Richard Anthony Brazier

Precambrian lithospheric controls on the development of the East African rift system

We propose a new kinematic framework for the East African rift system linking the development of the Eastern and Western rifts via stress transmission across the Archean Tanzania craton. The proposal is based on three observations. (1) A new map of uppermost-mantle P-wave velocities beneath Tanzania, in combination with the results of other seismic, gravity, heat-flow, and xenolith studies, reveals that the cratons thick, cold lithosphere has largely resisted modification by the Cenozoic extensional tectonism and has therefore behaved as a rigid tectonic block. (2) As the southward-propagating Eastern rift reached the craton margin ca. 12–10 Ma, the Western rift began to develop. (3) Subsequently, the Western rift enlarged into a 2500-km-long system of en echelon rift basins while development of the Eastern rift stalled where it ran into the craton. These observations suggest that when the Eastern rift reached the margin of the rigid craton ca. 12–10 Ma, the transmission of extensional stresses across the strong cratonic lithosphere caused rift faulting to develop in the weaker mobile-belt lithosphere on the west side of the craton. This new kinematic framework for rift development is consistent with models for both active and passive rifting in East Africa.

Focal mechanisms and the stress regime in NE and SW Tanzania, East Africa

[1] We report 12 new focal mechanisms from earthquakes in NE and SW Tanzania where the stress regime within the East African rift system is not well constrained. Focal mechanisms for events at the intersection of the Lake Tanganyika and Rukwa rifts in SW Tanzania indicate a complicated stress pattern with possible dextral strike-slip motion on some faults but oblique motion on others (either sinistral on NW striking faults or dextral on NE striking faults). Within the Rukwa rift, focal mechanisms indicate normal dip-slip motion with NE-SW opening. In NE Tanzania where the Eastern rift impinges on the margin of the Tanzania Craton, fault motions are consistent with a zone of distributed block faults and sub E-W extension. All twelve earthquakes likely nucleated within the crust.

Pn wave velocities beneath the Tanzania Craton and adjacent rifted mobile belts, east Africa

P wave travel times from regional earthquakes recorded by the Tanzania Broadband Seismic Experiment have been inverted for long wavelength (>100 km) Pn velocity variations beneath Tanzania using a generalized inverse algorithm. Pn velocities, on average, are 8.40 to 8.45 km/s beneath the center of the Tanzania Craton, 8.30–8.35 km/s beneath the terminus of the Eastern Branch of the rift system, and 8.35–8.40 km/s beneath the Western Branch. These velocities indicate that there are no broad (>100 km wide) thermal anomalies in the uppermost mantle beneath areas of rifting in Tanzania, and suggest that thermal anomalies present deeper in the mantle have not yet reached the base of the crust.

Source Parameters for Moderate Earthquakes in the Zagros Mountains with Implications for the Depth Extent of Seismicity

Six earthquakes within the Zagros Mountains with magnitudes between 4.9 and 5.7 have been studied to determine their source parameters. These events were selected for study because they were reported in open catalogs to have lower crustal or upper mantle source depths and because they occurred within an area of the Zagros Mountains where crustal velocity structure has been constrained by previous studies. Moment tensor inversion of regional broadband waveforms has been combined with forward modeling of depth phases on short-period teleseismic waveforms to constrain source depths and moment tensors. Our results show that all six events nucleated within the upper crust (<11 km depth) and have thrust mechanisms. This finding supports other studies that call into question the existence of lower crustal or mantle events beneath the Zagros Mountains.

Empirically Based Ground Truth Criteria for Seismic Events Recorded at Local Distances on Regional Networks with Application to Southern Africa

Wepresentanewapproachtoobtainingempiricallybased(EB)criteriafor estimating the epicentral location accuracy (i.e., ground truth, GT) of seismic events recorded at local distances on a regional network. The approach has been developed usinga jackknife resampling method appliedto carefully pickedPgphasearrivaltimes for GT reference events from several South African gold mines. The events were well recorded locally by Southern African Seismic Experiment (SASE) stations within the Archean Kaapvaal craton, an area of relatively simple crustal structure. The region- specific criteria obtained specify an EBGT395% level of epicentral accuracy if events are recorded on eight or more stations at distances less than the Pg=Pn crossover (215 km) when the stations have a primary azimuthal gap <202 degrees. In addition, when nine or more stations are used for event location and one of them is within 79 km of the event, we find that a focal depth accuracy of 4 km at the 95% confidence level can be obtained and that an accuracy of 6 km can be obtained if eight stations are used. This result illustrates that GT criteria commonly applied to global event catalogs can be relaxed if an accurate velocity model and carefully picked phase-arrival times are used for event locations. Consequently, it is likely that additional events can be added to GT compilations by developing EBGT criteria for other regional networks and using them to identify candidate GT events. For example, the EBGT criteria developed in this study, when applied to the SASE seismicity catalog, yields 10 new GT events.

From Craton to Rift: Empirically Based Ground-Truth Criteria for Local Events Recorded on Regional Networks

Region‐specific empirically based ground‐truth (EBGT) criteria used to estimate the epicentral‐location accuracy of seismic events have been developed for the Main Ethiopian Rift and the Tibetan plateau. Explosions recorded during the Ethiopia–Afar Geoscientific Lithospheric Experiment (EAGLE), the International Deep Profiling of Tibet, and the Himalaya (INDEPTH III) experiment provided the necessary GT0 reference events. In each case, the local crustal structure is well known and handpicked arrival times were available, facilitating the establishment of the location accuracy criteria through the stochastic forward modeling of arrival times for epicentral locations. In the vicinity of the Main Ethiopian Rift, a seismic event is required to be recorded on at least 8 stations within the local ![Graphic][1] crossover distance and to yield a network‐quality metric of less than 0.43 in order to be classified as EBGT595% (GT5 with 95% confidence). These criteria were subsequently used to identify 10 new GT5 events with magnitudes greater than 2.1 recorded on the Ethiopian Broadband Seismic Experiment (EBSE) network and 24 events with magnitudes greater than 2.4 recorded on the EAGLE broadband network. The criteria for the Tibetan plateau are similar to the Ethiopia criteria, yet slightly less restrictive as the network‐quality metric needs to be less than 0.45. Twenty‐seven seismic events with magnitudes greater than 2.5 recorded on the INDEPTH III network were identified as GT5 based on the derived criteria. When considered in conjunction with criteria developed previously for the Kaapvaal craton in southern Africa, it is apparent that increasing restrictions on the network‐quality metric mirror increases in the complexity of geologic structure from craton to plateau to rift. [1]: /embed/inline-graphic-1.gif

Journey to Antarctica: Modeling crustal structure with an earthquake and a genetic algorithm

In a previous work, we have used the genetic algorithm NSGA-II to generate a set of solutions to model the receiver functions and dispersion curves of several seismometer stations located in southern Africa. Now in continuation of applying the NSGA-II to seismic problems, we have used it to model the average velocity profiles along two-dimensional paths from a single seismic event to several stations across West Antarctica. The event was a rare continental earthquake of magnitude 5.6 that took place in West Antarctica near the Ross Ice Shelf during the austral winter of 2012. Data were collected from stations in the Global Seismic Network as well as a local network during the 2012–2013 field season. The seismograms were first modeled using a full body wave modeling code that generates synthetics based on a structure composed of layers with user-defined velocities, thicknesses, and densities. Those models then served as the starting models in NSGA-II, which created a set of solutions from which an average structure with error bounds was calculated for each station.

On a Common Mnemonic from Trigonometry

But how is it that the sines of these angles can be obtained via such a simple arithmetic sequence? The angles themselves occur in a pattern to be sure, but not such a simple one. We will show that θ (see the figure) is one of the special angles, 0, π/6, π/4, π/3, or π/2, precisely when the ratio |AD| |AC| is 0, 1/4, 1/2, 3/4, or 1, respectively. To put it another way, if we think of the point D as moving in a straight line from A to C then θ successively reaches each of the special angles π/6, π/4, and π/3 precisely when D is 1/4, 1/2, 3/4, or all of the way to C . (Clearly θ = {0, π/2} when D = {A,C}.) For clarity and without loss of generality we assume that |AC| = 1. Observe that triangles ABD and BCD are similar to each other since both are similar to triangle ABC. Thus by proportionality of similar triangles we have |BD| |AD| = 1−|AD| |BD| or