Evaluation of seismic hazard at Roermond, The Netherlands: A comparison of results after the 13 April 1992 earthquake

Mean return periods, RP, for the site of Roermond, The Netherlands, as calculated by different methods, are compared, and its quality evaluated by a simple two-tail test of hypothesis. Results show that RP values by the EGO-method are statistically more likely. They can be considered, despite their broad 90% probability intervals, and for the site and data used, more reliable, since the Roermond earthquake was not an unusual or surprising event for the Lower Rhine Embayment area, where earthquakes of comparable size have occurred since the 18th century.     

The Erzincan earthquake of 13 March 1992 in Eastern Turkey: tectonic aspects

The tectonic significance of the Erzincan earthquake of 13 March, 1992 in Eastern Turkey is discussed. The intersection of the North Anatolian and The East Anatolian strike-slip fault zones has resulted in formation of the Erzincan pull-apart basin and new seismically active fault branches on its northeastern side. Local concentrations of surface ruptures strike along the most active branches of the North Anatolian fault zone (N300W) for 62 km. They are usually open fractures with northeastern sides uplifted up to 20 cm and rarely with dextral offset up to 10 cm. These secondary ruptures manifest indirectly oblique seismic fault displacement corresponding to the Late Quaternary motion on the fault zone, although at the surface the dextral component has been suppressed relative to the vertical one.     

The Roer Valley Graben earthquake of 13 April 1992 and its seismotectonic setting

A moderately sized pure, normal dip-slip earthquake occurred in the Roer Valley Graben (RVG) near Roermond, The Netherlands on 13 April 1992 at 1h 20m UTC. This contribution presents an overview of the locations, fault-plane solutions and magnitudes obtained for the mainshock and the aftershocks by the different scientific groups involved in their analysis. The observed maximum intensity of VII is compared with that of other earthquakes in the region to illustrate the relatively low level of damage caused by the mainshock.
Using SH and Lg waves recorded at seven local and regional broadband stations, we determine a seismic moment of 1.4 × 10¹⁷ Nm, a static stress drop of 9.7 MPa and an average displacement of 33 cm over a rupture surface of approximately 11 km².
The seismotectonics of the region extending from the RVG to the city of Liège including the western part of the Rhenish Massif (WRM) and the eastern part of the Brabant Massif (EBM) is analysed based on the Roermond earthquake studies and data collected since 1985 by the Belgian seismic network. The geographical distribution of focal mechanism reveals four different seismotectonic regimes in this area. From stress tensor inversion we find that ₃ coincides with the minimum horizontal stress component in the RVG, the WRM and possibly in the EBM, while in the Liège region ₃ is approximately vertical. The minimum horizontal stress component shows a 30° rotation to the north in the WRM and the Liège region and possibly 50° in the EBM when compared with the minimum horizontal stress component in the RVG.     

The Erzincan earthquake of 13 March 1992 in Eastern Turkey

The 13 March 1992 Erzincan earthquake, M=6.8, occurred in the eastern half of the Erzincan basin. The largest aftershock took place near Pülümür on 15 March 1992. No clear surface breaks were observed, although teleseismic studies suggested that it was a strike-slip earthquake striking parallel to the North Anatolian fault, with a focus of approximately 10±2 km depth, 30 km rupture length, 95 cm of slip, and a 1.16×10²⁶ dyn.cm seismic moment. The aftershock distribution concentrated at an area of the intersection between the North Anatolian fault and the Ovacik fault. These results indicate that the previously suggested seismic gap along the North Anatolian fault, east of Erzincan, still remains unruptured.     

Tectonic position and geological and seismic manifestations of the Gorkha earthquake of April 25, 2015, in Nepal

The characteristics of sources of the Gorkha earthquake’s mainshock (April 25, 2015, Nepal) and strongest aftershock are given. Macroseismic data and examples of seismic dislocations are provided. The course of seismic energy release during the aftershock process is analyzed. The data on seismological precursors of the mainshock and the strongest aftershock of May 12, 2015, are presented, which allowed the aftershock to be predicted in a short-term interval.     

Landslides triggered by slipping-fault-generated earthquake on a plateau: an example of the 14 April 2010, Ms 7.1, Yushu, China earthquake

On 14 April 2010 at 07:49 (Beijing time), a catastrophic earthquake with Ms 7.1 struck Yushu County, Qinghai Province, China. A total of 2,036 landslides were interpreted from aerial photographs and satellite images, verified by selected field checking. These landslides cover about a total area of 1.194 km². The characteristics and failure mechanisms of these landslides are presented in this paper. The spatial distribution of the landslides is evidently strongly controlled by the locations of the main co-seismic surface fault ruptures. The landslides commonly occurred close together. Most of the landslides are small; there were only 275 individual landslide (13.5 % of the total number) surface areas larger than 1,000 m². The landslides are of various types. They are mainly shallow, disrupted landslides, but also include rock falls, deep-seated landslides, liquefaction-induced landslides, and compound landslides. Four types of factors are identified as contributing to failure along with the strong ground shaking: natural excavation of the toes of slopes, which mean erosion of the base of the slope, surface water infiltration into slopes, co-seismic fault slipping at landslide sites, and delayed occurrence of landslides due to snow melt or rainfall infiltration at sites where slopes were weakened by the co-seismic ground shaking. To analyze the spatial distribution of the landslides, the landslide area percentage (LAP) and landslide number density (LND) were compared with peak ground acceleration (PGA), distance from co-seismic main surface fault ruptures, elevation, slope gradient, slope aspect, and lithology. The results show landslide occurrence is strongly controlled by proximity to the main surface fault ruptures, with most landslides occurring within 2.5 km of such ruptures. There is no evident correlation between landslide occurrences and PGA. Both LAP and LND have strongly positive correlations with slope gradient, and additionally, sites at elevations between 3,800 and 4,000 m are relatively susceptible to landslide occurrence; as are slopes with northeast, east, and southeast slope aspects. Q₄ al-pl, N, and T₃ kn ¹ have more concentrated landslide activity than others. This paper provides a detailed inventory map of landslides triggered by the 2010 Yushu earthquake for future seismic landslide hazard analysis and also provides a study case of characteristics, failure mechanisms, and spatial distribution of landslides triggered by slipping-fault generated earthquake on a plateau.     

The 20 March 1992 South Aegean,Greece, earthquake (Ms= 5.3): possible anomalous effects

The earthquake (M_s= 5.3) of 20 March 1992 and its aftershocks, which occurred near the volcanic island complex of Milos, South Aegean, Greece, are studied on the basis of filed observations and instrumental data. The mainshock caused some building damage, the maximum intensity of VI+ (MM) being assigned to Triovasalos, Milos. Ground cracks, liquefaction in soil, landslides and rockfalls were observed in Milos. Liquefaction took place at an apparently anomalously long epicentral distance (D= 12 km) and is associated with unusually small earthquake magnitude. Abnormal animal behaviour was reported no longer than twelve hours before the mainshock. The b-value (= 1.02) of the G–R relation for the aftershock sequence, the exponentially decreasing number of aftershocks with time, and the difference (= 0.5) in magnitude between the mainshock and its largest aftershock imply that the origin of these earthquakes is tectonic and not associated with the volcanic field of Milos.     

Activity and distribution of geohazards induced by the Lushan earthquake,April 20, 2013

An Ms7.0 earthquake, focal depth 13 km, struck Lushan on April 20, 2013, caused 196 deaths and 21 missing, 13,484 injuries, and affected more than two million people. A field investigation was taken immediately after the quake, and the induced hazards were analyzed in comparison with the Wenchuan earthquake. We have identified 1,460 landslides and avalanches and four dammed lakes, which were generally small and concentrated on high elevation. Avalanches and rockfalls developed in cliffs and steep slopes of hard rocks, including Jinjixia of Baosheng Town and Dayanxia of Shuangshi Town, Lushan, and the K317 section the Xiaoguanzi section north to Lingguan Town along the provincial highway S210. Landslides were relatively less, mainly in moderate and small scales, developing in sandstone, shale, and loose colluviums. Only one single large landslide was observed to turn into debris slide-flow. Dammed lakes were formed by avalanches and landslides, all in small size and of low danger degree. The earthquake-induced hazards distributed in belt on the hanging wall along the faults, and their major controlling factors include tectonics, lithology, structure surface, and landform. More than 99 % landslides were within 30 km to the epicenter, and 678 within 10 km, accounting for 46 % of the total; about 50 % landslides were distributed on slopes between 35° and 55°, and 11 % on slope exceeding 75°; 60 % on slopes at the altitudes between 1,000 and 1,500 m, 77 % on slopes between 900 and 1,500 m; and 24 and 62 % in hard rocks and section between hard and soft rocks, respectively. Compared with the case of Wenchuan earthquake, both the number and extension of landslides and avalanches in Lushan earthquake-affected area are much smaller, only 5.53 % in number and 0.57 % in area. The earthquake has increased the instability of slope and potentiality of landslide and debris flow. Accordingly, the active period is expected to be relatively short comparing with that in Wenchuan earthquake-hit area. However, the insidious and concealed hazards bring difficulty for risk investigation.     

Revision of the earthquake catalogue and seismicity of Croatia, 1908–1992

The paper presents the results of Croatian earthquake catalogue revision for the period 1908–1992. The revised catalogue lists a total of 4853 events (4486 after 1908), of which 3700 are mainshocks. All primary data sources available to us (seismograms, phase onset time readings, macroseismic questionnaires…) were consulted in order to improve the quality and reliability of magnitude, intensity and location. Along with the most important earthquake parameters the catalogue contains entries that enable assessment of the reliability of location and bibliographic references. Using the temporal analysis of the maximum-likelihood estimate of the b -value in the frequency-magnitude relation, the catalogue is estimated to be complete for events with M ≥ 4.0 throughout the investigated period. The magnitude completeness threshold decreases to 3.8 after the middle of the century, and to about 3.6 in the last three decades. It is noted that the b-value variation with time is closely tied to the seismic activity, as well as that at present there seems to be a lack of seismic energy release corresponding to the M = 6.0–6.5 earthquake.     

The proposed solway firth barrages: A review of the geological and geotechnical aspects

The various barrages suggested for the Solway Firth are discussed. The specific geology of each of the proposed sites is reviewed and the inadequacy of present information emphasised. Geotechnical problems which might arise during and after construction of the barrages are summarized. The possible sources of various types of constructional materials are indicated. Finally, methods are suggested which would provide much of the information essential for an accurate assessment of the various proposals for the area.     

Prediction of the 6.6 Grevena-Kozani earthquake of May 13, 1995

Seismic Electric Signals (SES) were recorded by VAN-group on April 18–19, 1995, at Ioannina station; they resulted in an official prediction that was sent (two weeks before the earthquake occurrence) to the Greek authorities as well as to various International Institutes.The observation of these electrical variations was confirmed by Gruszow et al. (1996); however, they claim that these signals could be attributed to a (non determined) nearby artificial source with huge intensity (IL≈4 × 10⁴Am, for r ≈ 2 km. or 1.6 × 10⁵Am, for r ≈ 4 km). This claim is not valid, because, such an artificial source (cf. horizontal point current dipole) should have produced: (a) electrical field variations having amplitudes two orders of magnitude, larger than the observed ones; this is theoretically shown and experimentally verified and (b) magnetic field variations mainly on the horizontal field, while, in the present case, they have been recorded mainly on the vertical component.Furthermore, we show that the above SES obey the criteria, suggested by Varotsos and Lazaridou (1991), for discriminating SES from noise.     

Engineering geological conditions and the effects of the 1981 earthquake in Athens,Greece

The 1981 earthquakes, manifested in the Gulf of Corinth, brought serious structural damage to buildings in the city of Athens. Because of the observed concentration of damages in the Halandri district of Athens, an investigation of soil conditions was carried out. This included boreholes, measurement of physicomechanical properties of soil and weak rock units and determination of the depth to top-of-rock and thickness of recent deposits. Based on stratigraphy, the area studied was divided in five zones. For each zone, the total number of damaged buildings was determined and the percentages of damage to three-storey and multi-storey buildings were calculated. Seismic characteristics, acceleration and dominant period of soil and weak rock formations were estimated. From the above analysis, it is concluded that the dominant period of the soil, in the zones where most of the damage to multi-storey buildings was observed, is approximately equal to the dominant period of these buildings.     

The geological background of the May 12, 2008 Ms 8.0 Wenchuan catastrophic earthquake,Longmen Shan,Western China

The Longmen Shan fault zone is located at the particular boundary between the Triassic Songpan-Ganzi orogen of the Qinghai-Tibetan Plateau and the stable Sichuan basin of the Yangtze platform. There are four major active faults and three tectonic nappes in this region. According to an analysis of neotectonics and historical earthquakes, the Longmen Shan fault zone presents a high level of seismic hazard. The rupture system that hosted the Wenchuan earthquake is characterized by thrust and dextral strike-slip movement.     

Observations from the 12 October 1992 Dahshour earthquake in Egypt

An earthquake of local magnitude 5.3 (duration magnitudeM _d ) on the Richter Scale occurred at Dahshour, 18 km south of Cairo, Egypt, on Monday, 12 October 1992 at 3:09 pm (local time). Numerous aftershocks followed the main event during the following weeks with magnitude up to 4.3. The earthquake occurred in an area that has had no recent seismic activity, and affected many cities in Egypt. Many buildings and monuments were severely damaged or collapsed. Modern concrete skeletal structures suffered minor nonstructural damage. Earthquake physical damage was estimated at about one billion U.S. Dollars. The severity of the damage was mainly due to poor construction materials and detailing, aging, inferior workmanship, and inadequate maintenance. Egypt was generally considered to be an area of moderate seismic activity. In 1989, earthquake provisions were first introduced in the Egyptian Code of Practice for Reinforced Concrete Structures only. The earthquake clearly showed the urgent need for an assessment and rehabilitation program to mitigate seismic risk hazard in existing structures. In addition, future development planning, and earthquake preparedness strategies should implement lessons learned from the event. In this paper, an overview discussion about the observations from the 12 October earthquake is presented.