On the assessment of maximum earthquakes in the alps and adjacent areas

Strong tectonic earthquakes within the crust always occur on already existing faults, and they have the property of a shear rupture. Such earthquakes with surface-wave magnitudes M < 7 obviously have a geometric similarity. Because of this similarity and the validity of the Gutenberg and Richter's energy—magnitude relation, the expression M = 2 log₁₀ L + const., with L = focal length, is valid.The expression L_maxL^* for the maximum focal length, is also valid if L^* is the length of the rectilinear extent of the seismic line on which the maximum earthquake occurs. The bounds of L^* may be given by sharp bends and/or by traversing deep faults. Thus the maximum imaginable earthquake on a seismic line with the length L^* has the magnitude M_max = 2 log₁₀ L^* + const.For the investigated region — the Alps and adjacent areas — from the data of recent and historical strong earthquakes, it follows that M_max = 2 log₁₀ L^* + 1.7, if L^* is measured in kilometres. These limiting values lie in the centre-field of the magnitude range for maximum earthquakes, published by Shebalin in 1970. By the aid of this equation it is also possible to assess the upper limiting value of the accompanying maximum scale intensity.     

Earthquakes in Switzerland and surrounding regions during 2010

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2010. During this period, 407 earthquakes and 85 quarry blasts were detected and located in the region under consideration. With a total of only 19 events with M_L ≥ 2.5, the seismic activity in the year 2010 was below the average over the previous 35 years. The two most noteworthy earthquakes were the M_L 3.4 Barrhorn event near Sankt Niklaus (VS) and the M_L 3.0 event of Feldkirch, both of which produced shaking of intensity IV.     

Earthquakes in Switzerland and surrounding regions during 2006

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2006. During this period, 572 earthquakes and 91 quarry blasts were detected and located in the region under consideration. Of these earthquakes, two occurred in conjunction with the construction of the new Gotthard railway tunnel and 165 were induced artificially by the stimulation of a proposed geothermal reservoir beneath the city of Basel. With 20 events with M _L ≥ 2.5, five of which were artificially induced, the seismic activity in the year 2006 was far below the average over the previous 31 years. Nevertheless, six events were felt by the public, most prominently the strongest of the induced Basel events (M _L 3.4), which caused some non-structural building damage. Noteworthy are also the two earthquakes near Cortaillod (M _L 3.2), on the shore of Lake Neuchatel, and in Val Mora (M _L 3.5), between the Engadin and Val Müstair, as well as the 42 aftershocks of the M _L 4.9 Vallorcine earthquake, between Martigny and Chamonix, of September 2005. Editorial handling: Stefan Bucher     

Earthquakes in Switzerland and surrounding regions during 2004

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2004. During this period, 677 earthquakes and 96 quarry blasts were detected and located in the region under consideration. With 22 events with M_L≥2.5, the seismic activity in the year 2004 was close to the average over the last 30 years. As in previous years, most of the activity was concentrated in the Valais and in Graubünden. In addition, several moderate earthquakes occurred in the lower crust below the northern Alpine foreland. Unusual was that five earthquakes were sufficiently strong to cause ground shaking of intensity IV over large portions of the territory. Two were located in Switzerland (Liestal, M_L 3.8, and Brugg, M_L 4.0). The epicenters of the other three strong events were located outside Switzerland (Besan?on in the French Jura, M_L 4.8, Waldkirch in southern Germany, M_L 5.1, and Lago di Garda in northern Italy, M_L 5.3).     

Seismic slip rates,potential subsurface rupture areas and seismic potential of the Vienna Basin Transfer Fault

The Vienna Basin Transfer Fault (VBTF) is a slow active fault with moderate seismicity (I _max~8–9, M _max~5.7) passing through the most vulnerable regions of Austria and Slovakia. We use different data to constrain the seismic potential of the VBTF including slip values computed from the seismic energy release during the 20th century, geological data on fault segmentation and a depth-extrapolated 3-D model of a generalized fault surface, which is used to define potential rupture zones. The seismic slip of the VBTF as a whole is in the range of 0.22–0.31 mm/year for a seismogenic fault thickness of 8 km. Seismic slip rates for individual segments vary from 0.00 to 0.77 mm/year. Comparing these data to geologically and GPS-derived slip velocities (>1 mm/year) proofs that the fault yields a significant seismic slip deficit. Segments of the fault with high seismic slip contrast from segments with no slip representing locked segments. Fault surfaces of segments within the seismogenic zone (4–14 km depth) vary from 55 to 400 km². Empirical scaling relations show that these segments are sufficiently large to explain both, earthquakes observed in the last centuries, and the 4th century Carnuntum earthquake, for which archeo-seismological data suggest a magnitude of M ≥ 6. Based on the combination of all data (incomplete earthquake catalog, seismic slip deficits, locked segments, potential rupture areas, indications of strong pre-catalog earthquakes) we argue, that the maximum credible earthquake for the VBTF is in the range M _max = 6.0–6.8, significantly larger than the magnitude of the strongest recorded events (M = 5.7).     

Andean earthquakes triggered by the 2010 Maule,Chile (Mw 8.8) earthquake: Comparisons of geodetic,seismic and geologic constraints

The Maule, Chile, (M_w 8.8) earthquake on 27 February 2010 triggered deformation events over a broad area, allowing investigation of stress redistribution within the upper crust following a mega-thrust subduction event. We explore the role that the Maule earthquake may have played in triggering shallow earthquakes in northwestern Argentina and Chile. We investigate observed ground deformation associated with the M_w 6.2 (GCMT) Salta (1450 km from the Maule hypocenter, 9 h after the Maule earthquake), M_w 5.8 Catamarca (1400 km; nine days), M_w 5.1 Mendoza (350 km; between one to five days) earthquakes, as well as eight additional earthquakes without an observed geodetic signal. We use seismic and Interferometric Synthetic Aperture Radar (InSAR) observations to characterize earthquake location, magnitude and focal mechanism, and characterize how the non-stationary, spatially correlated noise present in the geodetic imagery affects the accuracy of our parameter estimates. The focal mechanisms for the far-field Salta and Catamarca earthquakes are broadly consistent with regional late Cenozoic fault kinematics. We infer that dynamic stresses due to the passage of seismic waves associated with the Maule earthquake likely brought the Salta and Catamarca regions closer to failure but that the involved faults may have already been at a relatively advanced stage of their seismic cycle. The near-field Mendoza earthquake geometry is consistent with triggering related to positive static Coulomb stress changes due to the Maule earthquake but is also aligned with the South America-Nazca shortening direction. None of the earthquakes considered in this study require that the Maule earthquake reactivated faults in a sense that is inconsistent with their long-term behavior.     

Earthquakes in Switzerland and surrounding regions during 2008

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2008. During this period, 451 earthquakes and 75 quarry blasts were detected and located in the region under consideration. The three strongest events occurred in the Valais, near Lac des Toules (M_L 3.6), and in Graubünden, near Ilanz (M_L 3.7) and Paspels (M_L 4.0). Although felt by the population, they were not reported to have caused any damage. However, with a total of only 15 events with M_L ≥ 2.5, the seismic activity in the year 2008 was far below the average over the previous 33 years.     

Magnitude conversion problem for the Turkish earthquake data

Earthquake catalogues which form the main input in seismic hazard analysis generally report earthquake magnitudes in different scales. Magnitudes reported in different scales have to be converted to a common scale while compiling a seismic data base to be utilized in seismic hazard analysis. This study aims at developing empirical relationships to convert earthquake magnitudes reported in different scales, namely, surface wave magnitude, M _S, local magnitude, M _L, body wave magnitude, m _b and duration magnitude, M _d, to the moment magnitude (M _w). For this purpose, an earthquake data catalogue is compiled from domestic and international data bases for the earthquakes occurred in Turkey. The earthquake reporting differences of various data sources are assessed. Conversion relationships are established between the same earthquake magnitude scale of different data sources and different earthquake magnitude scales. Appropriate statistical methods are employed iteratively, considering the random errors both in the independent and dependent variables. The results are found to be sensitive to the choice of the analysis methods.     

Earthquakes in Switzerland and surrounding regions during 2012

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2012. During this period, 497 earthquakes and 88 quarry blasts were detected and located in the region under consideration. With a total of only 13 events with M_L ≥ 2.5, the seismic activity in the year 2012 was far below the average over the previous 37 years. Most noteworthy were the earthquake sequence of Filisur (GR) in January with two events of M_L 3.3 and 3.5, the M_L 4.2 and M_L 3.5 earthquakes at a depth of 32 km below Zug in February and the M_L 3.6 event near Vallorcine in October. The epicentral intensity of the M_L 4.2 event close to Zug was IV, with a maximum intensity of V reached in a few areas, probably due to site amplification effects.     

Probabilities for the occurrences of medium to large earthquakes in northeast India and adjoining region

The return periods and occurrence probabilities related to medium and large earthquakes (M _w 4.0–7.0) in four seismic zones in northeast India and adjoining region (20°–32°N and 87°–100°E) have been estimated with the help of well-known extreme value theory using three methods given by Gumbel (1958), Knopoff and Kagan (1977) and Bury (1999). In the present analysis, the return periods, the most probable maximum magnitude in a specified time period and probabilities of occurrences of earthquakes of magnitude M ≥ 4.0 have been computed using a homogeneous and complete earthquake catalogue prepared for the period between 1897 and 2007. The analysis indicates that the most probable largest annual earthquakes are close to 4.6, 5.1, 5.2, 5.5 and 5.8 in the four seismic zones, namely, the Shillong Plateau Zone, the Eastern Syntaxis Zone, the Himalayan Thrusts Zone, the Arakan-Yoma subduction zone and the whole region, respectively. The most probable largest earthquakes that may occur within different time periods have been also estimated and reported. The study reveals that the estimated mean return periods for the earthquake of magnitude M _w 6.5 are about 6–7 years, 9–10 years, 59–78 years, 72–115 years and 88–127 years in the whole region, the Arakan-Yoma subduction zone, the Himalayan Thrusts Zone, the Shillong Plateau Zone and the Eastern Syntaxis Zone, respectively. The study indicates that Arakan-Yoma subduction zone has the lowest mean return periods and high occurrence probability for the same earthquake magnitude in comparison to the other zones. The differences in the hazard parameters from zone to zone reveal the high crustal heterogeneity and seismotectonics complexity in northeast India and adjoining regions.     

Statistical relations between source parameters for low and intermediate magnitudes (Friuli, 1976)

A data set of three-component short-period digital seismograms recorded in Friuli after the strong earthquake of 6th May 1976, allowed the local magnitude M_L and the seismic moment M₀ to be estimated in the range 0 < M_L < 2. The data set including the same parameters for the higher-magnitude Friuli events (M_L 5) shows two different slopes for the relation Log M₀ = CM_L + d for the two different ranges of M_L. One finds C ~ 1.0 (for 0 < M_L < 2) and C ~1.5 (for5 M_L 6.2), respectively.This implies that apparent stress release increases at low magnitudes, while it appears to be comparatively independent of the magnitude and to have an average value of about 100 bar for higher-magnitude earthquakes. Conversely, the fault dimensions do not appear to be magnitude-dependent for M_L < 2; for higher-magnitude events the linear fault dimensions range from about 1 km at M_L ~ 5 to about 12 km for the strong earthquake of 6th May 1976 (M_L = 6.2).     

Is earthquake activity along the French Atlantic margin favoured by local rheological contrasts?

The seismological study of recent seismic crises near Oleron Island confirms the coexistence of an extensional deformation and a transtensive regime in the Atlantic margin of France, which is different from the general western European stress field corresponding to a strike-slip regime. We argue that the switch of the principal stress axes σ₁/σ₂ in a NW–SE vertical plane is linked with the existence of crustal heterogeneities. Events of magnitude larger than 5 sometimes occur along the Atlantic margin of France, such as the 7 September 1972 (M_L = 5.2) earthquake near Oleron island and the 30 September 2002 (M_L = 5.7) Hennebont event in Brittany. To test the mechanism of local strain localization, we model the deformation of the hypocentral area of the Hennebont earthquake using a 3D thermo-mechanical finite element code. We conclude that the occurrence of moderate earthquakes located in limited parts of the Hercynian shear zones (as the often reactivated swarms near Oleron) could be due to local reactivation of pre-existing faults. These sporadic seismic ruptures are favoured by stress concentration due to rheological heterogeneities.     

Shear‐wave splitting and earthquake forecasting

Seismic shear‐wave splitting (SWS) monitors the low‐level deformation of fluid‐saturated microcracked rock. We report evidence of systematic SWS changes, recorded above small earthquakes, monitoring the accumulation of stress before earthquakes that allows the time and magnitude of impending large earthquakes to be stress‐forecast. The effects have been seen with hindsight before some 15 earthquakes ranging in magnitude from an M1.7 seismic swarm event in Iceland to the Ms7.7 Chi‐Chi Earthquake in Taiwan, including a successfully stress‐forecast of a M5.0 earthquake in SW Iceland. Characteristic increases in SWS time‐delays are observed before large earthquakes, which abruptly change to deceases shortly before the earthquake occurs. There is a linear relationship between magnitudes and logarithms of durations of both increases and decreases in SWS time‐delays before large impending earthquakes. However, suitably persistent swarms of small earthquakes are too scarce for routine stress‐forecasting. Reliable forecasting requires controlled‐source cross‐hole seismics between neighbouring boreholes in stress‐monitoring sites (SMS). It would be possible to stress‐forecast damaging earthquakes worldwide by a global network of SMS in real time.     

Seismically induced changes in groundwater levels and temperatures following the ML5.8 (ML5.1) Gyeongju earthquake in South Korea

Hydrogeological responses to earthquakes such as changes in groundwater level, temperature, and chemistry, have been observed for several decades. This study examines behavior associated with M_L 5.8 and M_L 5.1 earthquakes that occurred on 12 September 2016 near Gyeongju, a city located on the southeast coast of the Korean peninsula. The M_L 5.8 event stands as the largest recorded earthquake in South Korea since the advent of modern recording systems. There was considerable damage associated with the earthquakes and many aftershocks. Records from monitoring wells located about 135 km west of the epicenter displayed various patterns of change in both water level and temperature. There were transient-type, step-like-type (up and down), and persistent-type (rise and fall) changes in water levels. The water temperature changes were of transient, shift-change, and tendency-change types. Transient changes in the groundwater level and temperature were particularly well developed in monitoring wells installed along a major boundary fault that bisected the study area. These changes were interpreted as representing an aquifer system deformed by seismic waves. The various patterns in groundwater level and temperature, therefore, suggested that seismic waves impacted the fractured units through the reactivation of fractures, joints, and microcracks, which resulted from a pulse in fluid pressure. This study points to the value of long-term monitoring efforts, which in this case were able to provide detailed information needed to manage the groundwater resources in areas potentially affected by further earthquakes.

     

The 1780 seismic sequence in NE Sicily (Italy): shifting an underestimated and mislocated earthquake to a seismically low rate zone

The southernmost sector of the Italian peninsula is crossed by an almost continuous seismogenic belt capable of producing M ∼ 7 earthquakes and extending from the Calabrian Arc, through the Messina Straits, as far as Southeastern Sicily. Though large earthquakes occurring in this region during the last millennium are fairly well known from the historical point of view and seismic catalogues may be considered complete for destructive and badly damaging events (IX ≤ I _o ≤ XI MCS), the knowledge and seismic completeness of moderate earthquakes can be improved by investigating other kinds of documentary sources not explored by the classical seismological tradition. In this paper, we present a case study explanatory of the problem, regarding the Ionian coast between the Messina Straits and Mount Etna volcano, an area of North-eastern Sicily lacking evidence of relevant seismic activity in historical times. Now, after a systematic analysis of the 18th century journalistic sources (gazettes), this gap can be partly filled by the rediscovery of a seismic sequence that took place in 1780. According to the available catalogues, the only event on record for this year is a minor shock (I _o = VI MCS, M _w = 4.8) recorded in Messina on March 28, 1780. The newly discovered data allow to reinstate it as the mainshock (I _o = VII–VIII MCS, M _w = 5.6) of a significant seismic period, which went on from March to June 1780, causing severe damage along the Ionian coast of North-eastern Sicily. The source responsible for this event appears located offshore, 40-km south of the previous determination, and is consistent with the Taormina Fault suggested by the geological literature, developing in the low seismic rate zone at the southernmost termination of the 1908 Messina earthquake fault.     

The 2005 Heidelberg and Speyer earthquakes and their relationship to active tectonics in the central Upper Rhine Graben

We determine the source parameters of three minor earthquakes in the Upper Rhine Graben (URG), a Cenozoic rift, using waveforms from permanent and temporary seismological stations. Two shallow thrust-faulting events (M _L = 2.4 and 1.5) occurred on the rift shoulder just south of Heidelberg in March 2005. They indicate a possible movement along the sediment–crystalline interface due to tectonic loading from the near-by Odenwald. In February 2005, an earthquake with a normal-faulting mechanism occurred north of Speyer. This event (M _L = 2.8) had an unusual depth of about 22 km and a similar deep normal-faulting event occurred there in 1972 (M _L = 3.2). Other lower crustal events without fault plane solutions are known from 1981 and 1983. At such a depth, inside the lower crust, ductile behaviour instead of brittle faulting is commonly assumed and used for geodynamic modelling. Based on the newly available fault plane solutions we can confirm the brittle, extensional regime in the upper and lower crust in the central to northern URG indicated in earlier studies.     

Earthquakes in Switzerland and surrounding regions during 2011

This report of the Swiss Seismological Service summarizes the seismic activity in Switzerland and surrounding regions during 2011. During this period, 522 earthquakes and 92 quarry blasts were detected and located in the region under consideration. With a total of only 10 events with M _L????2.5, the seismic activity in the year 2011 was far below the average over the previous 36?years. Most noteworthy were the earthquake sequence of Sierre (VS) in January, with two events of M _L 3.3 and 3.2, the M _L 3.3 earthquake at a depth of 31?km below Bregenz, and the M _L 3.1 event near Delémont. The two strongest events near Sierre produced shaking of intensity IV.