If the financial markets were a horror show (and sometimes they are), investors know what would be behind that white hockey mask today. The killer stalking portfolios would be revealed as...rising interest rates.

So far this year, $16.3 billion has gone into fixed-income exchange-traded funds designed to have low sensitivity to interest rates, according to Bloomberg data. That's a 35 percent jump since the end of last year. And while articles like this one come out at least once a month, and there have been false alarms about rising rates, the ETF flows ––which many consider to be the smart money––show that investors are seriously creeped out.

The latest scare sending them into these (jargon alert) "short duration" ETFs is the nearly 40 percent jump in the 10-year Treasury yield to 2.14 percent. The move took place in just one month. Duration is a measure of how sensitive a bond is to an increase in interest rates. For example, if your bond ETF has a duration of 6 years, that means the price would go down six percent for a 1 percent increase in interest rates. Start doing the math on that and you can see why people are getting jittery.

Here is a look at the most popular ETFs in two areas where worried bond investors are trying to take shelter.

Investment Grade

The most popular bond ETF this year is the Vanguard Short-Term Bond ETF (BSV). It has seen more than $3 billion in new money flow in this year; the only ETFs that got more money specialize in Japanese equities. BSV has a duration of 2.7 years and a 12-month yield of 1.7 percent. Its portfolio of 1,500 investment-grade bonds breaks down to 74 percent government bonds and 26 percent corporate. It has a 0.10 percent annual expense ratio, or $10 in fees per $10,000 invested.

A short-duration corporate bond ETF that's attracting a lot of money is the iShares Barclays 1-3 Year Credit Bond Fund (CSJ). It holds 822 investment-grade corporate bonds and has a duration of 1.8 years and a yield of 1.4 percent. CSJ has seen inflows this year of just under $1 billion. That's in sharp contrast to the iShares iBoxx $ Investment Grade Corporate Bond Fund (LQD). That fund, which has a far longer duration of 7.6 years, has lost $1.9 billion in assets so far this year.

High Yield

The SPDR Barclays Short-Term High Yield ETF (SJNK) is a short-duration kid brother to the SPDR Barclays High Yield Bond ETF (JNK). The portfolio of $10 billion JNK, the largest high-yield bond ETF in the world, would lose 4.3 percent in value for a 1 percent rise in interest rates (a duration of 4.3 years). SJNK's portfolio has a duration of two years and a yield of 6.12 percent -- just below JNK’s 6.4 percent yield.The similar yields are one reason JNK has seen $1.7 billion in outflows this year.

SJNK's low duration explains why its total return has been better recently compared to JNK's -- 3 percent this year, versus 2.3 percent for JNK. In the past month, which has been tough for junk bonds, SJNK is down 0.98 percent, while JNK is down 2.41 percent. Of course, while these ETFs ratchet down interest rate risk, they still have the credit risk that comes with junk bond investing.

Also attracting the attention of investors fearful of rising rates is the PowerShares Senior Loan ETF (BKLN), which has seen 2.8 billion in new assets flow in this year. Senior loans are basically high-yield bonds with floating rates, which removes interest rate risk altogether. BKLN is yielding 4.6 percent and has returned 9 percent in the past year.