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Despite a moderate magnitude, M_w = 6.4, the 5 February 2016 Meinong, Taiwan, earthquake caused significant damage in Tainan City and the surrounding areas. Several seismograms display an impulsive S-wave velocity pulse with an amplitude of about 1 m s-1, which is similar to large S-wave pulses recorded for the past several larger damaging earthquakes, such as the 1995 Kobe, Japan, earthquake (M_w = 6.9) and the 1994 Northridge, California, earthquake (M_w = 6.7). The observed PGV in the Tainan area is about 10 times larger than the median PGV of M_w = 6.4 crustal earthquakes in Taiwan. We investigate the cause of the localized strong ground motions. The peak-to-peak ground-motion displacement at the basin sites near Tainan is about 35 times larger than that at a mountain site with a similar epicentral distance. At some frequency bands (0.9 - 1.1 Hz), the amplitude ratio is as large as 200. Using the focal mechanism of this earthquake, typical “soft” and “hard” crustal structures, and directivity inferred from the observed waveforms and the slip distribution, we show that the combined effect yields an amplitude ratio of 17 to 34. The larger amplitude ratios at higher frequency bands can be probably due to the effects of complex 3-D basin structures. The result indicates that even from a moderate event, if these effects simultaneously work together toward amplifying ground motions, the extremely large ground motions as observed in Tainan can occur. Such occurrences should be taken into consideration in hazard mitigation measures in the place with frequent moderate earthquakes.

International audience; Extreme rainfall events can trigger numerous landslides in mountainous areas and a prolonged increase of riversediment load. The resulting mass transfer at the Earth surface in turn induces stress changes at depth, whichcould be sufficient to trigger shallow earthquakes. The 2009 Morakot typhoon represents a good case study as itdelivered 3 m of precipitation in 3 days and caused some of the most intense erosion ever recorded. Analysis ofseismicity time-series before and after the Morakot typhoon reveals a systematic increase of shallow (i.e. 0-15 kmof depth) earthquake frequency in the vicinity of the areas displaying a high spatial density of landslides. Thisstep-like increase in frequency lasts for at least 2-3 years and does not follow an Omori-type aftershock sequence.Rather, it is associated to a step change of the Gutenberg-Richter b-value of the earthquake catalog. Both changesoccurred in mountainous areas of southwest Taiwan, where typhoon Morakot caused extensive landsliding. Thesespatial and temporal correlations strongly suggest a causal relationship between the Morakot-triggered landslidesand the increase of earthquake frequency and their associated b-value. We propose that the progressive removalof landslide materials from the steep mountain landscape by river sediment transport acts as an approximatelyconstant increase of the stress rate with respect to pre-typhoon conditions, and that this in turn causes a step-wiseincrease in earthquake frequency. To test this hypothesis, we investigate the response of a rate-and-state fault tostress changes using a 2-D continuum elasto-dynamic model. Consistent with the results of Ader et al. (2013), ourpreliminary results show a step-like increase of earthquake frequency in response to a step-like decrease of thefault normal stress. We also investigate the sensitivity of the amplitude and time-scale of the earthquake frequencyincrease to the amplitude of the normal stress change and to rheological parameters. Our study offers new insightson the potential influence of extreme erosional events on the short-time scale dynamics of faults and earthquakes

International audience; Extreme rainfall events can trigger numerous landslides in mountainous areas and a prolonged increase of river sediment load. The resulting mass transfer at the Earth surface in turn induces stress changes at depth, which could be sufficient to trigger shallow earthquakes. The 2009 Morakot typhoon represents a good case study as it delivered 3 m of precipitation in 3 days and caused some of the most intense erosion ever recorded. Analysis of seismicity time-series before and after the Morakot typhoon reveals a systematic increase of shallow (i.e. 0-15 km of depth) earthquake frequency in the vicinity of the areas displaying a high spatial density of landslides. This step-like increase in frequency lasts for at least 2-3 years and does not follow an Omori-type aftershock sequence. Rather, it is associated to a step change of the Gutenberg-Richter b-value of the earthquake catalog. Both changes occurred in mountainous areas of southwest Taiwan, where typhoon Morakot caused extensive landsliding. These spatial and temporal correlations strongly suggest a causal relationship between the Morakot-triggered landslides and the increase of earthquake frequency and their associated b-value. We propose that the progressive removal of landslide materials from the steep mountain landscape by river sediment transport acts as an approximately constant increase of the stress rate with respect to pre-typhoon conditions, and that this in turn causes a step-wise increase in earthquake frequency. To test this hypothesis, we investigate the response of a rate-and-state fault to stress changes using a 2-D continuum elasto-dynamic model. Consistent with the results of Ader et al. (2013), our preliminary results show a step-like increase of earthquake frequency in response to a step-like decrease of the fault normal stress. We also investigate the sensitivity of the amplitude and time-scale of the earthquake frequency increase to the amplitude of the normal stress change and to rheological parameters. Our study offers new insights on the potential influence of extreme erosional events on the short-time scale dynamics of faults and earthquakes.

Vertical ocean‐bottom seismograph (OBS) data at frequencies below 0.05 Hz are contaminated by noise induced by infragravity waves. We constructed the transfer function between pressure and velocity data from OBSs deployed in Taiwan waters to remove the wave pressure‐induced noise from seismic recordings. Data were analyzed from five portable broadband OBSs deployed each for 10 months at water depths from 1740 to 4600 m and from a cabled, shallow‐buried seismograph (EOS1) installed on the seafloor at 300 m depth. Removing long‐period noise from these OBS data improves the identification of teleseismic phases such as P, S, SS, Pdiff, and PKIKP that are otherwise ambiguous or unidentifiable. For EOS1, infragravity‐wave signals completely mask the P and S waveforms in the 10–50 s period band suitable for centroid moment tensor (CMT) solutions for most of the local events. Application of the transfer functions to predict and remove wave deformation yielded clean prominent P and S waveforms at these periods and aided in the CMT determination for small events jointly with land stations. The relative amplitudes of the wavenumber‐normalized transfer function for some of the OBSs are mostly determined by the thickness of the sediment at the OBS site.