Similar Epicentral Distance Research Articles

Sensitivities of diffracted PKKPab waves to the velocity structures in the lowermost mantle

SUMMARY Diffractions of PKKPab (${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$) along the core mantle boundary (CMB) have been observed well beyond its cutoff distance in recent studies, making it useful for improving the spatial sampling to constrain the lowermost mantle velocity structures. However, the diffractions of PKKPab waves may occur at one of the three CMB interaction points (core entry, underside reflection and exit), giving rise to uncertainties of the sampling region. Moreover, the sensitivity kernels of the non-geometrical ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ in the lowermost mantle are difficult to obtain with classical ray theory and can be expected to be more complicated than the typical banana-doughnut shape for direct arrivals. In this study, we address these two issues by analysing the finite-frequency delay-time sensitivity kernels of the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ waves computed by numerical algorithms based on full-wave theory. We find that the diffraction effects for the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ waves are most significant near their core entry and exit regions. For a dominant period of 1 s, the estimated widths of the first Fresnel zones on the mantle side of these two areas are about 60 km. To further investigate the sensitivities of ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ to different structures in the lowermost mantle, we conduct a series of 1D and 2D high-frequency (∼1 Hz) modelling experiments. Our results show that the travel times and amplitudes of the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ waves are sensitive to large-scale P-wave anomalies (with Vp perturbations of ± 2 per cent and thicknesses of more than 100 km) and small-scale ultra-low velocity zones (ULVZs; with Vp reduction of 5 per cent or 10 per cent and thickness of tens of kilometers). However, the slownesses of the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}\ $ waves remain nearly unchanged in the perturbed models. We explain this unexpected result by the differential delay-time sensitivity kernels for stations at similar epicentral distances. Our results demonstrate both the advantages and limitations of the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ waves in studying the structures at the base of the mantle.

Teleseismic P waves at the AlpArray seismic network: wave fronts, absolute travel times and travel-time residuals

Abstract. We present an extensive dataset of highly accurate absolute travel times and travel-time residuals of teleseismic P waves recorded by the AlpArray Seismic Network and complementary field experiments in the years from 2015 to 2019. The dataset is intended to serve as the basis for teleseismic travel-time tomography of the upper mantle below the greater Alpine region. In addition, the data may be used as constraints in full-waveform inversion of AlpArray recordings. The dataset comprises about 170 000 onsets derived from records filtered to an upper-corner frequency of 0.5 Hz and 214 000 onsets from records filtered to an upper-corner frequency of 0.1 Hz. The high accuracy of absolute and residual travel times was obtained by applying a specially designed combination of automatic picking, waveform cross-correlation and beamforming. Taking travel-time data for individual events, we are able to visualise in detail the wave fronts of teleseismic P waves as they propagate across AlpArray. Variations of distances between isochrons indicate structural perturbations in the mantle below. Travel-time residuals for individual events exhibit spatially coherent patterns that prove to be stable if events of similar epicentral distance and azimuth are considered. When residuals for all available events are stacked, conspicuous areas of negative residuals emerge that indicate the lateral location of subducting slabs beneath the Apennines and the western, central and eastern Alps. Stacking residuals for events from 90∘ wide azimuthal sectors results in lateral distributions of negative and positive residuals that are generally consistent but differ in detail due to the differing direction of illumination of mantle structures by the incident P waves. Uncertainties of travel-time residuals are estimated from the peak width of the cross-correlation function and its maximum value. The median uncertainty is 0.15 s at 0.5 Hz and 0.18 s at 0.1 Hz, which is more than 10 times lower than the typical travel-time residuals of up to ±2 s. Uncertainties display a regional dependence caused by quality differences between temporary and permanent stations as well as site-specific noise conditions.

Review of geological and seismotectonic investigations related to 1998 Mw5.6 and 2004 Mw5.2 earthquakes in Krn Mountains

A review of geological and seismotectonic investigations conducted in the two decades after the 12 April 1998 earthquake in Krn Mountains, according to its magnitude the strongest earthquake in Slovenia in the 20th century, is given. Many of these studies have wider scientific meaning than expected from the size of the earthquake. This was the first case in Slovenia that a strong earthquake was undoubtedly related to a particular fault. Seismotectonic studies of seismogenic Ravne fault revealed that it is an actively propagating strike-slip fault growing by interaction of individual right stepping fault segments and breaching of local transtensional step-over zones. Airborne laser scanning (LiDAR) of Idrija and Ravne faults, which resulted in high resolution bare earth digital elevation model, was in 2005 for the first time used to study surface expression of an active fault in Europe. Among the primary characteristics of the 1998 earthquake were extensive environmental effects expressed mainly as massive rockfalls. They were systematically documented and evaluated for intensity assessment using European Macroseismic Scale (EMS-98) and Environmental Seismic Intensity (ESI) scale introduced in 2007, because application of the data on damage to buildings was limited in sparsely populated high mountains epicentral area. These studies were pioneering due to novelty of both intensity scales, indicating their strong points and weaknesses. Large variations in damage to buildings in the upper Soča valley at similar epicentral distances pointed to strong site effects due to very heterogeneous glacial and fluvial deposits in sedimentary basins and valleys. Therefore, different seismic microzonation maps were prepared to evaluate the influence of soft sediments on seismic ground motion. Conducted studies fostered development of several earthquake geology research methods in Slovenia as tectonic geomorphology, evaluation of environmental seismic effects and seismotectonics. They had positive impact also on the university education in the fields of geophysics, seismology and structural geology.

Evidence for strong lateral seismic velocity variation in the lower crust and upper mantle beneath the California margin

Regional seismograms from earthquakes in Northern California show a systematic difference in arrival times across Southern California where long period (30–50 s) SH waves arrive up to 15 s earlier at stations near the coast compared with sites towards the east at similar epicentral distances. We attribute this time difference to heterogeneity of the velocity structure at the crust–mantle interface beneath the California margin. To model these observations, we propose a fast seismic layer, with thickness growing westward from the San Andreas along with a thicker and slower continental crust to the east. Synthetics generated from such a model are able to match the observed timing of SH waveforms better than existing 3D models. The presence of a strong upper mantle buttressed against a weaker crust has a major influence in how the boundary between the Pacific plate and North American plate deforms and may explain the observed asymmetric strain rate across the boundary.

A strong-motion hot spot of the 2016 Meinong, Taiwan, earthquake (Mw = 6.4)

Despite a moderate magnitude, M_w = 6.4, the 5 February 2016 Meinong, Taiwan, earthquake caused significant damage in Tainan City and the surrounding areas. Several seismograms display an impulsive S-wave velocity pulse with an amplitude of about 1 m s-1, which is similar to large S-wave pulses recorded for the past several larger damaging earthquakes, such as the 1995 Kobe, Japan, earthquake (M_w = 6.9) and the 1994 Northridge, California, earthquake (M_w = 6.7). The observed PGV in the Tainan area is about 10 times larger than the median PGV of M_w = 6.4 crustal earthquakes in Taiwan. We investigate the cause of the localized strong ground motions. The peak-to-peak ground-motion displacement at the basin sites near Tainan is about 35 times larger than that at a mountain site with a similar epicentral distance. At some frequency bands (0.9 - 1.1 Hz), the amplitude ratio is as large as 200. Using the focal mechanism of this earthquake, typical “soft” and “hard” crustal structures, and directivity inferred from the observed waveforms and the slip distribution, we show that the combined effect yields an amplitude ratio of 17 to 34. The larger amplitude ratios at higher frequency bands can be probably due to the effects of complex 3-D basin structures. The result indicates that even from a moderate event, if these effects simultaneously work together toward amplifying ground motions, the extremely large ground motions as observed in Tainan can occur. Such occurrences should be taken into consideration in hazard mitigation measures in the place with frequent moderate earthquakes.

Seismic activity of the Nevados de Chillán volcanic complex after the 2010 Mw8.8 Maule, Chile, earthquake

Couplings between large magnitude subduction zone earthquakes and the subsequent response of their respective volcanic arcs are generally accepted, but the mechanisms driving this coupling are not known. The 2010 Maule earthquake (Mw8.8) ruptured approximately 500km along strike in the south-central part of Chile, and provides an opportunity to investigate earthquake–volcano interactions. In an exploratory campaign, we deployed four broadband seismometers atop and surrounding the Nevados de Chillán volcanic complex because it is located directly behind one of the two primary slip patches of the Maule Earthquake. The data recorded (from December 2011 to April 2012) shows significant seismic activity, characterized by numerous volcano tectonic events and tremor episodes occurring within the volcanic complex. We recorded two strong aftershocks of the Maule earthquake (Mw 6.1 in January 2012, and Mw 7.1 in April 2012) and investigated the response of the volcano to the incoming seismic energy. We find that volcanic tremor increased significantly upon arrival of the seismic waves from the Mw 6.1 event, which was then followed a few hours later by a significant increase in volcano-tectonic events when tremors subsided. This delay between tremor and volcano-tectonic events suggests that fluid and/or magma migration (manifested as tremor) readjusted the local stress state that then induced the volcano-tectonic events. The increased activity persisted during the subsequent two weeks. In contrast, the Mw 7.1 event, which occurred at a similar epicentral distance from the volcano, did not produce any significant seismic response of the volcanic complex. Analysis of the particle velocity records of the two events shows that the volcanic system was perturbed in different ways because of the incidence angle of the incoming energy, inducing a back-elliptical vertical and fault parallel motion for the Mw 6.1 and no clear directional dependence for the Mw 7.1. This suggests dilatation-induced fluid migration within the complex, and a kinematic mechanism of the perturbation rather than the perturbation amplitude. Our results demonstrate the importance for continued monitoring of the arc behind Maule, with an increased seismometer and GPS array density to determine the style of deformation currently occurring in the arc.

On the behavior of site effects in central Mexico (the Mexican volcanic belt – MVB), based on records of shallow earthquakes that occurred in the zone between 1998 and 2011

Abstract. The Mexican volcanic belt (MVB) is a seismogenic zone that transects the central part of Mexico with an east–west orientation. The seismic risk and hazard of this seismogenic zone has not been studied in detail due to the scarcity of instrumental data as well as because seismicity in the continental regime of central Mexico is not too frequent. However, it is known that there are precedents of large earthquakes (Mw > 6.0) that have taken place in this zone. The valley of Mexico City (VM) is the sole zone, within the MVB, that has been studied in detail. Studies have mainly focused on the ground amplification during large events such as the 1985 subduction earthquake that occurred off coast of Michoacán. The purpose of this article is to analyze the behavior of site effects in the MVB zone based on records of shallow earthquakes (data not reported before) that occurred in the zone between 1998 and 2011. We present a general overview of site effects in the MVB, a classification of the stations in order to reduce the uncertainty in the data when obtaining attenuation parameters in future works, as well as some comparisons between the information presented here and that presented in previous studies. A regional evaluation of site effects and Fourier acceleration spectrum (FAS) shape was estimated based on 80 records of 22 shallow earthquakes within the MVB zone. Data of 25 stations were analyzed. Site effects were estimated by using the horizontal-to-vertical spectral ratio (HVSR) methodology. The results show that seismic waves are less amplified in the northeast sites of the MVB with respect to the rest of the zone and that it is possible to classify two groups of stations: (1) stations with negligible site amplification (NSA) and (2) stations with significant site amplification (SSA). Most of the sites in the first group showed small (<3) amplifications while the second group showed amplifications ranging from 4 to 6.5 at frequencies of about 0.35, 0.75, 15 and 23 Hz. With these groups of stations, average levels of amplification were contrasted for the first time with those caused by the subduction zone earthquakes. With respect to the FAS shapes, most of them showed similarities at similar epicentral distances. Finally, some variations of site effects were found when compared to those obtained in previous studies on different seismicity regions. These variations were attributed to the location of the source. These aspects help to advance the understanding about the amplification behavior and of the expected seismic risk on central Mexico due to large earthquakes within the MVB seismogenic zone.

Background and delayed-triggered swarms in the central Southern Alps, South Island, New Zealand

Low-magnitude earthquake swarms (ML ≤ 2.8), consisting of up to 47 events of similar waveforms, have been observed repeatedly in the central Southern Alps, a rapidly uplifting orogen bounded by the transpressive Alpine Fault in the South Island of New Zealand. We compare nine background swarms recorded between November 2008 and April 2010 with five delayed-triggered swarms that occurred after the MW 7.8 Dusky Sound and the MW 7.1 Darfield (Canterbury) earthquakes. The two types of swarms are similar in terms of the magnitudes, depths, focal mechanisms, and interevent times of the constituent microearthquakes, and appear to both involve the rupture of steeply dipping faults in highly fractured crust in a 10 km × 12 km area in the center of the SAMBA network. The delayed-triggered swarms occurred at similar epicentral distances (c. 4.5× the rupture length of the mainshocks) to the Dusky Sound and Darfield earthquakes, commenced shortly after the passage of the surface waves, continued for ∼5 and ∼2 days, respectively, and were followed in each case by a ≥2 day long quiescent period, which may suggest clock-advanced of faults in their failure-cycle. Triggering thresholds of ≥0.01 MPa proposed elsewhere are similar to the dynamic stress changes computed for the Southern Alps (≥0.09 MPa). However, as 98% of the locatable triggered events occurred several hours after the surface waves had passed, the dynamic stress changes associated with the surface waves themselves are unlikely to have triggered the earthquakes directly. Instead, we suggest that the locations and delays of the triggered swarms are more consistent with triggering by pore pressure diffusion.

Mechanism of Different Coseismic Water-Level Changes in Wells with Similar Epicentral Distances of Intermediate Field

Water-level changes at different monitoring stations are observed during the Wenchuan earthquake ( M S 8.0) in the Chinese mainland. In the intermediate field, we observed coseismic water-level changes of different amplitude in wells with similar epicentral distances. In order to study the mechanism of those coseismic water-level changes, we calculated the static strain change with the Okada dislocation model (Okada, 1992; Lin and Stein, 2004; Toda et al. , 2005). By comparing the calculated coseismic water-level change based on the poroelastic theory with the observed water-level change, we can judge whether the poroelastic theory can be applied to the aquifer of the well. From our research, we find the poroelastic theory can be applied to the area with epicentral distance≲1.5 fault rupture length. Bearing this in mind, we find that when the water-level change of those wells can be explained by the poroelastic theory and that the difference of the water-level change in wells with similar epicentral distances is mostly related to the difference of Skempton’s coefficient B . Otherwise, the water-level change may be induced by the transition of the seismic waves because it is usually larger than the one induced by the undrained dilatation and consolidation, and changes more gradually.

Azimuth-Dependent Site Amplification

The strong motion accelerograms recorded at two stations of SMART-1 array have been analyzed to investigate azimuth-dependent site amplification. Fifteen earthquakes which triggered both accelerographs at rock site E02 and soil site E01 were selected for this study. We measure the spectral ratio of E01 to E02 in a window which includes direct S waves. Since ground motions at the rock site are less modified by the site effects and E02 and E01 are only 2 km apart, the spectral ratio of the soil site to the rock site will cancel most of source and raypath effects and reflect the site amplification of the soil site. These earthquakes are divided into 10 groups. The earthquakes in the same group have similar epicentral distance and backazimuth. Our studies show that different groups have different spectral ratios and no systematic changes have been found among the groups. In the same group, all earthquakes were expected to have the similar spectral ratio. However, similar spectral ratios were only found in some groups over some limited frequency ranges. These discrepancies are interpreted as the results of near-source and non-linear effects.

Seismotectonics and analysis of earthquakes from the Hindukush region

Summary. Over 80 earthquakes, exclusively from the Hindultush focal region. which were recorded at the Gauribidanur seismic array (GBA) have been used in this study. These events have similar epicentral distances and a narrow azimuthal range from GBA but varying focal depths from 10 to 240ltm. A fault plane dipping steeply (75) in the north-west direction and striking N 66 E has been investigated on the basis of the spatial distribution of earthquakes in two vertical planes through 68 E and 32 N. Short period P-wave recordings up to 30 s were processed using the adaptive cross-correlation filtering technique. Slowness and azimuthal anomalies were obtained for first arrivals. These anomalies show positive as well as negative bias and are attributed to a steep velocity gradient in the upper mantle between the 400700 ltm depth range where the seismic rays have their maximum penetration. Relative time residuals between the stations of GBA owe their origin very near to the surface beneath the array. A search of the signals across the array revealed that most of the events occurring at shallower depths had complex signatures as compared to the deeper events. The structure near the source region, complicated source functions and the scattering confined to the crust-upper mantle near source are mainly responsible for the complexity of the Hindultush earthquakes as the transmission zone of the ray tubes from turning point to the recording station is practically the same.

A macroseismic study and the implications of structural damage of two recent major earthquakes in the Jordan Rift

abstract A detailed macroseismic study of the July 11, 1927 earthquake was carried out. A quantitative analysis of damage data provided a correlation for estimating (MM) intensities: I = 6.4 + 1.2 log (percentage of damaged houses). Using axis lengths and areas bounded by the ensuing isoseismal lines, the depth (16 to 28 km) of the event was estimated, and its probable epicenter located near Damiya bridge on the Jordan river. A comparison with the equivalent parameters, inferred from instrumental records, shows agreement between both sets of results and thus confirms the validity of the approach used in the macroseismic study. The same procedure of studying macroseismic data was applied to an earlier (January 1, 1837) destructive earthquake. It was found that this latter event orginated in the upper crust, eastward of Safed, with a 6.25 to 6.5 magnitude. The isoseismals of both these major earthquakes are elongated in a north-south direction, along the major structural trend in the area. The southern coastal plain of Israel seems generally less vulnerable to Jordan Rift Valley earthquakes than inland regions of similar epicentral distances, though local pockets of anomalous intensities are observed for both earthquakes.