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Recent efforts to characterize small (Mw

This study explores the effectiveness of local-distance (

Low-yield explosion monitoring requires the use of nearby short-period observations, which exhibit sensitivities to geologic heterogeneity and have low signal-to-noise ratios compared to larger events. In this study, we analyze 843 seismic events using nearly 475,000 individual observations to compute precise relative locations of small (1

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

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

Crustal attenuation across the East African plateau in Tanzania, an area of uplifted and rifted Precambrian crust, has been investigated using seismic data from regional earthquakes recorded by the 1994–1995 Tanzania broadband seismic experiment. We use 1 Hz Lg coda waves from the 17 events, together with the energy flux model of Frankel and Wennerberg (1987), to obtain estimates of intrinsic ( Q I ) and scattering ( Q S ) attenuation for East Africa. Q I values across the plateau are fairly uniform, ranging from a low of ∼300 to a high of ∼600. Q I values for the Tanzania craton, in the middle of the plateau, are similar to those for the mobile belts, which form the sides of the plateau. Q I of 300 to 600 is somewhat lower than the average crustal Q for Precambrian terrains elsewhere. Heat flow from the Tanzania craton and surrounding mobile belts is not elevated; therefore, we attribute the lower-than-average Q values not to elevated crustal temperatures, but instead to rift faults in the crust that are interconnected and filled with fluids. Q S ranges from ∼1000 in the mobile belts and along the eastern margin of the Tanzania craton to ∼2200 in the north central part of the plateau just south of Lake Victoria. We attribute the variability in Q S to scattering of Lg by surface topography, in particular, rift basins along the eastern side of the plateau.

We report full moment tensor solutions for 76 mine tremors with moment magnitudes (Mw) between 0.5 and 2.6 recorded by a network of 20 high-frequency geophones in a deep gold mine in South Africa. Source mechanisms convey important information on how in-mine stresses are relaxed, and understanding the nature of such mechanisms is essential for improving our assessment of rock mass response to mining. Our approach has consisted of minimizing the L2 norm of the difference between observed and predicted P, SV, and SH spectral amplitudes, with visually assigned polarities, to constrain all six independent components of the seismic moment tensor. Our results reveal the largest principal stresses in the mine are com- pressive, oriented near vertically, and relaxed through a mix of volumetric closure and normal faulting, consistent with a gravity-driven closure of the mined-out areas. Pre- vious moment tensor studies in deep mines had suggested that the distribution of seis- mic sources in terms of the volumetric-shear mix was bimodal. A bimodal distribution is compatible with our moment tensor solutions only for moment magnitudes above 2.2. Events in the 0:5

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 (

A local magnitude ( M L) scale and seismicity catalog for the Ethiopian Plateau have been developed using data collected by the 2000–2002 Ethiopia Broadband Seismic Experiment. Locations for 253 local and regional events have been obtained using P -wave arrival times recorded on four or more stations. For constructing the M L scale, waveforms were corrected for instrument response and convolved with the nominal Wood–Anderson torsion seismograph response. Maximum S -wave amplitudes were then picked on the horizontal components of ground motion (3218 total observations) and used in an inversion for event magnitudes, two model parameters, and 54 horizontal component station corrections. The distance correction obtained from the inversion is -log A =0.726log( r /100)+0.000558( r -100)+3.0, where r is the hypocentral distance in kilometers. Seven of the 253 events can be considered ground truth (GT) events, with epicentral locations accurate to within 5 km according to the GT5 local criteria of Bondar et al. (2004). In contrast to previously reported ground-motion attenuation for the Main Ethiopian Rift, we find relatively low ground-motion attenuation for the Ethiopian Plateau, reflecting variations in crustal structure between the Ethiopian Plateau and Main Ethiopian Rift.

A local magnitude scale is developed for Tanzania, East Africa, using data collected by the 1994/1995 Tanzania Broadband Seismic Experiment. The waveform data from 1189 local and regional earthquakes located within East Africa were corrected for instrument response and convolved with the nominal Wood-Anderson torsion seismograph response appropriate for the original definition of local magnitude. A total of 24,710 maximum S -wave amplitudes were incorporated into an iterative regression for individual event local magnitudes, 38 horizontal component station factors, and 2 linear distance-dependent factors in the log A 0 term of the equation for M L . The resulting distance correction, log A 0 , is given by -log A 0 = 0.776 log( r /17) + 0.000902( r − 17) + 2.0, where r is hypocentral distance in kilometers. This distance correction yields much less ground-motion attenuation than observed for southern California and is similar to that observed for local S waves and regional Lg waves propagating in eastern North America. Normalizing the distance correction using the standard constraint of 1-mm ground motion at 100 km for a magnitude 3.0 earthquake results in a bias of nearly half of a magnitude unit between m b and M L , with M L being larger. Normalizing using the constraint of 10 mm of motion at 17 km for M L 3.0 removes the bias in magnitude measures and indicates that structure within Tanzania is relatively high Q . The seismicity rate of Tanzania for 1994/1995 was examined using the Gutenberg-Richter seismicity distribution and is seen to follow the relation log N = 4.63 − 0.84 M L , where N is the number of earthquakes per year of local magnitude M L or greater. The catalog of events used in this study is seen to be complete to magnitude 2 to 2.5.