In the central Vanuatu arc, living and recenfiy deceased reef corals act as natural tide gauges which have allowed us to map vertical tectonic deformation patterns. As corals grow, the density of the aragonite coral skeletons varies on an annual cycle, producing annual growth bands similar to tree tings. Using coral growth bands, we can determine the year coral surfaces died due to emergence. We interpret four major coral emergence events as coseismic uplifts that occurred near the epicenters and times of large shallow earthquakes on January 5, 1946 (M S = 7.3), August 11, 1965 (M S = 7.5), October 27, 1971 (M S = 7.1) and December 29, 1973 (M S = 7.5). The 1965 and 1973 events caused maximum uplifts of 120 and 60 cm, respectively, in the frontal arc. Also related to these events are uplifts of 10 cm and 6 cm in the back arc on Pentecost and Maewo islands, which lie east of the volcanic chain and the primary forearc zones of uplift and subsidence. Similar secondary zones of uplift occurred with the great 1960 Chile and 1964 Alaska earthquakes. The amplitude of these secondary uplifts is significantly larger than that predicted by models having a single fault in an elastic half-space. However, the amount of secondary uplift is comparable to that predicted if the fault occurs in a plate of constant thickness overlying a viscoelastic half-space. At various places in 1957, 1969-1970, 1977, and 1978-1981 there was about 5-10 cm of emergence not associated with major earthquakes, which may indicate nonseismic tectonic uplift. However, oceanographically lowered sea levels, as in E1 N/rios, may have determined the times when corals died and recorded these events. Nevertheless, the accumulation of emergence, its persistence, the limited geographic extent of each event, and occurrence in areas of rapid Holocene uplift suggest that the causes of the uplifts are tectonic. These events suggest that in some areas a third or more of the total accumulated uplift in central Vanuatu takes place as aseismic motion. However, in some areas we find only coseismic emergence. In central Vanuatu, contemporary coseismic vertical deformation, Holocene uplift, and topography have remarkably similar patterns. This suggests that the mechanisms and processes causing vertical deformation have varied little over the last 106years. Apparently, the topography, structure, and seismotectonics are controlled by the subduction of the d'Entrecasteaux ridge, a major bathymetric feature underthrusting this part of the arc. The influence of this ridge may have been especially extensive because it migrates very slowly along the arc trend, and thus it interacts for a long time with a single portion of the arc system. Our previous studies of reef terraces indicated the existence of at least four seismotectonic arc segments or blocks along the Santo-Malekula interval of the arc, and our present results further support this conclusion. Each block has uplifted at different times, by different amounts, at different rates, and tilted in a different direction. Bo...
In November 2013, a series of earthquakes began along a mapped ancient fault system near Azle, Texas. Here we assess whether it is plausible that human activity caused these earthquakes. Analysis of both lake and groundwater variations near Azle shows that no significant stress changes were associated with the shallow water table before or during the earthquake sequence. In contrast, pore-pressure models demonstrate that a combination of brine production and wastewater injection near the fault generated subsurface pressures sufficient to induce earthquakes on near-critically stressed faults. On the basis of modelling results and the absence of historical earthquakes near Azle, brine production combined with wastewater disposal represent the most likely cause of recent seismicity near Azle. For assessing the earthquake cause, our research underscores the necessity of monitoring subsurface wastewater formation pressures and monitoring earthquakes having magnitudes of ∼M2 and greater. Currently, monitoring at these levels is not standard across Texas or the United States.
This study summarizes our investigation of the 17 May 2012 M W-RMT 4.8 earthquake nearTimpson, Texas, the largest earthquake recorded historically in eastern Texas. To identify preshocks and aftershocks of the 17 May event we examined the arrivals recorded at Nacogdoches (NATX) 30 km from the 17 May epicenter, at nearby USArray Transportable Array stations, and at eight temporary stations deployed between 26 May 2012 and mid-2013. At NATX we identified seven preshocks, the earliest occurring in April 2008. Reliably located aftershocks recorded by the temporary stations lie along a 6 km long NW-SE linear trend corresponding to a previously mapped basement fault that extends across the highest-intensity (MMI VII) region of the 17 May main shock. Earthquakes in this sequence are relatively shallow-with focal depths ranging from 1.6 to 4.6 km. Evidence supporting these depths include: hypocentral locations of exceptionally well-recorded aftershocks, S-P intervals at the nearest stations, and comparisons of synthetics and observed seismograms. IntroductionOn 17 May at 0812 UTC a M W-RMT 4.8 earthquake occurred near Timpson, Texas. The quake awoke numerous residents of Nacogdoches, Texas, 50 km to the southwest of Timpson and caused significant damage to chimneys, fireplaces, and brick veneer siding 5 km southwest of Timpson. The 17 May earthquake is the largest earthquake in the historical record in East Texas (Figure 1). This paper discusses this earthquake and the sequence of preshocks and aftershocks, including an M W-RMT 4.0 foreshock on 10 May 2012 at 1515 UTC and aftershocks occurring on 25 January 2013 at 701 UTC (m bLg 4.1) and 2 September 2013 at 1652 (m bLg 4.1) and 1851 (M W-RMT 4.3).Regional tectonics in eastern Texas is dominated predominately by salt bodies; however, the Mt. Enterprise fault zone, a system of approximately east-west trending Cretaceous-Paleogene faults, is situated north and west of the epicentral area (Figure 1) [see Ewing, 1990]. In the epicentral area of the 2012 earthquake Jackson [1982] and Geomap Company [2012] also indicate a northwest-southeast trending fault that is roughly parallel to and slightly east of the Rusk-Shelby county line.Seismicity was rare in this region prior to the events analyzed in this study. The nearest events discussed by Frohlich and Davis [2002] were a M4.0 8 January 1891 Rusk, Texas, event, 80 km to the west of the epicentral region and the M3.0 9 June 1981 Center, Texas, earthquake, 25 km to the southeast. However, some investigations have suggested the 1891 Rusk event might be spurious-a thunderstorm or a tornado-and the 1981 Center earthquake was only locatable because it was recorded by a temporary local network deployed between June 1981 and August 1982[Pennington and Carlson, 1984. This network also recorded a microearthquake occurring on 11 December 1981 and located 25 km west of the 2012 epicenter.FROHLICH ET AL.
In recent years, numerous small earthquakes have occurred near the town of Pecos in West Texas; however, when this activity began and whether it was caused by increased petroleum industry activity has been uncertain because prior to 2017 there were few permanent seismograph stations in the region. We identify and locate earthquakes using data recorded since 2000 at TXAR, a sensitive 10-station seismic array situated about 240 km south of Pecos. We thus show that in 2007, one earthquake occurred near Pecos, in 2009 several more occurred, and subsequently, activity has increased considerably, with more than 2000 events identified in 2017. A time-of-day and year-by-year analysis identifies geographic areas in West Texas where events are likely to be natural earthquakes and quarry blasts. However, for the Pecos events, annual seismicity rates increase along with annual volumes of petroleum production and fluid waste disposal, suggesting a causal link. Analysis of seismograms collected by the EarthScope Transportable Array indicates that the 2009 earthquakes had focal depths of 4.0-5.2 km below sea level, within or just below strata where petroleum is produced and/or wastewater is injected. The largest earthquake to date had magnitude M L 3.7, but the recent high activity rates suggest that greater magnitudes may be possible. For the years 2000-2017, we provide a catalog of 10,753 epicenters of seismic events recorded at TXAR.
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