Andrew V. Newman

Teleseismic estimates of radiated seismic energy: The E/M 0 discriminant for tsunami earthquakes

We adapt the formalism of Boatwright and Choy for the computation of radiated seismic energy from broadband records at teleseismic distances to the real-time situation when neither the depth nor the focal geometry of the source is known accurately. The analysis of a large data set of more than 500 records from 52 large, recent earthquakes shows that this procedure yields values of the estimated energy, EE, in good agreement with values computed from available source parameters, for example as published by the National Earthquake Information Center (NEIC), the average logarithmic residual being only 0.26 units. We analyze the energy-to-moment ratio by defining Θ = log10(EE/M0). For regular earthquakes, this parameter agrees well with values expected from theoretical models and from the worldwide NEIC catalogue. There is a one-to-one correspondence between values of Θ that are deficient by one full unit or more, and the so-called “tsunami earthquakes”, previously identified in the literature as having exceedingly slow sources, and believed due to the presence of sedimentary structures in the fault zone. Our formalism can be applied to single-station measurements, and its coupling to automated real-time measurements of the seismic moment using the mantle magnitude Mm should significantly improve real-time tsunami warning.

Geodetic and seismic constraints on some seismogenic zone processes in Costa Rica

New seismic and geodetic data from Costa Rica provide insight into seismogenic zone processes in Central America, where the Cocos and Caribbean plates converge. Seismic data are from combined land and ocean bottom deployments in the Nicoya peninsula in northern Costa Rica and near the Osa peninsula in southern Costa Rica. In Nicoya, inversion of GPS data suggests two locked patches centered at 14 ± 2 and 39 ± 6 km depth. Interplate microseismicity is concentrated in the more freely slipping intermediate zone, suggesting that small interseismic earthquakes may not accurately outline the updip limit of the seismogenic zone, the rupture zone for future large earthquakes, at least over the short (∼1 year) observation period. We also estimate northwest motion of a coastal “sliver block” at 8 ± 3 mm/yr, probably related to oblique convergence. In the Osa region to the south, convergence is orthogonal to the trench. Cocos-Caribbean relative motion is partitioned here, with ∼8 cm/yr on the Cocos-Panama block boundary (including a component of permanent shortening across the Fila Costena fold and thrust belt) and ∼1 cm/yr on the Panama block–Caribbean boundary. The GPS data suggest that the Cocos plate–Panama block boundary is completely locked from ∼10–50 km depth. This large locked zone, as well as associated forearc and back-arc deformation, may be related to subduction of the shallow Cocos Ridge and/or younger lithosphere compared to Nicoya, with consequent higher coupling and compressive stress in the direction of plate convergence.

Geodetic and seismic constraints on recent activity at Long Valley Caldera, California : evidence for viscoelastic rheology

Long Valley Caldera is an active volcanic region in east central California. Surface deformation on the resurgent dome within the caldera was an order of magnitude higher for the five-month period September 1997 through January 1998 compared to the previous three-year average. However, the location of the immediate (shallow) source of deformation remained essentially constant, 5‐7 km beneath the dome, near the top of a region of probable magma accumulation defined by seismic data. Similarly, although the rate of seismic moment release increased dramatically, earthquake locations remained similar to earlier periods. The rate of deformation increased exponentially between April‐May 1997 and late November 1997 with a time constant of ,55‐65 days, after which it decreased exponentially with about the same time constant. We develop a model consistent with these observations and also consistent with independent constraints on sub-surface rheology from thermal, geochemical and laboratory data. Deformation at sites on the resurgent dome most sensitive to the shallow deformation source are well fit by a model with a single pressure source at 6 km depth which experienced a pressure pulse that began in late 1996, peaked in November 1997, close to the time of major seismic moment release, and essentially ended in mid-1999. The pressure source in our model is surrounded by a 1 km thick “shell” of Maxwell viscoelastic material (shell viscosity 10 16 Pa s) within an elastic half space, and has peak values that are much lower than corresponding purely elastic half space models. The shell viscosity is characteristic of a weak, deformable solid, e.g. quartz-bearing country rock surrounding the magma chamber at temperatures in the range 500‐6008C, i.e. above the brittle‐ductile transition, and/or largely crystallized rhyolite near its solidus temperature of ,6708C, material that probably exists near the top of the zoned magma chamber at Long Valley. q 2001 Elsevier Science B.V. All rights reserved.

Tsunami earthquakes: the quest for a regional signal

Using the technique developed by Newman and Okal [J. Geophys. Res. 103 (1998) 26885], a dataset of digital records from 84 earthquakes is analyzed to investigate their source slowness in the quest for a possible regional signal in three subduction zones which experienced recent tsunami earthquakes (Nicaragua, 1992; Java, 1994; Peru, 1996). The dataset is augmented by analog seismograms from historical events, including major tsunamigenic earthquakes of the past 65 years. We fail to detect a regional trend for slowness, which suggests that the latter may be controlled on a more local scale by morphological structures of the subducting plate. No correlation is found between slowness and either depth, focal mechanism, or seismic moment. In Nicaragua, we document two slow historical earthquakes located on the slab down-dip from the 1992 shock. The most interesting results are in Peru, where a local area of slowness is tentatively defined around the source of the 1960 tsunami earthquake, and where both the 1996 and 1960 tsunami earthquakes occur at the intersection of the trench with major topographic features on the Nazca plate, the Mendana fracture zone and the Trujillo trough, respectively.

Should Memphis build for California's earthquakes?

State and local authorities in parts of the central U.S. that are at risk from earthquakes in the New Madrid Seismic Zone (NMSZ) are considering adopting a new building code that would increase the earthquake resistance of new buildings to levels similar to those in southern California. Here, we argue against this proposal on the dual grounds that the earthquake hazard has been overestimated, and that the costs of the proposed change are likely to far exceed the potential benefits. Instead, we recommend weighing the costs and benefits of alternative strategies that could yield reasonable seismic safety at significantly lower cost.

Tsunami Forecast by Joint Inversion of Real-Time Tsunami Waveforms and Seismic or GPS Data: Application to the Tohoku 2011 Tsunami

Correctly characterizing tsunami source generation is the most critical component of modern tsunami forecasting. Although difficult to quantify directly, a tsunami source can be modeled via different methods using a variety of measurements from deep-ocean tsunameters, seismometers, GPS, and other advanced instruments, some of which in or near real time. Here we assess the performance of different source models for the destructive 11 March 2011 Japan tsunami using model–data comparison for the generation, propagation, and inundation in the near field of Japan. This comparative study of tsunami source models addresses the advantages and limitations of different real-time measurements with potential use in early tsunami warning in the near and far field. The study highlights the critical role of deep-ocean tsunami measurements and rapid validation of the approximate tsunami source for high-quality forecasting. We show that these tsunami measurements are compatible with other real-time geodetic data, and may provide more insightful understanding of tsunami generation from earthquakes, as well as from nonseismic processes such as submarine landslide failures.

Seismology: Tectonic strain in plate interiors?

Arising from: R. Smalley Jr, M. A. Ellis, J. Paul & R. B. Van Arsdale 435, 1088–1090 (2005); R. Smalley et al. reply.It is not fully understood how or why the inner areas of tectonic plates deform, leading to large, although infrequent, earthquakes. Smalley et al. offer a potential breakthrough by suggesting that surface deformation in the central United States accumulates at rates comparable to those across plate boundaries. However, we find no statistically significant deformation in three independent analyses of the data set used by Smalley et al., and conclude therefore that only the upper bounds of magnitude and repeat time for large earthquakes can be inferred at present.

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.

A new seismically constrained subduction interface model for Central America

We provide a detailed, seismically defined three-dimensional model for the subducting plate interface along the Middle America Trench between Northern Nicaragua through to Southern Costa Rica. The model uses data from a weighted catalog of about 30,000 earthquake hypocenters compiled from nine catalogs to constrain the interface through a process we term the “Maximum Seismicity Method”. The method determines the average position of the largest cluster of microseismicity beneath an a priori functional surface above the interface. This technique is applied to all seismicity above 40 km depth, the approximate intersection of the hanging-wall Mohorovicic discontinuity, where seismicity likely lies along the plate interface. Below this depth, an envelope above 90% of seismicity approximates the slab surface. Because of station proximity to the interface, this model provides highest precision along the interface beneath the Nicoya Peninsula of Costa Rica, an area where marked geometric changes coincide with crustal transitions and topography observed seaward of the trench. The new interface is useful for a number of geophysical studies that aim to understand subduction zone earthquake behavior, geodynamic and tectonic development of convergent plate boundaries.

Detailed spatiotemporal evolution of microseismicity and repeating earthquakes following the 2012 Mw 7.6 Nicoya earthquake

We apply a waveform matching technique to obtain a detailed earthquake catalog around the rupture zone of the 5 September 2012 moment magnitude 7.6 Nicoya earthquake, with emphasis on its aftershock sequence. Starting from a preliminary catalog, we relocate ~7900 events using TomoDD to better quantify their spatiotemporal behavior. Relocated aftershocks are mostly clustered in two groups. The first is immediately above the major coseismic slip patch, partially overlapping with shallow afterslip. The second one is 50 km SE to the main shock nucleation point and near the terminus of coseismic rupture, in a zone that exhibited little resolvable afterslip. Using the relocated events as templates, we scan through the continuous recording from 29 June 2012 to 30 December 2012, detecting approximately 17 times more than template events. We find 190 aftershocks in the first half hour following the main shock, mostly along the plate interface. Later events become more scattered in location, showing moderate expansion in both along-trench and downdip directions. From the detected catalog we identify 53 repeating aftershock clusters with mean cross-correlation values larger than 0.9, and indistinguishably intracluster event locations, suggesting slip on the same fault patch. Most repeating clusters occurred within the first major aftershock group. Very few repeating clusters were found in the aftershock grouping along the southern edge of the Peninsula, which is not associated with substantial afterslip. Our observations suggest that loading from nearby afterslip along the plate interface drives spatiotemporal evolution of aftershocks just above the main shock rupture patch, while aftershocks in the SE group are to the SE of the observed updip afterslip and poorly constrained.