Stable Continental Region Earthquakes Research Articles

Complex rupture dynamics of the extremely shallow August 2020 M5.1 Sparta, North Carolina earthquake

On August 9, 2020, an Mw 5.1 earthquake ruptured the uppermost crust near the town of Sparta, North Carolina, creating the first co-seismic faulting surface rupture documented in the Eastern United States. Combining deep learning and matched filter earthquake detection, with differential-travel times relocation, we obtain a catalog of 1761 earthquakes, about 5.8 times the number of events listed in the standard USGS/NEIC catalog. The relocated seismicity revealed a complex fault structure with distinct planar alignments, supported by a moment tensor inversion with significant non-double-couple component. The Sparta mainshock with a centroid depth of 1.3 km is interpreted to have nucleated near the intersection of two main fault strands. The mainshock likely ruptured a blind strike-slip fault and a reverse fault associated with the identified surface rupture, both possibly part of a flower structure-like diffuse fault zone. Our observations highlight a complex behavior of extremely shallow earthquakes in stable continental regions.

Understanding seismicity and seismotectonics in a stable continental region (NW Iberian Peninsula): Implications for the nature of intraplate seismicity

Despite that earthquakes in stable continental regions (SCR) often cause more damage than interplate seismicity, they remain poorly understood. This is mainly because of the lower rate of intraplate seismicity and because of its different behaviour compared to the better-known seismicity at the plate boundary. Understand the characteristics of the intraplate seismicity is a challenge for the seismic risk studies. We study and characterise an SCR (NW Iberian Peninsula), which not only registers moderate instrumental intraplate seismicity, but also important historic seismicity and paleoseismic activity. To tackle some of the difficulties posed by intraplate seismicity, we analyse a wide and multidisciplinary data set (e.g., geological structures, seismicity, focal mechanisms, and geophysical data). Seismicity in this region is not associated with an old rift, but with inherited faults widely distributed throughout the region with a great variety of orientations. The reactivation kinematics of these faults are coherent with the current regional stresses. Instrumental seismicity is not associated with the large active faults nor with crustal limits. Seismicity is mainly clustered in swarms and sequences. Although seismic swarms present lower magnitudes, they are the most common. Based on swarms' characteristics (high b-values, upward spatiotemporal migration), reported mantellic CO2 in some thermal springs, and the reactivation of inherited steeply-dipping faults, we propose the migration of deep fluids through steeply-dipping fractured areas as the cause of the intraplate seismicity. These processes could increase the pore pressure and decrease the stresses necessary for the fault rupture in a fault-valve behaviour. In general, in intraplate context, the important control in the seismicity of the inherited fault systems favourable oriented under the current stress tensor is observed, and also the need for mechanisms that can decrease the effective stress for the fault ruptures. Mechanisms as hydrothermal fluids in arterial faults with fault-valve processes has been identified as an effective driver of intraplate seismicity, playing an important role in stability of tectonic faults. The large number and variety of these faults, that share the low strain rates in intraplate polyorogenic context, may explain the different characteristics of these intraplate regions compared with the interplate regions, as the “unanticipated” behaviour, variety of kinematics, the long quiescence periods without seismicity associated and erosion obliterating their morphotectonic expression.

Scaling relations between seismic moment and rupture area of earthquakes in stable continental regions

This article describes the development of scaling relations between seismic moment and rupture area of earthquakes in stable continental regions (SCRs). The article reviews the relations developed by Somerville and compares them with relations developed by other investigators. It also compares the scaling relations of SCR earthquakes with those in tectonically active continental regions (TCRs). Three different methods of estimating rupture area, based on aftershocks, slip models, and duration methods were used by Somerville to analyze the relation between seismic moment and rupture area, using earthquake source parameters compiled from published literature. For each category of data, the relations obtained were not significantly different from those obtained by constraining them to be self-similar (scale-invariant), so self-similar relations were adopted. The stress drops corresponding to these scaling relations range from 51 to 86 bars, with an average of 65 bars. This value is comparable with the value of 58 bars obtained by Leonard, and it is recommended that the Leonard scaling relations for SCR earthquakes be used for the NGA East Project. To a first approximation, the results of Somerville and those of Somerville et al. indicate that the rupture areas of SCR earthquakes are about half those of TCR earthquakes, and their stress drops are about 2.8 times higher. Allmann and Shearer find less of a difference, presumably because their intraplate category includes some earthquakes that we would assign to TCR instead of SCR. Their study indicates that the rupture areas of intraplate earthquakes are about two-thirds those of TCR earthquakes, and their stress drops are about 2 times higher.

The 23 February 1828 Belgian earthquake: a destructive moderate event typical of the seismic activity in Western Europe

We present a comprehensive study of the 23 February 1828 earthquake that caused severe damage in the central part of the present Belgian territory. We synthesised and critically reassessed the information provided by contemporaneous scientific studies and newspapers of the Low Countries. In addition, a systematic survey of official, private, and religious historical sources was undertaken to improve our knowledge of the earthquake effects in the most affected region. In total, we collected information for 185 localities in the present territory of Belgium, France, Germany, The Netherlands, Grand Duchy of Luxembourg, and southern England. The complex spatial repartition of earthquake testimony locations is related to the population density, the types of historical sources, and the regional geological context. Our 1828 earthquake intensity analysis demonstrates that the shaking strength diminished more slowly along the WSW-ENE strike in this part of the Brabant Massif compared to the surrounding Rhenish Massif and Paris Basin. Moreover, the northward increasing thickness of sediments on top of the Brabant Massif controlled the observed strong intensity attenuation. Using the intensity dataset, we located the earthquake at 50.70° N and 5.12° E, with uncertainty around 10 km. The magnitude is estimated as MW = 5.1 ± 0.3 and the average focal depth optimising the location inversion is 10 km with a possible range between 3 and 15 km. Our study illustrates the benefit of historical seismicity studies on the knowledge of the impact of rare destructive earthquakes in stable continental regions like Western Europe.

Surface deformation relating to the 2018 Lake Muir earthquake sequence, southwest Western Australia: new insight into stable continental region earthquakes

Abstract. A shallow Mw 5.3 earthquake near Lake Muir in the stable continental region (SCR) crust of southwest Western Australia on the 16 September 2018 was followed on the 8 November by a proximal Mw 5.2 event. Focal mechanisms produced for the events suggest reverse and strike-slip rupture, respectively. Field mapping, guided by Sentinel-1 InSAR data, reveals that the first event produced an approximately 3 km long and up to 0.4–0.6 m high west-facing surface rupture, consistent with reverse slip on a moderately east-dipping fault. The InSAR data also show that the surface scarp relates to a subsurface rupture ∼ 5 km long, bound at its northern and southern extremities by bedrock structures. The November event produced a surface deformation envelope that is spatially coincident with that of the September event but did not result in discrete surface rupture. Almost 900 aftershocks were recorded by a temporary seismometer deployment. Hypocentre locations correlate poorly with the rupture plane of their respective mainshocks but correlate well with regions of increased Coulomb stress. The spatial and temporal relationships between the Mw>5.0 events and their aftershocks reveals dependencies with implications for how other less well-documented SCR earthquake sequences could be interpreted. Furthermore, the September Mw 5.3 Lake Muir earthquake was the ninth event documented to have produced surface rupture in Australia in historical times. These nine ruptures are located exclusively in the Precambrian non-extended SCR rocks of central and western Australia, and none could have been identified and mapped using topographic signature prior to the historical event. Consistent, though fragmentary, evidence exists from analogous regions worldwide. Our analysis of the Lake Muir earthquake sequence therefore provides constraint on models describing mechanisms for strain accumulation and localized release as earthquakes in non-extended SRC crust.

The April 2017 Mw6.5 Botswana Earthquake: An Intraplate Event Triggered by Deep Fluids

AbstractLarge earthquakes in stable continental regions remain puzzling as; unlike at plate boundaries, they do not result from the local buildup of strain driven by plate tectonics. The 2017 Mw6.5, Bostwana normal faulting earthquake occurred in a region devoid from recent tectonic activity and where present‐day deformation is negligible. The depth of the event (29 ± 4 km), in a felsic lower crust where ductile deformation is expected, likely requires a transient pulse of fluids from a deep source to activate brittle faulting. The mainshock was preceded by two foreshock swarm‐like sequences that may be further evidence for fluid movement in a critically loaded fault network. Contrary to plate boundary events, the Mw6.5 Botswana earthquake did not require prior localized stress or strain accumulation. We propose that the crust in stable continental regions, even long after the last tectonic episode, constitutes a reservoir of elastic stress that can be released episodically, for instance, as a result of deep fluid migration.

Episodic activity of a dormant fault in tectonically stable Europe: The Rauw fault (NE Belgium)

Our knowledge about large earthquakes in stable continental regions comes from studies of faults that generated historical surface rupturing earthquakes or were identified by their recent imprint in the morphology. Here, we evaluate the co-seismic character and movement history of the Rauw fault in Belgium, which lacks geomorphological expression and historical/present seismicity. This 55-km-long normal fault, with known Neogene and possibly Early Pleistocene activity, is the largest offset fault west of the active Roer Valley Graben. Its trace was identified in the shallow subsurface based on high resolution geophysics. All the layers within the Late Pliocene Mol Formation (3.6 to 2.59Ma) are displaced 7m vertically, without growth faulting, but deeper deposits show increasing offset. A paleoseismic trench study revealed cryoturbated, but unfaulted, late glacial coversands overlying faulted layers of Mol Formation. In-between those deposits, the fault tip was eroded, along with evidence for individual displacement events. Fragmented clay gouge observed in a micromorphology sample of the main fault evidences co-seismic faulting, as opposed to fault creep. Based on optical and electron spin resonance dating and trench stratigraphy, the 7m combined displacement is bracketed to have occurred between 2.59Ma and 45ka. The regional presence of the Sterksel Formation alluvial terrace deposits, limited to the hanging wall of the Rauw fault, indicates a deflection of the Meuse/Rhine confluence (1.0 to 0.5Ma) by the fault's activity, suggesting that most of the offset occurred prior to/at this time interval. In the trench, Sterksel Formation is eroded but reworked gravel testifies for its former presence. Hence, the Rauw fault appears as typical of plate interior context, with an episodic seismic activity concentrated between 1.0 and 0.5Ma or at least between 2.59Ma to 45ka, possibly related to activity variations in the adjacent, continuously active Roer Valley Graben.

A new paradigm for large earthquakes in stable continental plate interiors

AbstractLarge earthquakes within stable continental regions (SCR) show that significant amounts of elastic strain can be released on geological structures far from plate boundary faults, where the vast majority of the Earth's seismic activity takes place. SCR earthquakes show spatial and temporal patterns that differ from those at plate boundaries and occur in regions where tectonic loading rates are negligible. However, in the absence of a more appropriate model, they are traditionally viewed as analogous to their plate boundary counterparts, occurring when the accrual of tectonic stress localized at long‐lived active faults reaches failure threshold. Here we argue that SCR earthquakes are better explained by transient perturbations of local stress or fault strength that release elastic energy from a prestressed lithosphere. As a result, SCR earthquakes can occur in regions with no previous seismicity and no surface evidence for strain accumulation. They need not repeat, since the tectonic loading rate is close to zero. Therefore, concepts of recurrence time or fault slip rate do not apply. As a consequence, seismic hazard in SCRs is likely more spatially distributed than indicated by paleoearthquakes, current seismicity, or geodetic strain rates.

Ground Motion Models Used in the 2014 U.S. National Seismic Hazard Maps

The National Seismic Hazard Maps (NSHMs) are an important component of seismic design regulations in the United States. This paper compares hazard using the new suite of ground motion models (GMMs) relative to hazard using the suite of GMMs applied in the previous version of the maps. The new source characterization models are used for both cases. A previous paper ( Rezaeian et al. 2014 ) discussed the five NGA-West2 GMMs used for shallow crustal earthquakes in the Western United States (WUS), which are also summarized here. Our focus in this paper is on GMMs for earthquakes in stable continental regions in the Central and Eastern United States (CEUS), as well as subduction interface and deep intraslab earthquakes. We consider building code hazard levels for peak ground acceleration (PGA), 0.2-s, and 1.0-s spectral accelerations (SAs) on uniform firm-rock site conditions. The GMM modifications in the updated version of the maps created changes in hazard within 5% to 20% in WUS; decreases within 5% to 20% in CEUS; changes within 5% to 15% for subduction interface earthquakes; and changes involving decreases of up to 50% and increases of up to 30% for deep intraslab earthquakes for most U.S. sites. These modifications were combined with changes resulting from modifications in the source characterization models to obtain the new hazard maps.

HISTORICAL SEISMICITY OF THE STABLE CONTINENTAL REGIONS (SCRs) IN THE ARABIAN PLATE(PRELIMINARY STUDY).

Historical earthquakes catalog, which were felt in the Stable Continental Regions (SCRs) of the Arabian plate for the period (600 to 1900 A.D.), was compiled. The spatial distribution of these earthquakes appears as clusters of events in the northeastern parts, and absence of events in the other parts. This may be attributed to concentration of the population centers in the northern parts relative to other parts and / or to that the seismicity level is very low. These events have a shallow focal depths. This is a characteristic feature of the SCRs. The cumulative strain release curve exhibits two periods of high seismicity separated by low seismicity period. There is a relative quite period extends from (1194 to 1900).This quite period may appear hiatus in seismic activity or in the documentation . The b–value in magnitude relationship was low and this low value is a characteristics feature of the SCRs earthquakes.

An Appraisal of the 2001 Bhuj Earthquake (Mw 7.7, India) Source Zone: Fractal Dimension and b Value Mapping of the Aftershock Sequence

We examined seismic characteristics, b value and fractal dimension of the aftershock sequence of the January 26, 2001 Bhuj earthquake (Mw 7.7) that occurred in the Kutch failed rift basin, western margin of the Stable Continental Region (SCR) of India. A total of about 2,000 events (M ≥ 2.0) were recorded within two and a half months, immediately after the main shock. Some 795 events were precisely relocated by simultaneous inversion. These relocated events are used for mapping the frequency-magnitude relation (b value) and fractal correlation dimension (Dc) to understand the seismic characteristics of the aftershocks and the source zone of the main shock. The surface maps of the b value and Dc reveal two distinct tectonic arms or zones of the V-shaped aftershock area, western zone and eastern zone. The b value is relatively higher (~1.6) in the western zone compared to a lower value (~1.4) in the eastern zone. The Dc map also shows a higher value (1.2–1.35) in the western zone compared to a lower Dc (0.80–1.15) in the eastern zone; this implies a positive correlation between Dc and b value. Two cross sections, E–W and N–S, are examined. The E–W sections show similar characteristics, higher b value and higher Dc in the western zone and lower in the eastern zone with depth. The N–S sections across the fault zones, however, show unique features; it imaged both the b and Dc characteristics convincingly to identify two known faults, the Kutch Mainland fault and the South Wagad fault (SWF), one stepping over the other with a seismogenic source zone at depth (20–35 km). The source zone at depth is imaged with a relatively lower b and higher Dc at the ‘fault end’ of the SWF showing a negative correlation. These observations, corroborated with the seismic tomography as well as with the proposed geological/tectonic model, shed a new light to our understanding on seismogenesis of the largest SCR earthquake in India in the recent years.

Accounts of the New Madrid Earthquakes: Personal Narratives across Two Centuries of North American Seismology

ArgumentThe New Madrid earthquakes shook much of North America in the winter of 1811–1812. Accounts of the New Madrid earthquakes originally were collected and employed as scientific evidence in the early nineteenth century. These early accounts were largely ignored when scientific instruments promised more quantitative and exact knowledge. Years later the earthquakes themselves became both more important and less understood because of changes in scientific models. Today, so-called intraplate or stable continental region earthquakes pose a significant problem in seismology. Historical accounts of the New Madrid events offer some of the most significant examples upon which researchers can draw and form the basis for debates over present public policy. The changing function of accounts, from narrative elements of widely-shared scientific discussion, to raw data, and back into wide-ranging conversation once again, demonstrates both deep ruptures and surprising continuities during two centuries of understanding the earth and its movement.

A record of stable continental region earthquakes from Western Australia spanning the late Pleistocene: Insights for contemporary seismicity

In plate boundary regions moderate to large earthquakes are often sufficiently frequent that fundamental seismic parameters such as the recurrence intervals of large earthquakes and maximum credible earthquake (Mmax) can be estimated with some degree of confidence. The same is not true for the Stable Continental Regions (SCRs) of the world. Large earthquakes are so infrequent that the data distributions upon which recurrence and Mmax estimates are based are heavily skewed towards magnitudes below M w5.0, and so require significant extrapolation up to magnitudes for which the most damaging ground-shaking might be expected. The rarity of validating evidence from surface rupturing palaeo-earthquakes typically limits the confidence with which these extrapolated statistical parameters may be applied. Herein we present a new earthquake catalogue containing, in addition to the historic record of seismicity, 150 palaeo-earthquakes derived from 60 palaeo-earthquake features spanning the last > 100 ka of the history of the Precambrian shield and fringing extended margin of southwest Western Australia. From this combined dataset we show that Mmax in non-extended-SCR is M7.25 ± 0.1 and in extended-SCR is M7.65 ± 0.1. We also demonstrate that in the 230,000 km 2 area of non-extended-SCR crust, the rate of seismic activity required to build these scarps is one tenth of the contemporary seismicity in the area, consistent with episodic or clustered models describing SCR earthquake recurrence. A dominance in the landscape of earthquake scarps reflecting multiple events suggests that the largest earthquakes are likely to occur on pre-existing faults. We expect these results might apply to most areas of non-extended-SCR worldwide.

Postglacial Faults in Fennoscandia: Targets for scientific drilling

During the last stages of the Weichselian glaciation (ca. 9,000–15,000 years B.P.), reduced ice loads and glacially affected stress fields resulted in active faulting in Fennoscandia with fault scarps up to 160 km long and up to 30 m high. These postglacial (PG) faults are usually SE dipping, SW–NE oriented thrusts, and represent reactivated, pre-existing crustal discontinuities. Postglacial faulting indicates that the glacio-isostatic compensation is not only a gradual viscoelastic phenomenon, but also includes unexpected violent earthquakes, suggestively larger than other known earthquakes in stable continental regions. We explore here possibilities and benefits for investigating, via scientific drilling, the characteristics of postglacial faults in northern Fennoscandia, including their structure and rock properties, present and past seismic activity and state of stress, as well as hydrogeology and associated deep biosphere. The research is anticipated to advance science in neotectonics, hydrogeology and deep biosphere studies, and provide important information for nuclear waste disposal, petroleum exploration on the Norwegian continental shelf and studies of mineral resources in PG fault areas.

Analysis of Aftershock and Foreshock Activity in Stable Continental Regions: Implications for Aftershock Forecasting and the Hazard of Strong Earthquakes

The Omori-law aftershock parameters for 13 earthquakes in stable continental regions (SCRs) globally are found to distribute in the same way as those for California aftershock sequences. Of 19 SCR mainshocks with M ≥ 6.0 since 1968, eight had their largest aftershock within five days of the mainshock and 11 within 30 days of the mainshock. The mean magnitude difference between the mainshock and the largest aftershock of these 19 SCR events is 1.4 ± .7 magnitude units, with a range from 0.3 to 3.6 magnitude units. From 1968 to 2003 the rate at which SCR earthquakes of M ≥ 4.5 worldwide were followed by a comparable or larger earthquake within the next 30 days is 5%. These statistics can be used to produce aftershock forecasts for strong SCR earthquakes and to estimate the chances that an SCR earthquake of M ≥ 4.5 will be followed by a larger seismic event within the next month.

Shallow Seismicity in Stable Continental Regions

A worldwide compilation of well-constrained fault ruptures and focal depths of earthquakes reveals that the Earth9s crust in many stable continental regions (SCRs) is characterized by a bimodal depth distribution with a very shallow upper crustal component. The distributions can vary in a) depth of the modes and b) strength of bimodality, probably due to intracrustal boundaries, differences in frictional and rheological properties, heat-flow densities, strain-rates, and tectonic forces or forces stemming from the surface. Overall, SCR ruptures and SCR earthquakes are confined within the upper third (0–10 km) and/or the lower third of the crust (20–35 km), while the midcrust (10–20 km) tends to be aseismic. Historical data indicate that some SCRs show very well-developed bimodal distributions of focal depths ( e.g. , North Alpine foreland basin in Europe, Kachchh basin in India), while others show weak to no developed bimodal distributions ( e.g. , Charlevoix seismic zone, New Madrid seismic zone in the central United States). Moreover, many large SCR earthquakes ( M w 4.5–8.0) nucleate on reverse faults and close to the surface ( m b e.g. , ground motion).

Relocation of Early and Late Aftershocks of the 2001 Bhuj Earthquake Using Joint Hypocentral Determination (JHD) Technique: Implication toward the Continued Aftershock Activity for more than Four Years

We employed layered model joint hypocentral determination (JHD) with station corrections to improve location identification for the 26 January, 2001 Mw 7.7 Bhuj early and late aftershock sequence. We relocated 999 early aftershocks using the data from a close combined network (National Geophysical Research Institute, India and Center for Earthquake Research Institute, USA) of 8–18 digital seismographs during 12–28 February, 2001. Additionally, 350 late aftershocks were also relocated using the data from 4–10 digital seismographs/accelerographs during August 2002 to December 2004. These precisely relocated aftershocks (error in the epicentral location<30 meter, error in the focal depth estimation < 50 meter) delineate an east-west trending blind thrust (North Wagad Fault, NWF) dipping (~ 45°) southward, about 25 km north of Kachchh main land fault (KMF), as the causative fault for the 2001 Bhuj earthquake. The aftershock zone is confined to a 60-km long and 40-km wide region lying between the KMF to the south and NWF to the north, extending from 2 to 45 km depth. Estimated focal depths suggest that the aftershock zone became deeper with the passage of time. The P- and S-wave station corrections determined from the JHD technique indicate that the larger values (both +ve and -ve) characterize the central aftershock zone, which is surrounded by the zones of smaller values. The station corrections vary from −0.9 to +1.1 sec for the P waves and from −0.7 to +1.4 sec for the S waves. The b-value and p-value of the whole aftershock (2001–2004) sequences of Mw ≥ 3 are estimated to be 0.77 ± 0.02 and 0.99 ± 0.02, respectively. The p-value indicates a smaller value than the global median of 1.1, suggesting a relatively slow decay of aftershocks, whereas, the relatively lower b-value (less than the average b-value of 1.0 for stable continental region earthquakes of India) suggests a relatively higher probability for larger earthquakes in Kachchh in comparison to other stable continental regions of the Indian Peninsula. Further, based on the b-value, mainshock magnitude and maximum aftershock magnitude, the Bhuj aftershock sequence is categorized as the Mogi's type II sequence, indicating the region to be of intermediate level of stresses and heterogeneous rocks. It is inferred that the decrease in p-value and increase in aftershock zone, both spatially as well as depth over the passage of time, suggests that the decay of aftershocks perhaps could be controlled by visco-elastic creep in the lower crust.

안정대륙권역의 중규모지진에 의한 근단층지반운동의 모델링

이 논문에서는 안정대륙권역(Stable Continental Regions, SCRs)에서의 중규모 지진에 의한 근단층지반운동(Near Fault Ground Motion, NFGM) 모델을 처음으로 제시한다. 근단층지반운동은 큰 진폭의 장주기 속도 펄스를 갖는 특징을 가지고 있다. 이 속도 펄스를 모델링하기 위해서는 그 주기와 진폭을 지진의 규모와 단층거리의 함수로 표현할 수 있어야 한다. 그런데 안정대륙권역에서는 관측 자료가 빈약하여 지진데이터로부터 이 관계식을 직접 유도하는 것은 어렵기 때문에 이 연구에서는 간접적인 접근법을 채택하였다. 속도 펄스의 주기와 진폭은 단층파열의 상승시간과 파열속도의 함수임이 알려져 있고 활성구조권역(Active Tectonic Regions, ATRs)에 속하는 미국 서부지역에서는 실험적 공식이 확립되어 있다. 안정대륙권역에서의 상승시간과 단층파열속도의 지진규모에 대한 함수관계는 WUS와 CEUS에서의 자료를 비교하여 도출하였다. 이 관계식들로부터 안정대륙권역에서의 NFGM의 속도 펄스의 주기와 진폭을 지진규모 및 단층 거리에 대한 관계식으로 유도하였다. 안정대륙권역에서의 NFGM의 가속도 시간이력은 추계학적으로 생성된 원역지진지반가속도에 새로운 관계식에 의한 속도 펄스를 중첩하여 얻어진다. 적용 예제로서 탄소성 단자유도 시스템의 근단층지반운동에 대한 응답을 분석하였다. This paper proposes a method for modeling new fault ground motion due to moderate size earthquakes in Stable Continental Regions (SCRs) for the first time. The near fault ground motion is characterized by a single long period velocity pulse of large amplitude. In order to model the velocity pulse, its period and peak amplitude need be determined in terms of earthquake magnitude and distance from the causative fault. Because there have been observed very few new fault ground motions, it is difficult to derive the model directly from the recorded data in SCRs. Instead an indirect approach is adopted in this work. The two parameters, the period and peak amplitude of the velocity pulse, are known to be functions of the rise time and the slip velocity. For Western United States (WUS) that belongs active tectonic regions, there art empirical formulas for these functions. The relations of rise time and slip velocity on the magnitude in SCRs are derived by comparing related data between Western United States and Central-Eastern United States that belongs to SCRs. From these relations, the functions of these pulse parameters for NFGM in SCRs can be expressed in terms of earthquake magnitude and distance. A time history of near fault ground motion of moderate magnitude earthquake in stable continental regions is synthesized by superposing the velocity pulse on the for field ground motion that is generated by stochastic method. As an demonstrative application, the response of a single degree of freedom elasto-plastic system is studied.

Peak displacement demand of small to moderate magnitude earthquakes in stable continental regions

A theoretical fault-slip model has been developed for predicting the notional peak displacement demand (PDD) of earthquakes based on a limiting natural period of 5 s, for application in stable continental regions (SCRs). The developed theoretical expression is simple and robust. Importantly, it envelops predictions arising from a number of existing empirical and seismological (stochastic) models included in the comparison. The notional PDD prediction has been made initially for hard rock crustal conditions and at a reference source-site distance of 30 km. Factors have accordingly been introduced to correct for different distances and geological conditions in completing the PDD prediction model. Assuming displacement-controlled behaviour, the predicted notional PDD may be compared with the displacement capacity of a structure, or component, for purposes of seismic stability assessment.

A Source Study of the Bhuj, India, Earthquake of 26 January 2001 (Mw 7.6)

We study the source time function (STF) and radiated seismic energy (E_R) of the M_w 7.6 Bhuj earthquake using the empirical Green's function (EGF) technique. Our estimations of the STF and E_R are based on teleseismic P waves and regional seismograms, respectively. We find that the STFs as a function of azimuth have a similar shape and nearly constant duration of 18 sec. This suggests that the rupture propagation was essentially radial. The STFs show a sharp rise in the first 6 sec. The E_R estimated from the EGF technique is 2.1 × 10^(23) erg. We find that E_R's computed from integration of corrected velocity-squared spectra of teleseismic P waves and regional seismograms are in excellent agreement with the ER obtained from the EGF technique. Since the seismic moment, M_0, is 3.4 × 10^(27) dyne cm, we obtain E_R/M_0 = 6.2 × 10^(-5). The radiation efficiency, η_R, during the Bhuj earthquake was low, about 0.23. The sharp rise of the STFs and η_R = 0.23 can be explained by Sato and Hirasawa's (1973) quasi-dynamic, circular source model with an effective stress of ∼ 300 bar and the ratio of rupture to shear-wave velocity, V_R/β, of ∼ 0.5. The corresponding estimate of slip velocity at the center of the fault is 156 cm/sec. V_R/β ∼ 0.5 is in reasonable agreement with the duration of the STF and the reported dimension of the aftershocks, as well as with the results of inversion of teleseismic body waves. The observations may also be explained by a frictional sliding model, with gradual frictional stress drop and significant dissipation of energy on the fault plane. This model requires an average dynamic stress drop of about 120 bar and V_R/β ∼ 0.7 to explain both the rapid rise in the first 6 sec of the STFs and, along with a static stress drop of 200 bar, the observed E_R/M_0. High static stress drop is a common feature of most crustal earthquakes in stable continental regions. An examination of the available data, however, does not suggest that most of them also have relatively low radiation efficiency.