Automated Detection of Long-Period Disturbances in Seismic Records; MouseTrap Code

Abstract

Sudden disturbances in strong‐motion acceleration records, referred to as baseline offsets, are well known (Boore et al. , 2002; Javelaud et al. , 2011). The physical nature of this phenomenon remains unclear, although some cases are well explained by a permanent ground tilt (Graizer, 2010; Javelaud et al. , 2012). The ground tilt may be caused by near‐field source effects or a local tilt at a station produced by seismic vibrations in highly heterogeneous substrata (i.e., strong gradients of material properties at small scales). Tilts are frequently observed in volcanic areas (Wielandt and Forbriger, 1999; Wiens et al. , 2005). Delorey et al. (2008) performed an experiment showing that tilting of the instrument produces a long‐period disturbance of the characteristic shape. In these cases, the disturbances predominate on horizontal components. The disturbances can also have a purely instrumental origin (Iwan et al. , 1985; Shakal and Petersen, 2001; Boore, 2003). The latter is a more suitable explanation in cases where the disturbances are also strong on the vertical component. Similar artifacts have also been reported on different types of broadband seismometers worldwide (Zahradnik and Plesinger, 2005; Pillet and Virieux, 2007; Delorey et al. , 2008). A strong disturbance is frequently characterized by a one‐sided pulse in raw output velocity. A weaker disturbance is often masked by high‐frequency content of the velocity record. However, it can be easily visible in the integrated output of a broadband instrument (raw displacement) in which it looks like a baseline step, the duration of which is equal to the seismometer corner period (Fig. 1a). Figure 1. (a) Example of a steplike disturbance in the integrated output (raw displacement) of the Nanometrics Trillium T40 seismometer. The disturbance is well fitted by the simulated instrument response to an acceleration step of amplitude A =8.7×10−7 m·s−2, azimuth …