Earthquake swarms, introduction to the records in Iran


Assistant Professor, Research Institute for Earth Sciences, Geological Survey of Iran, Tehran, Iran


*نگارنده رابط:           تلفن: 64592476-021      دورنگار: 66070511-021                  


Earthquakes usually occur not as separate individual events, but they usually constitute parts of an earthquake sequence with different but defined characteristics. Earthquake swarms include earthquake sequences, which start gradually and end gradually with no dominant earthquake in the sequence in terms of magnitude. Study of earthquake swarms of the world has started from about half a century ago. The first stages of these studies suggested that earthquake swarms are closely related to volcanic regions, and upwelling of pore fluids. It was also suggested that this kind of sequences occur in regions with strong strength gradients. Non-double-couple focal mechanisms are reported to be associated with earthquakes in geothermal regions and with earthquakes related to tensile faulting resulted from pore pressure in geothermal or volcanic areas. Some other various subordinate settings are also suggested for earthquake swarms. They include settings related to geothermal regions, thrusting, releasing stopovers, pull-apart basins and mud volcanoes. Some earthquake swarms are suggested to be triggered by a distant major earthquake in distant regions. Most of earthquake swarms are not accompanied by surface ruptures; however some exceptions have been reported from California in United States.
Due to their relatively small magnitudes, the earthquake swarms have been less considered by researchers in Iran, although some of them have caused damages and destructions. This research studies 24 events of earthquake swarms that have occurred during the historical and instrumental periods of seismic activity in Iran, and tries to investigate their general characteristics and geological settings. The earliest accounts of a historical earthquake swarm in Iran probably dates back to 1482 A.D., when a sequence of foreshocks jolted the western Makran region in southeastern Iran for about 3 months; the earthquakes culminated in a destructive earthquake of 1483 A.D. in Hormuz Straight. Two sequences of earthquakes in 1819 and 1856 A.D. caused destruction in the Tabriz region of northwestern Iran. One of the deadliest earthquakes in Zagros region (Qir-Karzin earthquake of 1972, Ms=6.9) was preceded by a sequence of small and moderate earthquakes which had started about one month earlier. Results of this research indicate that most of the earthquake swarms in Iran occur in Zagros (54%), then in southern central Alborz (17%) and Azarbayjan (17%) areas. The rest are observed within Central Iran, Makran and Eastern Iran. It is interesting that despite their seismotectonic activity, the Eastern Iran and Kopeh-Dagh regions are not associated with important earthquake swarms. This may be related to a rather continuous seismogenic layer as compared to other parts of the Iranian Plateau. Earthquake swarms in Iran are observed both independent and related to the larger main shocks. Duration of occurrence of the earthquakes depends on to the general size of their magnitudes, and varies between 2 days for earthquakes smaller than 2.3 to more than a year. The longest sequence in Iran occurred after the Western Marivan earthquake of 1946, which lasted for 398 days. Number of reported events within an earthquake swarm sequence varies between 6 and 236, and even reportedly 1200. Size (diameter) of the epicentral region in which earthquake swarms occur varies between 12 and 146 km. None of the studied events in Iran, even those close to the volcanic regions, show any clear relationships to volcanic activities or upwelling of pore fluids. The only exception may be the Arak earthquake swarm of 2012, where thermal spring in the region may suggest some relationships with upwelling of pore fluids. The b value for the events in the Arak area does not deviate very much from 1, despite the early suggestions by some researchers of values greater than one for earthquake swarms. Spatial distribution of the Arak sequence does not conform to any planar (tectonic) structure at depth. Depth distribution of the earthquake swarms in Iran indicates that all of them have occurred within the crust. We suggest detailed study of focal mechanisms for the events in earthquake swarms of Iran to discriminate events related to earthquake faults from those which may be related to fluid injection at depth. In some regions, such as Zagros, earthquake swarms may be related to the mechanical discontinuities within the crust. Statistical analysis of earthquake swarms in Iran indicates that the cumulative magnitude of earthquakes within a sequence does not exceed more than 0.5 unit over the largest event of the same sequence. Statistics on magnitude, number of events, spatial and temporal distribution characteristics of the earthquake swarms in this study provide a ground on which seismologists may investigate this type of seismic activity in more details, and discriminate between the earthquake swarms and mainshocks or foreshokes.