Lithospheric Deformation at the Northern Longitudinal Valley, in Light of Focal Mechanism Solutions of the 1990 and 2012 Earthquake Sequences

The Longitudinal Valley (LV) is the suture zone between the Eurasian plate (EUP) and the Philippine Sea plate (PSP). The northern tip of the LV (near Hualien city) is the junction point where the collision evolves northward to a subduction of the PSP under the EUP. As a result, a high seismic activity is observed along the LV. In the northern tip of the LV, we identified four distinct seismic clusters since 1990 based on the Central Weather Bureau (CWB). We restrict our effort on two of them that distributed on most of the northern part of the LV. The first seismic crisis is triggered by a doublet of events (ML 6.5 and 6.7) on 13rd December, 1990, and the second crisis occurs in 2012 with a main shock of ML 5.3 on 14th June. The 2012 seismic crisis may be re-activated at the southern segment of the 1990 earthquake sequence. For the seismic crisis in 1990, a campaign seismic network of 15 accelerometers – the Hualien Temporary Seismic Network (HTSN) is deployed 3 days after the first main shock during 2 months to detect the aftershocks. For the seismic crisis in 2012, the data set is recorded by Free Field Strong Earthquake Observation Network from Geophysical Database Management System, CWB. We retrieved focal mechanism solutions (FMS) of 50 aftershocks in 1990 and 37 earthquakes in 2012 with local magnitude ranging from 2.5 to 5.0 by waveform inversion using the near-field component of seismic waveforms. A modified version of the Program FMNEAR is adopted in this study. In this method, a double grid-search is applied upon determining FMS parameters while rake value is lately tuned by a simulated annealing algorithm. Waveform adjustments are improved by depth optimization and a specific 1D velocity model for each station. A modified version of the FMNEAR has been proven efficiently to retrieve FMS for the small-to-moderate earthquakes with a limited number of stations. For both of the earthquake sequences, focal depths are in average 10 km deeper than the depth provided by the CWB catalog. Both FMS from 1990 and 2012 clusters can be classified into three groups according to their mechanisms and P- and T-axis. The main groups (with a higher number of events) display homogeneous FMS, mostly reverse in type that we believe illustrate the behavior of the main-fault that generate both the doublet of 1990 and the 2012 events. FMS of these main group displays a similar strike as the valley and one of the nodal planes dip eastward as the plate boundary between the EUP and the PSP. For the other groups, show variety amongst FMS, are the fault branches of the main-fault. We conclude that the Longitudinal Valley Fault, the effective plate boundary, displays a variety FMS came from additional fault systems from North to South despite its segmentation and variation of activity.