Jacob A. Richardson

Multiscale postseismic behavior on a megathrust: The 2012 Nicoya earthquake, Costa Rica

The Nicoya Peninsula in northwest Costa Rica overlies a section of the subduction megathrust along the Middle America Trench. On 5 September 2012, a moment magnitude 7.6 megathrust earthquake occurred beneath a dense network of continuous GPS and seismic stations. Many of the GPS stations recorded the event at high rate, 1 Hz or better. We analyze the temporal and spatial evolution of surface deformation after the earthquake. Our results show that the main rupture was followed by significant afterslip within the first 3 h following the main event. The behavior of the surface displacement can be represented by relaxation processes with three characteristic times: 7, 70, and more than 400 days. We assume that the long relaxation time corresponds to viscoelastic relaxation and the intermediate relaxation time corresponds to afterslip on the main fault. The short relaxation time may represent a combination of rapid afterslip, poroelastic adjustment in the upper crust, or other processes. During the first few months that followed the earthquake, afterslip likely released a significant amount of slip deficit still present following the coseismic rupture, in particular updip of the rupture. Afterslip seems to be bounded updip by regions affected by slow slip events prior to the earthquake, suggesting that the two processes are influenced by different frictional properties.

Role of sills in the development of volcanic fields: Insights from lidar mapping surveys of the San Rafael Swell, Utah

Analysis of airborne and terrestrial lidar data demonstrates that >0.4 km 3 of magma cooled in sills at shallow (<1 km) depth in the now-eroded Pliocene San Rafael Swell distributed volcanic field, Utah (USA). The volumes of each of seven sills are estimated from three-dimensional (3-D) models of the lidar data and range from 10 –4 to 10 –1 km 3 . Directions of magma flow during emplacement are interpreted from precise sill thickness measurements and measurements of linear vertical offsets within the sills, helping to identify feeder conduits and dikes; 3-D map relationships derived from lidar data demonstrate that magma flowed into and out of sills from these active dikes and eruptive conduits. Mapped sill volumes account for >92% of intrusive material within the 50 km 2 study area. We conclude that sills played a significant role in modifying eruption dynamics during activity in San Rafael, and suggest that monitoring of sill inflation and deflation in active distributed volcanic fields may provide key information about unrest and potential eruption dynamics.

The Syrtis Major volcano, Mars: A multidisciplinary approach to interpreting its magmatic evolution and structural development

Very weak crustal magnetic fields over the Syrtis Major volcanic complex imply almost total thermal demagnetization via magmatic intrusions over a large area less than ~4 Ga. We fit a model of these intrusions and the resulting thermal demagnetization to maps of crustal magnetic field strength at 185 km altitude. The best fits are most consistent with a “dog bone”-shaped region of intrusive material, elongated approximately north-south, with an area of ~350,000 km2 and an inferred volume of ~4–19 × 106 km3. Such a large volume is best explained by a long-lived mantle plume beneath the Syrtis edifice. A free-air gravity anomaly high over the Syrtis Major caldera is consistent with dense mafic residue remaining at depth following crystal fractionation that produced the silicic magmas seen at the surface. The elongation of this region is consistent with ascent and north-south emplacement of magma enabled by structures parallel to and associated with the preexisting Isidis impact basin.