Olivier Lengliné

Extending Earthquakes' Reach Through Cascading

Earthquakes, whatever their size, can trigger other earthquakes. Mainshocks cause aftershocks to occur, which in turn activate their own local aftershock sequences, resulting in a cascade of triggering that extends the reach of the initial mainshock. A long-lasting difficulty is to determine which earthquakes are connected, either directly or indirectly. Here we show that this causal structure can be found probabilistically, with no a priori model nor parameterization. Large regional earthquakes are found to have a short direct influence in comparison to the overall aftershock sequence duration. Relative to these large mainshocks, small earthquakes collectively have a greater effect on triggering. Hence, cascade triggering is a key component in earthquake interactions.

A new estimation of the decay of aftershock density with distance to the mainshock

[1] We investigate how aftershocks are spatially distributed relative to the mainshock. Compared to previous studies, ours focuses on earthquakes causally related to the mainshock rather than on aftershocks of previous aftershocks. We show that this distinction can be made objectively but becomes uncertain at long time scales and large distances. Analyzing a regional earthquake data set, it is found that, at time t following a mainshock of magnitude m, the probability of finding an aftershock at distance r relative to the mainshock fault decays as r −g , where g is typically between 1.7 and 2.1 for 3 ≤ m <6 and is independent of m, for r less than 10 to 20 km and t less than 1 day. Uncertainties on this probability at larger r and t do not allow for a correct estimation of this spatial decay. We further show that a static stress model coupled with a rate‐and‐state friction model predicts a similar decay, with an exponent g = 2.2, in the same space and time intervals. This suggests that static stress changes could explain the repartition of aftershocks around the mainshock even at distances larger than 10 times the rupture length. Citation: Marsan, D., and O. Lengline (2010), A new estimation of the decay of aftershock density with distance to the mainshock, J. Geophys. Res., 115, B09302, doi:10.1029/2009JB007119.

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.

Fluid‐induced earthquakes with variable stress drop

The static stress drop of an earthquake, which quantifies the ratio of seismic slip to the size of the rupture, is almost constant over several orders of magnitudes. Although variations are often observed, it is difficult, however, to attribute these variations either to a well-defined phenomenon or simply to measurement uncertainty. In this study we analyze the static stress drop of earthquakes that occurred during a water circulation test in the Soultz-sous-Forets, France, geothermal reservoir in 2010. During this circulation test, 411 earthquakes were recorded, the largest event having a magnitude MD2.3. We show that several earthquakes in the reservoir can be combined into groups of closely located similar repeating waveforms. We infer that the amplitudes, and hence magnitudes, vary between the repeaters although the waveforms and spectra are both similar in shape. We measure similar corner frequencies for these events despite their different magnitudes, suggesting a similar rupture size. Our results imply that events at the same location may exhibit stress drop variations by as much as a factor of 300. We interpret that this variation in stress drop is caused by fluid pressure at the interface reducing the normal stress. We also hypothesize that the observed variations reflect a transition from stable to unstable slip on the imaged asperities.

Seismicity distribution and locking depth along the Main Marmara Fault, Turkey

The seismicity along the Main Marmara Fault (MMF) below the Marmara Sea is analyzed during the 2007–2012 period to provide insights on the recent evolution of this important regional seismic gap. High precision locations show that seismicity is strongly varying along strike and depth providing fine details of the fault behavior that are inaccessible from geodetic observations. The activity strongly clusters at the regions of transition between basins. The Central basin shows significant seismicity located below the shallow locking depth inferred from GPS measurements. Its b-value is low and the average seismic slip is high. All observations are consistent with a deep creep of this segment. On the contrary, the Kumburgaz basin at the center of the fault shows sparse seismicity with the hallmarks of a locked segment. In the eastern Marmara Sea, the seismicity distribution along the Princes Island segment in the Cinarcik basin, is consistent with the geodetic locking depth of 10 km and a low contribution to the regional seismic energy release. The assessment of the locked segment areas provide an estimate of the magnitude of the main forthcoming event to be about 7.3 assuming that the rupture will not enter significantly within creeping domains.

Uncovering the hidden signature of a magmatic recharge at Piton de la Fournaise volcano using small earthquakes

We apply a template matching method to detect and locate preeruptive earthquakes at Piton de la Fournaise volcano in 2014 and 2015. This approach enabled the detection of many events and unveiled persistent seismicity features through multiple eruptions. Shallow earthquakes define a ring-shaped structure beneath the main crater. The repetitive occurrence of events along this structure suggests that it corresponds to a preexisting zone of weakness within the edifice. We also show evidence of deep magma transfer in 2015. More than 5000 deep earthquakes define an upward migration immediately followed by the occurrence of shallow events leading to an eruption 20 days later. This suggests the creation of a hydraulic connection between the lower part of the volcanic system and a magma reservoir located near sea level. We can envisage than such replenishments of the shallow reservoir occurred in the past but were undetected because of limited deep earthquake detections.

Long-lasting seismic repeaters in the Central Basin of the Main Marmara Fault†

The Main Marmara Fault (MMF) which crosses the whole Marmara Sea is a significant seismic gap along the North Anatolian Fault. Here we show that nine long-lasting strike-slip seismic repeaters exist below the Central basin within the seismogenic zone, in a 10 km region where deep creep was previously suggested from the analysis of the local seismicity. The typical recurrence time is 8 months during the 2008-2015 period. The cumulative slip of the repeating sequence appears to be compatible with the regional geodetic slip rate if they are assumed to be part of a large single asperity (10 km). The repeaters also exhibit short term crises and are possibly related to bursts of creep.

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.

Induced seismicity in EGS reservoir: the creep route

BackgroundObservations in enhanced geothermal system (EGS) reservoirs of induced seismicity and slow aseismic slip ruptures on related faults suggest a close link between the two phenomena.MethodsWe base our approach on the case study of the EGS site of Soultz-sous-Forêts where seismicity has been shown in particular during the 1993 stimulation to be induced not only by fluid pressure increase during stimulation but also by aseismic creeping effects. We propose an interpretation of the field observations of induced seismicity using a laboratory experiment that explores, in great detail, the deformation processes of heterogeneous interfaces in the brittle-creep regime. We track the evolution of an interfacial crack over 7 orders of magnitude in time and 5 orders of magnitude in space using optical and acoustic sensors.ResultsWe show that a creep route for induced seismicity is possible when heterogeneities exist along the fault. Indeed, seismic event occurrences in time and space are in strong relation with the development of the aseismic motion recorded during the experiments. We also infer the statistical properties of the organization of the seismicity that shows strong space-time clustering.ConclusionsWe conclude that aseismic processes might drive seismicity besides the classical effects related to fluid pressure and show that a creep route for induced seismicity is possible.

Insights on earthquake triggering processes from early aftershocks of repeating microearthquakes

Characterizing the evolution of seismicity rate of early aftershocks can yield important information about earthquake nucleation and triggering. However, this task is challenging because early aftershock seismic signals are obscured by those of the mainshock. Previous studies of early aftershocks employed high-pass filtering and template matching but had limited performance and completeness at very short times. Here we take advantage of repeating events previously identified on the San Andreas Fault at Parkfield and apply empirical Greens function deconvolution techniques. Both Landweber and sparse deconvolution methods reveal the occurrence of aftershocks as early as few tenths of a second after the mainshock. These events occur close to their mainshock, within one to two rupture lengths away. The aftershock rate derived from this enhanced catalog is consistent with Omoris law, with no flattening of the aftershock rate down to the shortest resolvable timescale ∼0.3 s. The early aftershock rate decay determined here matches seamlessly the decay at later times derived from the original earthquake catalog, yielding a continuous aftershock decay over timescales spanning nearly 8 orders of magnitude. Aftershocks of repeating microearthquakes may hence be governed by the same mechanisms from the earliest time resolved here, up to the end of the aftershock sequence. Our results suggest that these early aftershocks are triggered by relatively large stress perturbations, possibly induced by aseismic afterslip with very short characteristic time. Consistent with previous observations on bimaterial faults, the relative location of early aftershocks shows asymmetry along strike, persistent over long periods.