John P. Loveless

Stress modulation on the San Andreas fault by interseismic fault system interactions

During the interseismic phase of the earthquake cycle, between large earthquakes, stress on faults evolves in response to elastic strain accumulation driven by tectonic plate motions. Because earthquake cycle processes induce non-local stress changes, the interseismic stress accumulation rate on one fault is influenced by the behavior of all nearby faults. Using a geodetically constrained block model, we show that the total interseismic elastic strain field generated by fault interactions within Southern California may increase stressing rates on the Mojave and San Bernardino sections of the San Andreas fault within the Big Bend region by as much as 38% relative to estimates from isolated San Andreas models. Assuming steady fault system behavior since the C.E. 1857 Fort Tejon earthquake, shear stress accumulated on these sections due only to interaction with faults other than the San Andreas reaches 1 MPa, ∼3 times larger than the coseismic and postseismic stress changes induced by recent Southern California earthquakes. Stress increases along Big Bend sections coincide with the greatest earthquake frequency inferred from a 1500-yr-long paleoseismic record and may affect earthquake recurrence intervals within geometrically complex fault systems, including the sections of the San Andreas fault closest to metropolitan Los Angeles.

Pervasive cracking of the northern Chilean Coastal Cordillera: New evidence for forearc extension

Despite convergence across the strongly coupled seismogenic interface between the South American and Nazca plates, the dominant neotectonic signature in the forearc of northern Chile is arc-normal extension. We have used 1 m resolution IKONOS satellite imagery to map nearly 37,000 cracks over an area of 500 km2 near the Salar Grande (21°S). These features, which are best preserved in a ubiquitous gypcrete surface layer, have both nontectonic and tectonic origins. However, their strong preferred orientation perpendicular to the plate convergence vector suggests that the majority owe their formation to approximate east-west extension associated with plate boundary processes such as interseismic loading, coseismic and postseismic strain, and long-term instability resulting from subduction erosion. Similar structures were formed during or shortly after the 1995 Mw = 8.0 earthquake near the city of Antofagasta, south of Salar Grande, and in conjunction with the 2001 Mw = 8.2–8.4 Arequipa, Peru, event. Cracks such as these may form in other forearcs but remain largely unexposed because of vegetative cover or marked fluvial erosion—factors that are absent in northern Chile as a result of its hyperarid climate.

Surface cracks record long-term seismic segmentation of the Andean margin

Understanding the long-term patterns of great earthquake rupture along a subduction zone provides a framework for assessing modern seismic hazard. However, evidence that can be used to infer the size and location of past earthquakes is typically erased by erosion after a few thousand years. Meter-scale cracks that cut the surface of coastal areas in northern Chile and southern Peru preserve a record of earthquakes spanning several hundred thousand years owing to the hyperarid climate of the region. These cracks have been observed to form during and/or shortly after strong subduction earthquakes, are preserved for long time periods throughout the Atacama Desert, demonstrate evidence for multiple episodes of reactivation, and show changes in orientation over spatial scales similar to the size of earthquake segments. Our observations and models show that crack orientations are consistent with dynamic and static stress fi elds generated by recent earthquakes. While localized structural and topographic processes infl uence some cracks, the strong preferred orientation over large regions indicates that cracks are primarily formed by plate boundary‐scale stresses, namely repeated earthquakes. We invert the crack-based strain data for slip along the well-known Iquique seismic gap segment of the margin and fi nd consistency with gravity anomaly‐based inferences of long-term earthquake slip patterns, as well as the magnitude and location of the November 2007 Tocopilla earthquake. We suggest that the meter-scale cracks can be used to map characteristic earthquake rupture segments that persist over many seismic cycles, which encourages future study of cracks and other small-scale structures to better constrain the persistence of asperities in other arid, tectonically active regions.

Upper Plate Reverse Fault Reactivation and the Unclamping of the Megathrust During the 2014 Northern Chile Earthquake Sequence

After 137 years without a great earthquake, the Mw 8.1 Pisagua event of 1 April 2014 occurred in the central portion of the southern Peru–northern Chile subduction zone. This megathrust earthquake was preceded by more than 2 weeks of foreshock activity migrating ∼3.5 km/day toward the mainshock hypocenter. This foreshock sequence was triggered by an Mw 6.7 earthquake on a reverse fault in the upper plate that strikes at a high angle to the trench, similar to well-documented reverse faults onshore. These margin-oblique reverse faults accommodate north-south shortening resulting from subduction across a plate boundary that is curved in map view. Reverse slip on the crustal fault unclamped the subduction interface, precipitating the subsequent megathrust foreshock activity that culminated in the great Pisagua earthquake. The combination of crustal reverse faults and a curved subduction margin also occurs in Cascadia and northeastern Japan, indicating that there are two additional localities where great megathrust earthquakes may be triggered by upper plate fault activity.

Creeping subduction zones are weaker than locked subduction zones

Faults that are fully or partially locked pose the greatest seismic hazard because they accumulate stress that can then be released in large earthquakes. In contrast, other faults continuously creep. The creeping versus locked behaviour is probably related to the frictional properties of the fault and the effective normal stress on the fault, but it is unclear whether locked faults are weaker or stronger than creeping faults. Here we use stress orientations in subduction zones from inversion of earthquake moment tensors, and find that geodetically determined creeping versus locked behaviour is correlated with the orientation of the subduction zone plate boundary fault relative to the principal stress axes. Globally, locked subduction zones appear well-oriented for failure, assuming a typical laboratory friction coefficient. Creeping subduction zones are more poorly oriented, implying a lower apparent friction coefficient, due to either low intrinsic friction or reduced effective normal stress. The spatial variations of stress orientation on the Japan Trench are similarly correlated with spatial variations in coupling, with creeping regions having a lower apparent friction coefficient than locked regions. The absolute strength of faults is influenced by the ambient fluid pressure, which is often elevated in subduction zones. This suggests low overall strength for locked subduction zone faults, and additional strength reduction in creeping zones that may be due to transient elevated fluid pressures.The faults in creeping segments of subduction zones are weaker than those in locked segments, according to analyses of stress orientations and GPS data from subduction zones globally.

Coseismic extension from surface cracks reopened by the 2014 Pisagua, northern Chile, earthquake sequence

The A.D. 2014 Pisagua earthquake sequence reactivated ancient surface cracks along the entire rupture length in the northern Chilean forearc. These subtle brittle strain features that are ∼50 km above the subduction zone interface in the hyperarid Atacama Desert record deformation from the single earthquake sequence. In this study we document how ancient cracks, formed during thousands of plate boundary earthquake cycles, were reopened during the 2014 earthquake sequence. We show that crack orientations along the rupture length reflect deformation from the M w 8.1 mainshock and from an M w 7.7 aftershock 100 km to the south, as documented by displacements calculated from continuous geodetic observations. We suggest that cracks form during the passage of surface waves, and repeated opening and closing enhance crack aperture. The orientation and opening of the oldest cracks in the forearc are indicative of the modal or most common rupture area of major megathrust earthquakes in the region. While the long-term preservation of cracks may be limited to northern Chile, similar features likely form during strong earthquakes at other subduction zones and represent permanent forearc deformation.

Super‐interseismic periods: Redefining earthquake recurrence

Precise geodetic measurements made over broad swaths of tectonically active regions record patterns of interseismic strain accumulation, providing key insights into the locus and timing of pending earthquakes. Recent studies of geodetic position time series, including that of Melnick et al. (2017), illustrate temporal variation in the pattern of interseismic deformation. These authors propose that the 2010 Mw = 8.8 Maule, Chile, earthquake enhanced coupling on the Andean subduction zone adjacent to the rupture, including on the portion of the megathrust that broke 5 years later in the Mw = 8.3 Illapel event.

Block motion changes in Japan triggered by the 2011 Great Tohoku Earthquake

Plate motions are governed by equilibrium between basal and edge forces. Great earthquakes may induce differential static stress changes across tectonic plates, enabling a new equilibrium state. Here we consider the torque balance for idealized circular plates and find a simple scalar relationship for changes in relative plate speed as a function of its size, upper mantle viscosity, and coseismic stress changes. Applied to Japan, the 2011 MW = 9.0 Tohoku earthquake generated coseismic stresses of 102 – 105 Pa that could have induced changes in motion of small (radius ∼ 100 km) crustal blocks within Honshu. Analysis of time-dependent GPS velocities, with corrections for earthquake cycle effects, reveals that plate speeds may have changed by up to ∼ 3 mm/yr between ∼ 3.75-year epochs bracketing this earthquake, consistent with an upper mantle viscosity of ∼ 5 × 1018 Pa·s, suggesting that great earthquakes may modulate motions of proximal crustal blocks at frequencies as high as 10−8 Hz.

Interseismic deformation and moment deficit along the Manila subduction zone and the Philippine Fault system

We examine interseismic coupling of the Manila subduction zone and fault activity in the Luzon area using a block model constrained by GPS data collected from 1998 to 2015. Estimated long-term slip rates along the Manila subduction zone show a gradual southward decrease from 90-100 mm/yr at the northwest tip of Luzon to 65-80 mm/yr at the southern portion of the Manila Trench. We provide two block models (Models A and B) to illustrate possible realizations of coupling along the Manila Trench, which may be used to infer future earthquake rupture scenarios. Model A shows a low coupling ratio of 0.34 offshore western Luzon and continuous creeping on the plate interface at latitudes 18°-19°N. Model B includes the North Luzon Trough Fault and shows prevalent coupling on the plate interface with a coupling ratio of 0.48. Both models fit GPS velocities well though they have significantly different tectonic implications. The accumulated strain along the Manila subduction zone at latitudes 15°-19°N could be balanced by earthquakes with composite magnitudes of Mw 8.8-9.2 assuming recurrence intervals of 500-1000 years. GPS observations are consistent with full locking of the majority of active faults in Luzon to a depth of 20 km. Inferred moments of large inland earthquakes in Luzon fall in the range of Mw 6.9-7.6 assuming a recurrence interval of 100 years.

Slip Distribution of the 2014 Mw=8.1 Pisagua, Northern Chile, Earthquake Sequence Estimated From Coseismic Fore-Arc Surface Cracks

The 2014 MW = 8.1 Iquique (Pisagua), Chile earthquake sequence ruptured a segment of the Nazca-South America subduction zone that last hosted a great earthquake in 1877. The sequence opened >3,700 surface cracks in the fore arc of decameter-scale length and millimeter-to centimeter-scale aperture. We use the strikes of measured cracks, inferred to be perpendicular to coseismically applied tension, to estimate the slip distribution of the mainshock and largest aftershock. The slip estimates are compatible with those based on seismic, geodetic, and tsunami data, indicating that geologic observations can also place quantitative constraints on rupture properties. The earthquake sequence ruptured between two asperities inferred from a regional-scale distribution of surface cracks, interpreted to represent a modal or most common rupture scenario for the northern Chile subduction zone. We suggest that past events, including the 1877 earthquake, broke the 2014 Pisagua source area together with adjacent sections in a through-going rupture.