Rupture process of the MacQuarie Ridge earthquake of May 23, 1989

Broadband body waves recorded at 15 digital seismic stations worldwide are used to study the rupture process of the May 23, 1989 Macquarie Ridge earthquake. The centroidal solution (strike 211°, dip 86°, rake 180°, and depth of 10 km below the seafloor) indicates shallow rupture with pure right-lateral strike-slip motion along the Pacific-Australia plate boundary, in agreement with motion predicted by plate tectonic models. The total seismic moment is 13.4x10²⁰ Nm, 80% of which was released in the first 24 s of the rupture process. Modeling favors a bilaterally propagating rupture with slightly different dip and rake for the northward and southward fault segments and similar moment release along both directions. The estimated fault length is quite short, about 90 km, and the derived stress drop of 180 bar and average displacement of 17 m are unusually high. The bathymetry in the epicentral region shows topographic segmentation of the ridge, possibly indicating fault segmentation which confines ruptures to short segments

The 1967 Caracas earthquake: fault geometry, direction of rupture propagation and seismotectonic implications

The fault plane orientation of the July 30, 1967, Caracas earthquake (Mw=6.6) has been a source of controversy for several years. This strike-slip event was originally thought to have occurred on an east-­west oriented fault plane, reflecting the relative motion between the Caribbean and South American plates. More recently, however, the complex seismic radiation from this event was interpreted as being indicative of a north-south striking fault that ruptured along three en echelon segments. In this study we synthesize evidence based on the intensity and damage reports, the distribution of aftershocks, and the results of a joint formal inversion of the P and SH waves and show that these data clearly indicate that the rupture of the 1967 earthquake occurred on an east-west trending fault system. Using a master event technique, the largest aftershock, which occurred 40 min after the main event, is shown to lie 50 km east of the epicenter of the mainshock. The epicentral distances of small aftershocks registered in Caracas, based on the S-P arrival time differences and the polarizations of the P waves, are also consistent with these events occurring on an east-west oriented fault system north of Caracas. A joint inversion of the teleseismic P and SH waves, recorded on long-period seismographs of the World-Wide Standardized Seismographic Network, shows that in a time frame of 65 s, four distinct bursts of seismic moment release (subevents) occurred, with a total seismic moment of 8.6 x 1018 Nm. The first three subevents triggered sequentially from west to east, in a direction that is almost identical to the east-west trending nodal planes of the source mechanisms. The average depth of these three subevents is 14 km. The fourth, and last identifiable, subevent of the sequence shows a reverse faulting mechanism with the nodal planes oriented roughly east­west. It occurred at a 21-km depth, about 50 km to the north of the fault zone defined by the strike-slip subevents. This fourth subevent appears to reflect compressional deformation of the southern Caribbean, possibly related to underthrusting along the proposed Curaçao trench. The complexity of the fault system causing the 1967 earthquake suggests that the relative motion along the Caribbean-South America plate boundary in central Venezuela is taken up over a broad, highly faulted, and highly stressed zone of deformation and not by a simple, major throughgoing fault

Seismicity and fault interaction, Southern San Jacinto Fault Zone and adjacent faults, southern California: Implications for seismic hazard

The southern San Jacinto fault zone is characterized by high seismicity and a complex fault pattern that offers an excellent setting for investigating interactions between distinct faults. This fault zone is roughly outlined by two subparallel master fault strands, the Coyote Creek and Clark-San Felipe Hills faults, that are located 2 to 10 km apart and are intersected by a series of secondary cross faults. Seismicity is intense on both master faults and secondary cross faults in the southern San Jacinto fault zone. The seismicity on the two master strands occurs primarily below 10 km; the upper 10 km of the master faults are now mostly quiescent and appear to rupture mainly or solely in large earthquakes. Our results also indicate that a considerable portion of recent background activity near the April 9, 1968, Borrego Mountain rupture zone (M_L=6.4) is located on secondary faults outside the fault zone. We name and describe the Palm Wash fault, a very active secondary structure located about 25 km northeast of Borrego Mountain that is oriented subparallel to the San Jacinto fault system, dips approximately 70° to the northeast, and accommodates right-lateral shear motion. The Vallecito Mountain cluster is another secondary feature delineated by the recent seismicity and is characterized by swarming activity prior to nearby large events on the master strand. The 1968 Borrego Mountain and the April 28, 1969, Coyote Mountain (M_L=5.8) events are examples of earthquakes with aftershocks and subevents on these secondary and master faults. Mechanisms from those earthquakes and recent seismic data for the period 1981 to 1986 are not simply restricted to strike-slip motion; dipslip motion is also indicated. Teleseismic body waves (long-period P and SH) of the 1968 and 1969 earthquakes were inverted simultaneously for source mechanism, seismic moment, rupture history, and centroid depth. The complicated waveforms of the 1968 event (M_o=1.2 × 10^(19) Nm) are interpreted in terms of two subevents; the first caused by right-lateral strike-slip motion in the mainshock along the Coyote Creek fault and the second by a rupture located about 25 km away from the master fault. Our waveform inversion of the 1969 event indicates that strike-slip motion predominated, releasing a seismic moment of 2.5 × 10^(17) Nm. Nevertheless, the right-lateral nodal plane of the focal mechanism is significantly misoriented (20°) with respect to the master fault, and hence the event is not likely to be associated with a rupture on that fault. From this and other examples in southern California, we conclude that cross faults may contribute significantly to seismic hazard and that interaction between faults has important implications for earthquake prediction

A seasonally modulated earthquake swarm near Maupin, Oregon

From December 2006 to November 2011, the Pacific Northwest Seismic Network (PNSN) reported 467 earthquakes in a swarm 60 km east of Mt Hood near the town of Maupin, Oregon. The swarm included 20 M[subscript D] ≥ 3.0 events, which account for over 80 per cent of the cumulative seismic moment release of the sequence. Relocation of 45 M[subscript D] ≥ 2.5 earthquakes and moment tensor analysis of nine 3.3 ≤ M[subscript w] ≤ 3.9 earthquakes reveals right-lateral strike-slip motion on a north-northwest trending, 70° west dipping, 1 km² active fault patch at about 17 km depth. The swarm started at the southern end of the patch and migrated to the northwest at an average rate of 1–2 m d⁻¹ during the first 18 months. Event migration was interrupted briefly in late 2007 when the swarm encountered a 10° fault bend acting as geometrical barrier. The slow migration rate suggests a pore pressure diffusion process. We speculate that the swarm was triggered by flow into the fault zone from upwards-migrating, subduction-derived fluids. Superimposed on the swarm is seasonal modulation of seismicity, with the highest rates in spring, which coincides with the maximum snow load in the nearby Cascade Mountains. The resulting surface load variation of about 4 × 10¹¹ N km⁻¹ arc length causes 1 cm annual vertical displacements at GPS sites in the Cascades and appears sufficient to modulate seismicity by varying normal stresses at the fault and fluid flow rates into the fault zone.This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by Oxford University Press on behalf of The Royal Astronomical Society. The published article can be found at: http://gji.oxfordjournals.org/.Keywords: Earthquake dynamics, Fractures and faults, Earthquake source observations, Continental tectonics: strike-slip and transform, Dynamics and mechanics of faultingKeywords: Earthquake dynamics, Fractures and faults, Earthquake source observations, Continental tectonics: strike-slip and transform, Dynamics and mechanics of faultin

Seismic crustal imaging using fin whale songs

Fin whale calls are among the strongest animal vocalizations that are detectable over great distances in the oceans. We analyze fin whale songs recorded at ocean-bottom seismometers in the northeast Pacific Ocean and show that in addition to the waterborne signal, the song recordings also contain signals reflected and refracted from crustal interfaces beneath the stations. With these data, we constrain the thickness and seismic velocity of the oceanic sediment and basaltic basement and the P-wave velocity of the gabbroic lower crust beneath and around the ocean bottom seismic stations. The abundant and globally available fin whale calls may be used to complement seismic studies in situations where conventional air-gun surveys are not available

Location and Source Parameters of the 19 June 1994 (\u3ci\u3eM\u3csub\u3eW\u3c/sub\u3e\u3c/i\u3e = 5.0) Offshore Petrolia, California, Earthquake

The MW = 5.0, 19 June 1994 offshore Petrolia, California, earthquake was well recorded by nine ocean-bottom hydrophones (OBH) and seismometers (OBS), providing an opportunity to precisely locate an earthquake in the tectonically active Mendocino triple junction region. Adding the offshore data improves the azimuthal station coverage and essentially removes the epicenter\u27s sensitivity to the choice of inversion parameters and velocity models. The hypocentral parameters, assuming an oceanic upper-mantle velocity of 7.9 km/sec, are 10:39:33.2 UTC for origin time, 40.376° N latitude, 124.441° W longitude, and a depth of 18.8 km. The moment-tensor solution obtained by modeling of low-frequency regional waveforms indicates predominantly strike-slip faulting with a north-south-trending P axis, as is typical for Gorda plate earthquakes, and confirms the depth estimate from the P-wave travel-time data

A Rapid Response Network to Record Aftershocks of the 2015 M 7.8 Gorkha Earthquake in Nepal

The Himalaya has experienced large damaging earthquakes over the past few centuries, most recently the damaging 25 April 2015 M 7.8 Gorkha earthquake in Nepal. Because of the continued earthquake risk presented by the continental collisional plate boundary at the Main Himalayan thrust and the high population densities in the region, collecting and processing data related to recent large earthquakes in this region is critically important for improving our understanding of the regional tectonics and earthquake hazard. Following the 2015 Gorkha earthquake, we deployed a National Science Foundation‐funded rapid‐response aftershock network known as the Nepal Array Measuring Aftershock Seismicity Trailing Earthquake network across the rupture area for 11 months beginning 7 weeks after the mainshock. The network consisted of 41 broadband and short‐period seismometers, and 14 strong‐motion sensors at 46 sites across eastern and central Nepal. The network spanned a region approximately 210 km along strike by 110 km across strike with a station spacing of 20–25 km. In this article, we report lessons learned from this deployment as well as details of the publicly accessible dataset including data recovery, data quality, and potential for future research

Mode of Slip and Crust–mantle Interaction at Oceanic Transform Faults

Oceanic transform faults, connecting offset mid-ocean spreading centres, rupture quasi-periodically in earthquakes up to about magnitude M 7.0 that are often preceded by foreshocks. In addition to seismic slip, a large portion of slip takes place as aseismic creep, which likely influences initiation of large earthquakes. Although oceanic transform faults are one of the major types of plate boundaries, the exact mode of slip and interaction between the seismic and aseismic motion remains unclear. Here we present a detailed model of the mode of slip at oceanic transform faults based on data acquired from a recent temporary deployment of ocean-bottom seismometers at the Blanco Transform Fault and existing regional and teleseismic observations. In the model, the crustal part of the fault is brittle and fully seismically coupled, while the fault in the mantle, shallower than the depth of the 600 °C isotherm, creeps partially and episodically. The creep activates small asperities in the mantle that produce seismic swarms. Both mantle and the crustal zones release most of the plate-motion strain during large-magnitude earthquakes. Large earthquakes appear to be preceded by a brief episode of shallow mantle creep, accompanied by seismic swarms, which explains the observation of foreshocks and shows that mantle creep likely influences initiation of large seismic events