Rates of sulfur dioxide and particle emissions from White Island volcano,New Zealand,and an estimate of the total flux of major gaseous species

Airborne correlation spectrometry (COSPEC) was used to measure the rate of SO₂ emission at White Island on three dates, i.e., November 1983, 1230 ± 300 t/d; November 1984, 320 ± 120 t/d; and January 1985, 350 ± 150 t/d (t = metric tons). The lower emission rates are likely to reflect the long-term emission rates, whereas the November 1983 rate probably reflects conditions prior to the eruption of December 1983. The particle flux in the White Island plume, as determined with a quartz crystal microbalance/cascade in November 1983, was 1.3 t/d, unusually low for volcanic plumes. The observed plume particles, as shown from scanning electron microscopy, include halite, native sulfur, and silicates and are broadly similar to other volcanic plumes.Gas analyses from high-temperature volcanic fumaroles collected from June 1982 through November 1984 werde used together with the COSPEC data to estimate the flux of other gas species from White Island. The rates estimated are indicative of the long-term volcanic emission, i.e., 8000–9000 t/d H₂O, 900–1000 t/d CO₂, 70–80 t/d HCl, 1.5–2 t/d HF, and about 0.2 t/d NH₃. The long-term thermal power output at White Island is estimated at about 400 MW.     

Anomalous radon emanation at local and regional distances preceding earthquakes in the New Madrid seismic zone and adjacent areas of the central mid-continent of North America, 1981–84

Anomalous soil-radon activity, including several spike-like surges over periods of 5, 2, and 2 1/2 months, and a year-long declining trend, preceded the most significant earthquakes of the central mid-continental region of North America during 1981 and early 1984. The 5-month period of erratic weekly radon activity, February–June 1981, was followed by a tremor of magnitudeM=4.0, 40 km from the monitoring site in the New Madrid seismic zone. An unusual earthquake swarm in central Arkansas, 160 km from the New Madrid seismic zone and 230 km from the monitoring site in the seismic zone, began in January 1982, shortly after a year-long declining trend in anomalous radon emanation. Earthquakes of magnitudes 4.5, 4.1, and 4.0 occurred at the swarm's outset in early 1982, fitting a pattern anticipated for the New Madrid seismic zone on the basis of the radon activity. Two periods of coincident peak radon emanation have since been observed in the Arkansas and New Madrid seismic regions, as have synchronous seismic pulses for the two separate areas.Two more recent periods of highly erratic soil-radon emanation, March–May 1983 and November–January 1984, were followed by a 4.3 earthquake in southwestern Illinois on 15 may 1983, and 3.5–3.6 tremors and swarm activity in the New Madrid seismic zone in late January and mid-February 1984. Prior to the 4.3 event, radon peaked at three widely separated monitoring sites 1–3 weeks before the tremor at distances of 120, 225, and 320 km from the epicentral region, the station at 225 km, in the New Madrid seismic zone, recording the longest period of anomalous radon activity. As for the recent 3.5–3.6 tremors of 1984, seismic activity of this magnitude had been anticipated for January or February on the basis of the amplitude of the November radon anomaly.These observations provide further evidence of (a) the existence of soil-radon anomalies precursory to the large earthquakes in this intraplate region, (b) the utility of such anomalies in anticipating events of small to moderate magnitudes for the region, and (c) the occurrence of regional-scale strain events prior to some of the larger mid-continental earthquakes.A very recent radon anomaly, the strongest yet to be detected in the seven years of monitoring in the mid-continental region, occurred in the New Madrid seismic zone from mid-February through mid-June 1984. A 4.0 earthquake occurred one month after a peak in the radon activity. The amplitude and duration of the anomaly suggest that a significant change in the state of stress or strain may have occurred in the mid-continental region during 1984.     

The October 4th, 1983 — Magnitude 4 earthquake in Phlegraean Fields: Macroseismic survey

On Oct. 4th, 1983 the area of Phlegraean Fields, near Naples (Southern Italy) was shaked by an earthquake of magnitude (M _L) 4.0 that caused some damage in the town of Pozzuoli and its surroundings. This seismic event was the largest one recorded during the recent (1982–84) inflation episode occurred in the Phlegraean volcanic area, and a detailed macroseismic reconstruction of the event was carried out.Failing macroseismic data on other earthquakes occurred in Phlegraean Fields, the attenuation law of the intensity as a function of the distance as obtained for the Oct. 4th earthquake was compared with those obtained for other volcanic areas in central Italy —i.e., Tolfa, Monte Amiata — in order to check the reliability of the results obtained for Phlegraean Fields.The Blake's model of the earthquake of Oct. 4th, 1983 does not agree with the experimental data because isoseismals contain areas larger than those shown by the model. This result has been interpreted as an effect of energy focusing due to a reflecting layer 6–8 km deep.     

A new fault rupture scenario for the 2003 Mw 6.6 Bam earthquake, SE Iran: Insights from the high-resolution QuickBird imagery and field observations

The December 26, 2003 M_w 6.6 Bam earthquake is one of the most disastrous earthquakes in Iran. QuickBird panchromatic and multispectral satellite imagery with 61 cm and 2.4 m ground resolution, respectively provide new insights into the surface rupturing process associated with this earthquake. The results indicate that this earthquake produced a 2–5 km-wide surface rupture zone with a complex geometric pattern. A 10-km-long surface rupture zone developed along the pre-existing Bam fault trace. Two additional surface rupture zones, each 2–5 km long, are oblique to the pre-existing Bam fault in angles of 20–35°. An analysis of geometric and geomorphic features also shows that movement on the Bam fault is mainly right-lateral motion with some compressional component. This interpretation is consistent with field investigations, analysis of aftershocks as well as teleseismic inversion. Therefore, we suggest that the 2003 Bam earthquake occurred on the Bam fault, and that the surface ruptures oblique to the Bam fault are caused by secondary faulting such as synthetic shears (Reidel shears). Our fault model for the Bam earthquake provides a new tectonic scenario for explaining complex surface deformations associated with the Bam earthquake.     

Longitudinal and temporal patterns of benthic coarse particulate organic matter in the Agüera stream (northern Spain)

Benthic coarse particulate organic matter (CPOM) was studied between November 92 and December 93 at four sites along the longitudinal gradient of the Agüera stream system (Northern Spain). CPOM was sorted in four main categories: leaves (several species), fruits and seeds, twigs and debris. Headwater site showed higher densities of total CPOM, leaves and twigs than downstream reaches, but no regular longitudinal pattern of change was noticed. The ranges of mean CPOM standing stock at the sampling sites were 20.5–74.1 g AFDW m^–2 (site B), 9.9–47.7 g AFDW m^–2 (site 5), 4.3–21.4g AFDW m^–2 (site 7) and 9.8–37.9 g AFDW m^–2 (site 9). The particulate matter at downstream sites was in a more advanced stage of breakdown probably as a result of processing and transport from upstream reaches. Leaves species composition of benthic CPOM clearly reflected the type of riparian vegetation at each site. The timing of inputs and the hydrologic regime appeared to act together influencing temporal dynamics of benthic CPOM. A gradual temporal change in species composition of benthic leaf litter was observed under natural mature deciduous forest: first alder, later chestnut and finally oak.     

Discharge of H2 from the Atotsugawa and Ushikubi Faults,Japan, and its relation to earthquakes

The concentration of H₂ in soil gases has been measured weekly at five stations on the Atotsugawa and Ushikubi faults in northern central Main Island, Japan, since 1981 in search of possible relationship with earthquakes. The observed H₂ concentration varies from lower than 1 ppm to 7.8% in time and place. When a large earthquake (M: 7.7, epicenter distance: 486 km) occurred on 26 May 1983, an outstanding discharge of H₂ was observed at all five stations, preseismically at three of them, and coseismically at the other two. Simultaneous H₂ emission was also observed at some stations in seven other occasions. These periods of unusual H₂ discharge nearly coincided with occurrences of major earthquakes in Japan, but not of local minor earthquakes along the Atotsugawa fault. This fault, being a deep fracture zone, may be sensitive to large-scale crustal stress changes which incidentally cause the major earthquakes. Increased H₂ may be produced by rock fracture caused by the increased stresses on the fault and by the earthquakes themselves. Local minor earthquakes along Atotsugawa fault with magnitude lower than 3 may be unable to cause sufficient rock fracture to produce significant H₂.     

Equatorial and low latitude ionosphere during intense geomagnetic storms

An investigation is made in order to analyse the role of neutral gas composition in the equatorial and low latitude ionosphere during intense geomagnetic storms. To this end data taken by the Dynamic Explorer 2 satellite at 280–300 km (molecular nitrogen N₂ and atomic oxygen O concentrations, electron density and vertical plasma drifts) are used. The sudden commencements of the events considered occurred at 11:38 UT on March 1, 1982, 18:41 UT on November 20, 1982 and 16:14 UT on February 4, 1983. Vertical plasma drifts are the most important contributor to the initial storm time response of the equatorial F region. Neutral composition changes (expressed as an increase in the molecular species, mainly N₂) possibly play a predominant role in the equatorial and low latitude (10–20°) decreases of electron density at heights near F2-region maximum during the main and recovery phases of intense geomagnetic storms. Delayed increases of electron density observed at daytime during the recovery phase may be also attributed to increases in atomic oxygen. At low latitudes possibly a combined effect of O increase and upward plasma drift due to enhanced equatorward winds is the responsible mechanism for the maintenance of enhanced electron density values.     

Earthquake activity in the Aswan region,Egypt

The November 14, 1981 Aswan earthquake (M _L= 5.7), which was related to the impoundment of Lake Aswan, was followed by an extended sequence of earthquakes, and is investigated in this study. Earthquake data from June 1982 to late 1991, collected from the Aswan network, are classified into two sets on the basis of focal depth (i.e., shallow, or deeper than 10 km). It is determined that (a) shallow seismicity is characterized by swarm activity, whereas deep seismicity is characterized by a foreshock-main shock-aftershock sequence; (b) the b value is equal to 0.77 and 0.99 for the shallow and deep sequences, respectively; and (c) observations clearly indicate that the temporal variations of shallow seismic activity were associated with a high rate of water-level fluctuation in Lake Aswan; a correlation with the deeper earthquake sequence, however, is not evident. These features, as well as the tomographic characteristics of the Aswan region (Awad andMizoue, this issue), imply that the Aswan seismic activity must be regarded as consisting of two distinct earthquake groups.We also relocated the largest 500 earthquakes to determine their seismotectonic characteristics. The results reveal that the epicenters are well distributed along four fault segments, which constitute a conjugate pattern in the region. Moreover, fault-plane solutions are determined for several earthquakes selected from each segment, which, along with the 14 November 1981 main shock, demonstrate a prominent E-W compressional stress.     

The fractal nature of the inhomogeneities in the lithosphere evidenced from seismic wave scattering

In this paper we show evidences of the fractal nature of the 3-D inhomogeneities in the lithosphere from the study of seismic wave scattering and discuss the relation between the fractal dimension of the 3-D inhomogeneities and that of the fault surfaces. Two methods are introduced to measure the inhomogeneity spectrum of a random medium: 1. the coda excitation spectrum method, and 2. the method of measuring the frequency dependence of scattering attenuation. The fractal dimension can be obtained from the inhomogeneity spectrum of the medium. The coda excitation method is applied to the Hindu-Kush data. Based on the observed coda excitation spectra (for frequencies 1–25 Hz) and the past observations on the frequency dependence of scattering attenuation, we infer that the lithospheric inhomogeneities are multiple scaled and can be modeled as a bandlimited fractal random medium (BLFRM) with an outer scale of about 1 km. The fractal dimension of the 3-D inhomogeneities isD ₃=31/2–32/3, which corresponds to a scaling exponent (Hurst number)H=1/2–1/3. The corresponding 1-D inhomogeneity spectra obey the power law with a powerp=2H+1=2–5/3. The intersection between the earth surface and the isostrength surface of the 3-D inhomogeneities will have fractal dimensionD ₁=1.5–1.67. If we consider the earthquake fault surface as developed from the isosurface of the 3-D inhomogeneities and smoothed by the rupture dynamics, the fractal dimension of the fault trace on the surface must be smaller thanD ₁, in agreement with recent measurements of fractal dimension along the San Andreas fault.     

Composition of hydrothermal fluids during the bradyseismic crisis which commenced at Phlegraean Fields in 1982

A systematic geochemical surveillance on the fumaroles of Solfatara and the boiling pools of Pisciarelli was carried out by discontinuous monitoring of the chemical composition of the emitted fluids during the Phlegraean Fields bradyseismic crisis which has begun in 1982. The fluids are considered to be produced by the ebullition of shallow aquifers receiving a convective gaseous inflow from the underlying magma chamber.Increased water vapor concentrations at a constant temperature of about 155 °C throughout the investigated period, along with the occurrence of ground deformations and seismic phenomena, are interpreted as resulting from an increased heat supply to the boiling water bodies.Dissolution processes and reactions with the confining rocks can alter the chemical composition of fluids escaping from magma to a large extent. Therefore it does not appear correct to consider the absolute values of any chemical constituent for geochemical surveillance without taking this modifying factor into account. Acid gases will be preferably absorbed by the above mentioned aquifers, while other species like H₂, N₂, O₂, CH₄, will instead increase their relative concentrations. Because of this, water vapour concentrations and the ratios H₂S/CO₂ and H₂/CH₄ in surface thermal manifestations appear to reflect better the varying extent of the observed phenomenon.On the basis of these parameters, and of both the upheaval rate and the intensity of seismic events, maximum values in the convective input of magmatic origin are estimated to have occurred at the beginning of the crisis and in September–October 1983.As long as water bodies at shallow depth are able to buffer the convective flow both thermally and chemically, no important volcanic activity can develop. When the absorbing capacity of these aquifers is exhausted, the increasing temperature and the changing characteristics of fluids towards a magmatic composition will indicate a higher probability of eruptive phenomena.CNR —Centro di studio per la mineralogia e la geochimica dei sedimenti.     

Modeling of the runup heights of the Hokkaido-Nansei-Oki tsunami of 12 July 1993

The Hokkaido-Nansei-Oki earthquake (M _w 7.7) of July 12, 1993, is one of the largest tsunamigenic events in the Sea of Japan. The tsunami magnitudeM _t is determined to be 8.1 from the maximum amplitudes of the tsunami recorded on tide gauges. This value is larger thanM _w by 0.4 units. It is suggested that the tsunami potential of the Nansei-Oki earthquake is large forM _w . A number of tsunami runup data are accumulated for a total range of about 1000 km along the coast, and the data are averaged to obtain the local mean heightsH _n for 23 segments in intervals of about 40 km each. The geographic variation ofH _n is approximately explained in terms of the empirical relationship proposed byAbe (1989, 1993). The height prediction from the available earthquake magnitudes ranges from 5.0–8.4 m, which brackets the observed maximum ofH _n , 7.7 m, at Okushiri Island.     

Geochemical surveillance of the Solfatara of Pozzuoli (Phlegraean Fields) during 1983

Geochemical surveillance of the Phlegraean Fields area has been intensified since 1983, in response to the increased uplift rate (brady-seismic activity).Fumarolic gases from Solfatara (Pozzuoli) were sampled and analyzed monthly. A Reducing Capacity (RC) monitoring unit was installed at Soffione, the most active fumarole in the Solfatara system.The preliminary analysis of the RC temporal variations suggest they are consistent with the rate of the seismic energy release.The composition of fumarolic gases indicates that the equilibrium temperature and pressure are higher than those of sampling.The observed variations in CH₄ content are explained as an increase of pressure (from 1982 to the end of 1983) of about 70% at an extimated depth of 3–3.5 km.Finally, during 1983, there were no geochemical indications of both rising magma and significant accumulation of energy at shallow depth.     

Preliminary results of seismogeochemical research in China

Several kinds of geochemical anomaly before strong earthquakes have been observed in China since 1966. They include changes in groundwater radon levels, ion content of water (Ca⁺², Mg⁺², Cl^–, SO ₄ ^–2 , F^–), dissolved gases (H₂, CO₂), and gases escaping from the aeration zone through abandoned dry wells (Ar, N₂, CO₂). The radon anomalies may be grouped as long-term and short-term anomalies. Most of the geochemical anomalies observed are characterized by a pattern of increase. The largest amplitude recorded was 37 times the base level. Preliminary study indicates that the types of seismogeochemical anomaly observed prior to strong earthquakes depend on tectonic, geologic, lithologic, and hydrogeological conditions at the monitoring station. Results obtained from modelling experiments on the mechanisms of some anomalies are given.     

甘肃三个台地磁日变赫斯特指数的时序特征

选用甘肃嘉峪关、兰州和天水地磁台2011—2013年的观测数据,重点研究地磁垂直向日变化波形下行段的赫斯特指数变化的时序特征。结果表明,在此3年内,嘉峪关台和兰州台地磁垂直向日变化波形下行段的赫斯特指数变化范围分别是0.08和0.06,天水台在2012年存在一个赫斯特指数幅度超过正常变化范围的过程,在此过程完成后的2013年7月22日,在距离天水130km的漳县和岷县交界发生M6.6地震。结合其他学者的研究结果,认为这一赫斯特指数的异常变化过程可能反映了区域性地球深部(下地壳与上地幔)热物质运移变化而引起地球局部居里面变化,也可能是地震孕育过程在地磁日变中的表征。这一发现有益于进一步研究地磁日变异常的机理、震磁关系以及地震预报的探索。     

Relation between hydrogen emission and seismic activities

Measurements of chemical composition of bubbles from a mineral spring at Yuya Spa situated close to the Median Tectonic Line, the longest active fault in Japan, showed that the periods of increased H₂ emission coincided with occurrences of the Ohno earthquake swarm nearby. Four cases of the coincidence without exception were observed in the last three years. The fluctuation of H₂ concentration ranges between <0.5 and 200 ppm, whereas other gases such as He, Ar, N₂, and CH₄ do not fluctuate much. The H₂ concentration is correlated with the energy released by the seismic activity. This field evidence, together with the results of laboratory experiments conducted bySugisaki et al. (1983), leads to the conclusion that H₂ observed at the mineral spring was produced by the reaction between groundwater and rock fractured in the seismic activities. The observation that H₂ in the mineral spring tends to appear prior to an earthquake suggests that microcracks may occur in rocks prior to earthquakes. The precursory emission of H₂ may be useful for earthquake prediction.     

Characteristics of earthquake ground motions and the H/V of microtremors in the southwestern part of Taiwan

The relationships between the spectral characteristics of earthquake ground motions and those of micro‐tremors are investigated using the observed data from a dense strong‐motion network consisting of 108 stations in the Yun‐Li, Chia‐Yi and Tai‐Nan areas in southwestern Taiwan. Many high‐quality recordings, including those of the 921 Chi‐Chi earthquake (M_w=7.6), the 1022 Chia‐Yi mainshock (M_L=6.4), the 1022 major aftershock (M_L=6.0), as well as some weak motion events are selected to evaluate site responses. Microtremor measurements are also performed at most ground motion stations. With many stations in the area located on an alluvium structure, however, it is difficult to find good reference stations on rock sites, which therefore necessitates the calculation of single‐station H/V ratios. The predominant frequencies obtained from H/V ratios are consistent with those from spectral ratios. The site characteristics between the strong and weak events are different, however. This implies that a nonlinear effect probably occurred with the strong‐motion events. The main peak in the H/V spectra of the microtremors is in good agreement with the first peak obtained from the spectra of earthquake ground motions. It is reasonable to claim that the main peak reflects the deep underground structure. On the basis of the H/V ratios of the microtremors, it is concluded that the lower predominant frequencies appear in the plain area, while the higher values are near the mountainous region. Copyright © 2002 John Wiley & Sons, Ltd.     

Characteristics of strong ground motion for typical Australian intra-plate earthquakes and their relationship with the recommended response spectra

Global epicentre maps show that the majority of earthquakes are inter-plate, although moderate to large earthquakes do occur intra-plate, i.e. within the plates. The seismicity of the Australian continent is typical of intra-plate environments and a magnitude M_L 6 earthquake has an average return period of about 5 years. Recordings of Australian intra-plate earthquakes are investigated here to characterise their frequency content, peak acceleration and duration.Due to lack of quality strong motion records of large intra-plate earthquakes at short distances, synthetic seismograms are commonly used for testing structural behaviour. An empirical Green's Function method (Geophys. Res. Lett., 5 (1978), 1–4; Proceedings of the Third International Microzonation Conference, Seattle, USA, vol. 1, (1982), pp. 447–458.) is chosen to simulate a large earthquake by summation in time of a number of smaller earthquakes or sub-events, each given a slightly different origin time to represent more realistically the propagation of a rupture along an assumed fault plane. In the first instance, recordings on rock of the magnitude M_L 2.3 aftershock of the 29 December 1989 Newcastle earthquake were used as sub-events to simulate the main shock of magnitude M_L 5.6. Validation studies for events recorded elsewhere in Australia are also considered.The response spectra of such synthetic events will be compared with the recommended spectra developed empirically from a statistical analysis of strong motion data for magnitude 5.4–6.5 intra-plate earthquakes recorded in other parts of the world and normalised to a peak ground velocity of 50 mm/s which is typical for a return period of 500 years in Australia (Australasian Structural Engineering Conference, Auckland, New Zealand, (1998), pp. 439–444.). Preliminary results from this comparison with the response spectra recommended for the Building Code of Australia show that the synthetic waveforms produced by this method are realistic and can be used to represent ground motion during typical Australian intra-plate earthquakes.     

Gravity observations along the eastern margin of the Tibetan Plateau and an application to the Lushan M S7.0 earthquake

This paper introduces relative and absolute gravity change observations in the eastern portion of the Tibetan Plateau. We analyze and discuss a change that occurred in 2010 in the gravity along the eastern margin of the plateau and the relationship between this change and the 2013 Lushan M _S7.0 earthquake. Our results show that: (1) before the Lushan M _S7.0 earthquake, gravity anomalies along the eastern margin of the Tibetan Plateau changed drastically. The Lushan earthquake occurred at the bend of the high gradient zone of gravity variation along the southern edge of the Longmenshan fault zone. (2) The 2013 Lushan earthquake occurred less than 100 km away from the epicenter of the 2008 Wenchuan earthquake. Lushan and Wenchuan are located at the center of a four-quadrant section with different gravity anomalies, which may suggest that restoration after the Wenchuan earthquake may have played a role in causing the Lushan earthquake. (3) A medium-term prediction based on changes in gravity anomalies was made before the Lushan M _S7.0 earthquake, in particular, a prediction of epicenter location.     

A global experimental model for gravity tides of the Earth

The long-term, continuous and high-quality tidal gravity data, recorded with the superconducting gravimeters (SGs) at 19 stations in the Global Geodynamics Project (GGP), were simultaneously used to investigate the global pattern of the tidal gravity variations. The atmospheric effects were removed from the gravity observations by using the simultaneous pressure records at the stations. A total of six global co-tidal models were employed to remove the loading effects of oceanic tides. The resonance parameters of the Earth's free core nutation (FCN), as well as the spheroidal constant components in the gravimetric factors of waves O₁ and M₂, were accurately retrieved. As a result, a global experimental model for gravity tides (GEMGT) was developed, considering the nearly diurnal resonance and the latitude-dependence of the gravimetric amplitude factors. The final results indicate that the mean discrepancy of the four main tidal waves (i.e. O₁, K₁, M₂ and S₂) between the GEMGT and SG observations is less than 0.2% on average. The GEMGT is in good agreement with the theoretical models based on the inelastic non-hydrostatic equilibrium Earth models [Dehant, V., Defraigne, P., Wahr, J., 1999. Tides for a convective Earth. J. Geophys. Res. 104, 1035–1058; Mathews, P.M., 2001. Love numbers and gravimetric factor for diurnal tides. J. Geodetic Soc. Jpn. 46 (4), 231–236] with a mean discrepancy less than 0.15%. However, the GEMGT is in closer accordance with the theoretical model given by Mathews [Mathews, P.M., 2001. Love numbers and gravimetric factor for diurnal tides. J. Geodetic Soc. Jpn. 46 (4), 231–236] for the diurnal tides while it is in closer agreement with one obtained by Dehant et al. [Dehant, V., Defraigne, P., Wahr, J., 1999. Tides for a convective Earth. J. Geophys. Res. 104, 1035–1058] for the semi-diurnal tides.     

f max and fault zone property of Lushan earthquake of 20 April 2013, Sichuan,China

In this study, we determined f _max from near-field accelerograms of the Lushan earthquake of April 20, 2013 through spectra analysis. The result shows that the values of f _max derived from five different seismography stations are very close though these stations roughly span about 100 km along the strike. This implies that the cause of f _max is mainly the seismic source process rather than the site effect. Moreover, according to the source–cause model of Papageorgiou and Aki (Bull Seism Soc Am 73:693–722, 1983), we infer that the cohesive zone width of the rupture of the Lushan earthquake is about 204 with an uncertainty of 13 m. We also find that there is a significant bulge between 30 and 45 Hz in the amplitude spectra of accelerograms of stations 51YAL and 51QLY, and we confirm that it is due to seismic waves’ reverberation of the sedimentary soil layer beneath these stations.