Philippe Jousset

Modelling the time-dependent frequency content of low-frequency volcanic earthquakes

Abstract Low-frequency volcanic earthquakes and tremor have been observed on seismic networks at a number of volcanoes, including Soufriere Hills volcano on Montserrat. Single events have well known characteristics, including a long duration (several seconds) and harmonic spectral peaks (0.2–5 Hz). They are commonly observed in swarms, and can be highly repetitive both in waveforms and amplitude spectra. As the time delay between them decreases, they merge into tremor, often preceding critical volcanic events like dome collapses or explosions. Observed amplitude spectrograms of long-period volcanic earthquake swarms may display gliding lines which reflect a time dependence in the frequency content. Using a magma-filled dyke embedded in a solid homogeneous half-space as a simplified volcanic structure, we employ a 2D finite-difference method to compute the propagation of seismic waves in the conduit and its vicinity. We successfully replicate the seismic wave field of a single low-frequency event, as well as the occurrence of events in swarms, their highly repetitive characteristics, and the time dependence of their spectral content. We use our model to demonstrate that there are two modes of conduit resonance, leading to two types of interface waves which are recorded at the free surface as surface waves. We also demonstrate that reflections from the top and the bottom of a conduit act as secondary sources that are recorded at the surface as repetitive low-frequency events with similar waveforms. We further expand our modelling to account for gradients in physical properties across the magma–solid interface. We also expand it to account for time dependence of magma properties, which we implement by changing physical properties within the conduit during numerical computation of wave propagation. We use our expanded model to investigate the amplitude and time scales required for modelling gliding lines, and show that changes in magma properties, particularly changes in the bubble nucleation level, provide a plausible mechanism for the frequency variation in amplitude spectrograms.

Elastic models for the magma intrusion associated with the 2000 eruption of Usu Volcano, Hokkaido, Japan

Abstract After 23 years of dormancy, Usu Volcano (Hokkaido, Japan) erupted on March 31, 2000. Many observations (seismicity, deformation rates, gravity data, groundwater level monitoring) show that the period of intense activity was short, starting abruptly, and continuing for ca. 5 months with a decreasing rate. Uplift was observed at two successive and separate locations at the time of the eruption. We obtained GPS and microgravity data at Usu Volcano for two intervals, the first from August 1996 to July 1998, once every 2–4 months, and the second in November 2000, 2 months after the end of the eruption. Between July 1998 and November 2000, the displacements and gravity variations are among the largest ever recorded on an active volcano in association with an eruption. We review three different elastic models commonly used in volcano-geodesy (sphere, fault system, fissure zone) and invert the high-quality data using each of these models. The combined inversion of GPS and microgravity data leads to the best solution in the least-squares sense. It is compatible with the intrusion of approximately 5×10 11 kg of new magma into the western part of Usu Volcano. This appears to have occurred in a subvertical fracture zone (about 2.4 km length, 0.1 km width) aligned in the east–west direction. The fracture zone is between 0.4 and 3.3 km depth with an extension of about 30 m. The fractures are likely to be filled with material having a density slightly higher than the density of old products of Mount Usu, i.e. about 2400 kg m −3 . This model is consistent with the locations and magnitudes of the earthquakes recorded during the period of intense seismic activity in April and May 2000. These earthquakes correspond to the boundaries of the intruded magma body. The model suggests that the two locations of uplift are not independent.

Relative Gravity Measurement Campaign during the 7th International Comparison of Absolute Gravimeters (2005)

Since the 1st International Comparison of Absolute Gravimeters (ICAG) and accompanying Relative Gravity Campaign (RGC) held at the BIPM in 1981, repeated ICAG-RGCs have been organized every four years. A total of 19 absolute gravimeters (AG) and 15 relative gravimeters (RG) participated in the 7th ICAG-RGC, which took place in 2005. Co-located absolute and relative gravity measurements as well as precision levelling measurements were carried out. The final version of the absolute g values of the 7th ICAG has been officially released recently. This paper is the final report of the 7th RGC and replaces the preliminary results published earlier. It covers the organization of the RGC and the data processing, analyses RG behaviour, computes g, δg and OAG (offset of AG) and discusses their uncertainties. In preparation for the BIPM key comparison ICAG-2009, a standard data-processing procedure has been developed and installed in the BIPM ICAG-RGC software package, GraviSoft. This was used for the final data processing.

Performance of two Scintrex CG3M instruments at the fourth international comparison of absolute gravimeters

Two Scintrex CG3M gravimeters were calibrated and compared with other relative meters (LaCoste-Romberg and Scintrex meters) at the fourth International Comparison of Absolute Gravimeters at S?vres, France (30 May to 2 June 1994). LaCoste-Romberg meters were used as reference. Three main experiments were carried out at the Bureau International des Poids et Mesures. First, a calibration of the two Scintrex CG3M meters on a new baseline of five points, spanning a range of about 8 mGal. It is shown that both Scintrex meters provide results similar to within 0,005 mGal, even in noisy places, and that Scintrex results are similar to those of LaCoste-Romberg instruments to within an accuracy better than 0,010 mGal. Second, measurements of vertical gradient at four points were carried out. The results for the Scintrex meters lie within 0,007 mGal/m and are similar to those of LaCoste-Romberg meters to within an accuracy of around 0,010 mGal/m. Third, a series of continuous records (each of about 10 minutes) was carried out with three LaCoste-Romberg and the Scintrex meters at four adjacent points. A repeatability of better than 0,005 mGal was obtained for one Scintrex meter. There is a difference of 0,010 mGal with LaCoste-Romberg data. These results confirm that the Scintrex meter is suitable for measuring small gravity differences, similar to those observed on active volcanoes.

Microgravimetric measurements at the 1994 International Comparison of Absolute Gravimeters

On the occasion of the fourth International Comparison of Absolute Gravimeters carried out at Sevres in 1994 an extensive series of microgravimetric measurements was organized. In total, fifteen LaCoste, four Scintrex CG-3M and one Sodin gravimeter measured, within a small network, vertical gravity gradients and a calibration baseline. The results show that the accuracy for single instruments is in the range of 3 μGal to 5 μGal in gravity difference, for the Scintrex and the LaCoste meters. Data from the series were also used to intercompare different ways of calibrating the gravimeter electrostatic feedback systems. The calibration platform of the Institut fur Angewandte Geodasie, (IFAG), Frankfurt, and the calibration lift of the Observatoire Royal de Belgique (ORB) were installed at Sevres and the results compared with those for the calibration line. This paper gives the first results and a review of the techniques used.

Post-eruptive volcanic dome evolution as revealed by deformation and microgravity observations at Usu volcano (Hokkaido, Japan)

Abstract Usu volcano (Hokkaido, Japan) is a dacitic volcano, known for its high production rate of lava domes and crypto-domes. It is thus a good target to study processes of volcanic dome evolution (upheaval and/or relaxation). We carried out repeated GPS and microgravity surveys on the three most recent domes of Mt. Usu (1910: Meiji Shinzan; 1943–1945: Showa-Shinzan and 1977–1982: Usu-Shinzan). The repeat period was 1 to 2 months and extended from October 1996 to June 1997. We also compare new data with results from former studies. More than 20 years after the start of Usu-Shinzan dome growth, there is still subsidence at a maximum rate of about 7 to 8 cm/year. The reasons for this subsidence are discussed. Repeated gravity surveys revealed an increase of gravity on the domes (about 60±10 microgal/year for Usu-Shinzan, about 15 microgal at Showa-Shinzan and 10 to 20 microgal for Meiji-shinzan); this gravity increase exceeds that expected due to subsidence. We discuss and interpret the excess gravity change in terms of a density increase in the edifice, caused by a combination of processes (contraction of the edifice, water level change, devesiculisation, cooling and magma intrusion). Quantification of these processes at Usu volcano may help to understand the processes of evolution at domes on other volcanoes such as Merapi (Indonesia), Unzen (Japan) or Montserrat (West Indies).

On the gravimetric contribution to watt balance experiments

It has been recommended that the relative standard uncertainty of the numerical value of the Planck constant required for the redefinition of the kilogram should not exceed 2???10?8. To reach this goal using experiments based on a watt balance, the free-fall acceleration (g) traceable to the SI, at a given point and a given time, needs to be known with a sufficiently small uncertainty well below 2???10?8. Reducing the uncertainty in g allows the other uncertainties related to the watt balance to be increased. Instead of a simultaneous operation of an absolute gravimeter with a watt balance, we propose an alternative approach and demonstrate that a standard uncertainty below 5??Gal (relative uncertainty of 5???10?9) is reachable under the conditions at BIPM. Further decreasing the uncertainty could significantly increase commitments in terms of personnel and equipment and would not significantly improve the uncertainty targeted for the BIPM watt balance experiment. A 5??Gal uncertainty might also satisfy the needs of other watt balance experiments underway or planned. In our approach we combine the following information: (1) the Key Comparison Reference Values obtained from the CCM.G-K1, a key comparison carried out in the frame of the International Comparison of Absolute Gravimeters in 2009 (ICAG2009); (2) the accurate gravity network established using the qualified absolute and relative gravimeters; (3) temporal gravity variations based on observed Earth-tide parameters and modelled effects of polar motion and atmospheric mass redistribution; (4) uncertainty estimates that account for non-modelled effects; (5) the option to carry out absolute gravity measurements once every one or two years with two or more gravimeters for monitoring the stability of the gravity field at the BIPM.

Accurate Gravimetry at the BIPM Watt Balance Site

Associated with the 8th International Comparison of Absolute Gravimeters (ICAG) at the Bureau International des Poids et Mesures (BIPM) in 2009, accurate gravity measurements were made to support the BIPM watt balance (WB) project, aiming at determining the Planck constant h for the future realization of a new definition of the kilogram based on the Planck constant and the accurate g values. The ICAG 2009 was organized as a metrological Key Comparison (KC) as defined by the Committee International des Poids et Mesures (CIPM) Mutual Recognition Arrangement (MRA). Its results will thus constitute a precise and consistent gravity reference in SI units which then can be used as the global basis for geodetic, geophysical and metrological observations of gravity. Four absolute and six relative gravimeters took part in the WB gravity campaign. Their results can therefore be converted to the international reference of the KC results, i.e., they are SI-traceable. The WB network is a 3-D grid over the site in the WB laboratory. In order to monitor the stability of the sites, repeated precise leveling has being carried out. In addition, a set of accurate Earth tide parameters were determined based on six months of gravimetric tidal observations. They will enable the tidal prediction, which is needed in order to use gravity values to produce the momentary values of the acceleration of free fall that are required by the WB experiment. We present in this paper the organization, the measurements, the data processing and the preliminary results.

High precision levelling supporting the International Comparison of Absolute Gravimeters

High precision levelling is an indispensable method used to monitor benchmark and terrain stability at the Bureau International des Poids et Mesures (BIPM). Associated with the International Comparison of Absolute Gravimeters (ICAG), levelling measurements were carried out repeatedly over the past decades. A local gravity field strongly depends on vertical terrain deformation. 1?cm displacement implies about 2??Gal change in the vertical gravity acceleration. The precision of absolute and relative gravimetry nowadays is about 1??Gal.At the beginning of the century, a strategy was outlined that the ICAG should be upgraded to a metrological Key Comparison of the CIPM MRA (Mutual Recognition Arrangement) recognized officially by the designated governmental organizations. As a result of this decision, BIPM site B was constructed and completed in Spring 2001. The site B pillar is 4?m ? 6?m ? 1.5?m in dimension and more than 80?ton in weight. Such a large, newly built concrete body produces local deformation due to its sinking or tilting. This in turn influences the local gravity field. Rigorous levelling measurements have been performed by the Bureau de Recherches G?ologiques et Mini?res (BRGM), France, since 2001 and repeated together with the 4-year ICAGs of 2001, 2005 and 2009. ICAG-2009 was characterized by becoming the first Comit? International des Poids et Mesures (CIPM) Key Comparison which supported meanwhile the BIPM watt balance (WB) project. The WB pillar was built in Spring 2009. The Institut G?ographique National (IGN), France, has hence been invited to participate in the levelling. The latter also measured the link between the BIPM local network and external stations of the French national height reference system IGN69.In this paper, we report the final results of the levelling programmes of 2001, 2005 and 2009 and compare the results. We investigate stability of the BIPM gravity-levelling stations. We conclude that the existing and the newly built stations can be considered as stable for the purposes of the ICAGs and the WB, although further repeat measurement may be required for the WB site.For the first time, detailed and complete levelling data and results are published. After 30 years of organizing and holding eight ICAGs, the BIPM will hand over the KC ICAG-2013 to another CIPM MRA designated institute. This paper serves as a technical and historical report of precision levelling, a sub-task of the ICAGs.

Multi-station Analysis of Surface Wave Dispersion Using Distributed Acoustic Fiber Optic Sensing

Multi-station analysis of surface wave dispersion to evaluate the near subsurface has been used in geotechnical applications for more than 15 years. A fiber optic cable used as array of distributed horizontal component seismic receivers for the analysis of surface wave dispersion is used in an extended setup here by the recording of active hammer blows. Their first analyses reveal low-frequency signal recordings with good continuity that can be used for further processing.