C. A. Mortera-Gutierrez

Tectonic significance of an earthquake sequence in the Zacoalco half-graben, Jalisco, Mexico

Abstract We studied a sequence of small earthquakes that occurred during the months of April and May of 1997, in Jalisco, southwestern Mexico. The earthquakes were located along a set of active faults that form the Zacoalco half-graben (La Lima fault system), west of Lake Chapala, within the rift–rift–rift triple junction. A total of 33 events were located, with magnitudes ranging from 1.5 to 3.5, recorded by a portable array of broadband seismographs. We identified two groups of events: one corresponding to a shallow normal fault, synthetic to La Lima fault system, and another group associated with a deeper fault. The events that occurred on the synthetic fault show normal faulting oriented on a NW–SE plane, dipping shallowly towards the SW. The other group of mechanisms showed either a normal fault oriented NW–SE and dipping steeply to the NE, or a very shallow-dipping normal fault, dipping to the SW. Earthquake distribution and fault plane solutions suggest that the Zacoalco half-graben developed from blocks that rotate as slip occurs on listric faults. These mechanisms could represent the type of motion expected for larger earthquakes in the area, like the one that occurred in 1568.

Rifting under steam—How rift magmatism triggers methane venting from sedimentary basins

During opening of a new ocean magma intrudes into the surrounding sedimentary basins. Heat provided by the intrusions matures the host rock creating metamorphic aureoles potentially releasing large amounts of hydrocarbons. These hydrocarbons may migrate to the seafloor in hydrothermal vent complexes in sufficient volumes to trigger global warming, e.g. during the Paleocene Eocene Thermal Maximum (PETM). Mound structures at the top of buried hydrothermal vent complexes observed in seismic data off Norway were previously interpreted as mud volcanoes and the amount of released hydrocarbon was estimated based on this interpretation. Here, we present new geophysical and geochemical data from the Gulf of California suggesting that such mound structures could in fact be edifices constructed by the growth of black-smoker type chimneys rather than mud volcanoes. We have evidence for two buried and one active hydrothermal vent system outside the rift axis. The vent releases several hundred degrees Celsius hot fluids containing abundant methane, mid-ocean-ridge-basalt (MORB)-type helium, and precipitating solids up to 300 m high into the water column. Our observations challenge the idea that methane is emitted slowly from rift-related vents. The association of large amounts of methane with hydrothermal fluids that enter the water column at high pressure and temperature provides an efficient mechanism to transport hydrocarbons into the water column and atmosphere, lending support to the hypothesis that rapid climate change such as during the PETM can be triggered by magmatic intrusions into organic-rich sedimentary basins.

A Seismogeodetic Amphibious Network in the Guerrero Seismic Gap, Mexico

The historical record of large subduction earthquakes in Guerrero, Mexico, reveals the existence of an ∼230 km length segment below the coast where no major rupture has occurred in the past 60 years. Reliable quantification of the hazard associated with such a seismic gap is urgently needed for risk mitigation purposes by means of state-of-the-art observations and modeling. In this article, we introduce and quantitatively assess the first seismogeodetic amphibious network deployed in Mexican and Central American soils that will provide the opportunity to achieve this goal in the near future. Deployed in 2017, the network is the result of a collaborative effort between Mexican and Japanese scientists. It consists of 15 onshore broadband and 7 ocean-bottom seismometers, 33 Global Positioning System (GPS) stations, 7 ocean-bottom pressure gauges, and 2 GPSacoustic sites, most of them installed within the Guerrero seismic gap. Initial data from the network revealed the occurrence of a 6-month-long slow-slip event in Guerrero, starting in May and ending in October 2017. To illustrate the performance of the various instruments, we also present the first ocean-bottom pressure and GPS-acoustic measurements in Mexico; the latter was obtained by means of an autonomousWave Glider vehicle. The ground motion of the devastating 19 September 2017 Mw 7.1 earthquake in central Mexico is presented as well. Nominal resolution of the seismogeodetic network is estimated through different synthetic inversion tests for tomographic imaging and the seismic coupling (or slow-slip) determination on the plate interface. The tests show that combined onshore and offshore instruments should lead to unprecedented results regarding the seismic potential (i.e., interface coupling) of the seismic gap and the Earth structure from the Middle America trench up to 70-km depth across the Guerrero state.