Thesis advisor: John E. Ebel / Studies of aftershock sequences in the Central Virginia Seismic Zone (CVSZ) provide critical details of the subsurface geologic structures responsible for past and (possibly) future earthquakes in an intraplate setting. The 23 August 2011 MW 5.8 Mineral, Virginia, earthquake, the largest magnitude event recorded in the CVSZ, caused widespread damage and generated a lengthy and well-recorded aftershock sequence. Over 1600 aftershocks were recorded using a dense network of seismometers in the four months following the mainshock, offering the unique opportunity to study the fault structure responsible for the post-main event seismicity. Previous work has not accurately determined the geometry of the fault structure or the migration of post-mainshock seismicity and association of the 2011 event with a known fault has been unsuccessful. In this study, relative locations of recorded aftershocks were calculated using a version of the double-difference location method outlined in Ebel et al. (2008) to generate an accurate model of the fault structure. The moment tensor inversion technique of Ebel and Bonjer (1990) was used to generate focal mechanisms of dozens of the aftershocks at various locations on the fault structure. Results from the double-difference and moment tensor inversion methods were used to map the structure responsible for the aftershock sequence in high resolution. The calculated fault structure has planes with similar strikes and dips as known faults and geologic structures in the CVSZ. In-depth analysis of this aftershock sequence provides seismologists with the opportunity to better understand the seismic hazards present in poorly understood intraplate seismic zones. / Thesis (MS) — Boston College, 2016. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Earth and Environmental Sciences.

Several mechanisms are proposed to be part of the earthquake triggering process, including static stress interactions and dynamic stress transfer. Significant differences of these mechanisms are particularly expected in the spatial distribution of aftershocks. However, testing the different hypotheses is challenging because it requires the consideration of the large uncertainties involved in stress calculations as well as the appropriate consideration of secondary aftershock triggering which is related to stress changes induced by smaller pre- and aftershocks. In order to evaluate the forecast capability of different mechanisms, I take the effect of smaller--magnitude earthquakes into account by using the epidemic type aftershock sequence (ETAS) model where the spatial probability distribution of direct aftershocks, if available, is correlated to alternative source information and mechanisms. Surface shaking, rupture geometry, and slip distributions are tested. As an approximation of the shaking level, ShakeMaps are used which are available in near real-time after a mainshock and thus could be used for first-order forecasts of the spatial aftershock distribution. Alternatively, the use of empirical decay laws related to minimum fault distance is tested and Coulomb stress change calculations based on published and random slip models. For comparison, the likelihood values of the different model combinations are analyzed in the case of several well-known aftershock sequences (1992 Landers, 1999 Hector Mine, 2004 Parkfield). The tests show that the fault geometry is the most valuable information for improving aftershock forecasts. Furthermore, they reveal that static stress maps can additionally improve the forecasts of off--fault aftershock locations, while the integration of ground shaking data could not upgrade the results significantly. In the second part of this work, I focused on a procedure to test the information content of inverted slip models. This allows to quantify the information gain if this kind of data is included in aftershock forecasts. For this purpose, the ETAS model based on static stress changes, which is introduced in part one, is applied. The forecast ability of the models is systematically tested for several earthquake sequences and compared to models using random slip distributions. The influence of subfault resolution and segment strike and dip is tested. Some of the tested slip models perform very good, in that cases almost no random slip models are found to perform better. Contrastingly, for some of the published slip models, almost all random slip models perform better than the published slip model. Choosing a different subfault resolution hardly influences the result, as long the general slip pattern is still reproducible. Whereas different strike and dip values strongly influence the results depending on the standard deviation chosen, which is applied in the process of randomly selecting the strike and dip values. / Verschiedene Mechanismen werden für das Triggern von Erdbeben verantwortlich gemacht, darunter statische Spannungsänderungen und dynamischer Spannungstransfer. Deutliche Unterschiede zwischen diesen Mechanismen werden insbesondere in der räumlichen Nachbebenverteilung erwartet. Es ist allerdings schwierig diese Hypothesen zu überprüfen, da die großen Unsicherheiten der Spannungsberechnungen berücksichtigt werden müssen, ebenso wie das durch lokale sekundäre Spannungsänderungen hervorgerufene initiieren von sekundären Nachbeben. Um die Vorhersagekraft verschiedener Mechanismen zu beurteilen habe ich die Effekte von Erdbeben kleiner Magnitude durch Benutzen des "epidemic type aftershock sequence" (ETAS) Modells berücksichtigt. Dabei habe ich die Verteilung direkter Nachbeben, wenn verfügbar, mit alternativen Herdinformationen korreliert. Bodenbewegung, Bruchgeometrie und Slipmodelle werden getestet. Als Aproximation der Bodenbewegung werden ShakeMaps benutzt. Diese sind nach großen Erdbeben nahezu in Echtzeit verfügbar und können daher für vorläufige Vorhersagen der räumlichen Nachbebenverteilung benutzt werden. Alternativ können empirische Beziehungen als Funktion der minimalen Distanz zur Herdfläche benutzt werden oder Coulomb Spannungsänderungen basierend auf publizierten oder zufälligen Slipmodellen. Zum Vergleich werden die Likelihood Werte der Hybridmodelle im Falle mehrerer bekannter Nachbebensequenzen analysiert (1992 Landers, 1999 Hector Mine, 2004 Parkfield). Die Tests zeigen, dass die Herdgeometrie die wichtigste Zusatzinformation zur Verbesserung der Nachbebenvorhersage ist. Des Weiteren können statische Spannungsänderungen besonders die Vorhersage von Nachbeben in größerer Entfernung zur Bruchfläche verbessern, wohingegen die Einbeziehung von Bodenbewegungskarten die Ergebnisse nicht wesentlich verbessern konnte. Im zweiten Teil meiner Arbeit führe ich ein neues Verfahren zur Untersuchung des Informationsgehaltes von invertierten Slipmodellen ein. Dies ermöglicht die Quantifizierung des Informationsgewinns, der durch Einbeziehung dieser Daten in Nachbebenvorhersagen entsteht. Hierbei wird das im ersten Teil eingeführte erweiterte ETAS Modell benutzt, welches statische Spannungsänderung zur Vorhersage der räumlichen Nachbebenverteilung benutzt. Die Vorhersagekraft der Modelle wird systematisch anhand mehrerer Erdbebensequenzen untersucht und mit Modellen basierend auf zufälligen Slipverteilungen verglichen. Der Einfluss der Veränderung der Auflösung der Slipmodelle, sowie Streich- und Fallwinkel der Herdsegmente wird untersucht. Einige der betrachteten Slipmodelle korrelieren sehr gut, in diesen Fällen werden kaum zufällige Slipmodelle gefunden, welche die Nachbebenverteilung besser erklären. Dahingegen korrelieren bei einigen Beispielen nahezu alle zufälligen Slipmodelle besser als das publizierte Modell. Das Verändern der Auflösung der Bewegungsmodelle hat kaum Einfluss auf die Ergebnisse, solange die allgemeinen Slipmuster noch reproduzierbar sind, d.h. ein bis zwei größere Slipmaxima pro Segment. Dahingegen beeinflusst eine zufallsbasierte Änderung der Streich- und Fallwinkel der Segmente die Resultate stark, je nachdem welche Standardabweichung gewählt wurde.

Performance-Based (PBE) and Consequence-Based (CBE) are new approaches to seismic design, evaluation and risk assessment, in which design criteria are devised to achieve stated performance objectives, and regional losses to civil infrastructure are mitigated through selective interventions for critical components of a civil infrastructure. These new approaches give engineers more flexibility in achieving performance goals but require substantial additional computational resources to fully achieve performance goals. As a step toward making such approaches feasible, this dissertation develops a number of computationally efficient methods for performing finite element-based structural system dynamic response analysis and reliability assessment. The Enhanced Uncoupled Modal Response History Analysis (EUMRHA) procedure developed herein is an efficient response analysis procedure to make the analysis of dynamic structural response to earthquakes in the nonlinear range less time-consuming. This technique is used to investigate the potential for aftershocks to cause additional damage to steel moment frame buildings, utilizing a technique designed to enhance the efficiency of Monte Carlo simulation in estimating low-probability events. Relatively simple probabilistic tools are proposed for purposes of rapid structural evaluation and condition assessment of damaged buildings. Finally, an analysis-based inspection scheme based on an associated probability model of connection damage is proposed for assessing the safety condition of existing buildings, and a procedure to assess the likely performance of an un-repaired building during a future earthquake is developed.

This dissertation explores the capability of dense seismic array data for imaging the rupture properties of earthquake sources using a method known as back-projection. Only within the past 10 or 15 years has implementation of the method become feasible through the development of large aperture seismic arrays such as the High Sensitivity Seismograph Network in Japan and the Transportable Array in the United States. Coincidentally, this buildup in data coverage has also been accompanied by a global cluster of giant earthquakes (Mw>8.0). Much of the material in this thesis is devoted to imaging the source complexity of these large events. In particular, evidence for rupture segmentation, dynamic triggering, and frequency dependent energy release is presented. These observations have substantial implications for evaluating the seismic and tsunami hazards of future large earthquakes. In many cases, the details of the large ruptures can only be imaged by the back-projection method through the addition of different data sets and incorporating additional processing steps that enhance low-amplitude signals. These improvements to resolution can also be utilized to study much smaller events. This approach is taken for studying two very different types of earthquakes. First, a global study of the enigmatic intermediate-depth (100-300 km) earthquakes is performed. The results show that these events commonly have sub-horizontal rupture planes and suggest dynamic triggering of multiple sub-events. From these observations, a hypothesis for the generation of intermediate-depth events is proposed. Second, the early aftershock sequences of the 2004 Mw 9.1 Sumatra-Andaman and 2011 Mw 9.0 Tohoku, Japan earthquakes are studied using the back-projection method. These analyses show that many events can be detected that are not in any local or global earthquake catalogues. In particular, the locations of aftershocks in the back-projection results of the 2011 Tohoku sequence fill in gaps in the aftershock distribution of the Japan Meteorological Agency catalogue. These results may change inferences of the behavior of the 2011 mainshock, as well as the nature of future seismicity in this region. In addition, the rupture areas of the largest aftershocks can be determined, and compared to the rupture area of the mainshock. For the Tohoku event, this comparison reveals that the aftershocks contribute significantly to the cumulative failure area of the subduction interface. This result implies that future megathrust events in this region can have larger magnitudes than the 2011 event. / Earth and Planetary Sciences

Although the knowledge and technology of seismic analysis and seismic risk assessment tools have rapidly advanced in the past several decades, current seismic design codes and damage estimation methods ignore the effect of successive earthquakes on structures. In light of recent strong seismic events, mainshock-damaged structures are shown to be more vulnerable to severe damage and collapse during subsequent events. The increase in vulnerability during aftershocks results in the likelihood of increased damage and loss-of-life and property. After a major earthquake, structural engineers must assess whether mainshock-damaged buildings can be re-occupied or not, with due consideration to the threat of aftershocks. The outcome of this post-earthquake inspection is utilized to quantifiably judge the current status of structures (so-called building tagging). This tagging criterion is closely related to the evaluation of the residual capacity of damaged buildings as well as the computation of the probability of being in a damage state after an aftershock (aftershock fragility). The increased vulnerability estimation associated with the additional damage plays a significant role in assessing potential losses to facilitate crucial decision making such as emergency response mobilization, inspection priority, recovery strategy, and re-occupancy decision. The main objective of this research is to develop a probabilistic framework for accounting for these increased vulnerabilities in terms of the extent of damage associated with mainshock ground motions. Aftershock fragility curves are developed accounting for both the uncertainty from the seismic hazard and the uncertainty from the structural capacity. This proposed approach also allows for the inherent variability, such as modeling characteristics associated with the design codes, present in non-ductile and ductile reinforced concrete frames found in California.

Communities on Vancouver Island, British Columbia face significant exposure to damaging earthquakes. This seismic risk arises not only from the Island’s proximity to crustal, sub-crustal and subduction earthquake sources in the Cascadia Subduction Zone and from their associated aftershock sequences, but also from environmental (natural and human-made) and social vulnerabilities in Vancouver Island communities and their current capacities to respond and recover from a large seismic event. Seeking to 1) assist community officials and the general public to better understand the scope of the earthquake risk on Vancouver Island; 2) raise awareness of the gaps in Vancouver Island’s risk assessment; 3) encourage and facilitate comprehensive seismic risk discussions at all levels of governance; and 4) offer quantitative data on which to base sound funding and policy decisions, this dissertation offers three new studies, presented in paper format, toward the comprehensive management of seismic risk on Vancouver Island. The first paper, reviews the components of risk and, building on international risk management standards and best practices, develops a new, comprehensive Disaster Risk Management (DRM) Framework for practitioners. This DRM Framework is then used to review existing knowledge of Vancouver Island’s seismic risk. A number of information gaps are identified, and two in particular, mainshock and aftershock hazard assessment, are targeted for further analysis. / Graduate

No description available.

九二一地震是台灣數十年來傷亡最大的地震，根據中央氣象局的研究發現九二一地震之後半年至一年內發生的地震，大多數都是由其引發的餘震，然而一個地震屬於主震、或是某個地震的餘震又該如何判斷呢？本文是以統計資料分析之觀點來區分主震與餘震，而不是利用相關地震學理論來區分主震與餘震，本文主要研究的是比較四種區分主震與餘震的方法：整體距離(Global Distance)、負相關(Negative Correlation)、最近鄰區(Nearest Neighbors)、視窗(Window)。四種地震散群方法所需要給定的參數：時間與空間參數，要如何選取與決定，本文則是利用台灣自1991年1月 1日至2003年12月31日之地震規模大於5.0以上的資料，定義地震減少比例(decreasing earthquake percent)來選取參數，以求出最適當的模型參數。套用選取得到的模型參數，利用電腦模擬地震來驗證比較方法的優劣，依據誤判主震(False Positive)、誤判餘震(False Negative)、分錯比例(Overall Error Rate)等準則比較各種地震散群方法的優劣，研究發現四種方法各有其優劣之處。 關鍵詞：主震、餘震、空間統計、最近鄰區、電腦模擬 / The Chi-Chi earthquake resulted in one of the greatest casualties for the past 100 years in Taiwan. According to the Central Weather Bureau in Taiwan, most of the earthquakes that occurred 6 months to 12 months after the Chi-Chi earthquake were the aftershocks. But in general, how do we classify if a certain earthquake is a main earthquake or aftershock? In this study, our interest is on the statistical methods for detecting whether an earthquake is a main earthquake. Four declustering methods are considered: Global Distance, Negative Correlation, Nearest Neighbors and Window. Taiwan earthquake data, with magnitude larger than 5 occurring between 1991 and 2003, were used to determine the parameters used in these four methods. Finally, a computer simulation is used to evaluate the performance of four methods, based on the results such as false positive and false negative, and overall Error Rate. Key Words: Decluster, Aftershock, Spatial Statistics, Nearest Neighbors, Simulation

Fault planes of large earthquakes incorporate inhomogeneous structures. This can be observed in teleseismic studies through the spatial distribution of slip and seismic moment release caused by the mainshock. Both parameters are often concentrated on patches on the fault plane with much higher values for slip and moment release than their adjacent areas. These patches are called asperities which obviously have a strong influence on the mainshock rupture propagation. Condition and properties of structures in the fault plane area, which are responsible for the evolution of such asperities or their significance on damage distributions of future earthquakes, are still not well understood and subject to recent geo-scientific studies. <br><br> In the presented thesis asperity structures are identified on the fault plane of the M_w=8.0 Antofagasta earthquake in northern Chile which occurred on 30th of July, 1995. It was a thrust-type event in the seismogenic zone between the subducting pacific Nazca plate and the overriding South American plate. In cooperation of the German Task Force for Earthquakes and the CINCA'95 project a network of up to 44 seismic stations was set up to record the aftershock sequence. The seaward extension of the network with 9 OBH stations increased significantly the precision of hypocenter determinations. They were distributed mainly on the fault plane itself around the city of Antofagasta and Mejillones Peninsula. <br><br> The asperity structures were recognized here by the spatial variations of local seismological parameters; at first by the spatial distribution of the seismic b-value on the fault plane, derived from the magnitude-frequency relation of Gutenberg-Richter. The correlation of this b-value map with other parameters like the mainshock source time function, the gravity isostatic residual anomalies, the aftershock radiated seismic energy distribution and the vp/vs ratios from a local earthquake tomograhpy study revealed some ideas about the composition and asperity generating processes. The investigation of 295 aftershock focal mechanism solutions supported the resulting fault plane structure and proposed a 3D similar stress state in the area of the Antofagasta fault plane. / Die Bruchflaeche grosser Erdbeben umfasst inhomogene Strukturen, die bisher hauptsaechlich in teleseismischen Untersuchungen nachgewiesen werden konnten. Haeufig werden begrenzte Bereiche auf einer Bruchflaeche beobachtet, die durch eine starke Konzentration des freigesetzten seismischen Moments und durch grosse Dislokationen gekennzeichnet sind. Diese Bereiche werden als 'asperities' bezeichnet, die offensichtlich starken Einfluss auf den Bruchverlauf des Hauptbebens ausueben. Beschaffenheit und Eigenschaften der Strukturen in einem Herdgebiet, die verantwortlich sind fuer die Bildung solcher 'asperities' und deren eventueller Bedeutung fuer Schadensverteilungen in zukuenftigen Erdbeben, sind Gegenstand aktueller geowissenschaftlischer Untersuchungen. <br><br> In der vorliegenden Arbeit werden 'asperity'-Strukturen auf der Bruchflaeche des M_w=8.0 Antofagasta Erdbebens vom 30. Juli 1995 im Norden Chiles identifiziert. Es handelt sich hierbei um ein typisches Subduktionsbeben mit Aufschiebungscharakter, das in der seismogenen Zone zwischen der abtauchenden pazifischen Nazca-Platte und der ueberschiebenden suedamerikanischen Platte stattfand. Durch die Zusammenarbeit der Deutschen Task Force fuer Erdbeben und dem sich waehrend des Bebens bereits vor Ort befindlichen CINCA '95 Projektgruppe, konnte ein bis zu 44 Stationen umfassendes seismologisches Netzwerk zur Registrierung der Nachbeben errichtet werden. Vor allem die seeseitige Erweiterung des Netzes durch 9 OBH Stationen trug zur hohen Praezision der Hypozentrenbestimmung der Nachbeben bei, die sich hauptsaechlich auf der Bruchflaeche und damit im Kuestenbereich um die Stadt Antofagasta und der noerdlich gelegenen Halbinsel Mejillones verteilten. <br><br> Die 'asperity'-Strukturen konnten mittels raeumlicher Variationen von lokalen seismologischen Parametern erkannt werden; zunaechst durch die Verteilung des seimologischen b-Wertes auf der Bruchflaeche aus der Magnituden-Haeufigkeitsbeziehung von Gutenberg-Richter. Durch die Korrelation dieser Verteilung mit Parametern wie der Momentenrate aus dem Hauptbeben, der isostatischen Restanomalien des Gravitationsfeldes, der Verteilung der abgestrahlten seismischen Energie durch die Nachbeben und der vp/vs-Verhaeltnisse aus einer lokalen Erdbebentomographie konnten Rueckschluesse auf die Beschaffenheit und damit den Bildungsprozess der asperities gezogen werden. Die Untersuchung der Herflaechenloesungen die fuer 295 Nachbeben bestimmt wurden, ergab eine indirekte Bestaetigung der gefundenen Strukturen und wies auf die Existenz eines 3D Spannungszustands im Bereich der Bruchflaeche des Antofagasta Bebens hin.

In current design of steel eccentrically braced frames (EBFs), the yielding link is coupled with the floor beam. This often results in oversized link elements, which leads to over-designed structures and foundations. In addition, the beams are expected to sustain significant damage through repeated inelastic deformations under design level earthquakes, and thus the structure may require extensive repair or need to be replaced. These drawbacks can be mitigated by designing EBFs with replaceable shear links. Two different replaceable link types with alternate section profiles, connection configurations, welding details and intermediate stiffener spacing were tested. A total of 13 cyclic quasi-static full-scale cyclic tests were performed, which included tests on eccentrically braced frames with the replaceable shear links, to study their inelastic seismic performance. The links exhibited a very good ductile behaviour, developing stable and repeatable yielding. Additional inelastic rotation capacity can be achieved with bolted replaceable links when allowing bolt bearing deformations to occur. The on-site replaceability of the link sections is confirmed even in the presence of residual deformations of 0.5% drift.