Kou-Cheng Chen

We collected a data set of mainshocks and their respective largest foreshocks of 38 earthquake sequences in Taiwan. The plot of local magnitude, M_L, of a mainshock (denoted by M_Lm) versus M_L of its largest foreshock (denoted by M_Lf) shows an increase in M_Lm with M_Lf. This indicates that for Taiwan’s earthquakes the bigger the largest foreshock is, the larger the mainshock is. The plot of the epicentral distance, Δ (in km), from the largest foreshock to the mainshock versus M_L of the mainshock exhibits a weak increase in Δ with M_Lm as Δ1 day.

An earthquake sequence that occurred in the Hualien area during April 7 to August 30, 2021 has been recognized as two swarms by the Central Weather Bureau. Its earthquakes with M_L ≤ 6.2 (M_L = local magnitude) and focal depths ≤ 25 km were located in an area from 23°46’ N to 24°04’ N and from 121°25’ E to 121°42’ E. The Morlet wavelet technique is applied to analyze the dominant periods of temporal variations in numbers of daily events for the earthquake sequence in two magnitude ranges, i.e., M_L ≥ 3 and M_L ≥ 4. Results show that the dominant periods are 30.8 and 38.0 days when M_L ≥ 3; while the dominant period does not exist when M_L ≥ 4. The fluctuation analysis technique in the natural time domain is used to study the memory effect in the swarm for two magnitude ranges, i.e., M_L ≥ 3 and M_L ≥ 4. Calculated results show that the memory effect is stronger for the time sequence of magnitudes than for that of inter-event times and higher for M_L ≥ 3 earthquakes than for M_L ≥ 4 events. Consequently, only the short-term corrected memory effect was operative in the earthquake sequence of the Hualien swarms.

The strong‐motion downhole array (SMDA) in Taipei basin is examined, and the data quality, glitches, and systemic errors in its data are discussed. This seismic network is an array of arrays: the SMDA comprises a total of 32 triggered strong‐motion acceleration seismometers spanning eight sites. Each site has one seismometer at the surface and an additional two to four seismometers each collocated at the individual boreholes. Polarity reversals, swapped components, clock desynchronization, and bad components (flat‐line signal or aseismic noise) have all been observed and are shown to be occasional issues. Signal‐to‐noise ratios are generally excellent. The lack of known orientations at depth is the primary issue regarding data quality of the SMDA. Orientations of each borehole seismometer were recorded at the time of installation, but those initial values are not reliable for subsequent events. Furthermore, redetermined azimuthal orientations are inconsistent from event to event, suggesting that the borehole seismometer orientations change over time. Orientation wander is not associated with instrument maintenance. An iterative method to reliably determine borehole seismometer orientations is introduced. Data from each borehole seismometer are rotated and compared with that from a collocated reference station on the surface with known orientation until a maximum correlation is reached. This method is reliable for most events, but may become unreliable for local events in which the incidence of incoming seismic waves is near vertical and shorter wavelengths introduce complicated wave propagation within Taipei basin. Temporal analysis of borehole seismometer orientations shows not only drifting of azimuthal orientations over time, but also erratic changes in orientation in multiples of 90°. This behavior is suggestive of frequent polarity reversals and/or swapped components on one or both of the two horizontal components of each borehole seismometer.

The March 31, 2002, Hualien offshore earthquake (M_L6.8, Mw7.0) caused minor damage near its epicenter but significant damage in Taipei, about 110 km from the epicenter. The recorded PGA values around the Taipei basin vary in a factor of 5. The high‐PGA values in the southeastern part of the basin correspond to the most significant damage area during the earthquake. Strong ground‐motion recordings from this event recorded at stations around the Taipei basin exhibit several distinct large‐amplitude seismic arrivals that can be interpreted as the PmP, SmP and SmS phases. The largest amplitude of the critically reflected S‐wave from the Moho (SmS) and the amplification of soft shallow sediments unique for each site are responsible for the maximum peak ground motion and the significant damage in the Taipei basin during the Hualien offshore earthquake.