2006 Yogyakarta earthquake
The 2006 Yogyakarta earthquake occurred at with a moment magnitude of 6.4 and a maximum MSK intensity of VIII in the Yogyakarta region of Java, Indonesia.
Several factors led to a disproportionate amount of damage and number of casualties for the size of the shock, with more than 5,700 dead, tens of thousands injured, and financial losses of Rp 29.1 trillion. With limited effects to public infrastructure and lifelines, housing and private businesses bore the majority of damage, and the United States' National Geophysical Data Center classified the total damage from the event as extreme.
Although Indonesia experiences very large thrust earthquakes offshore at the Sunda Trench, this was a large strike-slip event that occurred on the southern coast of Java near the city of Yogyakarta. Mount Merapi lies nearby, and during its many previous historical eruptions, large volume lahars and volcanic debris flowed down its slopes where settlements were later built. This unconsolidated material from the stratovolcano amplified the intensity of the shaking and created the conditions for soil liquefaction to occur. Inadequate construction techniques and poor quality materials contributed to major failures with unreinforced masonry buildings, though other styles fared better.
Tectonic setting
The islands of Indonesia constitute an island arc that is one of the world's most seismically active regions, with high velocity plate movement at the Sunda Trench, and considerable threats from earthquakes, volcanic eruptions, and tsunami throughout. Java, one of the five largest in the Indonesian archipelago, lies on the Sunda Shelf to the north of the Sunda Trench, which is a convergent plate boundary where the Indo-Australian plate is being subducted under the Eurasian plate. The subduction zone offshore Java is characterized by a northward dipping Benioff zone, frequent earthquakes and volcanic activity that influence the regional geography, and direct or indirect stress transfer that has affected the various onshore faults. Sedimentation is closely related to tectonics, and while the volume of offshore sediment at the trench decreases with distance from the Ganges-Brahmaputra Delta at the Bay of Bengal, the onshore accrual of sediments near the Special Region of Yogyakarta has been shaped by tectonic events.Earthquake
According to the United States Geological Survey, the shock occurred south-southeast of Yogyakarta at a depth of, but other institutions provided source parameters that were not in agreement. No information was present on the extent of the faulting or the direction of propagation and there was no link to the eruption of Mount Merapi. The USGS suggested that the focal mechanism was most likely associated with left-lateral slip on a NE trending strike-slip fault, as that is the orientation of the Opak Fault, but this has not been validated. No surface breaks were documented, but the location of the greatest damage that was caused does align with the Opak Fault as a possible source.A group of Japanese and Indonesian scientists visited the area in March 2007 and confirmed the lack of surface ruptures, and pointed out that any visible expression of the fault would likely have been rapidly destroyed due to the tropical climate, and have acknowledged the widely varying locations that were reported by the various seismological institutions. Their investigation resulted in a different scenario, with an unknown or newly formed NE trending fault as the origin of the shock. Evidence for one of the proposed faults was found in the form of alignment of portions of the Oyo River near the USGS' epicenter, which is parallel to the Nglipar fault in the Gunungkidul region. If the shock occurred in this area it could indicate the reactivation of a major fault system. The second proposed fault further to the east is nearly parallel to the Ngalang and Kembang faults that lie to the north of the Oyo River.
InSAR analysis
While the densely populated area that saw significant destruction is adjacent to the Opak River Fault, both the USGS and Harvard University placed the epicenter to the east of that fault. Few seismometers were operating in the region, but a group of temporary units that were set up following the mainshock recorded a number of aftershocks that were east of the Opak River Fault and were aligned along a zone striking N°50E. Due to the ambiguous nature of the available information on the source of the Yogyakarta earthquake, a separate group of Japanese and Indonesian scientists applied one of the first uses of interferometric synthetic aperture radar to determine the source fault. Several data sets were collected from an instrument on board the Advanced Land Observation Satellite and were compared to each other to show potential ground deformation patterns.A lack of any dislocation found on the images along the Opak River fault made evident the lack of movement along that fault, and though the aftershocks were occurring at a depth of, the deformation was distinct at the surface. The observed ground deformation that was detailed by the differential satellite images and Global Positioning System measurements was roughly east of the Opak River Fault, along a zone that passed through the USGS' epicenter, and delineated a NE trending vertical fault. The displacements were not more than and indicated left-lateral strike-slip motion as well as a component of reverse slip, and to the west of the Opak River Fault strong ground motion triggered subsidence of volcanic deposits from Mount Merapi.