Quaternary Deposits Affect Coseismic Offset on Thrust Faults: Evidence from the 2008 Mw 7.9 Wenchuan, China, Earthquake

Coseismic offset is an important parameter to determine the characteristics of surface ruptures produced by large earthquakes and has significant implications for understanding fault-zone mechanics. To date, most studies have focused on broad-wavelength variations in coseismic offset and their related mechanisms. However, high-frequency variations in coseismic offset have been less commonly reported due to difficulty in field identification. Here, we show that three sites have typical abrupt changes in coseismic offset within short distances along surface ruptures produced by the 2008 Mw 7.9 Wenchuan earthquake. The Bailu, Qingping, and Xiaoyudong sites on different segments of the seismogenic faults show that coseismic vertical offsets can vary from ∼0.6 to ∼1.7 m at neighboring locations. Moreover, the offset gradients at the three sites are estimated from ∼2.5 to ∼30.9 m/km. Based on geologic and geophysical investigations at the three sites, we suggest that Quaternary deposits are the primary factor affecting coseismic offset. Specifically, thick and loosely packed deposits are more likely to yield smaller coseismic offsets than thin and densely packed deposits. Finally, through a compilation of recent thrust-type earthquakes, we suggest that the coseismic vertical offset gradient for thrust faults can vary greatly, which requires caution in seismic hazard assessments when designing linear infrastructure projects and constraining slip rates at specific sites.

Coseismic surface rupture during the 2018 Mw 7.5 Palu earthquake, Sulawesi Island, Indonesia

Sulawesi Island is located at the triple junction between the converging Australian, Sunda, and Philippine plates. The magnitude (Mw) 7.5 Palu earthquake occurred on 28 September 2018 on Sulawesi Island and caused serious casualties. The causative fault of the Palu earthquake was the left-lateral, strike-slip Palu-Koro fault, which has a rapid slip rate. We experienced this earthquake in Palu City and conducted field investigations on coseismic surface ruptures 1 d after the earthquake. Field surveys revealed that the coseismic surface ruptures were characterized by left-lateral offset, en echelon tensional cracks, mole tracks within a narrow zone, and large areas of sand liquefaction that increased the damage and losses. We measured the coseismic displacements along surface ruptures and observed a maximum coseismic offset of ∼6.2 m. The rupture traces in the north Palu Basin near Palu City mark the previously unmapped Palu-Koro fault. Based on the field investigations, we determined the exact location of the Palu-Koro fault within the Palu Basin and found that the Palu-Koro fault zone can be divided into three branches: F1, F2, and F3, forming a typical flower structure.

Supplemental Material: Coseismic surface rupture during the 2018 Mw 7.5 Palu earthquake, Sulawesi Island, Indonesia

Table S1: Main coseismic offset locations of the Palu rupture zone; Table S2: Field investigation locations; Figure S1: UAV images and surface coseismic left-lateral offset distribution; Figure S2: Mirrored trench and interpretation; Figure S3: Relations between the surface rupture trace and the architecture of the Palu-Koro fault.

Supplemental Material: Coseismic surface rupture during the 2018 Mw 7.5 Palu earthquake, Sulawesi Island, Indonesia

Table S1: Main coseismic offset locations of the Palu rupture zone; Table S2: Field investigation locations; Figure S1: UAV images and surface coseismic left-lateral offset distribution; Figure S2: Mirrored trench and interpretation; Figure S3: Relations between the surface rupture trace and the architecture of the Palu-Koro fault.

Estimation of successive coseismic vertical offsets using coeval sedimentary events – application to the southwestern limit of the Sea of Marmara's Central Basin (North Anatolian Fault)

In the deep part of the Sea of Marmara (Turkey), the sedimentation developing upon the North Anatolian Fault is strongly influenced by the associated seismic activity, through gravity reworking (fluidized landslides) and tsunamis. Specific layers (homogenites + turbidites, HmTu), representing individual sedimentary events, have been characterized along three giant piston cores retrieved from the Çinarcik and Central (or Orta) basins. Pre-Holocene, nonmarine sediments, were analyzed, representing the last 12–17 kyr BP (before present). For a 2 kyr long interval, 11 events could be precisely correlated on both sides of the Central Basin's southwestern scarp. For each of them, based on the specific depositional process, the thickness difference between the two sites was considered as a direct estimation of the vertical component of a coeval coseismic offset. The homogenite (upper) component accounts for the major part of the thickness difference (ranging from 36 to 144 cm). These offsets were considered as likely representing dominantly vertical throws, along the transtensional southwestern boundary of the inner, pull-apart Central Basin. In terms of natural hazards, further investigations on this local behavior should rather be directed to tsunami genesis.