Abstract: The presence of Tectonic Earthquake Swarms (TES) and sequences in the north and northeast of the island of Puerto Rico in the northeastern Caribbean have been recorded by the Puerto Rico Seismic Network (PRSN) since it started operations in 1974. A great wealth of seismic data has been archived during this period such that at least 10 years of catalog data can be used to characterize the seismic activity in the Puerto Rico-Virgin Islands (PRVI) region. This research uses two declustering methods to identify cluster events in the PRVI block. The first method, known as Model Independent Stochastic Declustering (MISD), filters the catalog sub-set into cluster and background seismic events. The second method uses a spatio-temporal algorithm applied to the catalog in order to link the separate seismic events into clusters. After using these two methods, identified clusters were classified into either earthquake swarms or seismic sequences. Results have allowed to identify and classify 128 clusters categorized in 11 distinctive regions based on their centers, and their spatio-temporal distribution have been used to determine interplate dynamics. This analysis yields implication on the behavior of the plate interface between the North American and Caribbean plates along the Puerto Rico trench.
Poster:
Characterizing_the_Temporal_and_Spatial_Distribution_of_Earthquake_Swarms_in_the_Puerto_Rico___Virgin_Island_Block (1)
Abstract: The epicentral region of the April 16, 2016 Mw 7.8 Pedernales earthquake lies north of the intersection of the Carnegie ridge and the Ecuador subduction zone. This segment has ruptured on decadal time scales in a series of megathrust events, Mw 8.8 (1906), Mw 7.8 (1942), Mw 7.7 (1958), and Mw 8.2 (1979), suggesting that accumulated strain is released through large earthquakes clustered in space and time. The 2016 rupture coincides with the rupture area of the Mw 7.8 1942 earthquake. Seismic data recorded by a dense array of 82 stations from Ecuador’s national network and an international rapid response effort were processed and analyzed. Multiple combinations of detection and association algorithms were assessed to address the challenge of producing robust earthquake catalogs from aftershock sequences. Calibrated relocations using the Hypocentroidal Decomposition approach were determined for a subset of events for which we combine phase readings from local and temporary stations with regional and teleseismic phase readings from the NEIC. Preliminary aftershock locations through August 2017 (11502 events, ≥ 6 phases, errors <10km, Mc1.8) show a sharp downdip limit of aftershocks and are characterized by a series of event clusters. Two distinct streaks of aftershocks outline the north and south ends of the rupture, and a third streak occurs between the two patches of slip defined by finite fault models. Aftershocks north of the main rupture are sparse until an earthquake swarm ~2 months after the mainshock in the vicinity of Esmeraldas, the portion of the subduction zone that ruptured in 1958. Eight months after the mainshock, a series of aftershocks in the Atacames area, north of the main rupture, caused additional structural damage. The spatial distribution of background seismicity and aftershocks is quite similar suggesting that background seismicity likely includes long-lived aftershocks from the ’06, ‘42, and ’58 large earthquakes as well as swarms associated with slow-slip events.
Poster:
LSC_SSA_mtg_v4
Abstract: On April 16, 2016, a large part of the Ecuadorian coast was shacked by an Mw7.8 earthquake, affecting nearly 30,000 homes and public buildings. Understanding soil characteristics of these affected areas is a necessary and useful aid to reconstruction efforts. To this end, the Instituto Geofísico at the Escuela Politécnica Nacional (IG-EPN) undertook data collection work with the aim of generating a reliable and freely accessible database consisting of dynamic soil characteristics for 8 villages located in the provinces of Manabí and Esmeraldas (Ecuador, http://www.igepn.edu.ec/mapas/mapa-frecuencias-suelos-manabi). The study consisted in the installation of four seismic station arrays for approximately 7 days in each village, during which time various measurements of ambient noise and seismic profiles were made. Here we employ various methods to assess soil characteristics, including: single-sensor horizontal over vertical component (H/V) spectral ratios, relative spectral ratios at various sites of interest with respect to a reference site (SSR), and linear arrays for multi-channel analysis of surface waves (MASW). These experiments were conducted within the first six months after the earthquake. The fundamental soil frequency, approximate site amplification curves, and shear wave velocity profiles for the populations that were part of the study were all obtained. These parameters then allow us to identify which zones share similar response characteristics, which we then correlate with the Ecuadorian Construction Norms (NEC-15) to determine soil type. It is important to note that, while in almost all of the villages studied several peak frequencies are found, the clearest peaks are typically between 1 to 2 Hz, indicating predominant soil types of type D and E.
Poster:
JGBL_Poster
Abstract: Seismicity at lithospheric plate boundaries can be accompanied by magmatic processes, which play fundamental role in formation and differentiation of the Earth’s crust. These processes are connected with magmatic emplacement as a result of an ongoing magmatic activity at the lower crustal level. Though they are usually confined to lithospheric plate boundaries, they can also occur in intraplate setting, however there, they are a rare phenomenon not commonly observed or discussed. Seismic and seismological interpretation must be supplemented by multidisciplinary research helping in detection of such areas and leading to advanced tectonic implications. One of them is the western edge of the Eger Rift in central Europe as a part of the European Cenozoic Rift System. It is a geodynamic region abundant of repeated seismic swarms and mantle derived fluids emanating at the surface, Cenozoic volcanism, and neotectonic crustal movements at the intersections of major intraplate faults showing diverse phenomena and representing unique European intracontinental setting. Strong lateral variations of the lower crust documented from wide-angle data are supported by results from local seismicity and indicate two types of the magmatic emplacement at the crust-mantle level. Xenoliths from corresponding depths document the origin depths of the magma at the lower crust/upper mantle transition. Increased helium isotope ratios in CO2-rich gases evidence lithospheric mantle origin. The character of the lower crustal material enables to differentiate timing and tectonic setting of two episodes with different times of origin. As a result, spatial and temporal relations to recent geodynamic processes suggest active magmatic processes in the intracontinental setting in this area.
Poster:
A_poster_SSA_2018
Abstract: Northwestern Venezuela is located in a zone of complex deformation due to the Caribbean-South American plate interactions. Several models regarding the shallow and deep structure of the region have been proposed. Nevertheless, most of these studies are based on basin-scale observations, and they lack crustal observations in order to support the interpretation of lithospheric structures. In 2014, new deep seismic profiles have been acquired across the Merida Andes orogenic belt, and the adjacent sedimentary basins, with 2000 km of 2D seismic profiles. It is presented an integrated analysis of the 560 km long Northern Andes seismic profile, oriented in a NNW direction, covering areas from the Proterozoic basement of Guyana Shield to both Paleozoic and Mesozoic-Cenozoic terranes. The profile cuts perpendicular to the main Bocono strike-slip fault system, aiming to image the collision zones (sutures), the crustal and lithospheric structure, and its impact on the understanding of continental margin evolution. 13 land shots with explosive charges between 0.2 and 1.6 tons were recorded by 545 seismometers (Texans). 2-D P-wave models from first arrival tomography and ray tracing modeling show velocities between 3.2-5.7 km/s for sedimentary cover; 6.1-6.5 km/s for upper crust, and 6.7 to 7.2, and 7.8 to 8.4 km/s for lower crust and upper mantle, respectively. The most prominent features imaged were a crustal thinning beneath the Falcon basin due to a previous history as a back-arc basin. In addition, lateral variability suggests collisional process related to suture zones between both Proterozoic and Paleozoic, and Paleozoic and Mesozoic-Cenozoic provinces. No crustal root associated to the Merida Andes was observed, contrary to the two southernmost profiles, indicating that the Northern Andes profile is located north of the Andean crustal domain. Finally, the Caribbean slab was observed in a low-angle subduction context beneath NW South America.
Poster:
Mazuera_etal_Poster-SSA-Miami-2018
Abstract: Seismic risk is a crucial issue for South Caucasus, which is the main gateway between Asia and Europe. The goal of this work is to propose new methods and criteria for defining an overall approach aimed at assessing and mitigating seismic risk in Georgia. Seismic hazard assessment means the identification of zones of similar levels of earthquake hazard expressed with ground motion parameters ( acceleration, velocity, displacement, Intensity) . This abstract presents the results of preliminary study of seismic microzonation of the city of Gori (Georgia) that was done in the frame of CNR (National Research Council of Italy) project – Preliminary study for the assessment of seismic risk in strategic cities of “Georgia”. The city of Gori is the central of Shida Kartli region (Mkhare) and is located on both sides of Liachvi and Mtkvari rivers, in the 135 km2 around the Gori fortress. The seismic microzonation map of level 1 of Gori was carried out in the light of 1) already available data (i.e., topographic map and boreholes data), 2) results of new geological surveys and iii) geophysical measurements (i.e., MASW and noise measurements processed with HVSR technique). So we have done seismic free field investigations full of the city and over the most strategic buildings and important parts of Gori : the city hall, the hospital, the Rugby stadium, Castel of Gori and museums and central square. Our preliminary results highlights the presence of both stable zones susceptible to local amplifications and unstable zones susceptible to geological instability. Our results are directed to establish the set of actions aimed at risk mitigation before the initial onset of the emergency, and to the management of emergency once the seismic event has occurred. The products obtained will constitute the basic elements of an integrated system aimed at reducing the risk and improving the overall safety of people and infrastructure in Georgia.
Poster:
SSA_2018_Poster_Zura gogoladze
