Alan T. Linde

Triggering of volcanic eruptions

Although earthquakes and volcanic eruptions are each manifestations of large-scale tectonic plate and mantle motions, it is usually thought that the occurrences of these events are not directly related. There have been some studies, however, in which triggering of volcanic eruptions by earthquakes (remote from the volcano) has been proposed,. The 1992 Landers (southern California) earthquake caused triggered seismicity at very large distances, including the magmatically active Long Valley caldera region which also experienced a significant coincident deformation transient. Motivated by this demonstration of the ability of a distant earthquake to disturb a volcanic system, and the earlier studies of specific cases of eruption triggering, we examine here the historical record of eruptions and earthquakes to see if there are indeed significantly more eruptions immediately following large earthquakes. We find that within a day or two of large earthquakes there are many more eruptions within a range of 750 km than would otherwise be expected. Additionally, it is well known that volcanoes separated by hundreds of kilometres frequently erupt in unison; the characteristics of such eruption pairs are also consistent with the hypothesis that the second eruption is triggered by earthquakes associated with the first.

Fault failure with moderate earthquakes

Abstract High resolution strain and tilt recordings were made in the near-field of, and prior to, the May 1983 Coalinga earthquake (ML = 6.7, Δ = 51 km), the August 4, 1985, Kettleman Hills earthquake (ML = 5.5, Δ = 34 km), the April 1984 Morgan Hill earthquake (ML = 6.1, Δ = 55 km), the November 1984 Round Valley earthquake (ML = 5.8, Δ = 54 km), the January 14, 1978, Izu, Japan earthquake (ML = 7.0, Δ = 28 km), and several other smaller magnitude earthquakes. These recordings were made with near-surface instruments (resolution 10−8), with borehole dilatometers (resolution 10−10) and a 3-component borehole strainmeter (resolution 10−9). While observed coseismic offsets are generally in good agreement with expectations from elastic dislocation theory, and while post-seismic deformation continued, in some cases, with a moment comparable to that of the main shock, preseismic strain or tilt perturbations from hours to seconds (or less) before the main shock are not apparent above the present resolution. Precursory slip for these events, if any occurred, must have had a moment less than a few percent of that of the main event. To the extent that these records reflect general fault behavior, the strong constraint on the size and amount of slip triggering major rupture makes prediction of the onset times and final magnitudes of the rupture zones a difficult task unless the instruments are fortuitously installed near the rupture initiation point. These data are best explained by an inhomogeneous failure model for which various areas of the fault plane have either different stress-slip constitutive laws or spatially varying constitutive parameters. Other work on seismic waveform analysis and synthetic waveforms indicates that the rupturing process is inhomogeneous and controlled by points of higher strength. These models indicate that rupture initiation occurs at smaller regions of higher strength which, when broken, allow runaway catastrophic failure.

Slow earthquakes triggered by typhoons

The first reports on a slow earthquake were for an event in the Izu peninsula, Japan, on an intraplate, seismically active fault. Since then, many slow earthquakes have been detected. It has been suggested that the slow events may trigger ordinary earthquakes (in a context supported by numerical modelling), but their broader significance in terms of earthquake occurrence remains unclear. Triggering of earthquakes has received much attention: strain diffusion from large regional earthquakes has been shown to influence large earthquake activity, and earthquakes may be triggered during the passage of teleseismic waves, a phenomenon now recognized as being common. Here we show that, in eastern Taiwan, slow earthquakes can be triggered by typhoons. We model the largest of these earthquakes as repeated episodes of slow slip on a reverse fault just under land and dipping to the west; the characteristics of all events are sufficiently similar that they can be modelled with minor variations of the model parameters. Lower pressure results in a very small unclamping of the fault that must be close to the failure condition for the typhoon to act as a trigger. This area experiences very high compressional deformation but has a paucity of large earthquakes; repeating slow events may be segmenting the stressed area and thus inhibiting large earthquakes, which require a long, continuous seismic rupture.

Continuous Borehole Strain and Pore Pressure in the Near Field of the 28 September 2004 M 6.0 Parkfield, California, Earthquake: Implications for Nucleation, Fault Response, Earthquake Prediction, and Tremor

Near-field observations of high-precision borehole strain and pore pres- sure, show no indication of coherent accelerating strain or pore pressure during the weeks to seconds before the 28 September 2004 M 6.0 Parkfield earthquake. Minor changes in strain rate did occur at a few sites during the last 24 hr before the earth- quake but these changes are neither significant nor have the form expected for strain during slip coalescence initiating fault failure. Seconds before the event, strain is stable at the 10 � 11 level. Final prerupture nucleation slip in the hypocentral region is constrained to have a moment less than 2 � 10 12 Nm( M 2.2) and a source size less than 30 m. Ground displacement data indicate similar constraints. Localized rupture nucleation and runaway precludes useful prediction of damaging earthquakes. Coseismic dynamic strains of about 10 microstrain peak-to-peak were superimposed on volumetric strain offsets of about 0.5 microstrain to the northwest of the epicenter and about 0.2 microstrain to the southeast of the epicenter, consistent with right lateral slip. Observed strain and Global Positioning System (GPS) offsets can be simply fit with 20 cm of slip between 4 and 10 km on a 20-km segment of the fault north of Gold Hill (M 0 � 7 � 10 17 N m). Variable slip inversion models using GPS data and seismic data indicate similar moments. Observed postseismic strain is 60% to 300% of the coseismic strain, indicating incomplete release of accumulated strain. No measurable change in fault zone compliance preceding or following the earthquake is indicated by stable earth tidal response. No indications of strain change accompany nonvolcanic tremor events reported prior to and following the earthquake.

Improvement of Seismic Observation in the Ocean by Use of Seafloor Boreholes

We developed a long-term, high-quality seismic ocean floor borehole observatory system, the `Neath Seafloor Equipment for Recording Earth9s Internal Deformation (NEREID). Four NEREID borehole observatories were installed in the Japan Trench off-Sanriku area (JT1, JT2), in the northwestern Pacific Basin (WP2), and in the Philippine Sea (WP1). The borehole sensors are cemented in the borehole to assure good coupling of sensors to the ground as well as to avoid effects of water flow around the sensors, which may have been a problem in previous borehole installations. The NEREID seismic records from two of the observatories (JT1, WP2) were free from long-period noise due to turbulence in the seafloor boundary current or to water flowing around the sensor that is significant on the seafloor. The infragravity wave noise clearly observed around 0.01 Hz on the horizontal components was significantly higher in the JT1 seismometer in the sediment because of the low shear modulus of the sediment. Ocean waves of long wavelength cause the infragravity wave noise. It is thus necessary to install seismometers in boreholes below the sediments to reduce the infragravity wave noise.

Episodic aseismic earthquake precursors

Shallow earthquakes are generally believed to be brittle fractures in a stressed medium with rupture velocity at a speed close to that for shear waves. We know, however, that the Earth allows failure over a wide range of timescales. Creep events occur on the San Andreas fault system, and for some earthquakes the high-speed rupture is accompanied by a slower process1–5. Here we report on episodic slow events which occurred before the Japan Sea earthquake of magnitude 7.7 on 26 May 1983. A Sacks–Evertson borehole strain meter6 90 km from the earthquake recorded about 100 aseismic strain events in a five-month period before and immediately following the earthquake; none has been detected after the large aftershocks. The observed signals are consistent with a precursory redistribution of stress through aseismic slip on a deep extension of the main-shock fault plane. Such episodic aseismic events may provide a mechanism for relatively rapid stress concentration before earthquakes.

Slow earthquakes and great earthquakes along the Nankai trough

Abstract We have reexamined reports indicating that slow deformation occurred before the great Japan earthquakes of 1944 (Tonankai) and 1946 (Nankaido) and find that the observations are well founded. Although no quantitative models have previously been proposed to explain all of the relevant data we show that they are satisfied by a simple model for both earthquakes. The model, based on known properties of subduction zones, has slow slip on the subduction interface in an area deeper than the seismic rupture. If this model is correct and a similar physical situation holds for an anticipated Tokai earthquake, existing instruments will be able to reveal the pre-slip in real time. While differences among the deformation time series at different sites will provide strong constraints on the slow rupture propagation, these differences could result in delaying the recognition of a coherent event.

Analysis of deformation data at Parkfield, California: Detection of a long‐term strain transient

Analysis of more than a decade of high-quality data, particularly those from the two-color electronic distance meter (EDM), in the Parkfield, California, area reveals a significant transient in slip rate along the San Andreas Fault. This transient consists of an increase in fault slip rate of 3.3±0.9 mm/yr during 1993.0 to 1998.0. The most reliable fault creep instruments show a comparable increase in slip rate, suggesting that the deformation is localized to the fault which breaks the surface. There was also an increase in precipitation around 1993. It is unlikely, however, that this anomaly is due directly to hydrology, as its spatial distribution is what would be expected for increased slip on the San Andreas Fault. The increase in slip rate corresponds temporally to a dramatic increase in seismicity, including the four largest earthquakes in the period 1984–1999 that occurred along a 6-km segment of the fault just to the north of the EDM network. There was also a previously reported anomaly in borehole shear strain [Gwyther et al., 1996] that closely corresponds temporally to the transient in EDM data. Solely on the basis of EDM data the transient can be modeled as a slip event on a 10-km-long segment of the fault. The calculated shear strains from this model, however, are not consistent with the observed ones. A compatible model can be found if there is increased aseismic slip to the northwest in conjunction with the four earthquakes. Support for this northwestern slip is provided by a recent study of slip rate based on microearthquake activity. We speculate that this northwestern event served to load the fault to the southeast, with the stress being partially released by the observed slip.

Annual modulation of triggered seismicity following the 1992 Landers earthquake in California

The mechanism responsible for the triggering of earthquakes remains one of the least-understood aspects of the earthquake process. The magnitude-7.3 Landers, California earthquake of 28 June 1992 was followed for several weeks by triggered seismic activity over a large area, encompassing much of the western United States. Here we show that this triggered seismicity marked the beginning of a five-year trend, consisting of an elevated microearthquake rate that was modulated by an annual cycle, decaying with time. The annual cycle is mainly associated with several hydrothermal or volcanic regions where short-term triggering was also observed. These data indicate that the Landers earthquake produced long-term physical changes in these areas, and that an environmental source of stress—plausibly barometric pressure—might be responsible for the annual variation.

Prototype PBO instrumentation of CALIPSO project captures world-record lava dome collapse on Montserrat Volcano

This article is an update on the status of an innovative new project designed to enhance generally our understanding of andesitic volcano eruption dynamics and, specifically the monitoring and scientific infrastructure at the active Soufriere Hills Volcano (SHV), Montserrat. The project has been designated as the Caribbean Andesite Lava Island Precision Seismo-geodetic Observatory known as CALIPSO. Its purpose is to investigate the dynamics of the entire SHV magmatic system using an integrated array of specialized instruments in four strategically located ∼200-m-deep boreholes in concert with several shallower holes and surface sites. The project is unique, as it represents the first, and only such borehole volcano-monitoring array deployed at an andesitic stratovolcano.