Jeffrey J. McGuire

Foreshock sequences and short-term earthquake predictability on East Pacific Rise transform faults

East Pacific Rise transform faults are characterized by high slip rates (more than ten centimetres a year), predominately aseismic slip and maximum earthquake magnitudes of about 6.5. Using recordings from a hydroacoustic array deployed by the National Oceanic and Atmospheric Administration, we show here that East Pacific Rise transform faults also have a low number of aftershocks and high foreshock rates compared to continental strike-slip faults. The high ratio of foreshocks to aftershocks implies that such transform-fault seismicity cannot be explained by seismic triggering models in which there is no fundamental distinction between foreshocks, mainshocks and aftershocks. The foreshock sequences on East Pacific Rise transform faults can be used to predict (retrospectively) earthquakes of magnitude 5.4 or greater, in narrow spatial and temporal windows and with a high probability gain. The predictability of such transform earthquakes is consistent with a model in which slow slip transients trigger earthquakes, enrich their low-frequency radiation and accommodate much of the aseismic plate motion.

Predominance of Unilateral Rupture for a Global Catalog of Large Earthquakes

The manner in which an earthquake rupture propagates across a fault reflects both the initial properties of the fault and the dynamical stresses produced by the rupture. We quantify the propagation of an earthquake rupture using the second moments of the earthquake space-time distribution. In particular, the second moments provide a simple way to differentiate between approximately bilateral and predominantly unilateral ruptures. We determined the second moments for a catalog of M w ≥7 earthquakes that have occurred since 1994. The results show that approximately 80% of large shallow ruptures are predominantly unilateral. Our result agrees well with strong-motion inversions from previous studies on moderate and large earthquakes. The predominance of unilateral propagation in large earthquakes is explained by a simple characteristic earthquake model in which large events rupture an entire structurally defined fault segment and nucleation points are uniformly distributed along the fault. The predominance of unilateral rupture may also be enhanced by the dynamic stress field produced by contrasts in the elastic properties between the two sides of plate boundary faults. Manuscript received 16 November 2001.

Estimating Finite Source Properties of Small Earthquake Ruptures

Many of the most fundamental questions in earthquake science are currently limited by a lack of knowledge about small earthquake ruptures. Small earthquakes are difficult to study owing to the poor constraints placed on many of the interesting physical parameters by bandlimited, far-field, seismic data. Traditionally, dynamic models, such as an expanding circular crack, have been utilized to bridge the gap between the easily measurable quantities for small earthquakes and more interesting physical parameters such as stress drop and rupture velocity. Here I present a method for estimating the basic finite source properties of a rupture that is independent of any a priori model and utilizes the description of a finite source, the second moments, that far-field waves are inherently sensitive to. Application to two magnitude 5 events in southern California demonstrates the ability of an empirical Green9s function approach to estimating the second moments to resolve the fault-plane ambiguity, rupture length, and overall directivity. Additional results are presented for two example M 2.7 events from the creeping section of the San Andreas fault to examine the likely lower bound on event size that can be studied with surface seismometers. The creeping section earthquakes have very similar rupture areas but would be improperly interpreted as significantly different using the traditional methodology. One of these events presents a relatively clear interpretation of the velocity of rupture front propagation, which is about 0.8 of the Rayleigh speed, suggesting little difference in rupture velocity between it and typical large earthquakes.

Time-Domain Observations of a Slow Precursor to the 1994 Romanche Transform Earthquake

Low-frequency spectral anomalies have indicated that some large earthquakes are preceded by extended episodes of smooth moment release, but the reality of these slow precursors has been debated because they have not been directly observed in the time domain. High-gain seismograms from the 14 March 1994 Romanche Transform event (moment magnitude Mw 7.0) show a precursory ramp with a moment of 7 × 1018 newton meters beginning about 100 seconds before the arrival of the high-frequency P waves. This precursor was the initial phase of a slow component of slip that released nearly half of the total moment of the earthquake. Such behavior may be typical for large earthquakes on the oceanic ridge-transform system.

A Rogue Earthquake Off Sumatra

A magnitude 8.6 strike-slip earthquake within an oceanic plate raises fundamental questions about earthquake physics. The magnitude (Mw) 8.6 earthquake of 11 April 2012 off the coast of Sumatra is one for the record books. It is far and away the largest strike-slip earthquake in the instrumental record. The Mw 8.2 aftershock that occurred just over 2 hours later is also among the largest such earthquakes. Furthermore, the 11 April mainshock may be the largest intraplate earthquake ever recorded, although the location (see the figure) is consistent with the notion of a wide, diffuse plate boundary that bisects the Indo-Australian Plate near the Ninetyeast Ridge (1). The earthquakes are the latest in a series of large (Mw 8) intraplate strike-slip earthquakes in oceanic lithosphere (2). What do these earthquakes reveal about earthquake physics, and how might they change earthquake hazard assessment?

The March 9, 1994 (M w 7.6), deep Tonga earthquake: Rupture outside the seismically active slab

We investigate the rupture process of the March 9, 1994, M w 7.6 deep Tonga earthquake and its relationship to the background seismicity of the subducted Tonga slab. Variations in observed P and S wave pulse duration indicate that the rupture propagated to the NNE and extended well beyond the background seismicity. We inverted 47 P and SH waveforms, including regional broadband waveforms from the Southwest Pacific Seismic Experiment, using a method that solves for the focal mechanism change during the rupture and the distribution of moment release along the fault plane. The results indicate that significant moment release occurred in previously aseismic regions outside the active seismic zone and that the rupture terminated 10-20 km beyond the bounds of the previous seismic activity. A significant change in focal mechanism occurred when the rupture propagated into the previously aseismic region. Rupture along the near-vertical NNE striking nodal plane provides a somewhat better fit to the body waveforms than rupture along the near-horizontal nodal plane. This result, combined with the planar alignment of aftershocks and the general NNE directivity of the waveforms, provides strong evidence that the rupture occurred on the near-vertical plane. Thermal modeling of the Tonga slab indicates that the rupture terminated in material about 200°C warmer than the temperature that normally limits the occurrence of smaller earthquakes. Additionally, aftershocks seem to be suppressed in the outer regions of the rupture, which contain about half of the moment release but only 1 of the 15 well-located aftershocks. We suggest that slabs may be composed of an inner cold core, where seismic rupture initiates and small earthquakes occur, and a thermal halo of warmer material, which can sustain rupture and only a few aftershocks. The mechanism by which rupture propagates through the warmer material need not be similar to the process governing rupture nucleation in the cold slab core; nucleation may occur through a process limited to the cold core such as transformational faulting, and propagation through the warmer material may occur through ductile faulting or plastic instabilities. Isolated deep earthquakes in other subduction zones, such as the 1994 Bolivia event, may occur almost completely within the warmer zone, accounting for the lack of background seismicity and the dearth of aftershocks.

Greenland supraglacial lake drainages triggered by hydrologically induced basal slip

Water-driven fracture propagation beneath supraglacial lakes rapidly transports large volumes of surface meltwater to the base of the Greenland Ice Sheet. These drainage events drive transient ice-sheet acceleration and establish conduits for additional surface-to-bed meltwater transport for the remainder of the melt season. Although it is well established that cracks must remain water-filled to propagate to the bed, the precise mechanisms that initiate hydro-fracture events beneath lakes are unknown. Here we show that, for a lake on the western Greenland Ice Sheet, drainage events are preceded by a 6–12 hour period of ice-sheet uplift and/or enhanced basal slip. Our observations from a dense Global Positioning System (GPS) network allow us to determine the distribution of meltwater at the ice-sheet bed before, during, and after three rapid drainages in 2011–2013, each of which generates tensile stresses that promote hydro-fracture beneath the lake. We hypothesize that these precursors are associated with the introduction of meltwater to the bed through neighbouring moulin systems (vertical conduits connecting the surface and base of the ice sheet). Our results imply that as lakes form in less crevassed, interior regions of the ice sheet, where water at the bed is currently less pervasive, the creation of new surface-to-bed conduits caused by lake-draining hydro-fractures may be limited.

Influence of fore-arc structure on the extent of great subduction zone earthquakes

[1] Structural features associated with fore-arc basins appear to strongly influence the rupture processes of large subduction zone earthquakes. Recent studies demonstrated that a significant percentage of the global seismic moment release on subduction zone thrust faults is concentrated beneath the gravity lows resulting from fore-arc basins. To better determine the nature of this correlation and to examine its effect on rupture directivity and termination, we estimated the rupture areas of a set of Mw 7.5–8.7 earthquakes that occurred in circum-Pacific subduction zones. We compare synthetic and observed seismograms by measuring frequency-dependent amplitude and arrival time differences of the first orbit Rayleigh waves. At low frequencies, the amplitude anomalies primarily result from the spatial and temporal extent of the rupture. We then invert the amplitude and arrival time measurements to estimate the second moments of the slip distribution which describe the rupture length, width, duration, and propagation velocity of each earthquake. Comparing the rupture areas to the trench-parallel gravity anomaly (TPGA) above each rupture, we find that in 11 of the 15 events considered in this study the TPGA increases between the centroid and the limits of the rupture. Thus local increases in TPGA appear to be related to the physical conditions along the plate interface that favor rupture termination. Owing to the inherently long timescales required for forearc basin formation, the correlation between the TPGA field and rupture termination regions indicates that long-lived material heterogeneity rather than short timescale stress heterogeneities are responsible for arresting most great subduction zone ruptures.

A double seismic zone in New Britain and the morphology of the Solomon Plate at intermediate depths

We have studied the spatial configuration of earthquakes in the New Britain subduction zone by inverting regional and teleseismic P, PKP, and pP arrival times using a hypocentroidal decomposition method. A previously unreported intermediate depth double seismic zone exists along a small, 70 km long segment of the slab. In this region, centroid moment tensor mechanisms suggest that the upper zone of seismicity shows horizontal, along-strike compression and the lower zone shows downdip tension. This contrasts with most previously studied double seismic zones, which generally show downdip compression in the upper zone. We propose that along-strike, rather than downdip compressional stresses exist in the upper seismicity plane due to membrane and lateral bending stresses resulting from the shape of the New Britain trench, which undergoes a 40° change in strike between 148° and 152° longitude. The double seismic zone is bounded by a tear in the slab which accommodates the majority of the change in strike that the slab must undergo. These results suggest that double seismic zones can show very rapid lateral variations controlled by the local tectonics, and that along-strike membrane and bending stresses resulting from slab morphology can be significant in producing double seismic zones.

The 1994 Bolivia and Tonga events: Fundamentally different types of deep earthquakes?

The 1994 Bolivia and Tonga events show large differences in source and aftershock properties. The Bolivia earthquake, characterized by high stress drop, slow rupture propagation, and a weak aftershock sequence, appears similar to other large deep earthquakes that are isolated from active seismic zones, including the 1970 Colombia earthquake. In contrast, the 1994 Tonga event showed stress drop, rupture propagation velocities, and an aftershock sequence similar to that found for shallow earthquakes. The aftershock sequence of the 1994 Tonga event is many times stronger than has been observed for any other deep earthquake. A survey of deep earthquake aftershocks suggests that aftershock occurrence is correlated with the overall subduction zone magnitude-frequency relation (b-value). The largest deep events occur in isolated regions lacking smaller earthquakes, or in subduction zones characterized by anomalously low b-values. The 1994 Tonga event is an exception and is the largest deep earthquake to occur within an active subduction zone showing a large b-value, perhaps explaining the strong aftershock sequence. The Bolivia and Tonga events may represent end members of a population of deep earthquakes showing exceptional diversity in source properties, or they may represent two fundamentally different types of deep earthquakes, perhaps resulting from different physical mechanisms.