Earthquakes strongly cluster in space and time, driven both by earthquake-to-earthquake triggering and underlying physical processes, such as tectonic stress loading, increased pore pressure, etc. I explore both global and regional datasets to understand characteristics of these processes in different tectonic environments. I study global seismicity using intermediate-period (35--70~s) Rayleigh waves recorded by the global seismic network. Applying a surface wave detect method identifies about 1000 previously un-cataloged earthquakes from 1997 to 2009, most of which are located in the southern ocean. I further analyze a small number of these events that are located in Antarctica to understand glacial-related triggering processes. Absolute and differential travel-times measured from waveform cross-correlation are used to obtain refined locations. A single-force model is applied to the observed amplitudes at 50~Hz to obtain best-fitting force directions. Additionally, possible glacial calving events are identified from MODIS images. The combined results suggest that events on Vanderford and Ninnis glaciers are a result of calving processes. To understand the general characteristics of earthquake clustering from a large dataset of earthquakes, I analyze seismicity in southern California. I use a high-resolution earthquake catalog based on waveform cross-correlation to study the spatial- temporal distribution of earthquakes. Parameters based on event location, magnitude and occurrence time are computed for isolated seismicity clusters. Spatial migration behavior is modeled using a weighted-least-squares method. Aftershock-like event clusters do not exhibit significant spatial migration compared with earthquake swarms. Two triggering processes are considered for swarms: slow slip and fluid diffusion, which are distinguished based on a statistical analysis of event migration. The results suggest fluid-induced seismicity is found across southern California, particularly within geothermal areas. In the Salton Sea geothermal field (SSGF), a correlation between seismicity and fluid injection activities is seen. Spatial -temporal variations of earthquake stress drops are investigated in different regions, and a distance- dependence of stress drop from the injection source is found in the SSGF, suggesting the influence of increased pore pressure. Temporal variation of stress drops within mainshock source regions shows that foreshocks and earthquake swarms have lower stress drops than background seismicity and aftershocks. These results, combined with the spatial migration observed for some large foreshock sequences, suggests an aseismic transient process is likely involved in foreshock triggering

Induced seismicity linked to geothermal resource exploitation, hydraulic fracturing, and wastewater disposal is evolving into a global issue because of the increasing energy demand. Moderate to large induced earthquakes, causing widespread hazards, are often related to fluid injection into deep permeable formations that are hydraulically connected to the underlying crystalline basement. Using injection data combined with a physics-based linear poroelastic model and rate-and-state friction law, we compute the changes in crustal stress and seismicity rate in Oklahoma. This model can be used to assess earthquake potential on specific fault segments. The regional magnitude-time distribution of the observed magnitude (M) 3+ earthquakes during 2008-2017 is reproducible and is the same for the 2 optimal, conjugate fault orientations suggested for Oklahoma. At the regional scale, the timing of predicted seismicity rate, as opposed to its pattern and amplitude, is insensitive to hydrogeological and nucleation parameters in Oklahoma. Poroelastic stress changes alone have a small effect on the seismic hazard. However, their addition to pore-pressure changes can increase the seismicity rate by 6-fold and 2-fold for central and western Oklahoma, respectively. The injection-rate reduction in 2016 mitigates the exceedance probability of M5.0 by 22% in western Oklahoma, while that of central Oklahoma remains unchanged. A hypothetical injection shut-in in April 2017 causes the earthquake probability to approach its background level by ∼2025. We conclude that stress perturbation on prestressed faults due to pore-pressure diffusion, enhanced by poroelastic effects, is the primary driver of the induced earthquakes in Oklahoma.

While cancer is a serious health issue, there are very few genetic biomarkers that predict predisposition, prognosis, diagnosis, and treatment response. Recently, sequence variations that disrupt microRNA (miRNA)-mediated regulation of genes have been shown to be associated with many human diseases, including cancer. In an early example, a variant at one particular single nucleotide polymorphism (SNP) in a let-7 miRNA complementary site in the 3' untranslated region (3' UTR) of the KRAS gene was associated with risk and outcome of various cancers. The KRAS oncogene is an important regulator of cellular proliferation, and is frequently mutated in cancers. To discover additional sequence variants in the 3' UTR of KRAS with the potential as genetic biomarkers, we resequenced the complete region of the 3' UTR of KRAS in multiple non-small cell lung cancer and epithelial ovarian cancer cases either by Sanger sequencing or capture enrichment followed by high-throughput sequencing. Here we report a comprehensive list of sequence variations identified in cases, with some potentially dysregulating expression of KRAS by altering putative miRNA complementary sites. Notably, rs712, rs9266, and one novel variant may have a functional role in regulation of KRAS by disrupting complementary sites of various miRNAs, including let-7 and miR-181.

Several major fault-drilling projects have captured the interseismic and postseismic periods of earthquakes. However, near-field observations of faults immediately before and during an earthquake remain elusive due to the unpredictable nature of seismicity. The Scientific Exploration of Induced SeisMicity and Stress (SEISMS) workshop met in March 2017 to discuss the value of a drilling experiment where a fault is instrumented in advance of an earthquake induced through controlled fluid injection. The workshop participants articulated three key issues that could most effectively be addressed by such an experiment: (1) predictive understanding of the propensity for seismicity in reaction to human forcing, (2) identification of earthquake nucleation processes, and (3) constraints on the factors controlling earthquake size. A systematic review of previous injection experiments exposed important observational gaps in all of these areas. The participants discussed the instrumentation and technological needs as well as faults and tectonic areas that are feasible from both a societal and scientific standpoint.

The stem cells that maintain and repair the postnatal skeleton remain undefined. One model suggests that perisinusoidal mesenchymal stem cells (MSCs) give rise to osteoblasts, chondrocytes, marrow stromal cells, and adipocytes, although the existence of these cells has not been proven through fate-mapping experiments. We demonstrate here that expression of the bone morphogenetic protein (BMP) antagonist gremlin 1 defines a population of osteochondroreticular (OCR) stem cells in the bone marrow. OCR stem cells self-renew and generate osteoblasts, chondrocytes, and reticular marrow stromal cells, but not adipocytes. OCR stem cells are concentrated within the metaphysis of long bones not in the perisinusoidal space and are needed for bone development, bone remodeling, and fracture repair. Grem1 expression also identifies intestinal reticular stem cells (iRSCs) that are cells of origin for the periepithelial intestinal mesenchymal sheath. Grem1 expression identifies distinct connective tissue stem cells in both the bone (OCR stem cells) and the intestine (iRSCs).