China Earthquake Administration, this study investigates the upper crustal anisotropy by the shear-wave splitting analysis on small local earthquakes, discusses the deformation patterns in the upper crust in the north segment of the Xiaojiang faults, evaluates the stress distribution in the study area, and analyzes its relationship with the regional tectonic structure. Adopting the data processing technique of shear-wave splitting, a total of 875 effective records were obtained at 50 stations. The mean direction of polarizations of fast shear-wave (PFS) is 162° ± 44° in the study areaand the mean normalized time-delay is 4.96 ± 2.38 ms/km. Based on the spatial distribution of the PFS and the regional geologic structure, the study area is divided into two zones: the zone N and the zone S. The PFS in the zone N is scattered, but the dominant PFS direction is in NNW, which is consistent with the direction of the regional maximum principal compressive stress. In the zone N, there are a few smaller local areas (i.e., subzones A, B, C, and D) in which the orientations of the PFS are quite different from the surrounding area. In the zone S, the dominant directions at most stations are in nearly NS, consistent with the strike of the Xiaojiang fault. It reveals the detailed spatial distribution of seismic anisotropy in the upper crust, as well as in situ principal compressive stress, indicating the influence of the regional stress, the complex tectonic environment, and maybe also the impact of the South China block. It also reveals that there also might be an upper-crust scale of tectonic line at near 26°20′N under Xiaojiang faults, which coincides with the north-south tectonic boundary in the lithospheric anisotropy.

Designing monitoring networks for local earthquakes

Abstract Seismic networks are essential for monitoring local earthquakes in connection to industrial activities, including wastewater injection and CO2 sequestration. Because these networks are typically deployed for short periods of time at specific sites, it is beneficial to develop best practices for efficient and effective installation and monitoring. Such standards are available for regional earthquake and microseismic monitoring, but not readily available for local scale (tens of kilometer scale) monitoring. Once the region of interest has been determined, the key parameters for establishing a network are available monitoring station locations, site noise, and site utility. Networks should be designed based on the project monitoring goals for earthquake magnitude and density. Herein, a network was established at the Patterson and Hartland Oil Fields in western Kansas in association with a US Department of Energy CarbonSAFE carbon capture and sequestration research project. We employed observations from an analog local network, the Wellington Earthquake Monitoring Network in south-central Kansas, to assess site noise and estimate earthquake detection thresholds for the Patterson and Hartland fields. Noise from oil production facilities was evaluated, concluding that oil lease sites are suitable for monitoring small local earthquakes (M1-M3). The network was designed to have a magnitude of completeness of M1 while using station locations on existing field operator leases. Fifteen months of continuous network operation demonstrates reliable and efficient local earthquake monitoring and provides best practices recommendations for similar operations.

Network analysis of earthquake ground motion spatial correlation: a case study with the San Jacinto seismic nodal array

SUMMARYThe spatial correlation of earthquake ground motion intensity can be measured from strong motion data; however, the data used in past studies is sparsely sampled in space, and only the interstation distance was considered as a correlation variable. These limitations mean that we have only weak constraints on the true correlation structure of ground motion and that potentially important aspects of spatial correlation are unconstrained. In this study, we combine a large-N seismic array and graph analytics to explore this issue at a local scale using small local and regional earthquakes. Our result suggests site conditions, and how they interact with the incident seismic wavefield, strongly condition the spatial correlation of ground motion. Future progress in characterizing ground motion spatial variability will require dense wavefield measurements, either through nodal deployments, or perhaps distributed acoustic sensing measurements, of seismic wavefields. Aftershock sequences of major earthquakes would provide particularly data-rich targets of opportunity.

Interseismic Quiescence and Triggered Slip of Active Normal Faults of Kīlauea Volcano's South Flank During 2001–2018

AbstractThe mobile south flank of Kīlauea Volcano hosts two normal fault systems, the Koa'e fault system (KFS) and the Hilina fault system (HFS). In historical time, at least three M>6.5 earthquakes have occurred on the basal detachment of the Kīlauea Volcano's south flank, with the most recent being the 4 May 2018 M6.9 earthquake. Here we analyze kinematic Global Positioning System data collected from 2001 to 2017 and interferometric synthetic aperture radar data before, during, and after the 2018 M6.9 earthquake to determine the crustal motion across the HFS and KFS faults. Our results indicate that the HFS faults did not significantly slip during the interseismic period from 2007 to 2011. Despite its substantial magnitude, interferometric synthetic aperture radar (InSAR) data show that the 2018 M6.9 earthquake triggered subcentimeter level slip along sections of the previously mapped HFS branches. Up to 20 cm of offset occurred on what appears to be a newly formed (or previously unknown) fault near the eastern end of the HFS. During the 3 months following the M6.9 earthquake, up to ~30 cm of slip occurred along the KFS, which helps accommodate rapid large‐scale subsidence of Kīlauea's summit region as large volumes of summit reservoir magma fed the lower East Rift Zone eruption. The HFS appears to activate only in concert with large earthquakes on the basal detachment. The KFS, on the other hand, moves both seismically during small local earthquakes and aseismically in response to nearby earthquakes and caldera subsidence.

Moment tensor inversion of small-to-moderate size local earthquakes in Egypt

Well resolved moment tensor solutions reveal information about the sources of seismic waves. In this article, a waveform inversion technique, which uses waveform data from near-source seismic stations that belong to the Egyptian National Seismological Network (ENSN) is applied in order to estimate both focal mechanism and moment magnitude of small and moderate local earthquakes that occurred after 2010 till 2014 in and around Egypt. Fifty nine earthquakes with local magnitude (ML) ranging between 3.5 and 5.5 are analyzed. The earthquake focal mechanism obtained by waveform inversion was fundamentally validated by means of the basic technique of first onset polarities. Based on the obtained results, we identified several seismic sources in and around Egypt (i.e. Gulf of Aqaba, northern Gulf of Suez, southern Gulf of Suez-northern Red Sea, Aswan zone, Dahshour zone, and the passive continental margin). A combination of the newly estimated focal mechanisms and the previously published ones of earthquakes occurred before 2010 are used in determination of stress field for each relevant seismic source. A new scaling relationship between local magnitude (ML) and the estimated moment magnitude (Mw) has been developed using a linear regression. The results of this study may be used to improve the ENSN's stations configuration and the efficiency to record teleseismic and regional events.

Structural Health Monitoring of Buildings Using Smartphone Sensors

This article presents the results of a shaker test of the Millikan Library in Pasadena, California, using sensors inside smartphones to demonstrate their potential usage as a way to monitor health states of buildings. This approach to structural health monitoring could allow many more commercial and residential buildings to be monitored because it removes the cost prohibitive nature of traditional seismic arrays and the complexity of deploying the instruments. Recordings from the smartphones during the shaking show high correlation with those from a reference sensor in the building, illustrating that the phones can capture the shaking even when not fully coupled to the floor. The fundamental translational frequencies for the east–west and north–south directions and the torsional frequencies of the building can be extracted from single phone recordings. As we compare the displacement derived from the phone recording by double integration to that from the reference sensor, both phase and amplitude match well. Signal‐to‐noise ratio is improved further by stacking records from multiple phones. These test results demonstrate the ability to extract the fundamental translational and torsional frequencies, and absolute displacements from upper levels of buildings shaken by small local earthquakes. This work builds on the ongoing MyShake project—a global smartphone seismic network.

Dynamic triggering of small local earthquakes in the central Himalaya

AbstractWe present the first observation of remote dynamic triggering of local microearthquakes in central Himalaya caused by the teleseismic waves from the 2007 Mw 8.5 Sumatra earthquake that occurs ~4500 km away. We find small local earthquakes in the Kumaon‐Garhwal Himalaya triggered by teleseismic long‐period surface waves. Interestingly, an elevated level of seismicity persists for a week or so after the arrival of the teleseismic waves. The teleseismic waves impart ~9 kPa of peak dynamic stresses, suggesting that the Himalayan faults in this area are sensitive to small stress changes. This heightened and protracted seismicity indicates that the transient dynamic stresses may have triggered secondary processes, such as slow slip, that may be responsible for the persistence of this earthquake sequence. The region is thought to be close to a large damaging earthquake in the near future. This study provides improved constraints on the factors controlling the earthquake cycle.

A New Strategy for Weak Events in Sparse Networks: The First-Motion Polarity Solutions Constrained by Single-Station Waveform Inversion

Moment tensor determinations of small earthquakes are quite challenging. It is because their signal‐to‐noise ratio is satisfactory only at frequencies above the microseismic noise peak (∼0.2 Hz), and waveforms can be modeled only up to ∼1–2 Hz at relatively near stations (epicentral distance of a few kilometers). Therefore, the availability of high‐quality waveforms at near stations is the most critical issue. Dense local networks enable solving specific issues like non‐double‐couple (non‐DC) components (e.g., Vavrycuk, 2011), relation with tectonics (Serpetsidaki et al. , 2013), and/or even space–time clustering of moment tensors (Cesca et al. , 2014). On the contrary, records of weak events at very few local stations hardly allow us to get even DC components of moment tensors. For example, focal mechanism of two microearthquakes, M w 0.2 and 0.4, were obtained by the full‐waveform inversion in the frequency range below 4 Hz and distance below 3.7 km by Benetatos et al. (2013). Fojtikova et al. (2010) studied shallow earthquakes of M w 1.2–3.4 at 0.8–1.6 Hz. A probabilistic nonlinear inversion of local waveforms was developed by Weber (2009) and applied to weak events in Hungary. Various constraints for determining focal mechanisms of small local earthquakes monitored by sparse networks were adopted. For example, Nakamura (2002) suggested a joint use of the P ‐ and S ‐wave polarities. Li et al. (2011) combined the phase and amplitude waveform matching with the first‐motion P polarities and average S / P amplitude ratios. Delouis and Legrand (1999) emphasized inclusion of the near‐field Green’s function terms. Pinar et al. (2003) made tests according to which some single‐station inversions are feasible. Busfar and Toksoz (2013) gave examples of focal mechanisms calculated in good velocity models from less than four stations. Godano et al. (2010, 2011) studied …

A Stochastic Ground‐Motion Model for Switzerland

Abstract We present a stochastic ground‐motion model for Switzerland. A model for the far‐field Fourier amplitude spectra of earthquakes is first proposed, constrained by ground motions observed from small local and regional earthquakes in addition to macroseismic intensities observed from large damaging events in Switzerland. We then use this model in a stochastic simulation technique to generate predictions of peak ground acceleration and velocity and 5%‐damped response pseudo‐spectral acceleration. We facilitate the prediction of ground motions from finite‐fault sources through the use of the R EFF distance metric and show that magnitude and distance scaling‐behavior of events up to M w 7.5 is broadly consistent with Next Generation Attenuation and Japanese models. Ground‐motion prediction uncertainty is described in terms of between‐ and within‐event uncertainties through residual analysis of response spectra. Finally, single‐station sigma is derived, accounting for the prediction uncertainty at a given site without the ergodic assumption. Consistency of the model is emphasized through its compatibility with other seismic hazard products: the model is referenced to a generic rock profile that was developed by utilizing velocity profiles of the sites of seismic stations and is calibrated at higher magnitudes to the Swiss regional macroseismic intensity prediction model. Furthermore, the model is based on moment magnitudes from the recently developed Earthquake Catalogue of Switzerland 2009 (ECOS09). The well‐defined reference for the model means that it can easily be adapted for a site‐specific application, for instance, through the use of proxies such as quarter‐wavelength velocity and travel‐time average shear‐wave velocity in the upper 30 m ( V S 30 ).

Basement depths in the Parecis Basin (Amazon) with receiver functions from small local earthquakes in the Porto dos Gaúchos seismic zone

Abstract Receiver functions from small local earthquakes were used to determine sediment thicknesses in Porto dos Gauchos seismic zone (PGSZ), Parecis basin, Amazonian craton, Brazil. The high velocity contrast between basement and sediments (P-wave velocities of 6.1 and 3.0 km/s, respectively) favors the generation of clear P-to-S converted phases (Ps) seen in the radial component, and also S-to-P conversions (Sp) seen in the vertical component. A reference 1D velocity model determined with shallow refraction experiment in PGSZ helped to convert Ps–P time differences to basement depths at 15 stations deployed for aftershocks studies. The results of receiver function integrated with the shallow refraction reveal that the basement depths in the PGSZ increases from the basin border in the north up to about 600 m depth in the south. The basement topography, however, does not vary smoothly and a basement high with a steep topography was detected near the epicentral area. A 400 m elevation difference within 1.7 km distance suggests a possible border fault of a buried graben. This feature seems to be oriented roughly WSW-ENE and could indicate basement structures related to the seismicity of the Porto dos Gauchos Seismic Zone.

Coda wave attenuation in the Parecis Basin, Amazon Craton, Brazil: sensitivity to basement depth

Small local earthquakes from two aftershock sequences in Porto dos Gauchos, Amazon craton—Brazil, were used to estimate the coda wave attenuation in the frequency band of 1 to 24 Hz. The time-domain coda-decay method of a single backscattering model is employed to estimate frequency dependence of the quality factor (Q c) of coda waves modeled using \(Q_{\rm c} =Q_{\rm 0} f^\eta \), where Q 0 is the coda quality factor at frequency of 1 Hz and η is the frequency parameter. We also used the independent frequency model approach (Morozov, Geophys J Int, 175:239–252, 2008), based in the temporal attenuation coefficient, χ(f) instead of Q(f), given by the equation \(\chi (f)\!=\!\gamma \!+\!\frac{\pi f}{Q_{\rm e} }\), for the calculation of the geometrical attenuation (γ) and effective attenuation \((Q_{\rm e}^{-1} )\). Q c values have been computed at central frequencies (and band) of 1.5 (1–2), 3.0 (2–4), 6.0 (4–8), 9.0 (6–12), 12 (8–16), and 18 (12–24) Hz for five different datasets selected according to the geotectonic environment as well as the ability to sample shallow or deeper structures, particularly the sediments of the Parecis basin and the crystalline basement of the Amazon craton. For the Parecis basin \(Q_{\rm c} =(98\pm 12)f^{(1.14\pm 0.08)}\), for the surrounding shield \(Q_{\rm c} =(167\pm 46)f^{(1.03\pm 0.04)}\), and for the whole region of Porto dos Gauchos \(Q_{\rm c} =(99\pm 19)f^{(1.17\pm 0.02)}\). Using the independent frequency model, we found: for the cratonic zone, γ = 0.014 s − 1, \(Q_{\rm e}^{-1} =0.0001\), ν ≈ 1.12; for the basin zone with sediments of ~500 m, γ = 0.031 s − 1, \(Q_{\rm e}^{-1} =0.0003\), ν ≈ 1.27; and for the Parecis basin with sediments of ~1,000 m, γ = 0.047 s − 1, \(Q_{\rm e}^{-1} =0.0005\), ν ≈ 1.42. Analysis of the attenuation factor (Q c) for different values of the geometrical spreading parameter (ν) indicated that an increase of ν generally causes an increase in Q c, both in the basin as well as in the craton. But the differences in the attenuation between different geological environments are maintained for different models of geometrical spreading. It was shown that the energy of coda waves is attenuated more strongly in the sediments, \(Q_{\rm c} =(78\pm 23)f^{(1.17\pm 0.14)}\) (in the deepest part of the basin), than in the basement, \(Q_{\rm c} =(167\pm 46)f^{(1.03\pm 0.04)}\) (in the craton). Thus, the coda wave analysis can contribute to studies of geological structures in the upper crust, as the average coda quality factor is dependent on the thickness of sedimentary layer.

Space-time variations in the parameters of split S waves excited by local earthquakes in southern Kamchatka

An analysis of the distribution of split S-wave parameters due to small local earthquakes occurring down to a depth of 200 km beneath the PET IRIS station (in the town of Petropavlovsk-Kamchatskii) for the period 1993–2002 revealed an inhomogeneous distribution of anisotropic properties and time-dependent changes in the state of stress and strain in the sinking slab. Three time intervals have been identified from the behavior of the split-wave parameters: 1993–1995, 1996–1998, and 1999–2002. The most orderly orientation of fast-wave polarization azimuths at all depths and lower stresses in the earth was observed in 1996–1998. Regions of relatively stable and unstable parameter behavior consistent with increased and decreased P and S velocities have been identified. The central block down to a depth of 120 km and the lower part of the Benioff zone are classified as stable regions dominated by E-SE fast-wave azimuths. Unstable behavior of fast-wave azimuths was noticed for depths of 60 to 90 km and around the top of the Benioff zone. The highest values of S-wave delay time and regions where the delay times were migrating mostly occur at contacts between more rigid and weakened blocks.

Intrinsic attenuation and scattering of shear waves in the lithosphere of Kamchatka

Records of small local Kamchatka earthquakes were processed using Multiple Lapse Time Window Analysis (MLTWA). The method makes use of normalized integrals of 3D seismic energy density in several windows applied to an earthquake record that has been put through a bandpass filter. The intrinsic attenuation and scattering properties of the earth were estimated by choosing parameters that provide the best agreement between experimental and theoretical integrals as functions of hypocentral distance. The theoretical normalized integrals as functions of distance were computed using an analytical solution to the equation of seismic energy transport based on a simple model of isotropic scattering for scalar seismic waves excited by an impulse point source in an earth that is uniform as to scattering and intrinsic attenuation. The resulting estimates of attenuation and scattering parameters are similar to those derived in other studies of Kamchatka, as well as of other continents: America (California, US), Europe (the Canary Is., Spain, and Italy), and Asia (Japan).

Use of ANSS Strong-Motion Data to Analyze Small Local Earthquakes

Advances in seismometry and recording systems have greatly increased the range of signals that can be recorded by strong-motion instruments. This increased range has practical implications for observational weak-motion seismology, as we demonstrate in this study. Analog strong-motion instruments, the most common type in use until the 1990s, had an effective bandwidth from ∼ 0.1–0.2 Hz to 25–50 Hz and a dynamic range of 40 to 60 dB (Heaton et al. 1989; Trifunac and Todorovska 2001). By contrast, modern 16- and 24-bit digital strong-motion instruments have bandwidths from DC to 80 Hz (at 200 sps) and a dynamic range of 90 to 135 dB (Trifunac and Todorovska 2001). Another advance in strong-motion instrumentation during the past decade or so has been the development of recorders that can support continuous digital telemetry of the data. Historically, strong-motion data have been recorded on-site in a triggered mode. The triggers usually are set to record potentially damaging ground motions from moderate to large ( M > 4) local earthquakes. These data are used mostly by engineers for structural design and by seismologists for modeling fault rupture histories. The continuous telemetry of strong-motion data has been driven primarily by the need for reliable near-real-time information on potentially damaging ground shaking (Kanamori et al. 1997). Fortunately, the continuous telemetry also enables the recording of smaller signals that normally would not trigger the strong-motion instruments, which means that the full range of signals recorded by the modern accelerometers and digitizers can be investigated. With the deployment of the Advanced National Seismic System (ANSS; USGS 1999) beginning in the year 2000, the number of modern continuously telemetered strong-motion instruments throughout the United States has increased greatly. Before ANSS, the only dense network of such instruments in the United States was located in southern California, as part …

Separation of Qi and Qs from passive data at Mt. Vesuvius: A reappraisal of the seismic attenuation estimates

Seismic attenuation in the area of Mt. Vesuvius is reappraised by studying more than 400 S-coda envelopes of small local VT earthquakes recorded at Mt. Vesuvius from 1996 to 2002 at the three-component stations of OVO and BKE. The purpose is to obtain a stable separate estimate of intrinsic and scattering quality factors for shear waves. We investigate in the present paper four frequency bands, centered respectively at f c = 3, 6, 12 and 18 Hz with a bandwidth of 0.6 f c. Stacked coda envelopes are fit to the multiple scattering model according to the Zeng approximation in the hypothesis of constant velocity half space. Results show that the diffusion regime is a good approximation as the scattering attenuation (proportional to Q S − 1 , the inverse scattering-quality factor) is much stronger than the intrinsic dissipation (proportional to Q I − 1 ). Q S − 1 decreases with frequency while intrinsic attenuation is much less frequency-dependent. We also fit the stacked coda envelopes at BKE to the diffusion equation solved with the boundary condition of a 2 km thick diffusive layer over a homogeneous half space. Results show that the diffusivity, D, estimated in the assumption of reflecting boundary condition is greater than that estimated in the assumption of uniform half space, whereas the diffusivity estimated with the absorbing boundary condition is close to the estimate done in the assumption of half space. OVO station shows results different from those obtained at BKE and at a group of five stations located on Mt. Vesuvius for the frequency bands centered at 12 and 18 Hz. In these two bands, scattering attenuation at OVO is comparable to the intrinsic dissipation, and is much smaller than that measured at the other stations. We interpret this anomaly as due to an effect of strong lateral heterogeneity which modifies the redistribution of the seismic energy into the coda at OVO. A comparison of the results obtained using passive data (the present data set) and the active data obtained in the same area during TOMOVES experiment by Wegler (2004) show that the diffusivity estimated with shot data during TOMOVES is smaller of a factor greater than 4. This discrepancy is interpreted as due to different earth volumes sampled by the coda waves in the two cases.

Earthquake Relocation and Focal Mechanism Determination Using Waveform Cross Correlation, Nicoya Peninsula, Costa Rica

The Nicoya Peninsula in Costa Rica directly overlies the seismogenic zone of the Middle America Trench, making it an ideal location for geophysical investigations of shallow subduction zone earthquake processes. As part of the collaborative Costa Rica Seismogenic Zone Experiment (crseize), a seismic network consisting of 20 land and 14 ocean-bottom seismometers recorded small magnitude local earthquakes along the Nicoya Peninsula from December 1999 to June 2001. Previous studies have used these data to compute local earthquake locations and 3D velocity structure to identify plate boundary seismicity and to investigate seismogenic behavior. Here we utilize waveform cross-correlation and clustering techniques in an attempt to improve earthquake relocations and determine first-motion focal mechanisms to validate, refine, and expand on existing models. Due to the high quality of the original locations and the small cross-correlation P -wave arrival time adjustments, large differences between the previously determined and the cross-correlated earthquake locations are not observed. However, focal mechanism determinations using cross-correlated P waveforms are significantly enhanced. Approximately 90% of the focal mechanisms computed for events previously identified as interplate earthquakes are consistent with underthrusting. Focal mechanisms for continental intraplate events indicate dextral strike-slip motion in the central region and normal faulting at the southern tip of the peninsula. These motions may be associated with oblique convergence and seamount subduction, respectively. Within the subducting plate, steep P and T axes of earthquakes below 50 km depth are consistent with unbending of the slab.

The Observed Wander of the Natural Frequencies in a Structure

The Southern California Seismic Network (scsn) has recently installed seismic stations in two buildings on the Caltech campus (Millikan Library and the Broad Center). Continuous real-time accelerometer data from these structures are now freely available to the community. This dataset provides a new opportunity to observe, and better understand, the variances in the primary dynamic property of a building system, its natural frequencies. Historical data (triggered strong-motion records, ambient and forced vibration tests) from the well-studied Millikan Library show dramatic decreases in natural frequencies, attributed mainly to moderately large local earthquakes. The current forced vibration east–west fundamental frequency is 22% lower than that originally measured in 1968. Analysis of the new continuous data stream allows the examination of other previously unrecognized sources of measurable change in the fundamental frequencies, such as weather (wind, rain, and temperature), as well as nonlinear building vibrations from small local and moderate regional earthquakes. Understanding these nonlinear shifts is one of the long-term goals of real-time building instrumentation and is critical if these systems are to be used as a postearthquake damage assessment tool.

The Nile Valley of Egypt: A Major Active Graben that Magnifies Seismic Waves

The Nile valley and the Nile delta are part of the active rift that is probably connected with the Red Sea tectonism. This zone is characterized by small-to-moderate size earthquakes that have caused extremely severe damage to recent and historical constructions. The most vulnerable area along the Nile valley is the one of Cairo-Faiyoum. Small local and large distant earthquakes could be a source of huge socio-economic damage in this area. The loose soft alluvial sediments of the Nile Canyon are the main factor behind this potential damage because they may greatly amplify the ground motion, as demonstrated by strong ground motion modelling. The largest amplification is generally concentrated along the edges of the graben and occurs at frequencies between 1 Hz and 2 Hz. This may explain the huge damage caused by distant earthquakes during recent and historical times. The distribution of intensity values during the events of 1926 and 1992 is well correlated with the modelled spatial distribution of the spectral amplification.