Minimum Earthquake Magnitude Research Articles

An investigation of subaqueous failures triggered by the 1935 CE Mw 6.1 Témiscaming earthquake, Quebec, Canada, as an analogue for a paleoseismic study

Subaqueous mass transport deposits (MTDs) attributed to the Mw 6.1 1935 CE Témiscaming earthquake were mapped at 17 sub-bottom acoustic profile survey areas on 11 lakes near Témiscaming, Quebec. Distributed over about 1270 km2, MTDs are the product of shallow failures, up to several metres thick, that occurred along planar surfaces and involved primarily lacustrine sediments. Core samples of unfailed deposits indicate that the failure planes occurred within soft sediments at the top of a glaciolacustrine unit or at the base of overlying lacustrine deposits. Radioisotope dating of sediment samples from six coring sites on Tee and Kipawa lakes confirm that the MTDs are the product of failures triggered by the 1935 CE earthquake. To assess the application of such a mapping study to a paleoseismic investigation, the minimum magnitude of an earthquake that can generate an MTD distribution of 1270 km2 was extrapolated from a published empirical plot. The resulting magnitude of Mw/Ms 5.7-5.8 is less than the instrumental Mw 6.1 magnitude and deemed a reasonable estimation of minimum earthquake magnitude. The distribution of MTDs triggered by the 1935 CE earthquake forms the only such signature within the Témiscaming study area since roughly 8 ka cal BP. The lack of an analogous, older signature(s) is consistent with the absence of equivalent shaking to the 1935 CE earthquake over this period, but the actual timespan may be shorter and begin when gyttja deposits on slopes became thick enough to be prone to failure from such an event.

Regional assessment of prehistoric earthquake magnitudes in the South Carolina Coastal Plain

The magnitudes of prehistoric earthquakes that created liquefaction features (i.e., sandblows) in the South Carolina Coastal Plain (SCCP) are not exactly known. In this study, in situ geotechnical data, including cone penetration test (CPT) data with pore water pressure measurements, in the vicinity of discovered sandblows at Hollywood, Sampit, Gapway, and Fort Dorchester sites located in the SCCP were used with the cyclic stress method and Ground Motion Prediction Equations (GMPEs) to back-calculate the minimum earthquake magnitude (M) and peak ground acceleration (amax) to form a regional assessment of minimum amax-M in the Charleston area. Results indicate that the minimum earthquake magnitude of the prehistoric earthquakes associated with the Charleston Source ranges from 6.6 to 7.2 for the earthquake that occurred about 546 ± 17 years before present (B.P.) and from 6.2 to 6.7 for the earthquake that occurred about 5038 ± 166 years B.P. Results also show that the minimum earthquake magnitude for the prehistoric earthquakes with a source inferred to the Sawmill Branch fault and occurred at least 3500 years ago, ranges from 5.1 to 5.7.

What is the Smallest Earthquake Magnitude that Needs to be Considered in Assessing Liquefaction Hazard?

Probabilistic assessments of the potential impact of earthquakes on infrastructure entails the consideration of smaller magnitude events than those generally considered in deterministic hazard and risk assessments. In this context, it is useful to establish if there is a magnitude threshold below which the possibility of triggering liquefaction can be discounted because such a lower bound is required for probabilistic liquefaction hazard analyses. Based on field observations and a simple parametric study, we conclude that earthquakes as small as moment magnitude 4.5 can trigger liquefaction in extremely susceptible soil deposits. However, for soil profiles that are suitable for building structures, the minimum earthquake magnitude for the triggering of liquefaction is about 5. We therefore propose that in liquefaction hazard assessments of building sites, magnitude 5.0 be adopted as the minimum earthquake size considered, while magnitudes as low as 4.5 may be appropriate for some other types of infrastructure.

New dating of rapid vertical deformation of Santa Tecla Fault scarps (Mt. Etna volcano, Sicily) by lichenometry method

The eastern slope of Mt. Etna is characterised by shallow seismicity originating from normal-oblique faulting, Timpe Fault System, related to WNW-ESE regional extension.Recent research has demonstrated that in the fault population of Mt. Etna's eastern flank the minimum earthquake magnitude that will have a ground rupture effect is ca. 2.5. This system is characterised by high frequency seismic activity, due to thinned seismogenic crustal layer. This characteristic, together with the high density of the fault segments, does not always for identification of the segments responsible for the earthquake. The earthquakes, affecting the medium-lower eastern flank, have been historically reconstructed by macroseismic analysis and reported in a macroseismic database, and in recent decades by instrumental seismic registration, which provide the seismological parameters capable of evaluating focal mechanism, hypocentre and relative algorithms related to geometric parameters which control the growth of fault segments.In this paper, we present a methodology to evaluate the age of the rapid exhumation of the free-face fault plane of the NNW-SSE oriented normal fault segment named S. Tecla (Timpe Fault System). It consists of the measurement of the thalli species (Lichenometry method) in order to evaluate the parameters which characterise their growth. The seismic history of the S. Tecla Fault indicates eight certain events from 1865 to 2005 with 3.4 ÷ 4.7 Magnitude (De Guidi et al., 2012 and reference therein). We found evidence of two different recent rapid vertical deformation events at the base of the S. Tecla fault escarpment, the oldest 20 m long and 0.25 m in height, and the youngest with a 0.02 m high nude surface exposed. We have observed that there are thalli of Xanthoparmelia conspersa (Ehrh. Ex Ach.) Hale, colonizing part of the nude surface on the escarpment. The results highlight that the oldest thalli was dated at 43.7 years old, showing that rapid vertical deformation generated the surface where the thalli, after 4 years, took root. The displacement of this surface could be related to the seismic events occurring on 3rd August 1973 (3.8 M) in S. M. Ammalati area probably accompanied by intense post seismic deformation. The second and last event could be attributable to a 3.1 M seismic event occurring on 25th September 2014 (ISIDe, 2016).

Assessment of paleoseismic shaking that caused a sand blow at Fort Dorchester, SC

A sand blow from a paleo-earthquake was identified at Fort Dorchester, South Carolina in 2007. Geotechnical field investigations indicate the thickness of the source sand layer ranges from 1.2 to 3.1 m and the depth below the ground surface to the top of the layer ranges from 2.3 to 3.9 m. The layer is predominantly angular to subangular silty quartz sand with a fines content of 4 to 22% and the predominant mean grain diameter of 0.18–0.26 mm. The geotechnical data were used with paleoliquefaction evaluation methods and two independent relations to account for soil aging to estimate the minimum magnitude and peak ground acceleration of the prehistoric earthquake. Four regionally proper Ground Motion Prediction Equations were used in combination with the back-calculated peak ground accelerations to obtain proper estimation of a max -M. For a range of site-to-source distance from 4 to 10 km, the minimum earthquake magnitude and peak ground acceleration of prehistoric earthquakes at the Fort Dorchester site ranges from 5 to 6.3 and 0.19 to 0.39 g respectively.

Earthquake-triggered landslides along the Hyblean-Malta Escarpment (off Augusta, eastern Sicily, Italy) – assessment of the related tsunamigenic potential

Abstract. Eastern Sicily is affected by earthquakes and tsunamis of local and remote origin, which is known through numerous historical chronicles. Recent studies have put emphasis on the role of submarine landslides as the direct cause of the main local tsunamis, envisaging that earthquakes (in 1693 and 1908) did produce a tsunami, but also that they triggered mass failures that were able to generate an even larger tsunami. The debate is still open, and though no general consensus has been found among scientists so far, this research had the merit to attract attention on possible generation of tsunamis by landslides off Sicily. In this paper we investigate the tsunami potential of mass failures along one sector of the Hyblean-Malta Escarpment (HME). facing Augusta. The HME is the main offshore geological structure of the region running almost parallel to the coast, off eastern Sicily. Here, bottom morphology and slope steepness favour soil failures. In our work we study slope stability under seismic load along a number of HME transects by using the Minimun Lithostatic Deviation (MLD) method, which is based on the limit-equilibrium theory. The main goal is to identify sectors of the HME that could be unstable under the effect of realistic earthquakes. We estimate the possible landslide volume and use it as input for numerical codes to simulate the landslide motion and the consequent tsunami. This is an important step for the assessment of the tsunami hazard in eastern Sicily and for local tsunami mitigation policies. It is also important in view of tsunami warning system since it can help to identify the minimum earthquake magnitude capable of triggering destructive tsunamis induced by landslides, and therefore to set up appropriate knowledge-based criteria to launch alert to the population.

Application of GMPEs to estimate the minimum magnitude and peak ground acceleration of prehistoric earthquakes at Hollywood, SC

In this study, the minimum earthquake magnitude and peak ground acceleration required to initiate liquefaction at the time of prehistoric earthquakes that date back to 11,000years before present at the Hollywood site located in the South Carolina Coastal Plain were computed. The soils at this site are estimated to be about 120,000 to 130,000years old. In-situ geotechnical data, including cone penetration data with pore water pressure measurements, were used with empirical methods that account for the age of the soil deposit to back analyse the peak ground acceleration. Results were then combined with the corresponding values obtained using Ground Motion Prediction Equations (GMPEs) to obtain a proper estimation of amax-M of the prehistoric earthquakes at the Hollywood site. When the age of the earthquake was not considered, the minimum magnitude ranged from 5.8 to 6.5 and the corresponding peak ground acceleration ranged from 0.25 to 0.32g. When the age of the earthquake was considered, the minimum magnitude was found to be 0.2 to 0.3units lower depending on earthquake age and the GMPE model. For the most recent prehistoric earthquake with the age of 546±17, the minimum magnitude ranged from 5.6 to 6.3 with corresponding acceleration ranging from 0.21 to 0.28g.

New insights into the local crust structure of Mt. Etna volcano from seismological and morphotectonic data

The reconstruction of the extensional active tectonic structures on the eastern flank of Mt. Etna and of the related earthquakes, allowed the analysis of fault parameters for more than 50 episodes of coseismic ground rupture for the last 250years. The calculation of statistical relationships among earthquake magnitude, surface rupture length, surface fault displacement and their combination was performed. Moreover we found out that the minimum earthquake magnitude to have ground rupture effect is ca. 2.5. We were therefore able to hypothesize a reduced seismogenic layer (5–6km) under the lower eastern flank of the volcano.We also analysed structural and marine geology data to find further surface evidences of the volcano dynamics. We recognized for the first time a local component of uplift affecting the volcanic domain at least in the last 20ka with a rate up to 3.5mm/yr that is likely related to a volcanic dome effect.Coupling the two aspects, the seismological one and the morphostructural one, of our research we propose a local thinning of the brittle crust in this sector of Mt. Etna. The thermal effects of the uprising magma on the rheology of the crust rocks well explain all our observations.

IS THERE A NEAR-FIELD FOR SMALL-TO-MODERATE MAGNITUDE EARTHQUAKES?

The major hazard posed by earthquakes is often thought to be due to moderate to large magnitude events. However, there have been many cases where earthquakes of moderate and even small magnitude have caused very significant destruction when they have coincided with population centres. Even though the area of intense ground shaking caused by such events is generally small, the epicentral motions can be severe enough to cause damage even in well-engineered structures. Two issues are addressed here, the first being the identification of the minimum earthquake magnitude likely to cause damage to engineered structures and the limits of the near-field for small-to-moderate magnitude earthquakes. The second issue addressed is whether features of near-field ground motions such as directivity, which can significantly enhance the destructive potential, occur in small-to-moderate magnitude events. The accelerograms from the 1986 San Salvador (El Salvador) earthquake indicate that it may be unconservative to assume that near-field directivity effects only need to be considered for earthquakes of moment magnitude M 6.5 and greater.

Determination of minimum earthquake magnitude in evaluation of seismic hazards at nuclear power plant site (in Chinese) : Pan Hua & Zhang Xiaodong, Seismology & Geology, 15(4), 1993, pp 369–374

Determination of minimum earthquake magnitude in evaluation of seismic hazards at nuclear power plant site (in Chinese) : Pan Hua & Zhang Xiaodong, Seismology & Geology, 15(4), 1993, pp 369–374

Minimum Earthquake Magnitude Associated with Coseismic Surface Faulting

Rupture of the ground surface by faulting associated with shallow earthquakes is an important element to consider in the evaluation of fault activity. Observational data compiled in this report indicate that the minimum earthquake magnitude associa ted with reported sudden surface faulting is about M, 5. Considering that the epicentral areas of many earthquakes of M, 5 or less were not searched for evidence of surface faulting, the actual minimum magnitude may be smaller. A combined empirical and theoretical analysis suggests that under ideal c onditions, coseismic surface faulting of a few millimeters associated with earthquakes having moment magnitudes as small as 3 could be recognized by simple field methods. Several factors such as dimensions, depth, and orientation of the rupture surface together with observational conditions affect the development and subsequent recognition of surface faulting. Surface displacements ranging from a few millimeters to several decimeters have accompanied earthquakes having magnitudes between 5 and 6. The larger fault displacements and the earthquakes can damage structures, and that possibility should be considered in regions where shallow earthquakes of that size can occur. The generally small and short surface ruptures associated with such earthquakes may leave very little evidence in the topogra phy, stratigraphy, or near -surface structure, especially if the displacements are consistently small and the recurrence intervals for earthquakes are long. Such c onditions may explain why so few active faults have been recognized in some regions of infrequent shallow eart hquakes, such as eastern North America.