Heather R. DeShon

Control of seafloor roughness on earthquake rupture behavior

Earthquake rupture complexity is described for three recent large underthrusting earthquakes along the Costa Rican subduction zone, the 1983 Osa, 1990 Nicoya Gulf, and 1999 Quepos events. These earthquakes occurred in regions characterized by distinctly different morphologic features on the subducting plate. The 1990 and 1999 events occurred along linear projections of subducting seamount chains and had fairly simple earthquake rupture histories. Both events are interpreted as failure of the basal contact of closely spaced isolated seamounts acting as asperities. In contrast, the 1983 event occurred along the subducting Cocos Ridge and had a complex rupture history. Comparison of rupture characteristics of these large underthrusting earthquakes with size and location of subducting features provides evidence that seamounts can be subducted to seismogenic depths and that variations in seafloor bathymetry of the subducting plate strongly influence the earthquake rupture process.

Teleseismic Relocation and Assessment of Seismicity (1918–2005) in the Region of the 2004 Mw 9.0 Sumatra–Andaman and 2005 Mw 8.6 Nias Island Great Earthquakes

The M w 9.0 2004 Sumatra–Andaman Islands and M w 8.6 Nias Island great earthquake sequences have generated over 5000 catalog-reported earthquakes along ∼1700 km of the Sumatra–Andaman and western Sunda regions. Studies of prior regional seismicity have been limited to global catalog locations that often have poorly constrained epicenters and depths. Approximately 3650 teleseismically well- recorded earthquakes occurring in this region during the period 1918–2005 are relocated with special attention to focal depth. Reduced uncertainties of epicenters and depths in the region (on the order of 15 and 10 km, respectively) foster interpretation of focal mechanism data and provide additional details about the subducting Indian and Australian plates. The revised earthquake dataset reveals a sharp delineation between aftershocks of the 2004 and 2005 earthquakes near Simeulue Island and a steepening in slab dip from south to north. The downdip width of the aftershock zone of the 2004 M w 9.0 earthquake varies from ∼200 km at its northern end to ∼275 km at its southern end, and events located between 35 and 70 km focal depth occur more frequently in the southernmost section of this aftershock zone. Outer- rise and near-trench normal and strike-slip faulting earthquakes also increase in frequency following the 2004 and 2005 earthquakes. Earthquake swarms triggered along the Andaman backarc spreading center both north of Sumatra and near Siberut Island, 100 km south of the Nias Island aftershock sequence, illustrate the complex and variable nature of seismicity following these great earthquakes.

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.

Causal factors for seismicity near Azle, Texas

In November 2013, a series of earthquakes began along a mapped ancient fault system near Azle, Texas. Here we assess whether it is plausible that human activity caused these earthquakes. Analysis of both lake and groundwater variations near Azle shows that no significant stress changes were associated with the shallow water table before or during the earthquake sequence. In contrast, pore-pressure models demonstrate that a combination of brine production and wastewater injection near the fault generated subsurface pressures sufficient to induce earthquakes on near-critically stressed faults. On the basis of modelling results and the absence of historical earthquakes near Azle, brine production combined with wastewater disposal represent the most likely cause of recent seismicity near Azle. For assessing the earthquake cause, our research underscores the necessity of monitoring subsurface wastewater formation pressures and monitoring earthquakes having magnitudes of ∼M2 and greater. Currently, monitoring at these levels is not standard across Texas or the United States.

Geologic evolution of southern Rusalka Planitia, Venus

Geologic mapping of southern Rusalka Planitia, Venus, reveals interactions of volcanism, tectonism, and topography. We recognize three regional plains units (prR1, prR2, and prR3) based on crosscutting structural relations, embayment patterns, radar brightness, and surface roughness data. Delineation of secondary (tectonic) structures allows us to constrain the relative temporal relations between the three material units. Unit prRl, a radar dark smooth unit exposed in local topographic highs, hosts NE trending extension fractures. Low-viscosity lava flows of prR2, the most areally extensive unit, fill local topographic lows and the NE trending fractures. A shield-sourced lava unit, prR3, overlies prR2 on the basis of embayment relations and radar brightness. NW trending wrinkle ridges deform all three plains units and record regional contraction. Locally, flood lava flows that fill NE trending fractures are structurally inverted to form short, stepped NE trending wrinkle ridges. Map patterns indicate that prR2 comprises a thin layer (<50 m thick), much thinner than previous estimates of 1–3 km. Therefore previously proposed estimates of plains flood lava flow volumes and effusion rates are much too high. The local geologic history of southern Rusalka Planitia is inconsistent with global stratigraphy models. Our study supports the view of plains evolution occurring through discrete volcanic processes working at local and regional (but not global) scales.

Constraining the boundary between the Sunda and Andaman subduction systems: Evidence from the 2002 Mw 7.3 Northern Sumatra earthquake and aftershock relocations of the 2004 and 2005 great earthquakes

The 2004 M w 9.0 Sumatra-Andaman earthquake initiated along the Andaman subduction zone, north of the last great Sumatra earthquake along the Sunda Trench in 1861. During the 2005 M w 8.7 Banyak Islands earthquake, a portion of the 1861 rupture subsequently failed. The boundary between the 2004 and 2005 ruptures broadly coincides with local trench rotation and the southern edge of the Andaman microplate, which suggests structural control on fault segmentation. Aftershock relocations of the 2004 and 2005 earthquakes show little overlap, and the sharp boundary between the series locates near the 2002 M w 7.3 Northern Sumatra earthquake. We posit that these features represent the southern extent of the stable Andaman microplate, ∼50-100 km northwest of what was previously reported. Broadband analyses of the 2002 earthquake yield a bilateral rupture pattern that is used to model Coulomb stress changes near the 2004 hypocenter to assess stress interactions along adjacent fault segments.

Earthquake Relocation and Focal Mechanism Determination Using Waveform Cross Correlation, Nicoya Peninsula, Costa Rica

The Nicoya Peninsula in Costa Rica directly overlies the seismogenic zone of the Middle America Trench, making it an ideal location for geophysical investigations of shallow subduction zone earthquake processes. As part of the collaborative Costa Rica Seismogenic Zone Experiment (crseize), a seismic network consisting of 20 land and 14 ocean-bottom seismometers recorded small magnitude local earthquakes along the Nicoya Peninsula from December 1999 to June 2001. Previous studies have used these data to compute local earthquake locations and 3D velocity structure to identify plate boundary seismicity and to investigate seismogenic behavior. Here we utilize waveform cross-correlation and clustering techniques in an attempt to improve earthquake relocations and determine first-motion focal mechanisms to validate, refine, and expand on existing models. Due to the high quality of the original locations and the small cross-correlation P -wave arrival time adjustments, large differences between the previously determined and the cross-correlated earthquake locations are not observed. However, focal mechanism determinations using cross-correlated P waveforms are significantly enhanced. Approximately 90% of the focal mechanisms computed for events previously identified as interplate earthquakes are consistent with underthrusting. Focal mechanisms for continental intraplate events indicate dextral strike-slip motion in the central region and normal faulting at the southern tip of the peninsula. These motions may be associated with oblique convergence and seamount subduction, respectively. Within the subducting plate, steep P and T axes of earthquakes below 50 km depth are consistent with unbending of the slab.

Integration of Arrival‐Time Datasets for Consistent Quality Control: A Case Study of Amphibious Experiments along the Middle America Trench

We have integrated waveform and arrival‐onset data collected in Costa Rica as part of the National Science Foundation (NSF)‐sponsored Costa Rica Seismogenic Zone Experiment (CRSEIZE) and along central Costa Rica and Nicaragua as part of the German SFB 574 program. The five arrays, composed of different sensor types (one‐ and three‐component land and ocean bottom seismometers and hydrophones), were archived using different software packages (Antelope and SEISAN) and were automatically and manually picked using various quality criteria resulting in a disparate set of pick weights. We evaluate pick quality using automated arrival detection and picking algorithm based on the wavelet transform and Akaike information criterion picker. The consistency of the arrival information over various scales provides a basis for assigning a quality to the analyst pick. Approximately 31% of P arrival times and 26% of S times have been classified as high‐quality picks (quality 0–1). An additional 21% of P times and 27% of S arrivals are good quality (quality 2–3). The revised quality picks are mapped directly into new pick weights for inversion studies. We explore the effect of new weighting and removal of poor data by relocating hypocenters through a minimum 1D velocity model and conducting double‐difference local earthquake tomography (LET). Analysis of the hypocenter relocation and seismic velocity tomography results suggest that using the improved quality determinations have a greater effect on improving sharpness in the velocity images than on the magnitude of hypocentral movement. Online Material: Figures of waveforms, event statistics, and tomography; and tables of station and event parameters, station qualities, velocity model, and hypocentral parameters.

High‐resolution 3‐D P wave attenuation structure of the New Madrid Seismic Zone using local earthquake tomography

A three-dimensional (3-D), high-resolution P wave seismic attenuation model for the New Madrid Seismic Zone (NMSZ) is determined using P wave path attenuation (t*) values of small-magnitude earthquakes (MD < 3.9). Events were recorded at 89 broadband and short-period seismometers of the Cooperative New Madrid Seismic Zone Network and 40 short-period seismometers of the Portable Array for Numerical Data Acquisition experiment. The amplitude spectra of all the earthquakes are simultaneously inverted for source, path (t*), and site parameters. The t* values are inverted for QP using local earthquake tomography methods and a known 3-D P wave velocity model for the region. The four major seismicity arms of the NMSZ exhibit reduced QP (higher attenuation) than the surrounding crust. The highest attenuation anomalies coincide with areas of previously reported high swarm activity attributed to fluid-rich fractures along the southeast extension of the Reelfoot fault. The QP results are consistent with previous attenuation studies in the region, which showed that active fault zones and fractured crust in the NMSZ are highly attenuating.

Discriminating between natural versus induced seismicity from long-term deformation history of intraplate faults

Long-term fault slip history can diagnose natural versus induced earthquakes, independent of correlations with fluid injection. To assess whether recent seismicity is induced by human activity or is of natural origin, we analyze fault displacements on high-resolution seismic reflection profiles for two regions in the central United States (CUS): the Fort Worth Basin (FWB) of Texas and the northern Mississippi embayment (NME). Since 2009, earthquake activity in the CUS has increased markedly, and numerous publications suggest that this increase is primarily due to induced earthquakes caused by deep-well injection of wastewater, both flowback water from hydrofracturing operations and produced water accompanying hydrocarbon production. Alternatively, some argue that these earthquakes are natural and that the seismicity increase is a normal variation that occurs over millions of years. Our analysis shows that within the NME, faults deform both Quaternary alluvium and underlying sediments dating from Paleozoic through Tertiary, with displacement increasing with geologic unit age, documenting a long history of natural activity. In the FWB, a region of ongoing wastewater injection, basement faults show deformation of the Proterozoic and Paleozoic units, but little or no deformation of younger strata. Specifically, vertical displacements in the post-Pennsylvanian formations, if any, are below the resolution (~15 m) of the seismic data, far less than expected had these faults accumulated deformation over millions of years. Our results support the assertion that recent FWB earthquakes are of induced origin; this conclusion is entirely independent of analyses correlating seismicity and wastewater injection practices. To our knowledge, this is the first study to discriminate natural and induced seismicity using classical structural geology analysis techniques.