Jean-Luc Got

Streaks of microearthquakes along creeping faults

Crustal faults that produce most of their slip aseismically typically generate large numbers of small earthquakes. These events have generally been interpreted as coming from localized patches of the fault that undergo unstable (stick–slip) sliding, surrounded by larger regions of stable sliding (creep). In published catalogues the microearthquakes often appear to be distributed over large portions of the fault surface. By accurately locating large numbers of microearthquakes from faults of different orientations in California and Hawaii, we show here that instead the locations define highly concentrated streaks that are characteristically aligned in the direction of fault slip. The underlying cause of this structural organization of the fault surface remains to be determined.

A reinterpretation of seismicity associated with the January 1983 dike intrusion at Kilauea Volcano, Hawaii

In January 1983, a dike intrusion/fissure eruption generated a swarm of 375 magnitude 1 to 3 earthquakes along a 16-km segment of Kilaueas Middle East Rift Zone. We searched the Hawaiian Volcano Observatory catalog for multiplets of similar events from this region from 1980 through 1985 and obtained precise relative locations by waveform cross correlation. Over 150 of the intrusion earthquakes could be grouped into 14 multiplets of five or more events with sufficient similarity for accurate relocation. Some multiplets were active for only a few minutes during the downrift migration phase of the seismic swarm, consistent with generation near the propagating dike tip, while others were active for several days. The two multiplets nearest the origin of the seismic swarm include events from the preceding days and months. Most multiplets span only 50 to 100 m following relocation, are located at about 3 to 4 km depth, and appear to deepen downrift. The catalog depths of those earthquakes in multiplets and those not in multiplets are similar, suggesting that most of the recorded seismicity may have come from a very limited depth interval despite the fact that the dike breached the surface. By analogy with a mechanical model used to explain a similar clustering of background seismicity in the Upper East Rift in 1991, we infer that the earthquakes are generated in regions of high stress concentration immediately above Kilaueas deforming deep rift body. This conclusion is consistent with the depth of the top of the deep rift body inferred from geodetic data and with numerical calculations suggesting that a significant ambient differential stress is required for dikes to produce earthquakes larger than magnitude 1.

Seismicity and deformation induced by magma accumulation at three basaltic volcanoes

We analyzed the evolution of volcano-tectonic (VT) seismicity and deformation at three basaltic volcanoes (Kilauea, Mauna Loa, Piton de la Fournaise) during phases of magma accumulation. We observed that the VT earthquake activity displays an accelerating evolution at the three studied volcanoes during the time of magma accumulation. At the same times, deformation rates recorded at the summit of Kilauea and Mauna Loa volcanoes were not accelerating but rather tend to decay. To interpret these observations, we propose a physical model describing the evolution of pressure produced by the accumulation of magma into a reservoir. This variation of pressure is then used to force a simple model of damage, where damage episodes are equivalent to earthquakes. This model leads to an exponential increase of the VT activity and to an exponential decay of the deformation rate during accumulation phases. Seismicity and deformation data are well fitted by such an exponential model. The time constant, deduced from the exponential increase of the seismicity, is in agreement with the time constant predicted by the model of magma accumulation. This VT activity can thus be a direct indication of the accumulation of magma at depth, and therefore can be seen as a long-term precursory phenomenon, at least for the three studied basaltic volcanoes. Unfortunately, it does not allow the prediction of the onset of future eruptions, as no diverging point (i.e., critical time) is present in the model.

Changes in effective stress during the 2003―2004 Ubaye seismic swarm, France

[1] We study changes in effective stress (normal stress minus pore pressure) that occurred in the French Alps during the 2003–2004 Ubaye earthquake swarm. Two complementary data sets are used. First, a set of 974 relocated events allows us to finely characterize the shape of the seismogenic area and the spatial migration of seismicity during the crisis. Relocations are performed by a double‐difference algorithm. We compute differences in travel times at stations both from absolute picking times and from cross‐correlation delays of multiplets. The resulting catalog reveals a swarm alignment along a single planar structure striking N130°E and dipping 80°W. This relocated activity displays migration properties consistent with a triggering by a diffusive fluid overpressure front. This observation argues in favor of a deep‐seated fluid circulation responsible for a significant part of the seismic activity in Ubaye. Second, we analyze time series of earthquake detections at a single seismological station located just above the swarm. This time series forms a dense chronicle of +16,000 events. We use it to estimate the history of effective stress changes during this sequence. For this purpose we model the rate of events by a stochastic epidemic‐type aftershock sequence model with a nonstationary background seismic rate l0(t). This background rate is estimated in discrete time windows. Window lengths are determined optimally according to a new change‐point method on the basis of the interevent times distribution. We propose that background events are triggered directly by a transient fluid circulation at depth. Then, using rate‐and‐state constitutive friction laws, we estimate changes in effective stress for the observed rate of background events. We assume that changes in effective stress occurred under constant shear stressing rate conditions. We finally obtain a maximum change in effective stress close to −8 MPa, which corresponds to a maximum fluid overpressure of about 8 MPa under constant normal stress conditions. This estimate is in good agreement with values obtained from numerical modeling of fluid flow at depth, or with direct measurements reported from fluid injection experiments.

A damage model for volcanic edifices: Implications for edifice strength, magma pressure, and eruptive processes

Monitoring of large basaltic volcanoes, such as Piton de la Fournaise (La Reunion Island, France), has revealed preeruptive accelerations in surface displacements and seismicity rate over a period of between 1 h and several weeks before magma reaches the surface. Such eruptions are attributed to ruptures of pressurized magma reservoirs. Elastic models used to describe surface deformation would assume that accelerations in surface deformation are due to increases in reservoir pressure. This assumption requires changes in magma or pressure conditions at the base of the magma feeding system that are unrealistic over the observed timescale. Another possible cause for these accelerations is magma pressure in the reservoir weakening the volcanic edifice. In the present study, we modeled such weakening by progressive damage to an initially elastic edifice. We used an incremental damage model, with seismicity as a damage variable with daily increments. Elastic moduli decrease linearly with each damage increment. Applied to an initially elastic edifice with constant pressure at the base of the system, this damage model reproduces surface displacement accelerations quite well when damage is sufficient. Process dynamics is controlled by the damage parameter, taken as the ratio between the incremental rupture surface and the surface to be ruptured. In this case, edifice strength and magma reservoir pressure decrease with decreasing elastic moduli, whereas surface displacement accelerates. We discuss the consequences of pressure decreases in magma reservoirs.

Deformation and rupture of the oceanic crust may control growth of Hawaiian volcanoes

Hawaiian volcanoes are formed by the eruption of large quantities of basaltic magma related to hot-spot activity below the Pacific Plate. Despite the apparent simplicity of the parent process—emission of magma onto the oceanic crust—the resulting edifices display some topographic complexity. Certain features, such as rift zones and large flank slides, are common to all Hawaiian volcanoes, indicating similarities in their genesis; however, the underlying mechanism controlling this process remains unknown. Here we use seismological investigations and finite-element mechanical modelling to show that the load exerted by large Hawaiian volcanoes can be sufficient to rupture the oceanic crust. This intense deformation, combined with the accelerated subsidence of the oceanic crust and the weakness of the volcanic edifice/oceanic crust interface, may control the surface morphology of Hawaiian volcanoes, especially the existence of their giant flank instabilities. Further studies are needed to determine whether such processes occur in other active intraplate volcanoes.

Anisotropic scattering and travel time delay analysis in Kilauea volcano, Hawaii, earthquake coda waves

We study the S wave coda of microearthquake multiplets (earthquakes showing similar seismograms) recorded on the Kilauea volcano, and we analyze the direction of emission for wavelets composing the coda. Our data set consists of 71 small magnitude, well-relocated, clustered events that occurred in an area of about 1 km2 in the southern flank of Kilauea, recorded at hypocentral distances of 10 to 20 km. For each station, we apply a cross-spectral moving-window technique to each pair of multiplet records to estimate the travel time delays due to source location offsets. These delays are computed for consecutive windows, starting at the P wave arrival time and ending at 4 times the S wave travel time. They are next used to estimate the slowness vector for each window along the seismograms. Our analysis shows (1) that scattered waves are emitted anisotropically along some preferred directions as late as 4 times the S wave travel time, and (2) that for some stations, the azimuthal plane rotates during the seismogram with a clear travel time delay signature. Our analysis shows that travel time delay variation in the coda can be purely due to geometrical propagation effects. This conclusion is of special interest when trying to use travel time delays to infer temporal changes in the crust. Our results do not support a coda diffracted isotropically in the crust, in the magnitude range and at the hypocentral distances we investigated.

Long‐term mass transfer at Piton de la Fournaise volcano evidenced by strain distribution derived from GNSS network

Basaltic volcanoes are among the largest volcanic edifices on the Earth. These huge volcanoes exhibit rift zones and mobile flanks, revealing specific stress field conditions. In this paper, we present new deformation data issued from the Global Navigation Satellite Systems (GNSS) network installed on Piton de la Fournaise. Density of the GNSS stations allowed us to reach a sufficient resolution to perform a spatially significant analysis of strain at the scale of the active part of the volcano. Since 2007, summit inflation during preeruptive/eruptive sequences (summit extension/cone flanks contraction) alternates with summit deflation during posteruptive/rest periods (summit contraction/cone flanks extension) and generates a “pulsation” of the volcano. This volcano “pulsation” increases rock fracturing and damage, decreases the rock stiffness, and increases the medium permeability. The deformation regime of the mobile eastern flank evidences mass transfer in depth from the summit to the east. During the long-term summit deflation recorded between 2011 and 2014, the upper eastern flank extended steadily eastward whereas the lower eastern flank contracted. Simultaneous extension and eastward displacement of the upper eastern flank and eastward contraction of the middle and lower eastern flank contributes to build the Grandes Pentes relief, steeping the topographic slope. We relate the eastern flank topographic slope spatial variations to rock or basal friction angle changes. The lower flank contraction process is an evidence of its progressive loading by the upper eastern flank, which brings this flank closer to an eventual instability.

Changes in seismicity and stress loading on subduction faults in the Kanto region, Japan, 2011–2014

Seismic activity has increased in the Kanto region, Japan, following the 2011 M9.0 Tohoku earthquake. We here reassess this increase up to June 2014, to show that normal, Omori-like relaxation characterizes the activity on crustal faults as well as on the Philippine Sea plate, but not on the deeper Pacific plate. There repeating earthquakes display a twofold rate of occurrence (still ongoing in June 2014) as compared to the pre-Tohoku rate, suggesting enhanced creep. We compute the Coulomb stress changes on the upper locked portion of the Philippine Sea plate, which last ruptured in 1923. We find that this fault was little affected by either the coseismic, the postseismic, the accelerated creep, or the 2011 Boso silent slip event.

A Real-Time Procedure for Progressive Multiplet Relative Relocation at the Hawaiian Volcano Observatory

A real-time procedure has been set up to select and relocate relatively similar events recorded by the U.S. Geological Survey Hawaiian Volcano Observatory (HVO, Hawaii) seismic network at the Observatory. It allows the relocation of such similar events with an uncertainty whose order of magnitude is 50 m horizontally and 100 m vertically. As a first step, a systematic exploration of the volcano to find similar events within the seismicity recorded between years 1988 and 2000 has been undertaken and allows events to be gathered into large multiplets. A generalized relative relocation of these similar events has been performed, and a database has been built from the result of this relocation. Incoming earthquakes are routinely compared to this database and relocated, and the database is updated. Preliminary results are presented and discussed. Manuscript received 21 May 2001.