Determining the Slip Rate and Earthquake Recurrence Interval on the Tip of a Foreberg in the Gobi-Altai, Mongolia

––The Gobi-Altai, Mongolia, includes high mountain ranges that have accommodated the compressional stresses derived from the collision between the Eurasian and Indian Plates. The Gurvan Bogd, which is one of the main mountain ranges in the Gobi-Altai, is a restraining bend along the Bogd sinistral fault. Although surface ruptures did not form near the Artz Bogd during the Mw = 8.1 Gobi-Altai earthquake of 1957, it is still active, as evidenced by a growing topography (i.e., forebergs). Six foreberg ridges have formed in the foreland of the Artz Bogd, which are considered to be the result of surface deformation of alluvial fans due to thrusting. One stream has cut down to expose a foreberg tip, providing the opportunity to explore the slip evolution of the region. Here we map a growing fault structure related to blind thrusting. We identify five faulting events from an analysis of the outcrop and apply optically stimulated luminescence dating to the faulted sedimentary layers, yielding an average slip rate of 0.045 ± 0.007 m/kyr and an earthquake recurrence interval of 5.8 ± 0.5 kyr over the last ~32 kyr. Furthermore, the long-term (~600 kyr) uplift rate of the foreberg is 0.067 ± 0.007 m/kyr, as deduced by dividing the vertical displacement of the alluvial fan surface by the 10Be surface exposure ages of boulders on the fan. The discrepancy (20–30%) between these two deformation rates may be due to the different timescales they cover and an along-strike gradient in slip rate.

Paleo-earthquake Evidence and Earthquake Recurrence for Düzce Fault, Turkey

Paleoseismological trenching was performed along the Düzce fault providing some preliminary insight about its seismogenic behavior. Dating was based on radiocarbon analysis of peat samples collected from the trenches and suggested seven earthquakes have occurred since 1740 BC. Integrating date constraints of events exposed in the trenches suggests a periodical earthquake recurrence model. According to a linear sequential event serial that has minimum misfit determined by considering the probability curve limits of the sample dates, the earthquake recurrence interval is between 384 and 460 years (or possibly between AD 394 and 400). A probability curve was also calculated for the date of the last earthquake (1999 Düzce earthquake) considering the probability distributions of sample dates based on the same event serial. This probability-distribution-based method, similarly, predicted that the 1999 Düzce earthquake occurred between 1933–2005 (± 36 years) with a 68 % probability. After this verification. Using this method, it was estimated that the next earthquake along the Düzce fault has a 68 % probability of occurring between 2328–2392. According to this calculation, the earthquake recurrence interval is about 391 ± 34 years with a 68 % probability and the AD 967 historical earthquake likely ruptured the Düzce fault. Assuming an average slip of 350 cm (the average slip of the 1999 earthquake), the slip rate was estimated to be between 8.7–11.2 mm/a.

Paleoearthquake history of the Spili fault

The paleoearthquake activity on the Spili Fault is examined using a novel methodology that combines measurements of Rare Earth Elements (REE) and of in situ cosmogenic 36Cl on the exhumed fault scarp. Data show that the Spili Fault is active and has generated a minimum of five large-magnitude earthquakes over the last ~16500 years. The timing and, to a lesser degree, the slip-size of the identified paleoearthquakes was highly variable. Specifically, the two most recent events occurred between 100 and 900 years BP producing a cumulative displacement of 3.5 meters. The timing of the three older paleoearthquakes is constraint at 7300, 16300 and 16500 years BP with slip sizes of 2.5, 1.2 and 1.8 meters, respectively. The magnitude of the earthquakes that produced the measured co-seismic displacements, ranges from M 6.3-7.3 while the average earthquake recurrence interval on the Spili Fault is about 4200 years. The above data suggest that the Spili is among the most active faults on Crete and its earthquake parameters may be incorporated into the National Seismic Hazard Model.

Renewal of Tidal Forests in Washington State after a Subduction Earthquake in A.D. 1700

With few exceptions, today's tidal trees near Washington's Pacific coast postdate an earthquake that lowered the region by 1 m or more. The earthquake, which occurred in A.D. 1700, is the most recent to have ruptured much of the plate boundary at this central part of the Cascadia subduction zone. Because of the coseismic subsidence, lowland forests became tidal flats where thousands of trees died. Most of the trees killed were Sitka spruce (Picea sitchensis). In the centuries since the earthquake, tidal deposits have built new land that has been colonized by new Sitka spruce. All but several tens of the region's tidal spruce consequently postdate 1700, as shown by counts of annual rings in 121 of the largest spruce in tidal forests at Copalis River, Grays Harbor, and Willapa Bay. Forests began to return to each of these estuaries in the early 1700s and spread seaward in the late 1700s and 1800s. Annual rings in the oldest of the trees thus record a large fraction of the earthquake-recurrence interval that began with the 1700 earthquake.