Bogdan Enescu

Some premonitory phenomena of the 1995 Hyogo-Ken Nanbu (Kobe) earthquake: seismicity, b-value and fractal dimension

Abstract This paper presents a detailed and complex analyses of the spatio-temporal evolution of seismic activity between 1976 and 1998, in a broad area surrounding the epicentre of the 1995 Hyogo-ken Nanbu (Kobe) earthquake as well as in the source area. Various precursory changes in seismicity, such as quiescence followed by increased seismic activity, b-value and fractal dimension (correlation dimension, D2) anomalies appear approximately two years before the occurrence of the major event. An increased level of background seismicity is clearly observed after the occurrence of the Kobe earthquake, particularly in the northeast extension of the source area. All these ‘anomalous’ changes in seismic activity before and after the occurrence of the 1995 Hyogo-ken Nanbu earthquake appear in a relatively large region surrounding the epicentre of the main shock as well as in the source area.

Aftershock modeling based on uncertain stress calculations

[1] We discuss the impact of uncertainties in computed coseismic stress perturbations on the seismicity rate changes forecasted through a rate- and state-dependent frictional model. We aim to understand how the variability of Coulomb stress changes affects the correlation between predicted and observed changes in the rate of earthquake production. We use the aftershock activity following the 1992 M7.3 Landers (California) earthquake as a case study. To accomplish these tasks, we first analyze the variability of stress changes resulting from the use of different published slip distributions. We find that the standard deviation of the uncertainty is of the same size as the absolute stress change and that their ratio, the coefficient of variation (CV), is approximately constant in space. This uncertainty has a strong impact on the forecasted aftershock activity if a rate-and-state frictional model is considered. We use the early aftershocks to invert for friction parameters and the coefficient of variation by means of the maximum likelihood method. We show that, when the uncertainties are properly taken into account, the inversion yields stable results, which fit the spatiotemporal aftershock sequence. The analysis of the 1992 Landers sequence demonstrates that accounting for realistic uncertainties in stress changes strongly improves the correlation between modeled and observed seismicity rate changes. For this sequence, we measure a friction parameter Asn � 0.017 MPa and a coefficient of stress variation CV = 0.95.

Omori-Utsu Law c-Values Associated with Recent Moderate Earthquakes in Japan

We investigate the early aftershock activity associated with four moder- ate earthquakes (Mw 6.6-6.7) that occurred recently in Japan. For each aftershock sequence, we examine continuous high-pass filtered seismograms recorded at seismic stations nearby the main fault to identify as many early events as possible. The mag- nitude of these events is calibrated using aftershocks that are listed in the earthquake catalog of Japan Meteorological Agency (JMA). The analysis of the aftershock decay rates reveals a power-law time dependence with a scaling exponent close to 1.0 that starts from about one minute from the mainshock. Our results demonstrate that the c-value of the Omori-Utsu law is very small, although a lower bound is not estab- lished due to completeness problems in the first minute after the mainshock and sta- tistical fluctuations.

Recurrent slow slip event likely hastened by the 2011 Tohoku earthquake

Slow slip events (SSEs) are another mode of fault deformation than the fast faulting of regular earthquakes. Such transient episodes have been observed at plate boundaries in a number of subduction zones around the globe. The SSEs near the Boso Peninsula, central Japan, are among the most documented SSEs, with the longest repeating history, of almost 30 y, and have a recurrence interval of 5 to 7 y. A remarkable characteristic of the slow slip episodes is the accompanying earthquake swarm activity. Our stable, long-term seismic observations enable us to detect SSEs using the recorded earthquake catalog, by considering an earthquake swarm as a proxy for a slow slip episode. Six recurrent episodes are identified in this way since 1982. The average duration of the SSE interoccurrence interval is 68 mo; however, there are significant fluctuations from this mean. While a regular cycle can be explained using a simple physical model, the mechanisms that are responsible for the observed fluctuations are poorly known. Here we show that the latest SSE in the Boso Peninsula was likely hastened by the stress transfer from the March 11, 2011 great Tohoku earthquake. Moreover, a similar mechanism accounts for the delay of an SSE in 1990 by a nearby earthquake. The low stress buildups and drops during the SSE cycle can explain the strong sensitivity of these SSEs to stress transfer from external sources.

Rupture process of the 2014 Iquique Chile Earthquake in relation with the foreshock activity

The rupture process of the 2014 Iquique, Chile earthquake is inverted from teleseismic P wave data applying a novel formulation that takes into account the uncertainty of Greens function, which has been a major error source in waveform inversion. The estimated seismic moment is 1.5 × 1021 Nm (Mw = 8.1), associated with a 140 km long and 140 km wide fault rupture along the plate interface. The source process is characterized by unilateral rupture propagation. During the first 20 s, the dynamic rupture front propagated from the hypocenter to the large asperity located about 50 km southward, crossing a remarkably active foreshock area at high velocity (of about 3.0 km/s), but small and irregular seismic moment release rate. Our result may suggest that the 20 s long initial phase was influenced by the stress drop due to the foreshock activity near the main shock hypocenter. Moreover, the 2 week long swarm-like foreshock activity migrating roughly at 5 km/day toward the main shock hypocenter, and possibly associated slow slip, contributed to the stress accumulation prior to the Mw 8.1 megaquake. The main shock initial rupture phase might have triggered the rupture of the large asperity, which had large fracture energy.

Spatial analysis of the frequency-magnitude distribution and decay rate of aftershock activity of the 2000 Western Tottori earthquake

The b-value of the frequency-magnitude distribution and the parameters in the modified Omori law, describing the decay rate of aftershock activity, are investigated for more than 4000 aftershocks identified in the first four months after the Western Tottori earthquake (October 6, 2000). We used the JMA data catalog, containing aftershocks with magnitude larger than or equal to 2.0. The studied area is first divided into three areas: one region (A) corresponding to the main aftershock area and other two (B and C) corresponding to seismic activity probably triggered by the stress change caused by the main shock. For region A, the magnitude of completeness (Mc) decreases with time, from the largest value of 3.2 in the first two hours of the sequence, to 2.0, about four days after the main shock. Taking the threshold magnitude as 3.2, we estimated the b-value for the whole region A to be about 1.3 and p-value around 1. However, highly significant variations in both b and p values are found when analyzing their spatial distribution in region A. The seismic activity in the regions B and C started about 2.5 days after the main shock. The b-value for region B (Mc = 2) is 1.05. The decay rate of earthquake activity in Region B is well modeled by the modified Omori law and the p-value is found to be relatively low (0.83). The number of events in region C is too small for a meaningful study. The physical interpretation of the spatial variation of the parameters is not straight forward. However, the variation of b-value can be related to the stress distribution after the main shock, as well as the history of previous ruptures. Thus, the relatively low stress in the regions that have already experienced rupture is probably responsible for the larger value of b found in these areas. Regions with relatively low b-value, on the other hand, are probably regions under higher applied shear stress after the main shock. Alternatively, one can hypothesize that the areas that experienced slip are more fractured, favoring higher b-values. The larger p-values correlate well with the regions that experienced larger slip during the main shock, while small p-values are found generally in regions that have not ruptured recently. The variation of p-value can be related with the frictional heating produced during rupture. The crustal structure may explain some local features of b and p value spatial distribution. In order to verify our hypothesis we also analyzed the seismic activity that occurred before the Tottori earthquake, starting in 1978, using the data of DPRI, Kyoto University. It seems that the previous seismic activity associated with some moderate events in 1989, 1990 and 1997 had an influence on the following seismicity in the area—in particular on the spatial distribution of b and p values observed for the aftershocks of the Tottori earthquake. The aftershocks of the 1997 M5.5 earthquake have a larger p-value than previous aftershock sequences, while the b-value has a clear increase following the M5.5 event.

Impact of Earthquake Rupture Extensions on Parameter Estimations of Point-Process Models

Online Material: Sensitivity to scaling of the d-parameter and to inhomogeneous background activity, and possible correlation between the largest magnitude and es- timated α-value. Abstract Stochastic point processes are widely applied to model spatiotemporal earthquake occurrence. In particular, the epidemic type aftershock sequence (ETAS) model has been shown to successfully reproduce the short-term clustering of earth- quakes. An important parameter of the model is the α-value describing the scaling of the aftershock productivity with magnitude of the triggering earthquake according to 10 αM . Fitting of the space-dependent ETAS model to empirical data yields α-values that are typically much smaller than the scaling inverted from more simple stacking of aftershock sequences. We show by means of synthetic simulations that this is likely to result from assuming spatial isotropy of aftershock occurrence that in fact aligns along the mainshock rupture. We fit the space-dependent and space-independent ETAS mod- els to simulations where each earthquake is a line source with an empirical magnitude- length relation. Although the space-time model describes past activity quite well, it overestimates the forecasted earthquake rate. On the other hand, the application of the space-independent ETAS model predicts future seismicity well and can therefore be applied for forecasting purposes. Our test for the observed aftershock sequence fol- lowing the 1992 M 7.3 Landers earthquake supports these results.

Correlations of Seismicity Patterns in Southern California with Surface Heat Flow Data

We investigate the relations between properties of seismicity patterns in Southern California and the surface heat flow using a relocated earthquake catalog. We first search for earthquake sequences that are well separated in time and space from other seismicity and then determine the epidemic type aftershock sequence (ETAS) model parameters for the sequences with a sufficient number of events. We focus on the productivity parameter α of the ETAS model that quantifies the relative efficiency of an earthquake with magnitude M to produce aftershocks. By stacking sequences with relatively small and relatively large α values separately, we observed clear differences between the two groups. Sequences with a smaller α have a relatively large number of foreshocks and relatively small number of after- shocks. In contrast, more typical sequences with larger α have relatively few fore- shocks and larger number of aftershocks. The stacked premainshock activity for the more typical latter sequences has a clear increase in the day before the occurrence of the main event. The spatial distribution of the α values correlates well with the surface heat flow: areas of high heat flow are characterized by relatively small α, indicating that in such regions the swarm-type earthquake activity is more common. Our results are compatible with a damage rheology model that predicts swarm-type seismic activity in areas with relatively high heat flow and more typical foreshock- mainshock-aftershock sequences in regions with normal or low surface heat flow. The high variability of α in regions with either high or low heat flow values indicates that at local scales additional factors (e.g., fluid content and rock type) may influence the seismicity generation process.

Aftershock distribution of the 2004 Mid Niigata Prefecture Earthquake derived from a combined analysis of temporary online observations and permanent observations

The 2004 Mid Niigata Prefecture Earthquake (Mj = 6.8) occurred on 23 October 2004 in the northeastern part of the Niigata-Kobe Tectonic Zone where large contraction rates were observed. The mainshock was followed by an anomalously intense aftershock activity that included nine Mj ≥5.5 aftershocks. We deployed three temporary online seismic stations in the aftershock area from 27 October, combined data from the temporary stations with those from permanent stations located around the aftershock area, and determined the hypocenters of the mainshock and aftershocks with a joint hypocenter determination (JHD) technique. The resulting aftershock distribution showed that major events such as the mainshock, the largest aftershock (Mj = 6.5), the aftershock on 27 October (Mj = 6.1), etc. occurred on different fault planes that were located nearly parallel or perpendicular to each other. This might be due to heterogeneous structure in the source region. The strain energy was considered to have been enough accumulated on the individual fault planes. These features are probably a cause of the anomalous intensity of the aftershock activity.

Fluid-driven seismicity activation in northern Nagano region after the 2011 M9.0 Tohoku-oki earthquake

The dynamic triggering of earthquakes is well documented; however, the underlying physical mechanisms remain obscure. Here we analyze the seismicity in northern Nagano, central Japan, following the Tohoku-oki quake, until the occurrence 13 h later of an Mw6.2 local earthquake. We use waveform detection techniques to identify 17 times more earthquakes than those in the Japan Meteorological Agency catalog. The activation of seismicity in the epicentral region of the Mw6.2 event is weak and delayed, culminating with the occurrence of the moderate shock preceded by two small foreshocks. The seismicity activation to the south is shallower, abundant, and starts during the passage of Tohoku-oki surface waves of high dynamic stresses. The early activation occurs in areas of relatively high near-surface fluid temperature, indicating that the dynamic triggering is likely caused by excitation of geothermal crustal fluids. The Mw6.2 Northern Nagano earthquake might have been delay-triggered by fluid migration from a deep source.