بررسی اثر فعالیت‌های پیش لرزه‌ای در سیگنال‌های vlf ثبت شده در گیرنده تهران

پدیده‌های الکترومغناطیسی ازجمله اختلال در سیگنال‌های رادیویی (vlf, very low frequency) برای پیش‌بینی کوتاه‌مدت زمین‌لرزه از چند سال پیش در دنیا مورداستفاده قرارگرفته‌اند. سیگنال‌های vlf از مرز پایینی یونوسفر بازتاب می‌یابند بنابراین هر تغییری در منطقه d یونوسفر شرایط بازتاب امواج vlf را تغییر می‌دهد. احتمالاً یکی از عواملی که پارامترهای مختلف یونوسفر را تحت‌تاثیر قرار می‌دهد، فرآیندهای اطراف کانون زمین‌لرزه قبل از وقوع آن است. تاکنون چند فرضیه برای مکانیسم نفوذ انرژی از کانون زمین‌لرزه‌های با عمق کمتر از 40 کیلومتر به یونوسفر به‌صورت تئوری پیشنهاد شده است. یکی از مناسب‌ترین فرضیه‌ها نقش امواج گرانشی جوی در این رابطه است با این حال مدارک مشاهده‌ای زیادی وجود ندارد. در این تحقیق تغییرات مشاهده شده در سیگنال‌های vlf دریافت شده در گیرنده موسسه ژئوفیزیک دانشگاه تهران (teh)، به‌عنوان پیش‌نشانگر قبل از وقوع رخداد لرزه‌ای مورد بررسی قرار گرفته است. در بازه زمانی و مسیر‌های مورد مطالعه تنها زمین‌لرزه نسبتا بزرگ، زمین‌لرزه 5.8 mn سیرچ کرمان در تاریخ 22/07/2018 می‌باشد. نتایج نشان می‌دهد که اختلالات واضحی چند روز قبل از وقوع زمین‌لرزه سیرچ کرمان مشاهده می‌شود که پس از بررسی عوامل مختلف موثر بر سیگنال می‌توان آن را به‌عنوان یک پیش‌نشانگر برای این زمین‌لرزه در نظر گرفت. همچنین تحلیل طیفی سیگنال‌های vlf فرستنده vtx3 در جنوب هند که در گیرنده تهران دریافت شده‌اند، بررسی شده است و نتایج آن نقش نوسانات گرانشی جوی را به‌عنوان مکانیسم اتصال لیتوسفر - یونوسفر به‌صورت مشاهده‌ای تایید می‌کند.

Investigation of preseismic activity effect on recorded VLF signals in Tehran receiver

Electromagnetic phenomena such as disturbance of VLF (very low frequency) radio signals, have been used for prediction of shortterm earthquakes in the world from some years ago. VLF signals are reflected from the lower bound of the ionosphere, so any change in the ionosphere D region leads to changes in the conditions of the VLF wave propagation. One of the factors that influences the various parameters of the ionosphere is the processes surrounding the earthquake center before it occurs. So far, several hypotheses have been proposed for the mechanism of energy penetration from the earthquake to the ionosphere in theory. One of the most appropriate hypotheses is the role of atmospheric gravitational waves in this regard. However, there are not many observational evidence. In this study, after a review of variations in VLF signals received at the receiver of Tehran University’s Institute of Geophysics (TEH), significant disturbances were observed several days prior to the onset of the Mn 5.8 Sirch earthquake that occurred on 22 July, 2018, in Kerman province, southeastern Iran. These abnormalities appeared as decreasing in the night range and increasing in the amplitude of the day and were only in the VTX3TEH pathway, so they did not have any relation to the disorder in the transmitter or receiver. The association of these abnormalities with the factors affecting the ionosphere, including Solar flares, Lightning, Meteorological and geomagnetic activities was also studied and risk associated with these factors were rejected. These abnormalities began about four days before the earthquake and continued until the day of the earthquake; therefore, they are probably related to the precursor effects of the Sirch Kerman earthquake. Spectral analysis of signals was also performed and a 20 minutes harmonic was found in the spectrum of days before the earthquake. This period is not observed in the quiet days. This result, in addition to denying the probability of a geomagnetic effect on the signal, shows the effect of atmospheric gravitational waves in the lithosphereatmosphericionospheric coupling mechanism for this earthquake. Based on these observations, the radio anomaly under study can be considered as a precursor of the Sirch earthquake in Kerman. The 22 July 2018 Mn 5.8 Sirch earthquake disturbances in Kerman provide another evidence of radio frequency disturbances at the VLF frequency before the earthquake. However, for more accurate monitoring of these signals, regular monitoring of longterm data as well as the number of more receivers in the country is required. In this case three important features, an earthquake preindicator, the time, location and magnitude of the earthquake in VLF/LF anomalies, occur from about a week to a maximum of 10 days before the earthquakes (Biagi, 2009) and are suitable for shortterm forecasting, but still the exact time of earthquake is not clear. The disturbances do not appear for earthquakes with magnitude less than 5.5 and for earthquakes between 56 disturbances are less severe than earthquakes larger than 6, so this method may estimate the magnitude of the earthquake. Also, using an appropriate network coverage of the VLF/LF receivers and the use of appropriate processing methods, it is possible to locate somewhat an earthquake. Finally, it can be said that this new field of science is considered as a promising candidate for shortterm earthquake prediction to reduce earthquake damage in active area such as Iran.