Luis Rivera

Spatial heterogeneity of tectonic stress and friction in the crust

The complex geometry of faults, seismicity, and diversity of earthquake mechanisms suggest that the stress and strength in Earths crust are spatially heterogeneous. We investigated the degree of heterogeneity using the following two end-member models. In one end-member model, we assumed that the orientation of stress is uniform in the crust as is assumed in many stress inversion studies. In this model, the variability of earthquake mechanisms means that friction during faulting must vary for each event. We computed friction μ from the ratio of the resolved shear stress to the effective normal stress on the fault plane with the assumption of hydrostatic pore pressure. The values of μ vary over a large range from 0 to 1.5. In the other extreme model we assumed optimally oriented slip and a constant μ = 0.6, as is suggested by Byerlees law, for all the earthquakes, and determined the local stress orientation for each earthquake. The orientation of the stress changes drastically from one earthquake to another, and the assumption of uniform stress field commonly used in stress inversion is not warranted. An important conclusion is that a regionally uniform stress field and constant friction on optimally oriented faults are mutually exclusive. The actual situation in the crust is most likely to be intermediate between these two end-member models. From the existing data alone, we cannot determine the degree of heterogeneity uniquely, but both μ and the local stress field near earthquake faults are likely to vary substantially, and studies on earthquake rupture dynamics must take these heterogeneities into consideration.

Stress tensor analysis of the 1998–1999 tectonic swarm of northern Quito related to the volcanic swarm of Guagua Pichincha volcano, Ecuador

The phreatic activity and the subsequent dacitic dome growth in 1998–1999 at Guagua Pichincha volcano, Ecuador, were associated with two seismic swarms: one located in the northern part of Quito (population: 1,500,000) and another one, just below the active volcano, about 15–20 km SW from the first one. Quito swarm tectonic events have high frequencies (from 1 to 10–15 Hz). We registered more than 3200 events (among which 2354 events of 1.4≤M_L≤4.2) between June 1998 and December 1999 at the −2- and −17-km depth. The volcanic events below the Guagua Pichincha caldera have high (from 1 to 10–15 Hz) and low (less than 3 Hz) frequencies. Approximately, 130,000 events were registered between September 1998 and December 1999 at the +2.4- and −3.5-km depth. Here, we study the stress tensors of these two swarms deduced from the polarities of P first motions and compare them to the regional stress tensor deduced from CMT Harvard focal mechanisms. The Quito swarm stress tensor is relatively close to the regional stress tensor (the σ_1 axis was oriented N117°E close to the N102°E direction of the plate motion found by the GPS measurement, and σ_3 is nearly vertical). The difference may be due to the action of the closely active Guagua Pichincha volcano. The Guagua Pichincha stress tensor is very different from the regional tectonic one. The σ_1 axis of the volcano is oriented N214°E, almost perpendicular to the σ_1 of the swarm of Quito and σ_3 is almost horizontal. Even if these two tensors are different, they can be explained in a more general tectonic scheme. The almost horizontal direction of σ_3 just below the volcano is compatible with an extensional horizontal direction that may be expected in the shallow extrados part of a compressional region and consistent with an opening of the top of the Guagua Pichincha volcano. The movement of the fluids (magma, gas and/or groundwater) produced by the closely active Guagua Pichincha volcano seems to have an influence in the acceleration of the generation of seismic events.

The Iquique earthquake sequence of April 2014: Bayesian modeling accounting for prediction uncertainty

The subduction zone in northern Chile is a well-identified seismic gap that last ruptured in 1877. On 1 April 2014, this region was struck by a large earthquake following a two week long series of foreshocks. This study combines a wide range of observations, including geodetic, tsunami, and seismic data, to produce a reliable kinematic slip model of the Mw=8.1 main shock and a static slip model of the Mw=7.7 aftershock. We use a novel Bayesian modeling approach that accounts for uncertainty in the Greens functions, both static and dynamic, while avoiding nonphysical regularization. The results reveal a sharp slip zone, more compact than previously thought, located downdip of the foreshock sequence and updip of high-frequency sources inferred by back-projection analysis. Both the main shock and the Mw=7.7 aftershock did not rupture to the trench and left most of the seismic gap unbroken, leaving the possibility of a future large earthquake in the region.

Diagnosing Source Geometrical Complexity of Large Earthquakes

We investigated the possible frequency dependence of the moment tensor of large earthquakes by performing W phase inversions using teleseismic data and equally-spaced narrow, overlapping frequency bands. We investigated frequencies from 0.6 to 3.8xa0mHz. Our focus was on the variation with frequency of the scalar moment, the amount of non-double-couple, and the focal mechanism. We applied this technique to 30 major events in the period 1994–2013 and used the results to detect source complexity. Based on the results, we classed them into three groups according to the variability of the source parameters with frequency: simple, complex and intermediate. Twelve of these events fell into the simple category: Bolivia-1994, Kuril-1994, Sanriku-1994, Antofagasta-1995, Andreanoff-1996, Peru-2001, Sumatra-2004, Sumatra-2005, Tonga-2006, Sumatra-2007, Japan-2011, and the recent Sea of Okhotsk-2013. Seven exhibited significant complexity: Balleny-1998, Sumatra-2000, Indian Ocean-2000, Macquarie Island-2004, Sichuan-2008, and Samoa-2009. The remaining 11 events showed a moderate degree of complexity. Here, we discuss the results of this study in light of independent observations of source complexity, made by various investigators.

Full Moment Tensor Variations and Isotropic Characteristics of Earthquakes in the Gulf of California Transform Fault System

The full moment tensor is a mathematical expression of six independent variables; however, on a routine basis, it is a common practice to reduce them to five assuming that the isotropic component is zero. This constraint is valid in most tectonic regimes where slip occurs entirely at the fault surface (e.g. subduction zones); however, we found that full moment tensors are best represented in transform fault systems. Here we present a method to analyze source complexity of earthquakes of different sizes using a simple formulation that relates the elastic constants obtained from independent studies with the angle between the slip and the fault normal vector, referred to as angle

A note on the dynamic and static displacements from a point source in multilayered media

theta

Near-vertical multiple ScS phases and vertically averaged mantle properties

θ; this angle is obtained from the full moment tensors. The angle