Late Quaternary Slip Rate Research Articles

Paleoseismicity and neotectonics of the Cordillera Blanca Fault Zone, northern Peruvian Andes

The Cordillera Bianca fault zone is a major west dipping normal fault that bounds the west side of a 120‐ to 170‐km‐wide zone of active extension along the crest of the northern Peruvian Andes. The fault is approximately 210 km long and exhibits continuous geomorphic evidence of repeated late Pleistocene and Holocene displacements but has not been the source of historical or teleseismically recorded earthquakes. Trenching and mapping of fault scarps provide new information on earthquake recurrence, slip rate, timing of the most recent events, and Andean neotectonics. At Quebrada Queroccocha, 55 km from the south end of the zone, an 11,000‐ to 14,000‐year‐old moraine is displaced 12–15 m, and younger valley fill lacustrine and fluvial deposits are displaced 7.5–8 m. Scarp profiles, tectonic terraces, and trench exposures indicate five to seven scarp‐forming earthquakes of 2–3 m per event during the past 11,000–14,000 years at this location. Considering uncertainties in the number of events and in the age of the displaced moraine, the average recurrence interval is estimated to be 2440±1060 years. Detrital charcoal from colluviums that predate and postdate the most recent event yielded ages of 2480±65 and 750+80 14C years B.P., respectively, and stratigraphic relationships suggest that the event may be closer to the older date. The late Quaternary slip rate at Quebrada Queroccocha is 0.86–1.36 mm/yr and appears to be representative of much of the fault zone. At Pachma Bajo, located 30 km from the north end of the zone on a separate segment, displacement during each of the past two events was 2 to ≥ 3 m. The morphology of scarps in alluvial fan and debris flow deposits indicates recurrence intervals of about 1000–3000 years. Relationships between pre‐Inca walls and faulted deposits suggest that 1500–2000 years have elapsed since the most recent event. Fault behavior parameters, particularly displacement per event and fault segment length, show that the Cordillera Bianca fault zone is similar to seismogenic normal faults that nucleate M 7–7½ earthquakes at depths of approximately 15 km. The Quebrada Queroccocha slip rate and analysis of other normal faults in the region suggest cumulative late Quaternary extensional strain rates of about 1–4 mm/yr across the complete zone of Andean crestal extension.

Late Quaternary activity along the Lone Pine fault, eastern California

The Lone Pine fault is a north-trending secondary break of the Owens Valley fault zone, 1.4 km west of Lone Pine, California. This fault forms an east-facing scarp as much as 6.5 m high across an abandoned outwash fan of the Tioga (latest Pleistocene) glaciation. The fault experienced large right-lateral and smaller vertical displacement during the 1872 Owens Valley earthquake. Knowledge of the character and amount of slip at this site in and before 1872 is necessary for evaluations of earthquake hazard near Owens Valley and may help us to understand earthquakes along other parts of the eastern front of the Sierra Nevada and in the Great Basin. Scarp profiles indicate a 1- to 2-m component of dip slip in 1872; thus three 1872-type earthquakes could have created the scarp. This number of events is also indicated by desert-varnish patterns on boulders in the fault scarp, by scarp morphology, and by sediments near the fault. Horizontal offset of a relict channel on the fan is 12 to 18 m (3 earthquakes). Horizontal offset of a younger debris flow is 10 to 12 m (apparently 2 earthquakes). Average horizontal offset for each earthquake, including that of 1872, is 4 to 6 m. The age of the fan surface is bracketed by a 21 ka shoreline of former Lake Owens and by the time of abandonment of the fan, about 10 ka. An average recurrence interval for 3 earthquakes is 5,000 to 10,500 yr. The average recurrence interval, combined with the average oblique offset for each event of 4.3 to 6.3 m, gives an average late Quaternary slip rate of 0.4 to 1.3 mm/yr for the Lone Pine fault. If we add the average horizontal-slip component for the Lone Pine fault to the 1872 horizontal slip reported for the adjacent main Owens Valley trace of 2.7 to 4.9 m, the combined 1872 horizontal-slip component for the Owens Valley fault zone is approximately between 7 and 11 m, a large value. The associated horizontal-slip rate for the Owens Vallev fault zone is 0.7 to 2.2 mm/yr.

A Kinematic model of southern California

We propose a kinematic model for southern California based on late Quaternary slip rates and orientations of major faults in the region. Internally consistent motions are determined assuming that these faults bound rigid blocks. Relative to North America, most of California west of the San Andreas fault is moving parallel to the San Andreas fault through the Transverse Ranges and not parallel to the motion of the Pacific plate. This is accomplished by counterclockwise rotation of California south of the San Andreas fault and by the westward movement of central California north of the Gar lock fault. The velocities of the blocks are calculated along several paths in southern California that begin in the Mojave Desert and end off the California coast. A path that crosses the western Transverse Ranges accumulates the accepted relative North America‐Pacific plate velocity, whereas paths to the north and south result in a significant missing component of motion. This implies the existence of a zone of active deformation in southern California that is interpreted to include the western Transverse Ranges and northwest trending, predominately strike‐slip faults close to the coast both north and south of the Transverse Ranges. Strain on this system accounts for about a third of the total North America‐Pacific plate motion.

Quaternary faulting along the Caribbean-North American plate boundary in Central America

Recent detailed mapping along the Motagua fault zone and reconnaissance along the Chixoy—Polochic and Jocotán—Chamelecón fault zones provide new information regarding the nature of Quaternary deformation along the Caribbean—North American plate boundary in Central America. The southern boundary of the Motagua fault zone is defined by a major active left-slip fault that ruptured during the February 4, 1976 Guatemala earthquake. The recurrent nature of slip along the fault is dramatically demonstrated where stream terraces of the Río El Tambor show progressive left-slip and vertical (up-to-the-north) slip. Left-slip increases from 23.7 m (youngest mappable terrace) to 58.3 m (oldest mappable terrace) and vertical slip increases from 0.6 m to 2.5 m. The oldest mappable terrace crossed by the fault appears to be younger than 40,000 years and older than 10,000 years. Reconnaissance along the Chixoy—Polochic fault zone between Chiantla and Lago de Izabal has located the traces of a previously unmapped major active left-slip fault. Geomorphic features along this fault are similar to those observed along the active trace of the Motagua fault zone. Consistent and significant features suggestive of left-slip have so far not been observed along the Guatemala section of the Jocotán—Chamelecón fault zone. In Central America, the active Caribbean—North American plate boundary is comprised of the Motagua, Chixoy—Polochic, and probably the Jocotán—Chamelecón fault zones, with each accommodating part of the slip produced at the mid-Cayman spreading center. Similarities in geomorphic expression, apparent amount of left-slip, and frequency and magnitude of historical and instrumentally recorded earthquakes between the active traces of the Motagua and Chixoy—Polochic fault zones suggest a comparable degree of activity during Quaternary time; the sense and amount of Quaternary slip on the Jocotán—Chamelecón fault zone remain uncertain, although it appears to be an active earthquake source. Uplift of major mountain ranges on the north side of each fault zone reflects the small but consistent up-to-the-north vertical component (up to 5% of the lateral component) of slip along the plate boundary. Preliminary findings, based on offset stream terraces, indicate a late Quaternary slip rate along the Caribbean—North American plate boundary of between 0.45 and 1.8 cm/yr. Age dating of offset Quaternary terraces in Guatemala will allow refinement of this rate.